Developmental origin of the rat adenohypophysis prior to the formation of Rathke's pouch

Neural plate progenitors give rise to both anterior and posterior pituitary cells

The pituitary is the master neuroendocrine gland, which regulates body homeostasis. It consists of the anterior pituitary/adenohypophysis harboring hormones producing cells and the posterior pituitary/neurohypophysis, which relays the passage of hormones from the brain to the periphery. It is accepted that the adenohypophysis originates from the oral ectoderm (Rathke's pouch), whereas the neural ectoderm contributes to the neurohypophysis. Single-cell transcriptomics of the zebrafish pituitary showed that cyp26b1-positive astroglial pituicytes of the neurohypophysis and prop1-positive adenohypophyseal progenitors expressed common markers implying lineage relatedness. Genetic tracing identifies that, in contrast to the prevailing dogma, neural plate precursors of zebrafish (her4.3+) and mouse (Sox1+) contribute to both neurohypophyseal and a subset of adenohypophyseal cells. Pituicyte-derived retinoic-acid-degrading enzyme Cyp26b1 fine-tunes differentiation of prop1+ progenitors into hormone-producing cells. These results challenge the notion that adenohypophyseal cells are exclusively derived from non-neural ectoderm and demonstrate that crosstalk between neuro- and adeno-hypophyseal cells affects differentiation of pituitary cells.

Missing pieces of the pituitary puzzle: participation of extra-adenohypophyseal placode-lineage cells in the adult pituitary gland

The pituitary gland is a major endocrine tissue composing of two distinct entities, the adenohypophysis (anterior pituitary, cranial placode origin) and the neurohypophysis (posterior pituitary, neural ectoderm origin), and plays important roles in maintaining vital homeostasis. This tissue is maintained by a slow, consistent cell-renewal system of adult stem/progenitor cells. Recent accumulating evidence shows that neural crest-, head mesenchyme-, and endoderm lineage cells invade during pituitary development and contribute to the maintenance of the adult pituitary gland. Based on these novel observations, this article discusses whether these lineage cells are involved in pituitary organogenesis, maintenance, regeneration, dysplasia, or tumors.

The Neurod1/4-Ntrk3-Src pathway regulates gonadotrope cell adhesion and motility

Pituitary gonadotrope cells are essential for the endocrine regulation of reproduction in vertebrates. These cells emerge early during embryogenesis, colonize the pituitary glands and organize in tridimensional networks, which are believed to be crucial to ensure proper regulation of fertility. However, the molecular mechanisms regulating the organization of gonadotrope cell population during embryogenesis remain poorly understood. In this work, we characterized the target genes of NEUROD1 and NEUROD4 transcription factors in the immature gonadotrope αT3-1 cell model by in silico functional genomic analyses. We demonstrated that NEUROD1/4 regulate genes belonging to the focal adhesion pathway. Using CRISPR/Cas9 knock-out approaches, we established a double NEUROD1/4 knock-out αT3-1 cell model and demonstrated that NEUROD1/4 regulate cell adhesion and cell motility. We then characterized, by immuno-fluorescence, focal adhesion number and signaling in the context of NEUROD1/4 insufficiency. We demonstrated that NEUROD1/4 knock-out leads to an increase in the number of focal adhesions associated with signaling abnormalities implicating the c-Src kinase. We further showed that the neurotrophin tyrosine kinase receptor 3 NTRK3, a target of NEUROD1/4, interacts physically with c-Src. Furthermore, using motility rescue experiments and time-lapse video microscopy, we demonstrated that NTRK3 is a major regulator of gonadotrope cell motility. Finally, using a Ntrk3 knock-out mouse model, we showed that NTRK3 regulates gonadotrope cells positioning in the developing pituitary, in vivo. Altogether our study demonstrates that the Neurod1/4-Ntrk3-cSrc pathway is a major actor of gonadotrope cell mobility, and thus provides new insights in the regulation of gonadotrope cell organization within the pituitary gland.

Hormonal and pheromonal studies on amphibians with special reference to metamorphosis and reproductive behavior

In this article, we review studies which have been conducted to investigate the hormonal influence on metamorphosis in bullfrog (Rana catesbeiana) and Japanese toad (Bufo japonicus) larvae, in addition to studies conducted on the hormonal and pheromonal control of reproductive behavior in red-bellied newts (Cynops pyrrhogaster). Metamorphosis was studied with an emphasis on the roles of prolactin (PRL) and thyrotropin (TSH). The release of PRL was shown to be regulated by thyrotropin-releasing hormone (TRH) and that of TSH was evidenced to be regulated by corticotropin-releasing factor. The significance of the fact that the neuropeptide that controls the secretion of TSH is different from those encountered in mammals is discussed in consideration of the observation that the release of TRH, which stimulates the release of PRL, is enhanced when the animals are subjected to a cold temperature. Findings that were made by using melanin-rich cells of Bufo embryos and larvae, such as the determination of the origin of the adenohypophyseal primordium, identification of the pancreatic chitinase, and involvement of the rostral preoptic recess organ as the hypothalamic inhibitory center of α-melanocyte-stimulating hormone (α-MSH) secretion, are mentioned in this article. In addition, the involvement of hormones in eliciting courtship behavior in male red-bellied newts and the discovery of the peptide sex pheromones and hormonal control of their secretion are also discussed in the present article.

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

Mixed corticomedullary tumor of the adrenal gland

Mixed corticomedullary tumor (MCMT) of the adrenal gland is an extremely rare tumor characterized by an admixture of steroidogenic cells and chromaffin cells in a single tumor mass simultaneously producing adrenocortical hormones and catecholamines; it is associated with ectopic adrenocorticotropic hormone (ACTH) in some cases. We reviewed and summarized clinicopathological data of 28 MCMTs, including four metastatic tumors in 26 previous reports. These reports included 21 females and 7 males, and the average tumor sizes were 4.8 ± 2.5 cm and 12.6 ± 6.4 cm in the non-metastatic and metastatic groups, respectively (P<0.001). The clinical manifestations and laboratory data were as follows: Cushing or subclinical Cushing syndrome, 58% (14/24); hypertension, 71% (17/24); elevated adrenocortical hormones, 75% (18/24); elevated catecholamines, 75% (18/24); and ectopic ACTH, 71% (10/14). All four patients with metastatic MCMTs had poor prognoses and elevated adrenocortical hormone levels; however, only two patients had elevated catecholamine levels. Immunohistochemistry was essential for the pathologic diagnosis of MCMTs. In this study, using an improved technique, we detected ectopic ACTH-producing cells in the same paraffin-embedded sections reported to be negative in our previous reports. As MCMT is composed of cells with embryologically different origins, its pathogenesis has been explained by various hypotheses. We compared MCMT to the adrenal gland of birds and the early stage of human fetuses, in which nests of chromaffin cells and steroidogenic cells admix without the formation of cortex and medulla. MCMT is characterized by the immaturity of organogenesis and might be classified as an adrenal embryonal tumor.

New insights into the role and origin of pituitary S100β-positive cells

In the anterior pituitary, S100β protein (S100β) has been assumed to be a marker of folliculo-stellate cells, which are one of the non-hormone-producing cells existing in the parenchyma of the adult anterior lobe and are composed of subpopulations with various functions. However, recent accumulating studies on S100β-positive cells, including non-folliculo-stellate cells lining the marginal cell layer (MCL), have shown the novel aspect that most S100β-positive cells in the MCL and parenchyma of the adult anterior lobe are positive for sex determining region Y-box 2 (SOX2), a marker of pituitary stem/progenitor cells. From the viewpoint of SOX2-positive cells, the majority of these cells in the MCL and in the parenchyma are positive for S100β, suggesting that S100β plays a role in the large population of stem/progenitor cells in the anterior lobe of the adult pituitary. Reportedly, S100β/SOX2-double positive cells are able to differentiate into hormone-producing cells and various types of non-hormone-producing cells. Intriguingly, it has been demonstrated that extra-pituitary lineage cells invade the pituitary gland during prenatal pituitary organogenesis. Among them, two S100β-positive populations have been identified: one is SOX2-positive population which invades at the late embryonic period through the pituitary stalk and another is a SOX2-negative population that invades at the middle embryonic period through Atwell's recess. These two populations are likely the substantive origin of S100β-positive cells in the postnatal anterior pituitary, while S100β-positive cells emerging from oral ectoderm-derived cells remain unclear.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

Some aspects of the hypothalamic and pituitary development, metamorphosis, and reproductive behavior as studied in amphibians

In this review article, information about the development of the hypothalamo-hypophyseal axis, endocrine control of metamorphosis, and hormonal and pheromonal involvements in reproductive behavior in some amphibian species is assembled from the works conducted mainly by our research group. The hypothalamic and pituitary development was studied using Bufo embryos and larvae. The primordium of the epithelial hypophysis originates at the anterior neural ridge and migrates underneath the brain to form a Rathke's pouch-like structure. The hypothalamo-hypophyseal axis develops under the influence of thyroid hormone (TH). For the morphological and functional development of the median eminence, which is a key structure in the transport of regulatory hormones to the pituitary, contact of the adenohypophysis with the undeveloped median eminence is necessary. For the development of proopiomelanocortin-producing cells, contact of the pituitary primordium with the infundibulum is required. The significance of avascularization in terms of the function of the intermediate lobe of the pituitary was evidenced with transgenic Xenopus frogs expressing a vascular endothelial growth factor in melanotropes. Metamorphosis progresses via the interaction of TH, adrenal corticosteroids, and prolactin (PRL). We emphasize that PRL has a dual role: modulation of the speed of metamorphic changes and functional development of organs for adult life. A brief description about a novel type of PRL (1B) that was detected was made. A possible reason why the main hypothalamic factor that stimulates the release of thyrotropin is not thyrotropin-releasing hormone, but corticotropin-releasing factor is considered in light of the fact that amphibians are poikilotherms. As regards the reproductive behavior in amphibians, studies were focused on the courtship behavior of the newt, Cynops pyrrhogaster. Male newts exhibit a unique courtship behavior toward sexually developed conspecific females. Hormonal interactions eliciting this behavior and hormonal control of the courtship pheromone secretion are discussed on the basis of our experimental results.

SOX10-positive cells emerge in the rat pituitary gland during late embryogenesis and start to express S100β

In the pituitary gland, S100β-positive cells localize in the neurohypophysis and adenohypophysis but the lineage of the two groups remains obscure. S100β is often observed in many neural crest-derived cell types. Therefore, in this study, we investigate the origin of pituitary S100β-positive cells by immunohistochemistry for SOX10, a potent neural crest cell marker, using S100β-green fluorescence protein-transgenic rats. On embryonic day 21.5, a SOX10-positive cell population, which was also positive for the stem/progenitor cell marker SOX2, emerged in the pituitary stalk and posterior lobe and subsequently expanded to create a rostral-caudal gradient on postnatal day 3 (P3). Thereafter, SOX10-positive cells appeared in the intermediate lobe by P15, localizing to the boundary facing the posterior lobe, the gap between the lobule structures and the marginal cell layer, a pituitary stem/progenitor cell niche. Subsequently, there was an increase in SOX10/S100β double-positive cells; some of these cells in the gap between the lobule structures showed extended cytoplasm containing F-actin, indicating a feature of migration activity. The proportion of SOX10-positive cells in the postnatal anterior lobe was lower than 0.025% but about half of them co-localized with the pituitary-specific progenitor cell marker PROP1. Collectively, the present study identified that one of the lineages of S100β-positive cells is a SOX10-positive one and that SOX10-positive cells express pituitary stem/progenitor cell marker genes.

Gene tracing analysis reveals the contribution of neural crest-derived cells in pituitary development

The anterior pituitary originates from the adenohypophyseal placode. Both the preplacode region and neural crest (NC) derive from subdivision of the neural border region, and further individualization of the placode domain is established by a reciprocal interaction between placodal precursors and NC cells (NCCs). It has long been known that NCCs are present in the adenohypophysis as interstitial cells. A recent report demonstrated that NCCs also contribute to the formation of pericytes in the developing pituitary. Here, we attempt to further clarify the role of NCCs in pituitary development using P0‐Cre/EGFP reporter mice. Spatiotemporal analyses revealed that GFP‐positive NCCs invaded the adenohypophysis in a stepwise manner. The first wave was detected on mouse embryonic day 9.5 (E9.5), when the pituitary primordium begins to be formed by adenohypophyseal placode cells; the second wave occurred on E14.5, when vasculogenesis proceeds from Atwell's recess. Finally, fate tracing of NCCs demonstrated that NC‐derived cells in the adenohypophysis terminally differentiate into all hormone‐producing cell lineages as well as pericytes. Our data suggest that NCCs contribute to pituitary organogenesis and vasculogenesis in conjunction with placode‐derived pituitary stem/progenitor cells.

Identification of marker genes for pars tuberalis morphogenesis in chick embryo: expression of Cytokine-like 1 and Gap junction protein alpha 5 in pars tuberalis

The adenohypophysis is formed from the oral ectoderm and consists of the pars distalis (PD), pars intermedia, and pars tuberalis (PT). The mechanisms of PD development have been extensively studied, and the cellular differentiation of the PD is well understood. However, the morphogenesis and differentiation of the PT are still unclear, and the genes expressed during PT development remain largely unknown. We have explored genes specifically expressed in the PT during embryonic development and analyzed their spatiotemporal expression patterns. Microarray analysis of laser-captured PT and PD tissues obtained from chick embryos on embryonic day 10 (E10.0) has shown high expression of Cytokine-like 1 (CYTL1) and Gap junction protein alpha 5 (GJA5) genes in the PT. Detailed analysis of these spatiotemporal expression patterns during chick embryo development by in situ hybridization has revealed that CYTL1 mRNA first appears in the lateral head ectoderm and ventral head ectoderm at E1.5. The expression of CYTL1 moves into Rathke's pouch at E2.5 and is then localized in the PT primordium where it is continuously expressed until E12.0. GJA5 mRNA is transiently detected in the PT primordium from E6.0 to E12.0, whereas its expression is not detected in the PD during development. Thus, these genes might be involved in the regulation mechanisms of PT development and could be useful markers for PT development.

S100β-Positive Cells of Mesenchymal Origin Reside in the Anterior Lobe of the Embryonic Pituitary Gland

The anterior and intermediate lobes of the pituitary gland develop through invagination of the oral ectoderm and as they are endocrine tissues, they participate in the maintenance of vital functions via the synthesis and secretion of numerous hormones. We recently observed that several extrapituitary cells invade the anterior lobe of the developing pituitary gland. This raised the question of the origin(s) of these S100β-positive cells, which are not classic endocrine cells but instead comprise a heterogeneous cell population with plural roles, especially as stem/progenitor cells. To better understand the roles of these S100β-positive cells, we performed immunohistochemical analysis using several markers in S100β/GFP-TG rats, which express GFP in S100β-expressing cells under control of the S100β promoter. GFP-positive cells were present as mesenchymal cells surrounding the developing pituitary gland and at Atwell's recess but were not present in the anterior lobe on embryonic day 15.5. These cells were negative for SOX2, a pituitary stem/progenitor marker, and PRRX1, a mesenchyme and pituitary stem/progenitor marker. However, three days later, GFP-positive and PRRX1-positive (but SOX2-negative) cells were observed in the parenchyma of the anterior lobe. Furthermore, some GFP-positive cells were positive for vimentin, p75, isolectin B4, DESMIN, and Ki67. These data suggest that S100β-positive cells of extrapituitary origin invade the anterior lobe, undergoing proliferation and diverse transformation during pituitary organogenesis.

Stem cells in the canine pituitary gland and in pituitary adenomas

Cushing's disease (CD) or pituitary-dependent hypercortisolism is a common endocrinopathy in dogs, with an estimated prevalence of 1 or 2 in 1000 dogs per year. It is caused by an adrenocorticotropic hormone secreting adenoma in the pars distalis or pars intermedia of the pituitary gland. The pituitary gland is a small endocrine gland located in the pituitary fossa. In the postnatal individual, the hypothalamus-pituitary axis plays a central role in maintaining homeostatic functions, like control of metabolism, reproduction, and growth. Stem cells are suggested to play a role in the homeostatic adaptations of the adult pituitary gland, such as the rapid specific cell-type expansion in response to pregnancy or lactation. Several cell populations have been suggested as pituitary stem cells, such as Side Population cells and cells expressing Sox2 or Nestin. These cell populations are discussed in this review. Also, stem and progenitor cells are thought to play a role in pituitary tumorigenesis, such as the development of pituitary adenomas in dogs. There are limited reports on the role of stem cells in pituitary adenomas, especially in dogs. Further studies are needed to identify and characterize this cell population and to develop specific cell targeting therapeutic strategies as a new way of treating canine CD.

Angiogenesis in the intermediate lobe of the pituitary gland alters its structure and function

The pars distalis (PD) and the pars intermedia (PI) have the same embryonic origin, but their morphological and functional characteristics diverge during development. The PD is highly vascularized, whereas the highly innervated PI is essentially non-vascularized. Based on our previous finding that vascular endothelial growth factor-A (VEGF-A) is involved in vascularization of the rat PD, attempt was made to generate transgenic Xenopus expressing VEGF-A specifically in the melanotrope cells of the PI as a model system for studying the significance of vascularization or avascularization for the functional differentiation of the pituitary. The PI of the transgenic frogs, examined after metamorphosis, were distinctly vascularized but poorly innervated. The experimentally induced vascularization in the PI resulted in a marked increase in tissue volume and a decrease in the expression of both alpha-melanophore-stimulating hormone (α-MSH) and prohormone convertase 2, a cleavage enzyme essential for generating α-MSH. The transgenic animals had low plasma α-MSH concentrations and displayed incomplete adaptation to a black background. To our knowledge, this is the first report indicating that experimentally induced angiogenesis in the PI may bring about functional as well as structural alterations in this tissue.

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

Differentiation capacity of native pituitary folliculostellate cells and brain astrocytes

Pituitary folliculostellate (FS) cells are characterized by producing S100B protein, as do brain astrocytes. FS cells have some functions in the pituitary gland, i.e. scavenger functions, sustentacular cell activity through cytokines, and intercellular communication through gap junctions. However, the biological significances of FS cells, especially their differentiation capacities in the anterior pituitary gland, are still under discussion. To understand FS cells with new approaches, we generated a transgenic rat expressing GFP under S100b gene promoter, which regulates tissue-specific expression of S100b gene. Using the transgenic rat, we succeeded in inducing skeletal muscle cells from FS cells by culturing it in serum-free medium containing B-27 supplement, thyroid hormone (tri-iodothyronine), epidermal growth factor, and basic fibroblast growth factor. In this study, we also succeeded in inducing skeletal muscle cells from primary cultured astrocytes and astrocyte cell line, C6 cells. Hence, we concluded that pituitary FS cells have wide differentiation potential and have similar characteristics to astrocytes, which not only support cell activity but also support differentiation capacity.

Historical view of development of comparative endocrinology in Japan

This article describing a brief history of development of comparative endocrinology in Japan is contributed to the journal General and Comparative Endocrinology, in commemoration of the 50th anniversary of its publication. It covers significant works in the field of comparative endocrinology that have been done by Japanese endocrinologists, focusing those achieved during the past 70 years. The contents were arranged according to the taxonomical order of the experimental animals with which individual researchers or research groups have contributed to the acquisition of important knowledge in comparative endocrinology.

The Notch effector gene Hes1 regulates migration of hypothalamic neurons, neuropeptide content and axon targeting to the pituitary

Proper development of the hypothalamic-pituitary axis requires precise neuronal signaling to establish a network that regulates homeostasis. The developing hypothalamus and pituitary utilize similar signaling pathways for differentiation in embryonic development. The Notch signaling effector gene Hes1 is present in the developing hypothalamus and pituitary and is required for proper formation of the pituitary, which contains axons of arginine vasopressin (AVP) neurons from the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). We hypothesized that Hes1 is necessary for the generation, placement and projection of AVP neurons. We found that Hes1 null mice show no significant difference in cell proliferation or death in the developing diencephalon at embryonic day 10.5 (e10.5) or e11.5. By e16.5, AVP cell bodies are formed in the SON and PVN, but are abnormally placed, suggesting that Hes1 may be necessary for the migration of AVP neurons. GAD67 immunoreactivity is ectopically expressed in Hes1 null mice, which may contribute to cell body misplacement. Additionally, at e18.5 Hes1 null mice show continued misplacement of AVP cell bodies in the PVN and SON and additionally exhibit abnormal axonal projection. Using mass spectrometry to characterize peptide content, we found that Hes1 null pituitaries have aberrant somatostatin (SS) peptide, which correlates with abnormal SS cells in the pituitary and misplaced SS axon tracts at e18.5. Our results indicate that Notch signaling facilitates the migration and guidance of hypothalamic neurons, as well as neuropeptide content.

Twenty years of ISAREN: an amphibian biologist in Wonderland

The 6th International Symposium on Amphibian and Reptilian Endocrinology and Neurobiology (ISAREN), the former International Symposium on Amphibian Endocrinology (ISAE), was recently held in Berlin. ISAREN developed from two symposia on amphibian biology held in European countries in 1988-1990. In this article, the history of ISAREN was briefly stated. In addition, some of the topics of our researches carried out in collaboration with several groups, using various amphibian species during the past 20 years and/or presented in the past symposia were reviewed. The topics included the discovery of pancreatic chitinase, involvement of growth hormone in vitellogenin synthesis, changes of ANF-like immunoreactivity in the frogs sent into the space, discovery of a peptide sex-pheromone, origin of the epithelial pituitary, and hypothalamic regulation of thyroid-stimulating hormone.

Conserved expression of mouse Six1 in the pre-placodal region (PPR) and identification of an enhancer for the rostral PPR

All cranial sensory organs and sensory neurons of vertebrates develop from cranial placodes. In chick, amphibians and zebrafish, all placodes originate from a common precursor domain, the pre-placodal region (PPR), marked by the expression of Six1/4 and Eya1/2. However, the PPR has never been described in mammals and the mechanism involved in the formation of PPR is poorly defined. Here, we report the expression of Six1 in the horseshoe-shaped mouse ectoderm surrounding the anterior neural plate in a pattern broadly similar to that of non-mammalian vertebrates. To elucidate the identity of Six1-positive mouse ectoderm, we searched for enhancers responsible for Six1 expression by in vivo enhancer assays. One conserved non-coding sequence, Six1-14, showed specific enhancer activity in the rostral PPR of chick and Xenopus and in the mouse ectoderm. These results strongly suggest the presence of PPR in mouse and that it is conserved in vertebrates. Moreover, we show the importance of the homeodomain protein-binding sites of Six1-14, the Six1 rostral PPR enhancer, for enhancer activity, and that Dlx5, Msx1 and Pax7 are candidate binding factors that regulate the level and area of Six1 expression, and thereby the location of the PPR. Our findings provide critical information and tools to elucidate the molecular mechanism of early sensory development and have implications for the development of sensory precursor/stem cells.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

A laser pointer driven microheater for precise local heating and conditional gene regulation in vivo. Microheater driven gene regulation in zebrafish

Background

Tissue heating has been employed to study a variety of biological processes, including the study of genes that control embryonic development. Conditional regulation of gene expression is a particularly powerful approach for understanding gene function. One popular method for mis-expressing a gene of interest employs heat-inducible heat shock protein (hsp) promoters. Global heat shock of hsp-promoter-containing transgenic animals induces gene expression throughout all tissues, but does not allow for spatial control. Local heating allows for spatial control of hsp-promoter-driven transgenes, but methods for local heating are cumbersome and variably effective.

Results

We describe a simple, highly controllable, and versatile apparatus for heating biological tissue and other materials on the micron-scale. This microheater employs micron-scale fiber optics and uses an inexpensive laser-pointer as a power source. Optical fibers can be pulled on a standard electrode puller to produce tips of varying sizes that can then be used to reliably heat 20-100 μm targets. We demonstrate precise spatiotemporal control of hsp70l:GFP transgene expression in a variety of tissue types in zebrafish embryos and larvae. We also show how this system can be employed as part of a new method for lineage tracing that would greatly facilitate the study of organogenesis and tissue regulation at any time in the life cycle.

Conclusion

This versatile and simple local heater has broad utility for the study of gene function and for lineage tracing. This system could be used to control hsp-driven gene expression in any organism simply by bringing the fiber optic tip in contact with the tissue of interest. Beyond these uses for the study of gene function, this device has wide-ranging utility in materials science and could easily be adapted for therapeutic purposes in humans.

Genetic regulation of pituitary gland development in human and mouse

Normal hypothalamopituitary development is closely related to that of the forebrain and is dependent upon a complex genetic cascade of transcription factors and signaling molecules that may be either intrinsic or extrinsic to the developing Rathke's pouch. These factors dictate organ commitment, cell differentiation, and cell proliferation within the anterior pituitary. Abnormalities in these processes are associated with congenital hypopituitarism, a spectrum of disorders that includes syndromic disorders such as septo-optic dysplasia, combined pituitary hormone deficiencies, and isolated hormone deficiencies, of which the commonest is GH deficiency. The highly variable clinical phenotypes can now in part be explained due to research performed over the last 20 yr, based mainly on naturally occurring and transgenic animal models. Mutations in genes encoding both signaling molecules and transcription factors have been implicated in the etiology of hypopituitarism, with or without other syndromic features, in mice and humans. To date, mutations in known genes account for a small proportion of cases of hypopituitarism in humans. However, these mutations have led to a greater understanding of the genetic interactions that lead to normal pituitary development. This review attempts to describe the complexity of pituitary development in the rodent, with particular emphasis on those factors that, when mutated, are associated with hypopituitarism in humans.

Molecular and tissue interactions governing induction of cranial ectodermal placodes

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the "ectodermal placodes," together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFbeta superfamily members, and Wnts are discussed.

Detailed analysis of formation of chicken pituitary primordium in early embryonic development

The primordium of the mammalian adenohypophysis derived from Rathke's pouch (RP) is known to be formed by oral ectoderm invagination. However, in the early phase of pituitary development, the detailed process by which the oral ectoderm develops into the adenohypophysis remains largely unknown. Using high-resolution non-radiolabeled in situ hybridization and the BrdU and TUNEL methods, we have examined the detailed expression pattern of factors involved in the formation of RP of chicken and the changes in the mitotic and apoptotic cell regions in RP. In the chicken embryo, Sonic hedgehog (Shh) mRNA was initially expressed in the stomodeal plate but not in the oral ectoderm. After prospective diencephalon had detached from the oral ectoderm, another Shh-expressing region appeared in the most rostral part of the recess. LIM homeobox gene 3 (Lhx3) mRNA first appeared in the anterior area of Rathke's recess, and expression then spread to the caudal region. alphaGSU mRNA-expressing cells were observed at both ends of the Lhx3-expressing region, and thereafter the expression area moved to the posterior region. Furthermore, a close overlap was found between the proliferating region and Lhx3 mRNA-expressing area, and TUNEL-positive cells appeared in Seessel's pouch derived from the foregut. Thus, the primordium of the pituitary gland corresponding to the Lhx3-expressing region is surrounded by the Shh-expressing region, which appears in two steps, and the mass growth and invagination of RP of chicken result from the coordination of the dorsal extension of the anterior region and the ventral extension of the posterior region of RP.

Retention of genes involved in the adenohypophysis-mediated endocrine system in early vertebrates

The adenohypophysis of vertebrates receives peptide hormones from the hypothalamus and secretes hormones that regulate diverse physiologic processes in peripheral organs. The adenohypophysis-mediated endocrine system is widely conserved across vertebrates but not invertebrates. Phylogenetic analysis indicates that the emergence of this system coincided with two rounds of whole-genome duplication (2R-WGD) in early vertebrates, but direct evidence linking these events has been unavailable. We detected all human paralogons (series of paralogous regions) formed in early vertebrates as traces of 2R-WGD, and examined the relationship between 2R-WGD and the evolution of genes essential to the adenohypophysis-mediated endocrine system. Regarding genes encoding transcription factors (TFs) involved in the terminal differentiation into hormone-secreting cells in adenohypophyseal development, we showed that most pairs of these genes and their paralogs were part of paralogons. In addition, our analysis also indicated that most of the paralog pairs in families of adenohypophyseal hormones and their receptors were part of paralogons. These results suggest that 2R-WGD played an important role in generating genes encoding adenohypophyseal TFs, hormones, and their receptors for increasing the diversification of hormone repertoire in the adenohypophysis-mediated endocrine system of vertebrates.

Independent and dynamic reallocation of pitx gene expression during vertebrate evolution, with emphasis on fish pituitary development

Several transcription regulators play key roles during pituitary morphogenesis. Well known intrinsic signals of the adenohypophysis such as the K(50)paired-like homeodomain proteins regulate commitment, proliferation, differential specification and maintenance of adenohypophyseal cells. We have cloned and successively characterized the mRNA localization of three pitx gene-pairs and three of their splice variants in salmon, pitx1alpha, pitx1beta; pitx2alpha, pitx2beta; pitx3alpha, pitx3beta; pitx1alphash, pitx1betash and pitx2alphaA. The high level of conservation between the pitx paralog-pairs indicates that they likely arose from lineage-specific genome duplication. We also report the isolation of a pitx1 gene in zebrafish. Comparative ISH studies of zebrafish, salmon and mouse pitx genes indicate both conservation and divergence of spatial expression domains in vertebrates. Significant differences were observed between the expressions of pitx orthologs during pituitary development. We suggest that the ancestral pituitary expression at early and late events of morphogenesis is preserved in different species through complementary shuffling of expression between the distinct pitx members of the family. Moreover, ISH analysis of the pitx salmon repertoire shows rapid evolution in this lineage, differences in spatio-temporal expression are observed between gene duplicates.

Hypothalamic input is required for development of normal numbers of thyrotrophs and gonadotrophs, but not other anterior pituitary cells in late gestation sheep

To evaluate the hypothalamic contribution to the development of anterior pituitary (AP) cells we surgically disconnected the hypothalamus from the pituitary (hypothalamo-pituitary disconnection, HPD) in fetal sheep and collected pituitaries 31 days later. Pituitaries (n = 6 per group) were obtained from fetal sheep (term = 147 +/- 3 days) at 110 days (unoperated group) of gestation and at 141 days from animals that had undergone HPD or sham surgery at 110 days. Cells were identified by labelling pituitary sections with antisera against the six AP hormones. Additionally, we investigated the colocalization of glycoprotein hormones. The proportions of somatotrophs and corticotrophs were unchanged by age or HPD. Lactotrophs increased 80% over time, but the proportion was unaffected by HPD. Thyrotrophs, which were unaffected by age, increased 70% following HPD. Gonadotrophs increased with gestational age (LH+ cells 55%; FSH+ cells 19-fold), but this was severely attenuated by HPD. We investigated the possible existence of a reciprocal effect of HPD on multipotential glycoprotein-expressing cells. Co-expression of LH and TSH was extremely rare (< 1%) and unchanged over the last month of gestation or HPD. The increase of gonadotrophs expressing FSH only or LH and FSH was attenuated by HPD. Therefore, the proportions of somatotrophs, lactotrophs and corticotrophs are regulated independently of hypothalamic input in the late gestation fetal pituitary. In marked contrast, the determination of the thyrotroph and gonadotroph lineages over the same time period is subject to complex mechanisms involving hypothalamic factors, which inhibit differentiation and/or proliferation of thyrotrophs, but stimulate gonadotrophs down the FSH lineage. Development of a distinct population of gonadotrophs, expressing only LH, appears to be subject to alternative mechanisms.

How to innervate a simple gut: familiar themes and unique aspects in the formation of the insect enteric nervous system

Like the vertebrate enteric nervous system (ENS), the insect ENS consists of interconnected ganglia and nerve plexuses that control gut motility. However, the insect ENS lies superficially on the gut musculature, and its component cells can be individually imaged and manipulated within cultured embryos. Enteric neurons and glial precursors arise via epithelial-to-mesenchymal transitions that resemble the generation of neural crest cells and sensory placodes in vertebrates; most cells then migrate extensive distances before differentiating. A balance of proneural and neurogenic genes regulates the morphogenetic programs that produce distinct structures within the insect ENS. In vivo studies have also begun to decipher the mechanisms by which enteric neurons integrate multiple guidance cues to select their pathways. Despite important differences between the ENS of vertebrates and invertebrates, common features in their programs of neurogenesis, migration, and differentiation suggest that these relatively simple preparations may provide insights into similar developmental processes in more complex systems.

Molecular physiology of pituitary development: signaling and transcriptional networks

The pituitary gland is a central endocrine organ regulating basic physiological functions, including growth, the stress response, reproduction, metabolic homeostasis, and lactation. Distinct hormone-producing cell types in the anterior pituitary arise from a common ectodermal primordium during development by extrinsic and intrinsic mechanisms, providing a powerful model system for elucidating general principles in mammalian organogenesis. The central purpose of this review is to inspect the integrated signaling and transcriptional events that affect precursor proliferation, cell lineage commitment, terminal differentiation, and physiological regulation by hypothalamic tropic factors.

Evolutionary modification of mouth position in deuterostomes

In chordates, the oral ectoderm is positioned at the anterior neural boundary and is characterized by pituitary homeobox (Pitx) and overlapping Dlx and Six3 expressions. Recent studies have shown that the ectoderm molecular map is also conserved in hemichordates and echinoderms. However, the mouth develops in a more posterior position in these animals, in a domain characterized by Nkx2.1 and Goosecoid expression, in a manner similar to that observed in protostomes. Furthermore, BMP signaling antagonizes mouth development in echinoderms and hemichordates, but seems to promote oral ectoderm specification in chordates. Conversely, Nodal signaling appears to be required for oral ectoderm specification in sea urchins but not in chordates. The Nodal/BMP antagonism at work during ectoderm patterning thus seems to constitute a conserved feature in deuterostomes, and mouth relocation may have been accompanied by a change in the influence of BMP/Nodal signals on oral ectoderm specification. We suggest that the mouth primordium was located at the anterior neural boundary, in early chordate evolution. In extant chordate embryos, subsequent mouth positioning differ between urochordates and vertebrates, presumably as a consequence of surrounding tissues remodelling. We illustrate these morphogenetic movements by means of morphological data obtained by the confocal imaging of ascidian tailbud embryos, and provide a table for determining the tailbud stages of this model organism.

Equivalent progenitor cells in the zebrafish anterior preplacodal field give rise to adenohypophysis, lens, and olfactory placodes

Embryonic organizing centers secrete signaling molecules that instruct target cells about their position and future identity. Information about cell position in relation to sources of instructive signals and about precursor cell lineages is key to our understanding of developmental processes that restrict cell potency and determine cell fate. We review adenohypophysis, lens, and olfactory placode formation and how gene expression patterns, cell positions, and cell fates in the anterior neural plate and anterior placodal field correlate in zebrafish and other vertebrates. Single cell lineage analysis in zebrafish suggests that the majority of preplacodal cells might be specified for pituitary, lens, or olfactory placode by the end of gastrulation.

Positioning the extreme anterior in Xenopus: cement gland, primary mouth and anterior pituitary

The extreme anterior of the deuterostome embryo is unusual in that ectoderm and endoderm are directly juxtaposed, without intervening mesoderm. In all vertebrates, this region gives rise to the anterior pituitary, the primary mouth and, in most frogs, to the mucus-secreting cement gland. Using the frog Xenopus laevis as a paradigm, we suggest that, initially, the extreme anterior forms a homogenous domain characterized by expression of pitx genes. Subsequently, this domain becomes subdivided to form these three different structures under the influence of different inductive signals from surrounding tissues.

Terminal nerve in the mouse-eared bat (Myotis myotis): ontogenetic aspects

As in other mammals, ontogenesis of the terminal nerve (TN) in the mouse-eared bat (Myotis myotis) starts shortly after the formation of the olfactory placode, a derivative of the ectoderm. During development of the olfactory pit, proliferating neuroblasts thicken the placodal epithelium and one cell population migrates toward the rostroventral tip of the telencephalon. Here they accumulate in a primordial terminal ganglion, which successively divides into smaller units. Initial fibers of the TN can be distinguished from olfactory fibers in the mid-embryonic period. The main TN fiber bundle (mfb) originates from the anteriormost ganglion in the nasal roof, whereas one or more inconstant smaller fiber bundles (sfb) originate from one or more smaller ganglia in the basal part of the rostral nasal septum. The fibers of the mfb and sfbs join in the posterior quarter of the nasal roof before reaching the central ganglion (M) located in the meninges medial to the olfactory bulb. From the mid-fetal period onward, a thin TN fiber bundle with some intermingled perikarya connects M to the brain by penetrating its wall rostral to the olfactory tubercle. Additional smaller ganglia may occur in this region. The TN and its ganglia persist in postnatal and adult bats but the number of perikarya is reduced here. Moreover, the different potential functions of the TN are discussed briefly.

Distinctive localization of N- and E-cadherins in rat anterior pituitary gland

In the rat anterior pituitary gland, folliculo-stellate cells aggregate preferably to form pseudofollicles, and each type of hormone-producing cell shows adhesive affinity with particular types of heterologous hormone-producing cells. Distribution of cadherin types in the rat anterior pituitary was examined immunohistochemically to clarify the unique cell arrangements caused by homologous and heterologous affinities among cells. N- and E-cadherins were detected continuously along cell membranes, while P-cadherin was not. N- and E-cadherins showed distinct isolation in localization, with N-cadherins localized in hormone-producing cells of distal and intermediate lobes in various amounts, and E-cadherins limited to folliculo-stellate cells and marginal layer cells facing the residual lumen of Rathke's pouch. A similar distribution of cadherins was observed in cell clusters of primary cultured anterior pituitary cells. These findings suggest that differential expression of cell adhesion molecules may be partially responsible for localization of hormone-producing cells and folliculo-stellate cells.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

A three-dimensional atlas of pituitary gland development in the zebrafish

The pituitary gland is unique to Chordates, with significant variation within this group, offering an excellent opportunity to increase insight into phylogenetic relationships within this phylum. The structure of the pituitary in adult Teleosts (class: Osteichthyes) is quite different from that in other chordates and is also variable among members of the class. Therefore, a complete description of the structure and development of the pituitary in members of this class is a critical component to our overall understanding of this gland. An obvious teleost model organism is the zebrafish, Danio rerio, as a significant amount of work has been done on the molecular control of pituitary development in this fish. However, very little work has been published on the morphological development of the pituitary in the zebrafish; the present study aims to fill this void. The pituitary develops from cells on the rostrodorsal portion of the head and reaches its final position, ventral to the hypothalamus, as the cephalic flexure occurs and the jaws and mouth form. The pituitary placode is juxtaposed to cells that will form the olfactory vesicles, the stomodeum, and the hatching gland. The volume of the pituitary is greatest at 24 hours post fertilization (hpf). From 24 to 120 hpf, the pituitary decreases in height and width as it undergoes convergent extension, increasing in length with the axis. The adenohypophysis is a morphologically distinct structure by 24 hpf, whereas the neurohypophysis remains indistinct until 72 hpf. The findings of this study correlate well with the available molecular data.

Origin and development of GnRH neurons

Gonadotropin-releasing hormone (GnRH) is an essential decapeptide, with both endocrine and neuromodulatory functions in vertebrates. GnRH-containing cells of the forebrain were thought to originate in the olfactory placode and migrate to their central nervous system destinations, and those of the midbrain to arise locally from the neural tube. Here, the embryonic origins of GnRH cells are re-examined in light of recent data suggesting that forebrain GnRH cells arise from the anterior pituitary placode and cranial neural crest, from where they migrate to their final destinations. The emerging picture suggests that GnRH cells do not originate from the olfactory placodes, but arise from multiple embryonic origins, and transiently associate with the developing olfactory system as they migrate to ventral forebrain locations.

A modular cis-regulatory system controls isoform-specific pitx expression in ascidian stomodaeum

Pituitary homeobox (pitx) genes have been identified in vertebrates as critical molecular determinants of various craniofacial ontogenetic processes including pituitary organogenesis. Accordingly, a prominent conserved feature of pitx genes in chordates is their early expression in the anterior neural boundary (ANB) and oral ectoderm, also known as the stomodaeum. Here we used the ascidian model species Ciona intestinalis to investigate pitx gene organization and cis-regulatory logic during early stages of oral development. Two distinct Ci-pitx mRNA variants were found to be expressed in mutually exclusive embryonic domains. Ci-pitx and vertebrate pitx2 genes display remarkably similar exon usage and organization, suggesting ancestry of the pitx transcriptional unit and regulation in chordates. We next combined phylogenetic footprinting, transient transgenesis, and confocal imaging methods to study the Ci-pitx cis-regulatory system, with special emphasis on the regulation of isoform-specific ANB/stomodaeal expression. Among 10 conserved noncoding sequences (CNSs) interspersed in C. intestinalis and Ciona savignyi pitx loci, we identified two separate cis-regulatory modules (CRMs) that drive ANB/stomodaeal expression in complementary spatiotemporal patterns. We discuss the developmental relevance of these data that provide an entry point to investigate the gene regulatory networks (GRNs) that position and shape oral structures in chordates.

Endocrinology of protochordates

Large-scale gene duplications occurred early in the vertebrate lineage after the split with protochordates. Thus, protochordate hormones and their receptors, transcription factors, and signaling pathways may be the foundation for the endocrine system in vertebrates. A number of hormones have been identified including cionin, a likely ancestor of cholecytokinin (CCK) and gastrin. Both insulin and insulin-like growth hormone (IGF) have been identified in separate cDNAs in a tunicate, whereas only a single insulin-like peptide was found in amphioxus. In tunicates, nine distinct forms of gonadotropin-releasing hormone (GnRH) are shown to induce gamete release, even though a pituitary gland and sex steroids are lacking. In both tunicates and amphioxus, there is evidence of some components of a thyroid system, but the lack of a sequenced genome for amphioxus has slowed progress in the structural identification of its hormones. Immunocytochemistry has been used to tentatively identify a number of hormones in protochordates, but structural and functional studies are needed. For receptors, protochordates have many vertebrate homologs of nuclear receptors, such as the thyroid, retinoic acid, and retinoid X receptors. Also, tunicates have cell surface receptors including the G-protein-coupled type, such as β-adrenergic, putative endocannabinoid, cionin (CCK-like), and two GnRH receptors. Several tyrosine kinase receptors include two epidermal growth factor (EGF) receptors (tunicates) and an insulin/IGF receptor (amphioxus). Interestingly, neither steroid receptors nor a full complement of enzymes for synthesis of sex steroids are encoded in the Ciona genome. Tunicates appear to have some but not all of the necessary molecules to develop a vertebrate-like pituitary or complete thyroid system.

Embryonic development of the Drosophila corpus cardiacum, a neuroendocrine gland with similarity to the vertebrate pituitary, is controlled by sine oculis and glass

We have investigated the development of the Drosophila neuroendocrine gland, the corpus cardiacum (CC), and identified the role of regulatory genes and signaling pathways in CC morphogenesis. CC progenitors segregate from the blastoderm as part of the anterior lip of the ventral furrow. Among the early genetic determinants expressed and required in this domain are the genes giant (gt) and sine oculis (so). During the extended germ band stage, CC progenitor cells form a paired cluster of 6-8 cells sandwiched in between the inner surface of the protocerebrum and the foregut. While flanking the protocerebrum, CC progenitors are in direct contact with the neural precursors that give rise to the pars intercerebralis, the part of the brain whose neurons later innervate the CC. At this stage, the CC progenitors turn on the homeobox gene glass (gl), which is essential for the differentiation of the CC. During germ band retraction, CC progenitors increase in number and migrate posteriorly, passing underneath the brain commissure and attaching themselves to the primordia of the corpora allata (CA). During dorsal closure, the CC and CA move around the anterior aorta to become the ring gland. Signaling pathways that shape the determination and morphogenesis of the CC are decapentaplegic (dpp) and its antagonist short gastrulation (sog), as well as hedgehog (hh) and heartless (htl; a Drosophila FGFR homolog). Sog is expressed in the midventral domain from where CC progenitors originate, and these cells are completely absent in sog mutants. Dpp and hh are expressed in the anterior visceral head mesoderm and the foregut, respectively; both of these tissues flank the CC. Loss of hh and dpp results in defects in CC proliferation and migration. Htl appears in the somatic mesoderm of the head and trunk. Although mutations of htl do not cause direct effects on the early development of the CC, the later formation of the ring gland is highly abnormal due to the absence of the aorta in these mutants. Defects in the CC are also caused by mutations that severely reduce the protocerebrum, including tailless (tll), suggesting that additional signaling events exist between brain and CC progenitors. We discuss the parallels between neuroendocrine development in Drosophila and vertebrates.

Development of the adenohypophysis in the lamprey: evolution of epigenetic patterning programs in organogenesis

In gnathostomes, the adenohypophysis, a component of the hypothalamo-hypophysial complex, is believed to develop through hierarchically organized epigenetic interactions based primarily on the topographical relationships between tissues. From a comparison of developmental processes and gene expression patterns of pituitary-related genes between the agnathan species, lampreys and gnathostomes, we speculate on the evolutionary pathway of the vertebrate adenohypophysis. In the lamprey, this is derived from the nasohypophysial placode (NHP) that develops anterior to the oral ectoderm. The NHP can be identified by the expression of LjPitxA, before actual histogenesis, but it is initially distant from the future hypothalamic region. Subsequently, the NHP expresses both LjFgf8/17 and LjBmp2/4a gene transcripts, and grows caudally to establish a de novo contact with the hypothalamic region by the mid-pharyngula stage. Later, the NHP gives rise to both the adenohypophysis and an unpaired nasal organ. Thus, the topographical relationship between the NHP and the hypothalamic region is established secondarily in the lamprey, unlike gnathostomes in which the equivalent relationship appears early in development. Comparing the developmental pattern of the amphioxus homologue of the adenohypophysis, we hypothesize that a modification of the regulation of the growth factor encoding gene lies behind the evolutionary changes recognized as heterochrony and heterotopy, which leads to the gnathostome hypophysial developmental pattern.

Gonadotropin-releasing hormone (GnRH) cells arise from cranial neural crest and adenohypophyseal regions of the neural plate in the zebrafish, Danio rerio

The olfactory placodes generate the primary sensory neurons of the olfactory sensory system. Additionally, the olfactory placodes have been proposed to generate a class of neuroendocrine cells containing gonadotropin-releasing hormone (GnRH). GnRH is a multifunctional decapeptide essential for the development of secondary sex characteristics in vertebrates as well as a neuromodulator within the central nervous system. Here, we show that endocrine and neuromodulatory GnRH cells arise from two separate, nonolfactory regions in the developing neural plate. Specifically, the neuromodulatory GnRH cells of the terminal nerve arise from the cranial neural crest, and the endocrine GnRH cells of the hypothalamus arise from the adenohypophyseal region of the developing anterior neural plate. Our findings are consistent with cell types generated by the adenohypophysis, a source of endocrine tissue in vertebrate animals, and by neural crest, a source of cells contributing to the cranial nerves. The adenohypophysis arises from a region of the anterior neural plate flanked by the olfactory placode fields at early stages of development, and premigratory cranial neural crest lies adjacent to the caudal edge of the olfactory placode domain [Development 127 (2000), 3645]. Thus, the GnRH cells arise from tissue closely associated with the developing olfactory placode, and their different developmental origins reflect their different functional roles in the adult animal.

Occurrence of prohormone convertase-like substances in the neural complex cells of the ascidian Halocynthia roretzi

Our previous study on the distribution of adrenocorticotropin (ACTH)-like substances in the neural complex (cerebral ganglion, dorsal strand, and neural gland) of an ascidian Halocynthia roretzi revealed that some of the cells in the cerebral ganglion and the cells scattered along the dorsal strand were immunopositive with antiserum against ACTH. In order to ascertain whether these cells are equipped with prohormone convertases, we performed immunohistochemical studies on the neural complex by using antisera against PC1 and PC2. A considerable number of cells around the dorsal strand and a few cells in the neural ganglion were immunopositive with PC1 and/or PC2 antibodies. Immunoelectron microscopic study demonstrated that some granulated cells situated in the cerebral ganglion and along the dorsal strand contained PC1- or PC2-like substances within their secretory granules. Western blot analysis revealed the presence of 66-kDa PC1-like and 70-kDa PC2-like substances in the neural complex. Moreover, immunostaining of consecutive sections showed that the majority of the cells containing PC1- and/or PC2-like substances corresponded to the cells immunoreactive with antisera against ACTH and CLIP but not to those immunoreactive with an antiserum against PRL. Cells belonging to the neural gland neither contained electron-dense granules nor showed immunoreactivity with any antisera employed in this experiment. The possibility that some of the cells situated in the cerebral ganglion and along the dorsal strand are progenitors of vertebrate adenohypophyseal cells is discussed.

Multiple roles for Hedgehog signaling in zebrafish pituitary development

The endocrine-secreting lobe of the pituitary gland, or adenohypophysis, forms from cells at the anterior margin of the neural plate through inductive interactions involving secreted morphogens of the Hedgehog (Hh), fibroblast growth factor (FGF), and bone morphogenetic protein (BMP) families. To better understand when and where Hh signaling influences pituitary development, we have analyzed the effects of blocking Hh signaling both pharmacologically (cyclopamine treatments) and genetically (zebrafish Hh pathway mutants). While current models state that Shh signaling from the oral ectoderm patterns the pituitary after placode induction, our data suggest that Shh plays a direct early role in both pituitary induction and patterning, and that early Hh signals comes from adjacent neural ectoderm. We report that Hh signaling is necessary between 10 and 15 h of development for induction of the zebrafish adenohypophysis, a time when shh is expressed only in neural tissue. We show that the Hh responsive genes ptc1 and nk2.2 are expressed in preplacodal cells at the anterior margin of the neural tube at this time, indicating that these cells are directly receiving Hh signals. Later (15-20 h) cyclopamine treatments disrupt anterior expression of nk2.2 and Prolactin, showing that early functional patterning requires Hh signals. Consistent with a direct role for Hh signaling in pituitary induction and patterning, overexpression of Shh results in expanded adenohypophyseal expression of lim3, expansion of nk2.2 into the posterior adenohypophysis, and an increase in Prolactin- and Somatolactin-secreting cells. We also use the zebrafish Hh pathway mutants to document the range of pituitary defects that occur when different elements of the Hh signaling pathway are mutated. These defects, ranging from a complete loss of the adenohypophysis (smu/smo and yot/gli2 mutants) to more subtle patterning defects (dtr/gli1 mutants), may correlate to human Hh signaling mutant phenotypes seen in Holoprosencephaly and other congenital disorders. Our results reveal multiple and distinct roles for Hh signaling in the formation of the vertebrate pituitary gland, and suggest that Hh signaling from neural ectoderm is necessary for induction and functional patterning of the vertebrate pituitary gland.