Programmed cell death in the interdigital tissue of the fetal mouse limb is apoptosis with DNA fragmentation

Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1

Apoptosis occurs during development when a separation of tissues is needed. Synovial joint formation is initiated at the presumptive site (interzone) within a cartilage anlagen, with changes in cellular differentiation leading to cavitation and tissue separation. Apoptosis has been detected in phalangeal joints during development, but its role and regulation have not been defined. Here, we use a mouse model of brachydactyly type A1 (BDA1) with an IhhE95K mutation, to show that a missing middle phalangeal bone is due to the failure of the developing joint to cavitate, associated with reduced apoptosis, and a joint is not formed. We showed an intricate relationship between IHH and interacting partners, CDON and GAS1, in the interzone that regulates apoptosis. We propose a model in which CDON/GAS1 may act as dependence receptors in this context. Normally, the IHH level is low at the center of the interzone, enabling the "ligand-free" CDON/GAS1 to activate cell death for cavitation. In BDA1, a high concentration of IHH suppresses apoptosis. Our findings provided new insights into the role of IHH and CDON in joint formation, with relevance to hedgehog signaling in developmental biology and diseases.

A review of polydactyly and its inheritance: Connecting the dots

OBJECTIVE

This study collects what is known about the inheritance underpinnings of syndromic and non-syndromic polydactylies and highlights dactyly presentations with unknown genetic roots. This review summarizes the current information and genetics-enhanced understanding of polydactyly.

BACKGROUND

There is a frequency of 0.37 to 1.2 per 1000 live births for polydactyly, which is also known as hyperdactyly. It is characterized by the presence of extra fingers. Polydactyly is caused by a failure in limb development, specifically the patterning of the developing limb bud. The phenotypic and genetic variability of polydactyly makes its etiology difficult to understand. Pre-axial polydactyly, central polydactyly (axial), and postaxial polydactyly are all examples of non-syndromic polydactyly (ulnar). An autosomal dominant disorder with varying penetrance that is mostly passed down via limb development patterning abnormalities.

METHOD

A comprehensive search of MEDLINE/PubMed and other databases was followed by an evaluation of the relevant papers, with a particular focus on those published between 2000 and 2022.

RESULTS

Of 747 published article related to Polydactyly from MEDLINE/PubMed search, 43 were from the last 10 years and were the focus of this review.

CONCLUSION

Polydactyly is one of the most frequent congenital hand malformations. PAP is more common than PPD, whereas central polydactyly is very uncommon.

Genetic targeting of Card19 is linked to disrupted NINJ1 expression, impaired cell lysis, and increased susceptibility to Yersinia infection

Cell death plays a critical role in inflammatory responses. During pyroptosis, inflammatory caspases cleave Gasdermin D (GSDMD) to release an N-terminal fragment that generates plasma membrane pores that mediate cell lysis and IL-1 cytokine release. Terminal cell lysis and IL-1β release following caspase activation can be uncoupled in certain cell types or in response to particular stimuli, a state termed hyperactivation. However, the factors and mechanisms that regulate terminal cell lysis downstream of GSDMD cleavage remain poorly understood. In the course of studies to define regulation of pyroptosis during Yersinia infection, we identified a line of Card19-deficient mice (Card19lxcn) whose macrophages were protected from cell lysis and showed reduced apoptosis and pyroptosis, yet had wild-type levels of caspase activation, IL-1 secretion, and GSDMD cleavage. Unexpectedly, CARD19, a mitochondrial CARD-containing protein, was not directly responsible for this, as an independently-generated CRISPR/Cas9 Card19 knockout mouse line (Card19Null) showed no defect in macrophage cell lysis. Notably, Card19 is located on chromosome 13, immediately adjacent to Ninj1, which was recently found to regulate cell lysis downstream of GSDMD activation. RNA-seq and western blotting revealed that Card19lxcn BMDMs have significantly reduced NINJ1 expression, and reconstitution of Ninj1 in Card19lxcn immortalized BMDMs restored their ability to undergo cell lysis in response to caspase-dependent cell death stimuli. Card19lxcn mice exhibited increased susceptibility to Yersinia infection, whereas independently-generated Card19Null mice did not, demonstrating that cell lysis itself plays a key role in protection against bacterial infection, and that the increased infection susceptibility of Card19lxcn mice is attributable to loss of NINJ1. Our findings identify genetic targeting of Card19 being responsible for off-target effects on the adjacent gene Ninj1, disrupting the ability of macrophages to undergo plasma membrane rupture downstream of gasdermin cleavage and impacting host survival and bacterial control during Yersinia infection.

Cadmium sulfate induces apoptosis in planarians

With rapid socio-economic development, heavy metal pollution in water has become common and affects both environment and human health. Cadmium (Cd) has been recognized as one of the heavy metals which cause acute or chronic toxic effects if ingested. Although its toxicity is undisputed, the underlying molecular mechanisms in vivo are not fully understood. Planarians, a model organism famous for their regenerative prowess, have long been utilized to study the effects of chemical exposure. In this study, we observed apoptosis with TUNEL assay in planarians induced by cadmium sulfate (CdSO₄) in a dose-dependent manner. The apoptosis-related genes were detected with quantitative RT-PCR. Significant changes in c-Myc, P53, and BcL-2 were indicated, which may play a partial role in the regulation of the process of apoptosis in the planarians. H&E staining showed that Cd had obvious biological toxicity in the planarians. Here, new insights on metal toxicity mechanisms are provided, contributing to understand how CdSO₄ induces the pathological and physiological processes of apoptosis in the living bodies. Meanwhile, planarians are proved to be a freshwater pollution indicator and toxicological research model.

Cell death in the developing vertebrate limb: A locally regulated mechanism contributing to musculoskeletal tissue morphogenesis and differentiation

Our aim is to critically review current knowledge of the function and regulation of cell death in the developing limb. We provide a detailed, but short, overview of the areas of cell death observed in the developing limb, establishing their function in morphogenesis and structural development of limb tissues. We will examine the functions of this process in the formation and growth of the limb primordia, formation of cartilaginous skeleton, formation of synovial joints, and establishment of muscle bellies, tendons, and entheses. We will analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death. Considering the prolonged plasticity of progenitors to escape from the death process, we will discuss a new biological perspective that explains cell death: this process, rather than secondary to a specific genetic program, is a consequence of the tissue building strategy employed by the embryo based on the formation of scaffolds that disintegrate once their associated neighboring structures differentiate.

We examine the functions of cell death in the formation and growth of the limb primordia.

We analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death.

Considering the prolonged plasticity of progenitors to escape from the death process and the absence of defined genetic program in their regulation we propose that cell death is a consequence of the tissue building strategy employed by the embryo regulated by epigenetic factors .

TRAIL receptor signaling: From the basics of canonical signal transduction toward its entanglement with ER stress and the unfolded protein response

The cytokine tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the large TNF superfamily that can trigger apoptosis in transformed or infected cells by binding and activating two receptors, TRAIL receptor 1 (TRAILR1) and TRAIL receptor 2 (TRAILR2). Compared to other death ligands of the same family, TRAIL induces apoptosis preferentially in malignant cells while sparing normal tissue and has therefore been extensively investigated for its suitability as an anti-cancer agent. Recently, it was noticed that TRAIL receptor signaling is also linked to endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). The role of TRAIL receptors in regulating cellular apoptosis susceptibility therefore is broader than previously thought. Here, we provide an overview of TRAIL-induced signaling, covering the core signal transduction during extrinsic apoptosis as well as its link to alternative outcomes, such as necroptosis or NF-κB activation. We discuss how environmental factors, transcriptional regulators, and genetic or epigenetic alterations regulate TRAIL receptors and thus alter cellular TRAIL susceptibility. Finally, we provide insight into the role of TRAIL receptors in signaling scenarios that engage the unfolded protein response and discuss how these findings might be translated into new combination therapies for cancer treatment.

Hydroxyurea embryotoxicity is enhanced in P53-deficient mice

Hydroxyurea, a ribonucleotide reductase inhibitor, is a potent teratogen in mice, causing severe limb and skeletal defects. The exposure of gestation day nine murine embryos to hydroxyurea elicits an early embryonic stress response that involves activation of the P53 transcription factor. The impact of this P53 activation on the embryotoxicity of hydroxyurea- is not known. The goal of this study was to test the hypothesis that P53 acts to suppress hydroxyurea embryotoxicity. Trp53^+/- timed pregnant mice were treated with saline or hydroxyurea (200 or 400 mg/kg) on gestation day nine; fetuses were examined for viability and external and skeletal malformations on gestation day eighteen. Neither the deletion of Trp53 nor hydroxyurea treatment significantly affected fetal growth although a trend towards a decrease in fetal weights was observed in Trp53^-/- fetuses. However, hydroxyurea induced a significantly higher incidence of malformations and resorptions in Trp53^-/- fetuses compared to their wildtype littermates. Thus, fetal P53 genotype is an important determinant of the effects of hydroxyurea on organogenesis-stage embryos.

Genetic Overview of Syndactyly and Polydactyly

Syndactyly and polydactyly-respectively characterized by fused and supernumerary digits-are among the most common congenital limb malformations, with syndactyly presenting at an estimated incidence of 1 in 2,000-3,000 live births and polydactyly at a frequency of 1 in approximately 700-1,000 live births. Despite their relatively regular manifestation in the clinic, the etiologies of syndactyly and polydactyly remain poorly understood because of their phenotypic and genetic diversity. Further, even though concrete knowledge of genotypic links has been established for some variants of syndactyly and polydactyly, there appears to be no single comprehensive published summary of all syndromic and nonsyndromic syndactyly and polydactyly presentations, and there is decidedly no resource that maps all syndromic and nonsyndromic syndactylies and polydactylies to their genetic bases. This gap in the literature problematizes comprehensive carrier screening and prenatal diagnosis and complicates novel diagnostic attempts. This review thus attempts to collect all that is known about the genetic bases of syndromic and nonsyndromic syndactylies and polydactylies, as well as to highlight the dactyly manifestations for which no genetic bases are as yet known. Then, having established a summation of existing and missing knowledge, this work briefly outlines the diagnostic techniques that a genetics-reinforced understanding of syndactyly and polydactyly could inform.

Apoptosis during embryonic tissue remodeling is accompanied by cell senescence

This study re-examined the dying process in the interdigital tissue during the formation of free digits in the developing limbs. We demonstrated that the interdigital dying process was associated with cell senescence, as deduced by induction of β-gal activity, mitotic arrest, and transcriptional up-regulation of p21 together with many components of the senescence-associated secretory phenotype. We also found overlapping domains of expression of members of the Btg/Tob gene family of antiproliferative factors in the regressing interdigits. Notably, Btg2 was up-regulated during interdigit remodeling in species with free digits but not in the webbed foot of the duck. We also demonstrate that oxidative stress promoted the expression of Btg2, and that FGF2 and IGF1 which are survival signals for embryonic limb mesenchyme inhibited Btg2 expression. Btg2 overexpression in vivo and in vitro induced all the observed changes during interdigit regression, including oxidative stress, arrest of cell cycle progression, transcriptional regulation of senescence markers, and caspase-mediated apoptosis. Consistent with the central role of p21 on cell senescence, the transcriptional effects induced by overexpression of Btg2 are attenuated by silencing p21. Our findings indicate that cell senescence and apoptosis are complementary processes in the regression of embryonic tissues and share common regulatory signals.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

α6 integrin transactivates insulin-like growth factor receptor-1 (IGF-1R) to regulate caspase-3-mediated lens epithelial cell differentiation initiation

The canonical mitochondrial death pathway was first discovered for its role in signaling apoptosis. It has since been found to have a requisite function in differentiation initiation in many cell types including the lens through low level activation of the caspase-3 protease. The ability of this pathway to function as a molecular switch in lens differentiation depends on the concurrent induction of survival molecules in the Bcl-2 and IAP families, induced downstream of an IGF-1R/NFκB coordinate survival signal, to regulate caspase-3 activity. Here we investigated whether α6 integrin signals upstream to this IGF-1R-mediated survival-linked differentiation signal. Our findings show that IGF-1R is recruited to and activated specifically in α6 integrin receptor signaling complexes in the lens equatorial region, where lens epithelial cells initiate their differentiation program. In studies with both α6 integrin knock-out mice lenses and primary lens cell cultures following α6 integrin siRNA knockdown, we show that IGF-1R activation is dependent on α6 integrin and that this transactivation requires Src kinase activity. In addition, without α6 integrin, activation and expression of NFκB was diminished, and expression of Bcl-2 and IAP family members were down-regulated, resulting in high levels of caspase-3 activation. As a result, a number of hallmarks of lens differentiation failed to be induced; including nuclear translocation of Prox1 in the differentiation initiation zone and apoptosis was promoted. We conclude that α6 integrin is an essential upstream regulator of the IGF-1R survival pathway that regulates the activity level of caspase-3 for it to signal differentiation initiation of lens epithelial cells.

The epidemiology, genetics and future management of syndactyly

Syndactyly is a condition well documented in current literature due to it being the most common congenital hand defect, with a large aesthetic and functional significance.There are currently nine types of phenotypically diverse non-syndromic syndactyly, an increase since the original classification by Temtamy and McKusick(1978). Non-syndromic syndactyly is inherited as an autosomal dominant trait, although the more severe presenting types and sub types appear to have autosomal recessive and in some cases X-linked hereditary.Gene research has found that these phenotypes appear to not only be one gene specific, although having individual localised loci, but dependant on a wide range of genes and subsequent signalling pathways involved in limb formation. The principal genes so far defined to be involved in congenital syndactyly concern mainly the Zone of Polarizing Activity and Shh pathway.Research into the individual phenotypes appears to complicate classification as new genes are found both linked, and not linked, to each malformation. Consequently anatomical, phenotypical and genotypical classifications can be used, but are variable in significance, depending on the audience.Currently, management is surgical, with a technique unchanged for several decades, although future development will hopefully bring alternatives in both earlier diagnosis and gene manipulation for therapy.

Autophagy and apoptosis in planarians

Adult planarians are capable of undergoing regeneration and body remodelling in order to adapt to physical damage or extreme environmental conditions. Moreover, most planarians can tolerate long periods of starvation and during this time, they shrink from an adult size to, and sometimes beyond, the initial size at hatching. Indeed, these properties have made them a classic model to study stem cells and regeneration. Under such stressful conditions, food reserves from the gastrodermis and parenchyma are first used up and later the testes, copulatory organs and ovaries are digested. More surprisingly, when food is again made available to shrunken individuals, they grow back to adult size and all their reproductive structures reappear. These cycles of growth and shrinkage may occur over long periods without any apparent impairment to the individual, or to its future maturation and breeding capacities. This plasticity resides in a mesoderm tissue known as the parenchyma, which is formed by several differentiated non-proliferating cell types and only one mitotically active cell type, the neoblasts, which represent approximately 20-30% of the cells in the parenchyma. Neoblasts are generally thought to be somatic stem-cells that participate in the normal continuous turnover of all cell types in planarians. Hence, planarians are organisms that continuously adapt their bodies (morphallaxis) to different environmental stresses (i.e.: injury or starvation). This adaptation involves a variety of processes including proliferation, differentiation, apoptosis and autophagy, all of which are perfectly orchestrated and tightly regulated to remodel or restore the body pattern. While neoblast biology and body re-patterning are currently the subject of intense research, apoptosis and autophagy remain much less studied. In this review we will summarize our current understanding and hypotheses regarding where and when apoptosis and autophagy occur and fulfil an essential role in planarians.

Sculpturing digit shape by cell death

Physiological cell death is a key mechanism that ensures appropriate development and maintenance of tissues and organs in multicellular organisms. Most structures in the vertebrate embryo exhibit defined areas of cell death at precise stages of development. In this regard the areas of interdigital cell death during limb development provide a paradigmatic model of massive cell death with an evident morphogenetic role in digit morphogenesis. Physiological cell death has been proposed to occur by apoptosis, cellular phenomena genetically controlled to orchestrate cell suicide following two main pathways, cytochrome C liberation from the mitochondria or activation of death receptors. Such pathways converge in the activation of cysteine proteases known as caspases, which execute the cell death program, leading to typical morphologic changes within the cell, termed apoptosis. According to these findings it would be expected that caspases loss of function experiments could cause inhibition of interdigital cell death promoting syndactyly phenotypes. A syndactyly phenotype is characterized by absence of digit freeing during development that, when caused by absence of interdigital cell death, is accompanied by the persistence of an interdigital membrane. However this situation has not been reported in any of the KO mice or chicken loss of function experiments ever performed. Moreover histological analysis of dying cells within the interdigit reveals the synchronic occurrence of different types of cell death. All these findings are indicative of caspase alternative and/or complementary mechanisms responsible for physiological interdigital cell death. Characterization of alternative cell death pathways is required to explain vertebrate morphogenesis. Today there is great interest in cell death via autophagy, which could substitute or act synergistically to the apoptotic pathway. Here we discuss what is known about physiological cell death in the developing interdigital tissue of vertebrate embryos, paying special attention to the avian species.

Pharmacokinetics and derivation of an anticancer dosing regimen for PAC-1, a preferential small molecule activator of procaspase-3, in healthy dogs

PAC-1 is a preferential small molecule activator of procaspase-3 and has potential to become a novel and effective anticancer agent. The rational development of PAC-1 for translational oncologic applications would be advanced by coupling relevant in vitro cytotoxicity studies with pharmacokinetic investigations conducted in large mammalian models possessing similar metabolism and physiology as people. In the present study, we investigated whether concentrations and exposure durations of PAC-1 that induce cytotoxicity in lymphoma cell lines in vitro can be achievable in healthy dogs through a constant rate infusion (CRI) intravenous delivery strategy. Time- and dose-dependent procaspase-3 activation by PAC-1 with subsequent cytotoxicity was determined in a panel of B-cell lymphoma cells in vitro. The pharmacokinetics of PAC-1 administered orally or intravenously was studied in 6 healthy dogs using a crossover design. The feasibility of maintaining steady state plasma concentration of PAC-1 for 24 or 48 h that paralleled in vitro cytotoxic concentrations was investigated in 4 healthy dogs. In vitro, PAC-1 induced apoptosis in lymphoma cell lines in a time- and dose-dependent manner. The oral bioavailability of PAC-1 was relatively low and highly variable (17.8 ± 9.5%). The achievement and maintenance of predicted PAC-1 cytotoxic concentrations in normal dogs was safely attained via intravenous CRI lasting for 24 or 48 h in duration. Using the dog as a large mammalian model, PAC-1 can be safely administered as an intravenous CRI while achieving predicted in vitro cytotoxic concentrations.

Palatal seam disintegration: to die or not to die? that is no longer the question

Formation of the medial epithelial seam (MES) by palatal shelf fusion is a crucial step of palate development. Complete disintegration of the MES is the final essential phase of palatal confluency with surrounding mesenchymal cells. In general, the mechanisms of palatal seam disintegration are not overwhelmingly complex, but given the large number of interacting constituents; their complicated circuitry involving feedforward, feedback, and crosstalk; and the fact that the kinetics of interaction matter, this otherwise simple mechanism can be quite difficult to interpret. As a result of this complexity, apparently simple but highly important questions remain unanswered. One such question pertains to the fate of the palatal seam. Such questions may be answered by detailed and extensive quantitative experimentation of basic biological studies (cellular, structural) and the newest molecular biological determinants (genetic/dye cell lineage, gene activity, kinase/enzyme activity), as well as animal model (knockouts, transgenic) approaches. System biology and cellular kinetics play a crucial role in cellular MES function; omissions of such critical contributors may lead to inaccurate understanding of the fate of MES. Excellent progress has been made relevant to elucidation of the mechanism(s) of palatal seam disintegration. Current understanding of palatal seam disintegration suggests epithelial-mesenchymal transition and/or programmed cell death as two most common mechanisms of MES disintegration. In this review, I discuss those two mechanisms and the differences between them.

Incudomalleal joint formation: the roles of apoptosis, migration and downregulation

Background

The middle ear of mammals is composed of three endochondrial ossicles, the stapes, incus and malleus. Joints link the malleus to the incus and the incus to the stapes. In the mouse the first arch derived malleus and incus are formed from a single Sox9 and Type II collagen expressing condensation that later subdivides to give rise to two separate ossicles. In contrast the stapes forms from a separate condensation derived from the second branchial arch. Fusion of the malleus and incus is observed in a number of human syndromes and results in conductive hearing loss. Understanding how this joint forms during normal development is thus an important step in furthering our understanding of such defects.

Results

We show that the developing incudomalleal joint is characterised by a lack of proliferation and discrete areas of apoptosis. Apoptosis has been suggested to aid in the removal of pre-cartilaginous cells from the joint region, allowing for the physical separation of the cartilaginous elements, however, we show that joint initiation is unaffected by blocking apoptosis. There is also no evidence of cell migration out of the presumptive joint region, as observed by labelling of joint and ossicle cells in culture. Using Type II collagen lacZ reporter mice, however, it is evident that cells in the presumptive joint region remain in place and downregulate cartilage markers.

Conclusion

The malleus and incus first appear as a single united condensation expressing early cartilage markers. The incudomalleal joint region forms by cells in the presumptive joint region switching off cartilage markers and turning on joint markers. Failure in this process may result in fusion of this joint, as observed in human syndromes such as Branchio-Oto-Renal Syndrome or Treacher Collins Syndrome.

Cell death in health and disease

Cell death is clearly an important factor in development, homeostasis, pathology and in aging, but medical efforts based on controlling cell death have not become major aspects of medicine. There are several reasons why hopes have been slow to be fulfilled, and they present indications for new directions in research. Most effort has focused on the machinery of cell death, or the proximate effectors of apoptosis and their closely associated and interacting proteins. But cells have many options other than apoptosis. These include autophagy, necrosis, atrophy and stepwise or other alternate means of self-disassembly. The response of a cell to a noxious or otherwise intimidating signal will depend heavily on the history, lineage and current status of the cell. Many metabolic and other processes adjust the sensitivity of cells to signals, and viruses aggressively attempt to regulate the death of their host cells. Another complicating factor is that many deathassociated proteins may have functions totally unrelated to their role in cell death, generating the possibility of undesirable side effects if one interferes with them. In the future, the challenge will be more to understand the challenge to the cell from a more global standpoint, including many more aspects of metabolism, and work toward alleviating or provoking the challenge in a targeted fashion.

Apoptosis regulates notochord development in Xenopus

The notochord is the defining characteristic of the chordate embryo and plays critical roles as a signaling center and as the primitive skeleton. In this study we show that early notochord development in Xenopus embryos is regulated by apoptosis. We find apoptotic cells in the notochord beginning at the neural groove stage and increasing in number as the embryo develops. These dying cells are distributed in an anterior to posterior pattern that is correlated with notochord extension through vacuolization. In axial mesoderm explants, inhibition of this apoptosis causes the length of the notochord to approximately double compared to controls. In embryos, however, inhibition of apoptosis decreases the length of the notochord and it is severely kinked. This kinking also spreads from the anterior with developmental stage such that, by the tadpole stage, the notochord lacks any recognizable structure, although notochord markers are expressed in a normal temporal pattern. Extension of the somites and neural plate mirrors that of the notochord in these embryos, and the somites are severely disorganized. These data indicate that apoptosis is required for normal notochord development during the formation of the anterior-posterior axis, and its role in this process is discussed.

The response of costal cartilage to mechanical injury in mice

BACKGROUND

The healing potential of articular cartilage in response to injury is poor, because articular cartilage lacks blood vessels or perichondrium. Costal cartilage is covered with a vascularized perichondrium, which is known to have chondrogenic potential. The purpose of this study was to determine whether costal cartilage can heal in response to mechanical injury.

METHODS

Sixty-five ICR mice were used. Under anesthesia, the left tenth costal cartilage was dissected using microscissors. At 1 day and at 1, 2, 3, and 12 weeks after injury, the mice were killed and paraffin sections were prepared. Safranin O staining, in situ hybridization of type II collagen, and immunostaining for CD44 were performed. Localization of cell proliferation was performed using immunohistochemistry with bromodeoxyuridine monoclonal antibody. In situ detection of apoptosis (deoxynucleotidyl transferase-mediated dUTP nick end labeling) was performed using an Apop Tag Kit.

RESULTS

From 1 to 2 weeks after costal cartilage injury, bromodeoxyuridine-positive cells were observed in the perichondrium. Two weeks after injury, the dissected cartilage fragments had combined with newly formed safranin O-positive tissue. Type II collagen mRNA was strongly expressed in the cells of the newly formed tissue. Apoptosis was detected in newly formed cartilaginous tissue at 1 and 2 weeks after injury. The cartilage fragments failed to unite at 12 weeks after injury. CD44 immunoreactivity was detected on the surface of the cavity between the cartilage fragments.

CONCLUSION

Although the dissected fragments of costal cartilage can combine with newly formed cartilaginous tissue temporarily, they fail to unite ultimately.

Birth and death of cells in limb development: A mapping study

Cell death and cell proliferation are basic cellular processes that need to be precisely controlled during embryonic development. The developing vertebrate limb illustrates particularly well how correct morphogenesis depends on the appropriate spatial and temporal balance between cell death and cell proliferation. Precise knowledge of the patterns of cell proliferation and cell death during limb development is required to understand how their modifications may contribute to the generation of the great diversity of limb phenotypes that result from spontaneous mutations or induced genetic manipulations. We have performed a comprehensive analysis of the patterns of cell death, assayed by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL), and cell proliferation, assayed by anti-phosphorylated histone H3 immunohistochemistry, in consecutive sections of forelimbs and hindlimbs covering an extensive period of chick and mouse limb development. Our results confirm and expand previous reports and show common and specific areas of cell death for each species. Mitotic cells were found scattered in a uniform distribution across the early limb bud, with the exception of the areas of cell death in which mitotic cells were scarce. At later stages, mitotic cells were seen more abundantly in the digital tips. The aim of the present study was to satisfy the need for organized data sets describing these processes, which will allow the side-by-side comparison between the two major model organisms of limb development, i.e., the mouse and the chick.

Developmental abnormalities in rat embryos leading to tibial ray deficiencies induced by busulfan

BACKGROUND

Little is known about the developmental changes associated with tibial ray deficiencies. The aim of this study was to detect cell death, proliferation, and gene expression that result in tibial ray deficiencies.

METHODS

We induced tibial ray deficiencies in rat embryos using a teratogenic agent (busulfan) and observed the developmental changes in 1126 hindlimbs. We performed Nile blue staining, whole mount in situ hybridization for fibroblast growth factor 8 (Fgf8), bone morphogenetic protein 4 (Bmp4) and Sonic hedgehog (Shh), terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) and assessment of cell proliferation by 5-bromo-2'-deoxy-uridine (BrdU)/anti-BrdU immunohistochemistry.

RESULTS

In situ hybridization showed reductions in Fgf8 and Bmp4 expression. Histological examination showed a delay of mesenchymal condensation, increased mesenchymal cell death, decreased mesenchymal cell proliferation, and a reduction in the number of mesenchymal cells. These abnormalities may cause hypoplasia of the limb. Bmp4 expression was markedly reduced in the anterior mesenchyme. Shh was expressed in the posterior mesenchyme. We suggest that the posterior skeletal elements may be fully formed owing to Shh expression, but the anterior skeletal elements may be underdeveloped owing to an intense reduction of Bmp4 expression in the anterior mesenchyme, causing hypoplasia of the tibial ray.

CONCLUSIONS

The combined effects of increased cell death, decreased cell proliferation, reduction of Fgf8 expression, and intense reduction of Bmp4 expression in the anterior mesenchyme may play an important role in the development of tibial ray deficiency induced by busulfan.

Disruption of actin cytoskeleton and anchorage-dependent cell spreading induces apoptotic death of mouse neural crest cells cultured in vitro

In vertebrate embryos, neural crest cells emigrate out of the neural tube and contribute to the formation of a variety of neural and nonneural tissues. Some neural crest cells undergo apoptotic death during migration, but its biological significance and the underlying mechanism are not well understood. We carried out an in vitro study to examine how the morphology and survival of cranial neural crest (CNC) cells of the mouse embryo are affected when their actin cytoskeleton or anchorage-dependent cell spreading is perturbed. Disruption of actin fiber organization by cytochalasin D (1 microg/ml) and inhibition of cell attachment by matrix metalloproteinase-2 (MMP-2; 2.0 units/ml) were followed by morphologic changes and apoptotic death of cultured CNC cells. When the actin cytoskeleton was disrupted by cytochalasin D, the morphologic changes of cultured CNC cells preceded DNA fragmentation. These results indicate that the maintenance of cytoskeleton and anchorage-dependent cell spreading are required for survival of CNC cells. The spatially and temporally regulated expression of proteinases may be essential for the differentiation and migration of neural crest cells.

beta-Catenin expression during vascular development and degeneration of avian mesonephros

beta-Catenin is a structural component of adherens junctions, a regulator of the Wnt signalling pathway and a transcriptional co-activator with a key role in vascular patterning. The avian mesonephros is a transitory embryonic kidney that is used in the study of vascular development and degeneration. Here we examine beta-catenin expression in this model during vascular development and degeneration. Quail embryos with developing or degenerating mesonephros were studied, on day 6 (30HH) or day 11 of incubation (40HH), respectively. QH1 whole mounts of developing mesonephros revealed numerous angioblast-like cells situated in the paramesonephric duct that seem to invade the mesonephros. Although these cells did not express beta-catenin, the surrounding periductal mesenchymal cells translocated high levels of beta-catenin into the nucleus. In contrast, degenerating mesonephros were devoid of angioblast-like cells and beta-catenin was lower than in the developing mesonephros. beta-Catenin was significantly reduced in the glomerular capillary tuffs, indicating that it was particularly down-regulated in the vascular system. No sex-related differences in beta-catenin expression were observed in degenerating mesonephros. Furthermore, two special populations of glomerular and peritubular endothelial cells were observed in degenerating mesonephros: one translocating beta-catenin into the nucleus and the other in apoptosis that did not translocate it. In conclusion, our results indicate that the paramesonephric duct is a potential new vasculogenetic pathway, and suggest that beta-catenin plays a role in the fate of mesonephric endothelial cells.

Programmed cell death in the embryonic vertebrate limb

The developing limb bud provides one of the best examples in which programmed cell death exerts major morphogenetic functions. In this work, we revise the distribution and the developmental significance of cell death in the embryonic vertebrate limb and its control by the BMP signalling pathway. In addition, paying special attention to the interdigital apoptotic zones, we review current data concerning the intracellular death machinery implicated in mesodermal limb apoptosis.

Selective activation of the MEK-ERK pathway is regulated by mechanical stimuli in forming joints and promotes pericellular matrix formation

It is well established that local modification of extracellular matrix (ECM) hyaluronan composition is vital in the regulation of cell behavior. Indeed, the formation of articulating chick joint cavities, which requires mechanical stimuli derived from skeletal movement, is dependent upon the accumulation of an ECM rich in hyaluronan (HA). However, the mechanisms responsible for such precise mechano-dependent regulation of cell behavior and the formation of a HA-rich ECM remain undefined. Here we show that extracellular-regulated kinase 1/2 (ERK1/2) is selectively activated in cells at sites of cavity formation and activity diminished by in ovo immobilization that induces cartilaginous fusion across presumptive joint interzones. In vitro analyses offer mechanistic support for the role of mechanical stimuli in promoting a MEK-dependent activation of ERK1/2. In addition, our direct regulation of ERK1/2 phosphorylation status via modulation of its up-stream "classical cascade" activator either pharmacologically or by transfection with dominant negative or constitutively active Mek confirms the essential role for ERK1/2 activation in the elaboration of HA-rich pericellular matrices. Together, our findings demonstrate that the MEK-ERK pathway, regulated by mechanical stimuli, controls HA-rich matrix assembly. The precision of ERK1/2 activation selectively distinguishing cells at the joint line suggests that it directly contributes to the loss of tissue cohesion essential for generating HA-rich cavities between joint elements during their development.

Opposing roles of two isoforms of the Prx1 homeobox gene in chondrogenesis

The Prx1 homeobox gene is critical for cartilage and bone development as suggested by previous expression studies and demonstrated by gene targeting. However, neither approach assessed the individual roles of the two isoforms Prx1a and Prx1b. In this study, Western blot analysis demonstrates that, in the early stages of chondrogenesis, during mesenchymal condensation, only Prx1a is expressed. Higher level Prx1b expression is concomitant with the formation of a defined perichondrium. Prx1a overexpression in limb micro mass cultures results in an increase in the number of prechondrogenic condensations and cartilage nodules, whereas overexpression of Prx1b results in a decrease. Prx1a increases the percentage of proliferating cells in micro mass cultures and decreases apoptosis. The Prx1b isoform does not alter proliferation, but it does increase apoptosis, which is opposite of Prx1a. These results suggest that the Prx1a:Prx1b ratio and the alternative splicing mechanism that generates these two isoforms are critical in controlling chondrogenesis.

Development of the articular cavity in the rat temporomandibular joint with special reference to the behavior of endothelial cells and macrophages

Previous developmental studies on the temporomandibular joint (TMJ) have proposed several hypotheses on the formation of its articular cavity. However, detailed information is meager. The present study examined the formation process of the articular cavity in the rat TMJ by immunocytochemistry for CD31, RECA-1, and ED1, which are useful cellular markers for endothelial cells and monocyte/macrophage lineages, respectively. The upper articular cavity formation had begun by embryonic day 21 (E21) and was completed at postnatal day 1 (P1) in advance of the lower cavitation; the latter took place from P1 to P3. The occurrence and distribution pattern of the CD31-, RECA-1-, and ED1-positive cells differed between the upper and lower articular cavity-forming areas: the ED1-positive cells exclusively occurred in the area of the prospective upper articular cavity prior to its formation, while no ED1-positive cell appeared in the lower cavity-forming area. In contrast, the CD31- and RECA-1-positive endothelial cells were restricted to the lower cavity-forming area (never the prospective upper cavity) at E19 and diminished thereafter. Throughout the cavity formation, we failed to find any apoptotic cells in the cavity formation area, indicating no involvement of apoptosis in the cavity formation in TMJ. The present findings on the behaviors of endothelial cells and ED1-positive cells show a possibility of different mechanism in the cavity formation between the upper and lower articular cavities in the rat TMJ. The appearance of ED1-reactive cells and temporal vascularization may play crucial roles in the upper and lower articular cavity formation, respectively.

Spatiotemporal distribution of apoptosis during normal cloacal development in mice

To understand normal cloacal developmental processes, serial sagittal sections of mouse embryos were made every 6 hrs from embryonic day 11.5 (E11.5) to E13.5. During cloacal development to form the urogenital sinus and anorectal canal, fusion of the urorectal septum with the cloacal membrane was not observed, and the ventral and dorsal parts of the cloaca were continuously connected by the canal until disappearance of the cloacal membrane to open the vestibule formed by the urogenital sinus and anorectal canal to the outside at E13.5. Ventral shifting of the dorsal part of the cloaca was observed until E12.5. The dorsal part was transformed in accordance with ventral shifting. In addition, apoptosis was seen to occur around the dorsal part. However, from E12.25, apoptotic cells are observed in a linear arrangement in the urorectal septum just ventral to the peritoneal cavity. Interestingly, extension of this line reaches the area of the cloacal membrane disintegrated by apoptosis. The present findings suggest that in the early stages (until E12.0), distribution of apoptosis in mesenchyme around the dorsal part of the cloaca might be strongly related to the transformation and ventral shifting of this part. Conversely, the apoptosis pattern in urorectal septum mesenchyme in later stages (from E12.0) might be involved in transformation of the urorectal septum and disintegration of the cloacal membrane.

Developmental failure of phalanges in the absence of growth/differentiation factor 5

Growth/differentiation factor 5 (GDF5) is a member of the bone morphogenetic protein (BMP) family, which has been implicated in several skeletogenic events including cartilage and bone formation. To study the role of GDF5, we analyzed digit development in brachypodism (bp) mice, which carry functional null mutations of the Gdf5 gene and exhibit a reduction in the length of digit bones and loss of the middle phalanges. In situ detection of apoptosis and whole-mount detection of cell death showed abnormal apoptosis in the developing phalanges of bp mice. In situ hybridization in bp mice showed overexpression of Gdf5 mRNA in the developing phalanges where apoptotic cells were increased. In addition, bp mice exhibited excessive apoptosis in the interdigital regions. The condensed mesenchymal cells were progressively decreased in the developing phalanges and failed to form cartilage models of the middle phalanges. These findings show that excessive apoptosis in the absence of GDF5 results in developmental failure of the phalanges. We conclude that GDF5 is essential for maintenance and growth of the developing phalanges.

Growth and apoptosis during larval forelimb development and adult forelimb regeneration in the newt ( Notophthalmus viridescens)

Many of the genes involved in the initial development of the limb in higher vertebrates are also expressed during regeneration of the limb in urodeles such as Notophthalmus viridescens. These similarities have led researchers to conclude that the regeneration process is a recapitulation of development, and that patterning of the regenerate mimics pattern formation in development. However, the developing limb and the regenerating limb do not look similar. In developing urodele forelimbs, digits appear sequentially as outgrowths from the limb palette. In regeneration, all the digits appear at once. In this work, we address the issue of whether regeneration and development are similar by examining growth and apoptosis patterns. In contrast to higher vertebrates, forelimb development in the newt, N. viridescens, does not use interdigital apoptosis as the method of digit separation. During adult forelimb regeneration, apoptosis seems to play an important role in wound healing and again during cartilage to bone turnover in the advanced digits and radius/ulna. However, similar to forelimb development, demarcation of the digits in adult forelimb regeneration does not involve interdigital apoptosis. Outgrowth, rather than regression of the interdigital mesenchyme, leads to the individualization of forelimb digits in both newt development and regeneration.

Growth/differentiation factor 5 enhances chondrocyte maturation

Growth/differentiation factor 5 (GDF5) is required for limb mesenchymal cell condensation and joint formation during skeletogenesis. Here, we use a model consisting of long-term, high-density cultures of chick embryonic limb mesenchymal cells, which undergo the entire life history of chondrocyte development, to examine the effects of GDF5 overexpression on chondrocyte maturation. Exposure to GDF5 significantly enhanced chondrocyte hypertrophy and maturation, as determined by the presence of alkaline phosphatase activity, collagen type X protein production, and the presence of a sulfated proteoglycan-rich extracellular matrix. Histologic analysis also revealed an increase in cell volume and cellular encasement in larger lacunae in GDF5-treated cultures. Taken together, these results support a role for GDF5 in influencing chondrocyte maturation and the induction of hypertrophy in the late stages of embryonic cartilage development, and provide additional mechanistic insights into the role of GDF5 in skeletal development.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Development of synovial joints

Synovial joints arise through two main processes. In long bone elements, cartilaginous differentiation occurs across the locations of the prospective joints that then segment secondarily. This process occurs through the development of a noncartilaginous region known as the interzone. The interzone becomes an important signaling center to the opposing elements, which can regulate growth through such factors as GDF-5. The interzone also expresses bone morphogenetic proteins (BMPs) and their antagonists, such as noggin. Overexpression of BMPs, or the loss of noggin leads to joint fusions. The interzone also expresses Wnt-14, which appears to be specific for this region in the developing anlagen, and regulates its nonchondrogenic nature. Cavitation of the joint follows, driven by selective high-level synthesis of hyaluronan by interzone cells and presumptive synovial cells. In addition, as the interzone disperses during cavity enlargement, data are now accruing that suggest that both the synovium and articular cartilage develop from this population. Finally, the development of articular cartilage progresses through appositional growth driven by a progenitor/stem cell subpopulation that resides in the articular surface. The individual elements of the skeleton are connected together at regions termed joints or articulations. Classically, there are three broad categories of joints: immovable joints (syntharthroses); mixed articulations, in which the range of movement is limited (amphiarthroses); and the movable, or synovial, joints (diarthroses). This review concentrates on the development of the synovial joints.

Current and future genetic screening for male infertility

Although much of male infertility is currently unexplained, it is likely that underlying defects in critical genes or entire gene pathways are responsible. Because powerful technologies exist to bypass severe male-factor infertility, improving the diagnosis of genetic infertility is important for the infertile couple, not only to explain the problem but also to inform them of conditions potentially transmissible to offspring.

Morgue mediates apoptosis in the Drosophila melanogaster retina by promoting degradation of DIAP1

Inhibitor of apoptosis proteins (IAPs) provide a critical barrier to inappropriate apoptotic cell death through direct binding and inhibition of caspases. We demonstrate that degradation of IAPs is an important mechanism for the initiation of apoptosis in vivo. Drosophila Morgue, a ubiquitin conjugase-related protein, promotes DIAP1 down-regulation in the developing retina to permit selective programmed cell death. Morgue complexes with DIAP1 in vitro and mediates DIAP1 degradation in a manner dependent on the Morgue UBC domain. Reaper (Rpr) and Grim, but not Hid, also promote the degradation of DIAP1 in vivo, suggesting that these proteins promote cell death through different mechanisms.

Programmed cell death takes flight: genetic and genomic approaches to gene discovery in Drosophila

Programmed cell death (PCD) is an essential and wide-spread physiological process that results in the elimination of cells. Genes required to carry out this process have been identified, and many of these remain the subjects of intense investigation. Here, we describe PCD, its functions, and some of the consequences when it goes awry. We review PCD in the model system, the fruit fly, Drosophila melanogaster, with a particular emphasis on cell death gene discovery resulting from both genetics and genomics-based approaches.

TGF-beta is required for programmed cell death in interdigital webs of the developing mouse limb

During limb formation massive cell death in the mesenchyme of the interdigital spaces accompanies the formation of free digits. Members of the transforming growth factor beta (TGF-) superfamily were discussed to play a key role in cell-cell interactions, important in the regulation of programmed cell death (PCD). TGF-beta itself is believed to be involved in epithelial-mesenchymal interactions. Here, we demonstrate that PCD is significantly reduced in interdigital spaces of the developing limbs of Tgfbeta2-/-Tgfbeta3-/- double knockouts. The regression of interdigital webs seems to be doses-dependent as interdigital mesenchyme is at least partly reduced in Tgfbeta2-/-Tgfbeta3+/- mutants, whereas interdigital zones of Tgfbeta2-/-Tgfbeta3-/- double knockouts reveal only minimal signs of regression. We conclude that TGF- is a critical extrinsic regulator of PCD.

Spatiotemporal and tissue specific distribution of apoptosis in the developing chick heart

To investigate spatial and temporal distributions of apoptosis in the embryonic chick heart and its relation to different tissue types, we examined apoptosis in the embryonic chick heart from Hamburger and Hamilton stage 17 through 3 days after hatching. MF20 antibody, alpha-smooth muscle actin (SMA) antibody and EAP-300 antibody were applied to delineate specific cell types. During early development of the embryonic chick heart, very few apoptotic cells were detected. The first distinctive zone of apoptosis was observed in the outflow tract at stage 25. This focus was most prominent during septation of the pulmonary artery from the aorta (i.e., between stages 28 and 29), and diminished to virtually background level by stage 32, except in the subconal regions. Subsequently, remarkable apoptosis appeared in the atrioventricular cushions by stage 26, peaked at stages 29-31, and dropped significantly thereafter. Characteristic distribution patterns of apoptotic cells were also detected in the cardiac conduction tissues, including the His bundle, the bundle branches, and the ventricular trabeculae. After stage 36, cell death dropped to background level, except in developing coronary vessels. MF20 and TUNEL double staining revealed that apoptosis in cardiomyocytes was limited to a few specific regions, much less than in cushion tissues. SMA and TUNEL double staining demonstrated that vascular structures were the major foci of apoptosis from stage 40 to 44, whereas adjacent perivascular Purkinje cells displayed significantly less cell death at these stages. The characteristic spatiotemporal locations of apoptosis parallel the morphologic changes and tissue differentiation during heart development, suggesting that apoptosis is crucial to the transformation of the heart from a simple tube to a complex multichambered pump.

In utero exposure to brief hyperthermia interferes with the production and migration of neocortical neurons and induces apoptotic neuronal death in the fetal mouse brain

To investigate the pathogenetic mechanisms of brain maldevelopment induced by maternal hyperthermia, we exposed pregnant ICR mice to 43 degrees C for 12.5 min on day 13.5 or 14.5 of gestation and examined the proliferation and migration of neuronal precursor cells in the telencephalon of their fetuses. The brain weight was significantly decreased in heat-stressed fetuses when examined at 72 h after treatment. Histological examination revealed that the thickness of the neopallium, especially that of the intermediate (migratory) zone and the cortical plate, was decreased in the heated group. BrdU/anti-BrdU immunohistochemistry showed that cell proliferation in the matrix cell zone was suppressed for up to 8 h after hyperthermia and that the migration of BrdU-labeled neurons from the matrix cell zone to the primordial cortex was decelerated significantly. In addition, apoptotic cell death which is rarely observed in the brain of control animals increased in the brain of heat-stressed fetuses at 8-12 h after treatment. Thus, it seems that brief hyperthermia at critical stages of neuronal differentiation can interfere with the production and migration of neuronal precursor cells and result in abnormal brain development and neurobehavioural disturbances.

Differential tissue growth and patterns of cell death in mouse limb autopod morphogenesis

Programmed cell death (PCD) is considered one of the most important cellular processes in the morphogenesis of organs and tissues during animal development. Although the embryonic limb has been established as a classic model for the study of PCD, detailed studies on this process' contribution to morphogenesis are still lacking. In the present work, using modern computer-aided techniques, we estimated the contribution of PCD to mouse limb morphogenesis. For the detection of apoptotic cell death, we stained whole embryonic limbs with acridine orange or, in some instances, used the TUNEL technique, and visualized the tissues by confocal laser scanning microscopy. We found that cell death patterns are dynamic during limb development, and occur in gradients oriented with the main limb axes, anteroposterior, dorsoventral and distoproximal. Interdigital apoptosis in the autopod was initially detected at the most distal region, and then more proximally as development proceeded. Interestingly, we found that digit separation is more pronounced on the dorsal side, contrary to what is expected from the apoptotic cell distribution, which shows more abundant cell death in the ventral region. Using 2-D and 3-D models, we found that most digit individualization occurs rather by digit growth than by interdigital cell death. Therefore, digits do not mainly individualize by degeneration of preformed interdigital tissue, but probably by a dynamic balance between proliferation and cell death, reducing interdigital growth, which results in protrusion of digits. We determined the expression pattern of fgf-8 during the period of digit individualization, as the product of this gene could participate in defining the limb growth pattern. Initially, fgf-8 expression was coincident with the apical ectodermal ridge, but when cell death was first detected in the interdigits, fgf-8 expression became restricted to the tip of the growing digits. Therefore, FGF-8 could be one of the factors responsible for differential digit-interdigit growth, and might also act as a survival factor on interdigital tissue. We also found that the expression patterns of rar-beta, bmp-2, bmp-4, bmp-7, msx-1, and msx-2 genes, proposed to be involved in the activation of interdigital cell death, did not overlap with, or were not highly expressed in the major zones of cell death in the developing limb.

A bipotential precursor population for pancreas and liver within the embryonic endoderm

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.

Wnt-14 plays a pivotal role in inducing synovial joint formation in the developing appendicular skeleton

The long bones of the vertebrate appendicular skeleton arise from initially continuous condensations of mesenchymal cells that subsequently segment and cavitate to form discrete elements separated by synovial joints. Little is known, however, about the molecular mechanisms of joint formation. We present evidence that Wnt-14 plays a central role in initiating synovial joint formation in the chick limb. Wnt-14 is expressed in joint-forming regions prior to the segmentation of the cartilage elements, and local misexpression of Wnt-14 induces morphological and molecular changes characteristic of the first steps of joint formation. Induction of an ectopic joint-like region by Wnt-14 suppresses the formation of the immediately adjacent endogenous joint, potentially providing insight into the spacing of joints.