Topographical appearance of adenohypophysial cells with special reference to the development of the portal system

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.

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.

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.

Three-dimensional ultrastructural analyses of anterior pituitary gland expose spatial relationships between endocrine cell secretory granule localization and capillary distribution

Endocrine and endothelial cells of the anterior pituitary gland frequently make close appositions or contacts, and the secretory granules of each endocrine cell tend to accumulate at the perivascular regions, which is generally considered to facilitate secretory functions of these cells. However, three-dimensional relationships between the localization pattern of secretory granules and blood vessels are not fully understood. To define and characterize these spatial relationships, we used scanning electron microscopy (SEM) three-dimensional reconstruction method based on focused ion-beam slicing and scanning electron microscopy (FIB/SEM). Full three-dimensional cellular architectures of the anterior pituitary tissue at ultrastructural resolution revealed that about 70% of endocrine cells were in apposition to the endothelial cells, while almost 30% of endocrine cells were entirely isolated from perivascular space in the tissue. Our three-dimensional analyses also visualized the distribution pattern of secretory granules in individual endocrine cells, showing an accumulation of secretory granules in regions in close apposition to the blood vessels in many cases. However, secretory granules in cells isolated from the perivascular region tended to distribute uniformly in the cytoplasm of these cells. These data suggest that the cellular interactions between the endocrine and endothelial cells promote an uneven cytoplasmic distribution of the secretory granules.

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.

PRRX1- and PRRX2-positive mesenchymal stem/progenitor cells are involved in vasculogenesis during rat embryonic pituitary development

We have recently shown that cells positive for the paired-related homeobox transcription factors PRRX1 and PRRX2 occur in the rat pituitary, and that they are derived from two different origins: pituitary-derived cells positive for stem cell marker SOX2 and extra-pituitary-derived cells negative for SOX2. In this study, we have further characterized the PRRX1- and PRRX2-positive cells that originate from extra-pituitary cells. Immunohistochemical analyses were performed with specific antibodies against PRRX1 and PRRX2 in order to clarify their roles in pituitary vasculogenesis. PRRX1- and PRRX2-positive cells were found in Atwell's recess and at the periphery of the pituitary on embryonic day 15.5 (E15.5). Several PRRX1-positive cells then invaded the anterior lobe, together with a few PRRX2-positive cells, on E16.5. Some PRRX1-positive cells were also positive for mesenchymal stem cell marker NESTIN. Moreover, some PRRX1/NESTIN double-positive cells showed characteristics of vascular endothelial cells with an Isolectin-B4-binding capacity. PRRX1 co-localized with vascular smooth muscle cell/pericyte marker α-smooth muscle actin in the deep area of Atwell's recess. We confirmed the presence of PRRX2/NESTIN double-positive cells at an entry area in Atwell's recess and at the periphery of the pituitary, but PRRX2 did not co-localize with Isolectin B4 or α-smooth muscle actin. These data suggest that PRRX1- and PRRX2-positive mesenchymal stem/progenitor cells are present at the periphery of the embryonic pituitary and at the entry from Atwell's recess and participate in pituitary vasculogenesis by differentiation into vascular endothelial cells and pericytes, whereas the presence of PRRX2 indicates much higher stemness than PRRX1.

Rapid transition of NESTIN-expressing dividing cells from PROP1-positive to PIT1-positive advances prenatal pituitary development

We recently reported that the quantitative and qualitative transition of stem/progenitor cells occurs by the acquisition of a novel mechanism in the terminal differentiation during postnatal development of the anterior pituitary. We hypothesised that this novel mechanism is an alteration of a cell supply system accompanying proliferation of the progenitor cells. In the present study, we examined the proliferation activities of progenitor cells by indication of the expression of Nestin, a marker of rapidly dividing progenitor cells, aiming to verify our hypothesis and to resolve another outstanding issue regarding whether the Nestin gene is expressed in the pituitary. We found that NESTIN-positive dividing cells certainly exist in the pituitary through all stages of development. Almost all of the PROP1-positive progenitor cells express Nestin in early embryonic pituitary development. Thereafter, Nestin-expressing dividing cells involved in the cell supply system transfer from PROP1-positive progenitor cells to committed progenitor cells, such as PIT1-positive cells, on neonatal pituitary development. Furthermore, our data, together with the findings of previous studies on cell lineage tracing analyses using Nestin-Cre mice derived by the central nervous system (CNS)-specific Nestin promoter, suggest that at least two regulation systems for Nestin-expression exist in the pituitary, with the majority of these not being CNS-specific.

Temporospatial gene expression of Prx1 and Prx2 is involved in morphogenesis of cranial placode-derived tissues through epithelio-mesenchymal interaction during rat embryogenesis

Paired-related homeobox transcription factors, PRX1 and PRX2, are verified to play essential roles in limb, heart and craniofacial development by analyses of knockout animals. Their gene expression in the embryonic primordia derived from the mesoderm and neural crest is confirmed by in situ hybridization. Nevertheless, a detailed localization of PRX1 and PRX2 was not carried out because of a lack of specific antibodies for each factor. We have previously confirmed the presence of PRX proteins in rat embryonic pituitary by using an antibody that recognizes both PRX1 and PRX2. However, the pituitary originates in the cranial placodes, not the mesoderm or neural crest. In this study, we analyze the temporospatial distribution of PRX1 and PRX2 with novel antibodies specific for each factor, together with a stem/progenitor marker SOX2 (sex-determining region Y-box 2) in the primordia formed by epithelio-mesenchymal interaction. We observe immunoreactive signals of both PRX proteins in rat embryo, showing a similar pattern to that obtained by in situ hybridization. In early embryogenesis, PRX proteins are not co-localized with SOX2 but PRX2 and/or PRX1-positive cells are present in the border or periphery of SOX2-positive primordia originating in the cranial placode. During advanced embryogenesis, either PRX2-positive cells become condensed in the border of SOX2-positive cells or PRX1 and/or PRX2 become co-localized with SOX2. Our results suggest that PRX proteins, especially PRX2, play a role in the morphogenesis of the primordial tissues formed by the epithelio-mesenchymal interaction and that neural crest cells contribute to the morphogenesis of tissues derived from the cranial placode.

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.

Paired-related homeodomain proteins Prx1 and Prx2 are expressed in embryonic pituitary stem/progenitor cells and may be involved in the early stage of pituitary differentiation

We recently cloned a paired-related homeodomain protein Prx2 as a novel factor in the pituitary. In the present study, we investigated the ontogenic profiles of Prx2 and another cognate Prx1 in the rat embryonic pituitary. Quantitative real-time polymerase chain reaction showed low expression of Prx2 and a marked increase of Prx1 on rat embryonic day (E)20.5. Immunohistochemical analyses using an antibody that recognises both proteins, with the aim of investigating their roles in pituitary organogenesis, demonstrated that PRXs first appear in the Rathke's pouch on E13.5 in the pituitary stem/progenitor cells expressing Prop1 and Sox2. After E16.5, the number of Prx-expressing cells was increased in both anterior and intermediate lobes. SOX2(+) stem/progenitor cells in the intermediate lobe started to produce PRXs, and PRX(+) /SOX2(+) /PROP1(+) -cells were present on the anterior side of the marginal cell layer and were scattered in the parenchyma of the anterior lobe. On the other hand, PRX(+) -cells negative for PROP1 and SOX2 were located in the anterior lobe. Analysis of the relationship with pituitary endocrine cells revealed that a part of PRX(+) /PROP1(-) /SOX2(-) -cells in the anterior lobe co-expressed all types of hormones. The proportion of co-localisation of PRXs and hormones was highest on the day each hormone first appeared. These data indicate that PRXs are produced in the pituitary progenitor cells and may play roles in the process of terminal differentiation during early pituitary organogenesis. An in vitro small interfering RNA-knockdown experiment in the pituitary-derived cell line, TtT/GF, revealed that PRX1 and PRX2 play roles in proliferation by different mechanisms because knockdown of Prx2, but not Prx1, induced the p21 expression. Furthermore, immunohistochemical analysis demonstrated that 76% of PRXs(+) cells were positive for a cell proliferation marker Ki67 in the E18.5 pituitary. This is the first report of the involvement of PRX1 and PRX2 in organogenesis of tissue originating from the ectoderm other than the mesoderm.

Postnatal ontogeny of the glucocorticoid receptor in the hippocampus

Corticosteroid hormones are important intrinsic factors that not only mediate the response to stress but also largely contribute to the main physiological processes. The biological actions of these steroids involve, first of all, the activation of specific receptors, namely mineralocorticoid (MR) and glucocorticoid (GR) receptors. These two receptor types govern a flexible and well-balanced mechanism that leads to the often opposing changes in the cell. The hippocampus is the central part of the extrahypothalamic feedback loop in the control of the hypothalamic-pituitary-adrenal (HPA) axis activity. The coexpression of both MR and GR in the hippocampus serves a coordinated response to corticosteroids in the hippocampal neurons, thereby mediating the neuronal excitability, stress response, and behavioral adaptation. Each receptor type reveals distinct ontogenetic pattern over the postnatal period. This review addresses the issues relating to postnatal development of the HPA axis and especially the hippocampal expression of the GR proteins in intact and prenatally stressed rats.

Regional differences in the pituitary distribution of luteinizing hormone in the gonadectomized and proestrous female rat

Previous data have shown that regional differences in the presence of anterior pituitary luteinizing hormone (LH) generally correlate with the comparable disparities in distribution of gonadotropes throughout the gland. In female rats, the differences are apparent over the estrous cycle, but are more prominent during the hours preceding the proestrus surge of LH. The current experiments examined (1) if such regional disparities are present throughout the surge window, (2) if differences are mirrored by release of LH in vitro and (3) if the appearance of regional differences is altered in ovariectomized females. Results showed that a comparative elevation in the rostral portion of the pituitary during the pre-surge period diminishes and finally disappears concurrent with the rise in circulating LH. This increase in rostral LH concentrations is reflected in this region by a comparable effect in vitro on stimulated LH secretion from pituitary fragments, although the effect is somewhat diminished by referencing release against tissue concentrations of LH present in a contralateral rostral fragment. Ovariectomies conducted at 1500h on proestrus, at a time when a significant regional difference has faded, resulted in a prompt increase in LH across all areas of the pituitary, and the emergence of a marked augmentation in rostral concentrations over the ensuing 72h. The effect was not seen when ovariectomies were performed on estrus. These data show that, while a regional disparity in anterior pituitary LH is present as circulating concentrations of estradiol rise prior to the LH surge, the removal of this steroid feedback at a time when LH synthesis is normally amplified accentuates the difference between the rostral region and other areas of the pituitary.

Gene expression of vascular endothelial growth factor-A in the pituitary during formation of the vascular system in the hypothalamic-pituitary axis of the rat

Techniques involving fluorescein-5-isothiocyanate-conjugated gelatin injection, immunohistochemistry, and in situ reverse transcription/polymerase chain reaction (RT-PCR) revealed a close relationship between vascular endothelial growth factor (VEGF)-A-expressing cells and microvessels in the hypothalamic-pituitary axis of the rat. In situ RT-PCR clearly indicated the presence of VEGF-A mRNA-expressing cells in the pars tuberalis and in the pars distalis both at embryonic day 15.5 (E15.5) and in later developmental stages. The primary capillaries extended along the developing pars tuberalis, whereas the portal vessels penetrated into the pars distalis at E15.5 and subsequently expanded into the lobe to connect with the secondary capillary plexus, emerging in the pars distalis. At the same time, several VEGF-A-positive cells appeared in the pars distalis. These VEGF-A-positive cells were found to correspond to a portion of adrenocorticotropin (ACTH) cells by dual-staining for in situ RT-PCR and immunohistochemistry, suggesting that some ACTH cells have the potential to produce VEGF-A. Thus, the present study suggests that VEGF-A is involved in the development of the primary capillaries and in the vascularization of the pars distalis, but not in the portal vessels since the formation of portal vessels begins at E13.5, before the appearance of VEGF-A in the rostral region of the pars distalis.

Developmental changes in proliferative activity of cells of the murine Rathke's pouch

To clarify proliferative activity in the cells of the Rathke's pouch of the rat, we studied the labeling index using bromodeoxyuridine-immunohistochemistry. Rat fetuses were removed 1 h after transplacental injection of bromodeoxyuridine on day 11.5-21.5 of gestation, and were subsequently used to examine cellular proliferation. Although the labeling index within the Rathke's pouch was 30% on day 12.5, it decreased with development and by day 18.5 of gestation had a value of about 5%. The labeling index within Rathke's pouch was not homogeneous throughout the entire pouch, but tended to be higher in regions where cells were more densely packed. This heterogeneous pattern of distribution of labeling index values continued until day 15. On that day, immunoreactive ACTH cells first appeared in the region where the labeling index was low. From day 17 of gestation, the uneven distribution of the labeling index became vague and, simultaneously, the distribution of ACTH cells became homogeneous throughout the pouch. It was concluded that proliferation of the epithelium of Rathke's pouch is heavily involved in the growth of the pouch until at least the appearance of ACTH cells.

Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors

Development of the anterior pituitary gland involves proliferation and differentiation of ectodermal cells in Rathke's pouch to generate five distinct cell types that are defined by the trophic hormones they produce. A detailed ontogenetic analysis of specific gene expression has revealed novel aspects of organogenesis in this model system. The expression of transcripts encoding the alpha-subunit common to three pituitary glycoprotein hormones in the single layer of somatic ectoderm on embryonic day 11 established that primordial pituitary cell commitment occurs prior to formation of a definitive Rathke's pouch. Activation of Pit-1 gene expression occurs as an organ-specific event, with Pit-1 transcripts initially detected in anterior pituitary cells on embryonic day 15. Levels of Pit-1 protein closely parallel those of Pit-1 transcripts without a significant lag. Unexpectedly, Pit-1 transcripts remain highly expressed in all five cell types of the mature pituitary gland, but the Pit-1 protein is detected in only three cell types--lactotrophs, somatotrophs, and thyrotrophs and not in gonadotrophs or corticotrophs. The presence of Pit-1 protein in thyrotrophs suggests that combinatorial actions of specific activating and restricting factors act to confine prolactin and growth hormone gene expression to lactotrophs and somatotrophs, respectively. A linkage between the initial appearance of Pit-1 protein and the surprising coactivation of prolactin and growth hormone gene expression is consistent with the model that Pit-1 is responsible for the initial transcriptional activation of both genes. The estrogen receptor, which has been reported to be activated in a stereotypic fashion subsequent to the appearance of Pit-1, appears to be capable, in part, of mediating the progressive increase in prolactin gene expression characteristic of the mature lactotroph phenotype. This is a consequence of synergistic transcriptional effects with Pit-1, on the basis of binding of the estrogen receptor to a response element in the prolactin gene distal enhancer. These data imply that both transcriptional and post-transcriptional regulation of Pit-1 gene expression and combinatorial actions with other classes of transcription factors activated in distinct temporal patterns, are required for the mature physiological patterns of gene expression that define distinct cell types within the anterior pituitary gland.

Ontogeny of pituitary responsiveness to corticotropin-releasing hormone in rat

The ontogeny of the pituitary's responsiveness to synthetic rat corticotropin-releasing hormone (CRH) in the late prenatal and early postnatal periods of rats was studied by a superfusion system using whole pituitaries. A significant increase of immunoreactive beta-endorphin (IR-beta-Ep) secretion in response to 10(-10) M CRH but not to 10(-11) M CRH was observed in pituitaries from the 15th day of gestation, the earliest day that we tested, whereas 10(-11) M CRH stimulated IR-beta-Ep release from the pituitaries of 17.5-day-old fetuses. Dose-related IR-beta-Ep secretions induced by 10(-12) M to 10(-10) M CRH were observed in pituitaries of 19.5- and 21.5-day-old fetuses, and 1-, 3- and 9-day-old newborn pups. CRH stimulated not only IR-beta-Ep and IR-adrenocorticotropic hormone (ACTH) but also IR-alpha-melanocyte-stimulating hormone (IR-alpha-MSH) secretions from fetal pituitaries. The content of IR-CRH in the hypothalamic extract from 15-day-old fetus was 6.6 +/- 3.6 pg/hypothalamus (mean +/- S.E.M.) and it gradually increased to reach 212.7 +/- 20.3 pg/hypothalamus on the 21.5th day of gestation. However, the content of IR-CRH in the hypothalamus dramatically decreased just after birth and then rapidly increased again from the 5th day after birth. These data indicate that the responsiveness of corticotrophs to CRH is already present on the 15th day of gestation, when the content of IR-CRH in the hypothalamus is extremely low and that the amount of hypothalamic IR-CRH dramatically dropped for several days just after birth in rats.

The development and morphogenesis of the human pituitary gland

In order to clarify the environmental factors which are involved in the development of the primordium of the pituitary gland such as cell-cell interactions, a three-dimensional reconstruction of this organ and its surrounding tissues was carried out. Pituitary material was obtained from human fetuses mainly during the period of organogenesis. Rathke's diverticulum was found to stretch rostrally from the stomodeal epithelium to the middle of the mesoderm, and already by the 5th week of fetal growth, it was clearly seen to be involved with the diencephalon. The area of contact between Rathke's pouch and the diencephalon gradually moved from the rostral to caudal regions and, after 13 weeks of development, had a position similar to that found in the newborn infant. Among the cells forming Rathke's pouch, it was found that the closer their relationship was to the diencephalon, the greater were their epithelial characteristics. When the relationship of such cells to the diencephalon was weaker, their differentiation to endocrine cells occurred earlier. Immunohistochemically, that portion of the pituitary primordium which has a close relationship with the diencephalon, later to become the pars intermedia, showed an adrenocorticotropic hormone (ACTH) immunoreactivity later than that of the pars anterior. On the other hand, in the 21st fetal week, nearly all of the cells of the pars intermedia were found to be ACTH-positive. This finding is thought to indicate a close connection between the physical contact between the brain (diencephalon) and the pituitary primordium and the development of the pars intermedia; the differentiation of ACTH cells. The surface of the epithelium of Rathke's cavity continues to increase at least until the 21st fetal week, so the growth of the epithelium of Rathke's pouch is thought to be heavily involved in the growth of the primordium of the pituitary gland in the early stages of development.

Permeability of the blood-brain barrier in the median eminence during the perinatal period in rats

An electron microscopic study has been carried out in order to examine the permeability of the blood-brain barrier in the median eminence of perinatal rats. After several minutes, intravascularly injected electron-dense tracers (lanthanum nitrate; horseradish peroxidase, 40000 MW1, ferritin, 500000 MW) pass the capillary wall, the perivascular space, and become incorporated into neurosecretory axons and basal processes of tanycytes both in fetuses and young rats. In the case of immature capillaries, the materials diffuse freely through the endothelial cells, and to a lesser extent are transferred via occasional plasmalemmal vesicles and fenestrae. As the maturation of capillaries proceeds, their permeability via plasmalemmal vesicles and fenestrae increases considerably due to a gradual rise of the number of these structures. The plasmalemmata of the differentiated endothelial cells become impermeable to all of the tracers. Only ionic lanthanum appears to penetrate through transendothelial channels and intercellular junctions between adjacent endothelial cells.

Ontogenetic appearance of immunoreactive endocrine cells in rat pancreatic islets

Chronological development of immunoreactive, pancreatic endocrine cells was immunohistochemically studied in rats. The first immunoreaction occurs for glucagon on day 11.5 and for insulin on day 12.5 of gestation, respectively, in the cells located within the cap-like or tubular pancreatic primordium derived from the gut wall. Immunoreactive somatostatin cells appear first at the periphery of primitive islets on day 15.5. On day 18.5, the cells of the primitive islets obtain their definite arrangement and the islets are now separated from the tissue of the exocrine pancreas. Decapitation or encephalectomy performed on day 16.5 embryos fails to influence the ensuing further development of endocrine pancreas. This suggests that the hypothalamus or pituitary does not play an essential role in the histogenesis of the pancreatic islets.