Interdigital cell death function and regulation: New insights on an old programmed cell death model

Macrophages allocate before apoptosis initiation and produce reactive oxygen species during interdigital phagocytosis

During programmed cell death (PCD), it is commonly accepted that macrophages are recruited by apoptotic cells to complete cell degradation. Interdigital cell death, a classical model of PCD, contributes to digit individualization in limbs of mammals and other vertebrates. Here, we show that macrophages are present in interdigits before significant cell death occurs and remain after apoptosis inhibition. The typical interdigital phagocytic activity was not observed after a partial depletion of macrophages and was markedly reduced by engulfment/phagosome maturation inhibition, as detected by its association with high lysosomal activity. β-galactosidase activity in this region was also coupled with phagocytosis, against its relationship with cellular senescence. Interdigital phagocytosis correlated with high levels of reactive oxygen species (ROS), common in embryo regions carrying abundant cell death, suggesting that macrophages are the major source of ROS. ROS generation was dependent on NADPH oxidases and blood vessel integrity, but not directly associated with lysosomal activity. Therefore, macrophages prepattern regions where abundant cell death is going to occur, and high lysosomal activity and the generation of ROS by an oxidative burst-like phenomenon are activities of phagocytosis.

The pathogenic mechanism of syndactyly type V identified in a Hoxd13Q50R knock-in mice

Syndactyly type V (SDTY5) is an autosomal dominant extremity malformation characterized by fusion of the fourth and fifth metacarpals. In the previous publication, we first identified a heterozygous missense mutation Q50R in homeobox domain (HD) of HOXD13 in a large Chinese family with SDTY5. In order to substantiate the pathogenicity of the variant and elucidate the underlying pathogenic mechanism causing limb malformation, transcription-activator-like effector nucleases (TALEN) was employed to generate a Hoxd13Q50R mutant mouse. The mutant mice exhibited obvious limb malformations including slight brachydactyly and partial syndactyly between digits 2-4 in the heterozygotes, and severe syndactyly, brachydactyly and polydactyly in homozygotes. Focusing on BMP2 and SHH/GREM1/AER-FGF epithelial mesenchymal (e-m) feedback, a crucial signal pathway for limb development, we found the ectopically expressed Shh, Grem1 and Fgf8 and down-regulated Bmp2 in the embryonic limb bud at E10.5 to E12.5. A transcriptome sequencing analysis was conducted on limb buds (LBs) at E11.5, revealing 31 genes that exhibited notable disparities in mRNA level between the Hoxd13Q50R homozygotes and the wild-type. These genes are known to be involved in various processes such as limb development, cell proliferation, migration, and apoptosis. Our findings indicate that the ectopic expression of Shh and Fgf8, in conjunction with the down-regulation of Bmp2, results in a failure of patterning along both the anterior-posterior and proximal-distal axes, as well as a decrease in interdigital programmed cell death (PCD). This cascade ultimately leads to the development of syndactyly and brachydactyly in heterozygous mice, and severe limb malformations in homozygous mice. These findings suggest that abnormal expression of SHH, FGF8, and BMP2 induced by HOXD13Q50R may be responsible for the manifestation of human SDTY5.

Extensive elimination of acinar cells during normal postnatal pancreas growth

While programmed cell death plays important roles during morphogenetic stages of development, post-differentiation organ growth is considered an efficient process whereby cell proliferation increases cell number. Here we demonstrate that early postnatal growth of the pancreas unexpectedly involves massive acinar cell elimination. Measurements of cell proliferation and death in the human pancreas in comparison to the actual increase in cell number predict daily elimination of 0.7% of cells, offsetting 88% of cell formation over the first year of life. Using mouse models, we show that death is associated with mitosis, through a failure of dividing cells to generate two viable daughters. In p53-deficient mice, acinar cell death and proliferation are reduced, while organ size is normal, suggesting that p53-dependent developmental apoptosis triggers compensatory proliferation. We propose that excess cell turnover during growth of the pancreas, and presumably other organs, facilitates robustness to perturbations and supports maintenance of tissue architecture.

Epithelial apoptotic pattern emerges from global and local regulation by cell apical area

Geometry is a fundamental attribute of biological systems, and it underlies cell and tissue dynamics. Cell geometry controls cell-cycle progression and mitosis and thus modulates tissue development and homeostasis. In sharp contrast and despite the extensive characterization of the genetic mechanisms of caspase activation, we know little about whether and how cell geometry controls apoptosis commitment in developing tissues. Here, we combined multiscale time-lapse microscopy of developing Drosophila epithelium, quantitative characterization of cell behaviors, and genetic and mechanical perturbations to determine how apoptosis is controlled during epithelial tissue development. We found that early in cell lives and well before extrusion, apoptosis commitment is linked to two distinct geometric features: a small apical area compared with other cells within the tissue and a small relative apical area with respect to the immediate neighboring cells. We showed that these global and local geometric characteristics are sufficient to recapitulate the tissue-scale apoptotic pattern. Furthermore, we established that the coupling between these two geometric features and apoptotic cells is dependent on the Hippo/YAP and Notch pathways. Overall, by exploring the links between cell geometry and apoptosis commitment, our work provides important insights into the spatial regulation of cell death in tissues and improves our understanding of the mechanisms that control cell number and tissue size.

Hnrnpk is essential for embryonic limb bud development as a transcription activator and a collaborator of insulator protein Ctcf

Proper development of the limb bud relies on the concordance of various signals, but its molecular mechanisms have not yet been fully illustrated. Here we report that heterogeneous nuclear ribonucleoprotein K (hnRNPK) is essential for limb bud development. Its ablation in the limb bud results in limbless forelimbs and severe deformities of the hindlimbs. In terms of mechanism, hnRNPK functions as a transcription activator for the vital genes involved in the three regulatory axes of limb bud development. Simultaneously, for the first time we elucidate that hnRNPK binds to and coordinates with the insulator protein CCCTC binding factor (CTCF) to maintain a three-dimensional chromatin architecture. Ablation of hnRNPK weakens the binding strength of CTCF to topologically associating domain (TAD) boundaries, then leading to the loose TADs, and decreased interactions between promoters and enhancers, and further decreased transcription of developmental genes. Our study establishes a fundamental and novel role of hnRNPK in regulating limb bud development.

Modeling the Differentiation of Embryonic Limb Chondroprogenitors by Cell Death and Cell Senescence in High Density Micromass Cultures and Their Regulation by FGF Signaling

Considering the importance of programmed cell death in the formation of the skeleton during embryonic development, the aim of the present study was to analyze whether regulated cell degeneration also accompanies the differentiation of embryonic limb skeletal progenitors in high-density tridimensional cultures (micromass cultures). Our results show that the formation of primary cartilage nodules in the micromass culture assay involves a patterned process of cell death and cell senescence, complementary to the pattern of chondrogenesis. As occurs in vivo, the degenerative events were preceded by DNA damage detectable by γH2AX immunolabeling and proceeded via apoptosis and cell senescence. Combined treatments of the cultures with growth factors active during limb skeletogenesis, including FGF, BMP, and WNT revealed that FGF signaling modulates the response of progenitors to signaling pathways implicated in cell death. Transcriptional changes induced by FGF treatments suggested that this function is mediated by the positive regulation of the genetic machinery responsible for apoptosis and cell senescence together with hypomethylation of the Sox9 gene promoter. We propose that FGF signaling exerts a primordial function in the embryonic limb conferring chondroprogenitors with their biological properties.

Evolutionary genetics of flipper forelimb and hindlimb loss from limb development-related genes in cetaceans

Background

Cetacean hindlimbs were lost and their forelimb changed into flippers characterized by webbed digits and hyperphalangy, thus allowing them to adapt to a completely aquatic environment. However, the underlying molecular mechanism behind cetacean limb development remains poorly understood.

Results

In the present study, we explored the evolution of 16 limb-related genes and their cis-regulatory elements in cetaceans and compared them with that of other mammals. TBX5, a forelimb specific expression gene, was identified to have been under accelerated evolution in the ancestral branches of cetaceans. In addition, 32 cetacean-specific changes were examined in the SHH signaling network (SHH, PTCH1, TBX5, BMPs and SMO), within which mutations could yield webbed digits or an additional phalange. These findings thus suggest that the SHH signaling network regulates cetacean flipper formation. By contrast, the regulatory activity of the SHH gene enhancer—ZRS in cetaceans—was significantly lower than in mice, which is consistent with the cessation of SHH gene expression in the hindlimb bud during cetacean embryonic development. It was suggested that the decreased SHH activity regulated by enhancer ZRS might be one of the reasons for hindlimb degeneration in cetaceans. Interestingly, a parallel / convergent site (D42G) and a rapidly evolving CNE were identified in marine mammals in FGF10 and GREM1, respectively, and shown to be essential to restrict limb bud size; this is molecular evidence explaining the convergence of flipper-forelimb and shortening or degeneration of hindlimbs in marine mammals.

Conclusions

We did evolutionary analyses of 16 limb-related genes and their cis-regulatory elements in cetaceans and compared them with those of other mammals to provide novel insights into the molecular basis of flipper forelimb and hindlimb loss in cetaceans.

Genetic determinants of syndactyly: perspectives on pathogenesis and diagnosis

The formation of the digits is a tightly regulated process. During embryogenesis, disturbance of genetic pathways in limb development could result in syndactyly; a common congenital malformation consisting of webbing in adjacent digits. Currently, there is a paucity of knowledge regarding the exact developmental mechanism leading to this condition. The best studied canonical interactions of Wingless‐type–Bone Morphogenic Protein–Fibroblast Growth Factor (WNT–BMP–FGF8), plays a role in the interdigital cell death (ICD) which is thought to be repressed in human syndactyly. Animal studies have displayed other pathways such as the Notch signaling, metalloprotease and non-canonical WNT-Planar cell polarity (PCP), to also contribute to failure of ICD, although less prominence has been given. The current diagnosis is based on a clinical evaluation followed by radiography when indicated, and surgical release of digits at 6 months of age is recommended. This review discusses the interactions repressing ICD in syndactyly, and characterizes genes associated with non-syndromic and selected syndromes involving syndactyly, according to the best studied canonical WNT-BMP-FGF interactions in humans. Additionally, the controversies regarding the current syndactyly classification and the effect of non-coding elements are evaluated, which to our knowledge has not been previously highlighted. The aim of the review is to better understand the developmental process leading to this condition.

Force-generating apoptotic cells orchestrate avian neural tube bending

Apoptosis plays an important role in morphogenesis, and the notion that apoptotic cells can impact their surroundings came to light recently. However, how this applies to vertebrate morphogenesis remains unknown. Here, we use the formation of the neural tube to determine how apoptosis contributes to morphogenesis in vertebrates. Neural tube closure defects have been reported when apoptosis is impaired in vertebrates, although the cellular mechanisms involved are unknown. Using avian embryos, we found that apoptotic cells generate an apico-basal force before being extruded from the neuro-epithelium. This force, which relies on a contractile actomyosin cable that extends along the apico-basal axis of the cell, drives nuclear fragmentation and influences the neighboring tissue. Together with the morphological defects observed when apoptosis is prevented, these data strongly suggest that the neuroepithelium keeps track of the mechanical impact of apoptotic cells and that the apoptotic forces, cumulatively, contribute actively to neural tube bending.

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Apoptotic cells are force-generating cells in the avian neural tube

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Apoptotic force drives the upward movement of the nucleus and nuclear fragmentation

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Apoptotic cells cumulatively impact the neighboring tissue

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Apoptotic force mechanical impact participates in progressive bending of the neural tube

Macrophages and the maintenance of homeostasis

There have been many chapters written about macrophage polarization. These chapters generally focus on the role of macrophages in orchestrating immune responses by highlighting the T-cell-derived cytokines that shape these polarizing responses. This bias toward immunity is understandable, given the importance of macrophages to host defense. However, macrophages are ubiquitous and are involved in many different cellular processes, and describing them as immune cells is undoubtedly an oversimplification. It disregards their important roles in development, tissue remodeling, wound healing, angiogenesis, and metabolism, to name just a few processes. In this chapter, we propose that macrophages function as transducers in the body. According to Wikipedia, “A transducer is a device that converts energy from one form to another.” The word transducer is a term used to describe both the “sensor,” which can interpret a wide range of energy forms, and the “actuator,” which can switch voltages or currents to affect the environment. Macrophages are able to sense a seemingly endless variety of inputs from their environment and transduce these inputs into a variety of different response outcomes. Thus, rather than functioning as immune cells, they should be considered more broadly as cellular transducers that interpret microenvironmental changes and actuate vital tissue responses. In this chapter, we will describe some of the sensory stimuli that macrophages perceive and the responses they make to these stimuli to achieve their prime directive, which is the maintenance of homeostasis.

Cracking the Cell Death Code

Cell death is an invariant feature throughout our life span, starting with extensive scheduled cell death during morphogenesis and continuing with death under homeostasis in adult tissues. Additionally, cells become victims of accidental, unscheduled death following injury and infection. Cell death in each of these occasions triggers specific and specialized responses in the living cells that surround them or are attracted to the dying/dead cells. These responses sculpt tissues during morphogenesis, replenish lost cells in homeostasis to maintain tissue/system function, and repair damaged tissues after injury. Wherein lies the information that sets in motion the cascade of effector responses culminating in remodeling, renewal, or repair? Here, we attempt to provide a framework for thinking about cell death in terms of the specific effector responses that accompanies various modalities of cell death. We also propose an integrated threefold "cell death code" consisting of information intrinsic to the dying/dead cell, the surroundings of the dying cell, and the identity of the responder.

Epithelial Migration and Non-adhesive Periderm Are Required for Digit Separation during Mammalian Development

The fusion of digits or toes, syndactyly, can be part of complex syndromes, including van der Woude syndrome. A subset of van der Woude cases is caused by dominant-negative mutations in the epithelial transcription factor Grainyhead like-3 (GRHL3), and Grhl3^-/-mice have soft-tissue syndactyly. Although impaired interdigital cell death of mesenchymal cells causes syndactyly in multiple genetic mutants, Grhl3^-/- embryos had normal interdigital cell death, suggesting alternative mechanisms for syndactyly. We found that in digit separation, the overlying epidermis forms a migrating interdigital epithelial tongue (IET) when the epithelium invaginates to separate the digits. Normally, the non-adhesive surface periderm allows the IET to bifurcate as the digits separate. In contrast, in Grhl3^-/- embryos, the IET moves normally between the digits but fails to bifurcate because of abnormal adhesion of the periderm. Our study identifies epidermal developmental processes required for digit separation.

Role of Notch Signaling in Leg Development in Drosophila melanogaster

Notch pathway plays diverse and fundamental roles during animal development. One of the most relevant, which arises directly from its unique mode of activation, is the specification of cell fates and tissue boundaries. The development of the leg of Drosophila melanogaster is a fine example of this Notch function, as it is required to specify the fate of the cells that will eventually form the leg joints, the flexible structures that separate the different segments of the adult leg. Notch activity is accurately activated and maintained at the distal end of each segment in response to the proximo-distal patterning gene network of the developing leg. Region-specific downstream targets of Notch in turn regulate the formation of the different types of joints. We discuss recent findings that shed light on the molecular and cellular mechanisms that are ultimately governed by Notch to achieve epithelial fold and joint morphogenesis. Finally, we briefly summarize the role that Notch plays in inducing the nonautonomous growth of the leg. Overall, this book chapter aims to highlight leg development as a useful model to study how patterning information is translated into specific cell behaviors that shape the final form of an adult organ.

In vivo detection of programmed cell death during mouse heart development

Despite the great progress on the cell biology of programmed cell death (PCD), its incidence and exact time course during embryonic and particular heart development are still unclear. This is also due to the lack of models enabling to directly identify and monitor PCD cells at different time points in vivo. Herein we report generation of transgenic murine embryonic stem cell and mouse models expressing secreted Annexin V-YFP under control of the CAG promoter. This enables to visualize and quantify PCD in vitro and in vivo during embryonic development. At early embryonic stages we found Annexin V-YFP⁺ fluorescent cells in known areas of PCD, such as the otic ring and at the site of neural tube closing, underscoring its specificity for detection of PCD. We have focused our detailed analysis primarily on PCD in the embryonic heart for a better understanding of its role during development. Our findings reveal that PCD peaks at early stages of cardiogenesis (E9.5-E13.5) and strongly decreases thereafter. Moreover, the PCD cells in the heart are predominantly cardiomyocytes, and an unexpected area of prominent cardiac PCD are the ventricular trabeculae (E9.5-E14.5). Thus, the sA5-YFP mouse line provides novel insight into the incidence and relevance of cardiac PCD during embryonic development ex- and in vivo.

A Review of the Genetics and Pathogenesis of Syndactyly in Humans and Experimental Animals: A 3-Step Pathway of Pathogenesis

Embryology of normal web space creation and the genetics of syndactyly in humans and experimental animals are well described in the literature. In this review, the author offers a 3-step pathway of pathogenesis for syndactyly. The first step is initiated either by the overactivation of the WNT canonical pathway or the suppression of the Bone Morphogenetic Protein (BMP) canonical pathway. This leads to an overexpression of Fibroblast Growth Factor 8 (FGF8). The final step is the suppression of retinoic acid in the interdigital mesenchyme leading to suppression of both apoptosis and extracellular matrix (ECM) degradation, resulting in syndactyly.

Spontaneous Cancers, But Not Many Induced Ones in Animals, Resemble Semi-New Organisms that Possess a Unique Programmed Cell Death Mode Different from Apoptosis, Senescent Death, Necrosis and Stress-Induced Cell Death

There are four basic cell death modes in animals, i.e. physiological senescent death (SD) and apoptosis as well as pathological necrosis and stress-induced cell death (SICD). There have been numerous publications describing “apoptosis” in cancer, mostly focused on killing cancer cells using radio- or chemo-therapy, with few on exploring how cancer cells die naturally without such treatments. Spontaneous benign or malignant neoplasms are immortal and autonomous, but they still retain some allegiance to their parental tissue or organ and thus are still somewhat controlled by the patient's body. Because of these properties of immortality, semi-autonomy, and semi-allegiance to the patient's body, spontaneous tumors have no redundant cells and resemble “semi-new organisms” parasitizing the patients, becoming a unique tissue type possessing a hitherto unannotated cell death mode besides SD, apoptosis, necrosis and SICD. Particularly, apoptosis aims to expunge redundant cells, whereas this new mode does not. In contrast to spontaneous tumors, many histologically malignant tumors induced in experimental animals, before they reach an advanced stage, regress after withdrawal of the inducer. This mortal and non-autonomous nature disqualifies these animal lesions as authentic neoplasms and as semi-new organisms but makes them a good tissue type for apoptosis studies. Ruminating over cell death in spontaneous cancers and many inauthentic tumors induced in animals from these new slants makes us realize that “whether cancer cells undergo apoptosis” is not an easy question with a simple answer. Our answer is that cancer cells have an uncharacterized programmed cell death mode, which is not apoptosis.

Evaluating the Remote Control of Programmed Cell Death, with or without a Compensatory Cell Proliferation

Organisms and their different component levels, whether organelle, cellular or other, come by birth and go by death, and the deaths are often balanced by new births. Evolution on the one hand has built demise program(s) in cells of organisms but on the other hand has established external controls on the program(s). For instance, evolution has established death program(s) in animal cells so that the cells can, when it is needed, commit apoptosis or senescent death (SD) in physiological situations and stress-induced cell death (SICD) in pathological situations. However, these programmed cell deaths are not predominantly regulated by the cells that do the dying but, instead, are controlled externally and remotely by the cells' superior(s), i.e. their host tissue or organ or even the animal's body. Currently, it is still unclear whether a cell has only one death program or has several programs respectively controlling SD, apoptosis and SICD. In animals, apoptosis exterminates, in a physiological manner, healthy but no-longer needed cells to avoid cell redundancy, whereas suicidal SD and SICD, like homicidal necrosis, terminate ill but useful cells, which may be followed by regeneration of the live cells and by scar formation to heal the damaged organ or tissue. Therefore, “who dies” clearly differentiates apoptosis from SD, SICD and necrosis. In animals, apoptosis can occur only in those cell types that retain a lifelong ability of proliferation and never occurs in those cell types that can no longer replicate in adulthood. In cancer cells, SICD is strengthened, apoptosis is dramatically weakened while SD has been lost. Most published studies professed to be about apoptosis are actually about SICD, which has four basic and well-articulated pathways involving caspases or involving pathological alterations in the mitochondria, endoplasmic reticula, or lysosomes.

An essential role for the nuclear protein Akirin2 in mouse limb interdigital tissue regression

The regulation of interdigital tissue regression requires the interplay of multiple spatiotemporally-controlled morphogen gradients to ensure proper limb formation and release of individual digits. Disruption to this process can lead to a number of limb abnormalities, including syndactyly. Akirins are highly conserved nuclear proteins that are known to interact with chromatin remodelling machinery at gene enhancers. In mammals, the analogue Akirin2 is essential for embryonic development and critical for a wide variety of roles in immune function, meiosis, myogenesis and brain development. Here we report a critical role for Akirin2 in the regulation of interdigital tissue regression in the mouse limb. Knockout of Akirin2 in limb epithelium leads to a loss of interdigital cell death and an increase in cell proliferation, resulting in retention of the interdigital web and soft-tissue syndactyly. This is associated with perdurance of Fgf8 expression in the ectoderm overlying the interdigital space. Our study supports a mechanism whereby Akirin2 is required for the downregulation of Fgf8 from the apical ectodermal ridge (AER) during limb development, and implies its requirement in signalling between interdigital mesenchymal cells and the AER.

Cooperation of BMP and IHH signaling in interdigital cell fate determination

The elaborate anatomy of hands and feet is shaped by coordinated formation of digits and regression of the interdigital mesenchyme (IM). A failure of this process causes persistence of interdigital webbing and consequently cutaneous syndactyly. Bone morphogenetic proteins (BMPs) are key inductive factors for interdigital cell death (ICD) in vivo. NOGGIN (NOG) is a major BMP antagonist that can interfere with BMP-induced ICD when applied exogenously, but its in vivo role in this process is unknown. We investigated the physiological role of NOG in ICD and found that Noggin null mice display cutaneous syndactyly and impaired interdigital mesenchyme specification. Failure of webbing regression was caused by lack of cell cycle exit and interdigital apoptosis. Unexpectedly, Noggin null mutants also exhibit increased Indian hedgehog (Ihh) expression within cartilage condensations that leads to aberrant extension of IHH downstream signaling into the interdigital mesenchyme. A converse phenotype with increased apoptosis and reduced cell proliferation was found in the interdigital mesenchyme of Ihh mutant embryos. Our data point towards a novel role for NOG in balancing Ihh expression in the digits impinging on digit-interdigit cross talk. This suggests a so far unrecognized physiological role for IHH in interdigital webbing biology.

Understanding the relationship between cell death and tissue shrinkage via a stochastic agent-based model

Cell death, a process which can occur both naturally and in response to insult, is both a complex and diverse phenomenon. Under some circumstances, dying cells actively contract and cause their neighbors to rearrange and maintain tissue integrity. Under other circumstances, dying cells leave behind gaps, which results in tissue separation. A better understanding of how the cellular scale features of cell death manifest on the population scale has implications ranging from morphogenesis to tumor response to treatment. However, the mechanistic relationship between cell death and population scale shrinkage is not well understood, and computational methods for studying these relationships are not well established. Here we propose a mechanically robust agent-based cell model designed to capture the implications of cell death on the population scale. In our agent-based model, algorithmic rules applied on the cellular level emerge on the population scale where their effects are quantified. To better quantify model uncertainty and parameter interactions, we implement a recently developed technique for conducting a variance-based sensitivity analysis on the stochastic model. From this analysis and subsequent investigation, we find that cellular scale shrinkage has the largest influence of all model parameters tested, and that by adjusting cellular scale shrinkage population shrinkage varies widely even across simulations which contain the same fraction of dying cells. We anticipate that the methods and results presented here are a starting point for significant future investigation toward quantifying the implications of different mechanisms of cell death on population and tissue scale behavior.

Diphtheria Toxin-Induced Cell Death Triggers Wnt-Dependent Hair Cell Regeneration in Neonatal Mice

Cochlear hair cells (HCs), the sensory cells that respond to sound, do not regenerate after damage in adult mammals, and their loss is a major cause of deafness. Here we show that HC regeneration in newborn mouse ears occurred spontaneously when the original cells were ablated by treatment with diphtheria toxin (DT) in ears that had been engineered to overexpress the DT receptor, but was not detectable when HCs were ablated in vivo by the aminoglycoside antibiotic neomycin. A variety of Wnts (Wnt1, Wnt2, Wnt2b, Wnt4, Wnt5a, Wnt7b, Wnt9a, Wnt9b, and Wnt11) and Wnt pathway component Krm2 were upregulated after DT damage. Nuclear β-catenin was upregulated in HCs and supporting cells of the DT-damaged cochlea. Pharmacological inhibition of Wnt decreased spontaneous regeneration, confirming a role of Wnt signaling in HC regeneration. Inhibition of Notch signaling further potentiated supporting cell proliferation and HC differentiation that occurred spontaneously. The absence of new HCs in the neomycin ears was correlated to less robust Wnt pathway activation, but the ears subjected to neomycin treatment nonetheless showed increased cell division and HC differentiation after subsequent forced upregulation of β-catenin. These studies suggest, first, that Wnt signaling plays a key role in regeneration, and, second, that the outcome of a regenerative response to damage in the newborn cochlea is determined by reaching a threshold level of Wnt signaling rather than its complete absence or presence.

SIGNIFICANCE STATEMENT

Sensory HCs of the inner ear do not regenerate in the adult, and their loss is a major cause of deafness. We found that HCs regenerated spontaneously in the newborn mouse after diphtheria toxin (DT)-induced, but not neomycin-induced, HC death. Regeneration depended on activation of Wnt signaling, and regeneration in DT-treated ears correlated to a higher level of Wnt activation than occurred in nonregenerating neomycin-treated ears. This is significant because insufficient regeneration caused by a failure to reach a threshold level of signaling, if true in the adult, has the potential to be exploited for development of clinical approaches for the treatment of deafness caused by HC loss.

Dinosaurs, Chameleons, Humans, and Evo-Devo Path: Linking Étienne Geoffroy's Teratology, Waddington's Homeorhesis, Alberch's Logic of "Monsters," and Goldschmidt Hopeful "Monsters"

Since the rise of evo-devo (evolutionary developmental biology) in the 1980s, few authors have attempted to combine the increasing knowledge obtained from the study of model organisms and human medicine with data from comparative anatomy and evolutionary biology in order to investigate the links between development, pathology, and macroevolution. Fortunately, this situation is slowly changing, with a renewed interest in evolutionary developmental pathology (evo-devo-path) in the past decades, as evidenced by the idea to publish this special, and very timely, issue on "Developmental Evolution in Biomedical Research." As all of us have recently been involved, independently, in works related in some way or another with evolution and developmental anomalies, we decided to join our different perspectives and backgrounds in the present contribution for this special issue. Specifically, we provide a brief historical account on the study of the links between evolution, development, and pathologies, followed by a review of the recent work done by each of us, and then by a general discussion on the broader developmental and macroevolutionary implications of our studies and works recently done by other authors. Our primary aims are to highlight the strength of studying developmental anomalies within an evolutionary framework to understand morphological diversity and disease by connecting the recent work done by us and others with the research done and broader ideas proposed by authors such as Étienne Geoffroy Saint-Hilaire, Waddington, Goldschmidt, Gould, and Per Alberch, among many others to pave the way for further and much needed work regarding abnormal development and macroevolution.

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.

Spatiotemporal distribution of proliferation, proapoptotic and antiapoptotic factors in the early human limb development

Involvement of proliferation and apoptosis in the human limb development was analyzed electronmicroscopically and immunohistochemically in histological sections of 8 human embryos, 4(th) -10(th) week old, using apoptotic (caspase-3, AIF, BAX), anti-apoptotic (Bcl-2) and proliferation (Ki-67) markers, and TUNEL method. The data were analyzed by Mann-Whitney test, Kruskal-Wallis and Dunn's post hoc test. Initially, developing human limbs consisted of mesenchymal core and surface ectoderm with apical ectodermal ridge (AER). During progression of development, strong proliferation activity gradually decreased in the mesenchyme (from 78% to 68%) and in the epithelium (from 62% to 42%), while in the differentiating finger cartilages proliferation was constantly low (26-7%). Apoptotic caspase-3 and AIF-positive cells characterized mesenchyme and AER at earliest stages, while during digit separation they appeared in interdigital mesenchyme as well. Strong Bcl-2 expression was observed in AER, subridge mesenchyme and phalanges, while BAX expression charaterized limb areas undergoing apoptosis. Ultrastructurally, proliferating cells showed mitotic figures, while apoptotic cells were characterized by nuclear fragmentation. Macrophages were observed around the apoptotic cells. We suggest that intense proliferation enables growth and elongation of human limb primordia, and differential growth of digits. Both caspase-3 and AIF-dependant pathways of cell death control the extent of AER and numer of cells in the subridge mesenchyme at earliest developmental stages, as well as process of digit separation at later stages of limb development. Spatio-temporal co-expresson of Bcl-2 and BAX indicates their role in suppression of apoptosis and selective stimulation of growth during human limb morphogenesis.

Reflections on the role of senescence during development and aging

New and stimulating results have challenged the concept that cellular senescence might not be synonymous with aging. It is indisputable that during aging, senescent cell accumulation has an impact on organismal health. Nevertheless, senescent cells are now known to display physiological roles during embryonic development, during wound healing repair and as a cellular response to stress. The fact that senescence has been found in cells that did not attain their maximal round of replications, nor have metabolic alterations or DNA damage, also challenges the paradigm that senescence is cellular aging, and it is in favor of the idea that cellular senescence is a phenomenon that has a function by itself. Therefore, in order to understand this phenomenon it is important to analyze the relationship between senescence and other cellular responses that have many features in common, such as apoptosis, cancer and autophagy, particularly highlighting their role during development and adulthood.

A Viability Approach for Robustness Measurement, Organizational Autopoiesis, and Cell Turnover in a Multicellular System

In this article, we use the potential of computational biology to highlight the key role of cell apoptosis for studying some tissue's properties through in silico experiments of morphogenesis. Our morphogenesis model is a new approach focusing on the deterministic program within cells that controls their placement and their differentiation at the beginning of the embryogenesis. Indeed, when the tissue is made by just a few pair of cells, we consider that cellular mechanisms are related neither to the influence of mechanical forces nor to the spread of chemicals. Dynamics are based on spatial and logical choices, the other factors being involved when the tissue contains a large number of cells. We had established a mathematical formulation of such a model and had enlightened the link between phenotype (cell placement and cell differentiation) and genotype (cell program) at the early embryogenesis. Indeed, that work allowed for generating any early tissue and the associated program that designs it. We propose now to study and assess some properties of these tissues for further selection and classification purposes. More precisely, we present in this article novel methods to measure tissue robustness based on the backward morphogenesis of our model. We also show some implementations of their self-maintenance properties, on the one hand to deal with environment disturbances through autopoiesis and on the other hand to achieve a dynamical steady state which ensures tissue renewal.

BMPs are direct triggers of interdigital programmed cell death

During vertebrate embryogenesis the interdigital mesenchyme is removed by programmed cell death (PCD), except in species with webbed limbs. Although bone morphogenetic proteins (BMPs) have long been known to be players in this process, it is unclear if they play a direct role in the interdigital mesenchyme or if they only act indirectly, by affecting fibroblast growth factor (FGF) signaling. A series of genetic studies have shown that BMPs act indirectly by regulating the withdrawal of FGF activity from the apical ectodermal ridge (AER); this FGF activity acts as a cell survival factor for the underlying mesenchyme. Other studies using exogenous factors to inhibit BMP activity in explanted mouse limbs suggest that BMPs do not act directly in the mesenchyme. To address the question of whether BMPs act directly, we used an interdigit-specific Cre line to inactivate several genes that encode components of the BMP signaling pathway, without perturbing the normal downregulation of AER-FGF activity. Of three Bmps expressed in the interdigital mesenchyme, Bmp7 is necessary for PCD, but Bmp2 and Bmp4 both have redundant roles, with Bmp2 being the more prominent player. Removing BMP signals to the interdigit by deleting the receptor gene, Bmpr1a, causes a loss of PCD and syndactyly, thereby unequivocally proving that BMPs are direct triggers of PCD in this tissue. We present a model in which two events must occur for normal interdigital PCD: the presence of a BMP death trigger and the absence of an FGF survival activity. We demonstrate that neither event is required for formation of the interdigital vasculature, which is necessary for PCD. However, both events converge on the production of reactive oxygen species that activate PCD.

Hand/foot splitting and the 're-evolution' of mesopodial skeletal elements during the evolution and radiation of chameleons

Background

One of the most distinctive traits found within Chamaeleonidae is their split/cleft autopodia and the simplified and divergent morphology of the mesopodial skeleton. These anatomical characteristics have facilitated the adaptive radiation of chameleons to arboreal niches. To better understand the homology of chameleon carpal and tarsal elements, the process of syndactyly, cleft formation, and how modification of the mesopodial skeleton has played a role in the evolution and diversification of chameleons, we have studied the Veiled Chameleon (Chamaeleo calyptratus). We analysed limb patterning and morphogenesis through in situ hybridization, in vitro whole embryo culture and pharmacological perturbation, scoring for apoptosis, clefting, and skeletogenesis. Furthermore, we framed our data within a phylogenetic context by performing comparative skeletal analyses in 8 of the 12 currently recognized genera of extant chameleons.

Results

Our study uncovered a previously underappreciated degree of mesopodial skeletal diversity in chameleons. Phylogenetically derived chameleons exhibit a ‘typical’ outgroup complement of mesopodial elements (with the exception of centralia), with twice the number of currently recognized carpal and tarsal elements considered for this clade. In contrast to avians and rodents, mesenchymal clefting in chameleons commences in spite of the maintenance of a robust apical ectodermal ridge (AER). Furthermore, Bmp signaling appears to be important for cleft initiation but not for maintenance of apoptosis. Interdigital cell death therefore may be an ancestral characteristic of the autopodium, however syndactyly is an evolutionary novelty. In addition, we find that the pisiform segments from the ulnare and that chameleons lack an astragalus-calcaneum complex typical of amniotes and have evolved an ankle architecture convergent with amphibians in phylogenetically higher chameleons.

Conclusion

Our data underscores the importance of comparative and phylogenetic approaches when studying development. Body size may have played a role in the characteristic mesopodial skeletal architecture of chameleons by constraining deployment of the skeletogenic program in the smaller and earliest diverged and basal taxa. Our study challenges the ‘re-evolution’ of osteological features by showing that ‘re-evolving’ a ‘lost’ feature de novo (contrary to Dollo’s Law) may instead be due to so called ‘missing structures’ being present but underdeveloped and/or fused to other adjacent elements (cryptic features) whose independence may be re-established under changes in adaptive selective pressure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0464-4) contains supplementary material, which is available to authorized users.

Programmed cell death in aging

Programmed cell death (PCD) pathways, including apoptosis and regulated necrosis, are required for normal cell turnover and tissue homeostasis. Mis-regulation of PCD is increasingly implicated in aging and aging-related disease. During aging the cell turnover rate declines for several highly-mitotic tissues. Aging-associated disruptions in systemic and inter-cell signaling combined with cell-autonomous damage and mitochondrial malfunction result in increased PCD in some cell types, and decreased PCD in other cell types. Increased PCD during aging is implicated in immune system decline, skeletal muscle wasting (sarcopenia), loss of cells in the heart, and neurodegenerative disease. In contrast, cancer cells and senescent cells are resistant to PCD, enabling them to increase in abundance during aging. PCD pathways limit life span in fungi, but whether PCD pathways normally limit adult metazoan life span is not yet clear. PCD is regulated by a balance of negative and positive factors, including the mitochondria, which are particularly subject to aging-associated malfunction.

Radiological imaging of congenital hand anomalies--a 6-year single-centre experience and what the hand surgeons want to know

OBJECTIVE

Congenital hand anomalies present a rare but important physical and emotional challenge for children and parents. Radiological imaging is important for accurate diagnosis, to aid decision making and to monitor changes in the growing hand. The goal of any treatment is to help the child achieve his/her maximum potential, to provide a useful hand with attention to cosmesis. We investigated the range of congenital hand anomalies imaged in a tertiary referral centre. We examined the timing of imaging and the key clinical questions.

METHODS

The radiology imaging system was searched retrospectively for radiographs of congenital hand anomalies over a 6-year period. The images were reviewed and patient demographics, diagnosis and other imaging recorded.

RESULTS

Over 6 years, 85 patients had imaging. Twenty-three patients had bilateral problems and 11 had recognised syndromes. The most common abnormalities imaged were duplicated thumbs (28 %), followed by syndactyly (18 %). Children were first imaged as early as 1 day old, with the median age of initial imaging 12 months.

CONCLUSIONS

Thumb duplication and syndactyly are the most common conditions for which radiographs are requested at our hospital, although overall syndactyly is considered the most common congenital hand anomaly. For a variety of reasons, children are often imaged very early, before review by the Specialist in Children's Hand Surgery (despite surgery being unlikely before 1 year of age.) We discuss the classification systems and specific issues that hand surgeons want to know from the radiologists.

Retinoic acid-independent expression of Meis2 during autopod patterning in the developing bat and mouse limb

BACKGROUND

The bat has strikingly divergent forelimbs (long digits supporting wing membranes) and hindlimbs (short, typically free digits) due to the distinct requirements of both aerial and terrestrial locomotion. During embryonic development, the morphology of the bat forelimb deviates dramatically from the mouse and chick, offering an alternative paradigm for identifying genes that play an important role in limb patterning.

RESULTS

Using transcriptome analysis of developing Natal long-fingered bat (Miniopterus natalensis) fore- and hindlimbs, we demonstrate that the transcription factor Meis2 has a significantly higher expression in bat forelimb autopods compared to hindlimbs. Validation by reverse transcriptase and quantitative polymerase chain reaction (RT-qPCR) and whole mount in situ hybridisation shows that Meis2, conventionally known as a marker of the early proximal limb bud, is upregulated in the bat forelimb autopod from CS16. Meis2 expression is localised to the expanding interdigital webbing and the membranes linking the wing to the hindlimb and tail. In mice, Meis2 is also expressed in the interdigital region prior to tissue regression. This interdigital Meis2 expression is not activated by retinoic acid (RA) signalling as it is present in the retained interdigital tissue of Rdh10 (trex/trex) mice, which lack RA. Additionally, genes encoding RA-synthesising enzymes, Rdh10 and Aldh1a2, and the RA nuclear receptor Rarβ are robustly expressed in bat fore- and hindlimb interdigital tissues indicating that the mechanism that retains interdigital tissue in bats also occurs independently of RA signalling.

CONCLUSIONS

Mammalian interdigital Meis2 expression, and upregulation in the interdigital webbing of bat wings, suggests an important role for Meis2 in autopod development. Interdigital Meis2 expression is RA-independent, and retention of interdigital webbing in bat wings is not due to the suppression of RA-induced cell death. Rather, RA signalling may play a role in the thinning (rather than complete loss) of the interdigital tissue in the bat forelimb, while Meis2 may interact with other factors during both bat and mouse autopod development to maintain a pool of interdigital cells that contribute to digit patterning and growth.

Vascular patterning regulates interdigital cell death by a ROS-mediated mechanism

Blood vessels serve as key regulators of organogenesis by providing oxygen, nutrients and molecular signals. During limb development, programmed cell death (PCD) contributes to separation of the digits. Interestingly, prior to the onset of PCD, the autopod vasculature undergoes extensive patterning that results in high interdigital vascularity. Here, we show that in mice, the limb vasculature positively regulates interdigital PCD. In vivo, reduction in interdigital vessel number inhibited PCD, resulting in syndactyly, whereas an increment in vessel number and distribution resulted in elevation and expansion of PCD. Production of reactive oxygen species (ROS), toxic compounds that have been implicated in PCD, also depended on interdigital vascular patterning. Finally, ex vivo incubation of limbs in gradually decreasing oxygen levels led to a correlated reduction in both ROS production and interdigital PCD. The results support a role for oxygen in these processes and provide a mechanistic explanation for the counterintuitive positive role of the vasculature in PCD. In conclusion, we suggest a new role for vascular patterning during limb development in regulating interdigital PCD by ROS production. More broadly, we propose a double safety mechanism that restricts PCD to interdigital areas, as the genetic program of PCD provides the first layer and vascular patterning serves as the second.

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Interdigital cell death in the embryonic limb is associated with depletion of Reelin in the extracellular matrix

Interdigital cell death is a physiological regression process responsible for sculpturing the digits in the embryonic vertebrate limb. Changes in the intensity of this degenerative process account for the different patterns of interdigital webbing among vertebrate species. Here, we show that Reelin is present in the extracellular matrix of the interdigital mesoderm of chick and mouse embryos during the developmental stages of digit formation. Reelin is a large extracellular glycoprotein which has important functions in the developing nervous system, including neuronal survival; however, the significance of Reelin in other systems has received very little attention. We show that reelin expression becomes intensely downregulated in both the chick and mouse interdigits preceding the establishment of the areas of interdigital cell death. Furthermore, fibroblast growth factors, which are cell survival signals for the interdigital mesoderm, intensely upregulated reelin expression, while BMPs, which are proapototic signals, downregulate its expression in the interdigit. Gene silencing experiments of reelin gene or its intracellular effector Dab-1 confirmed the implication of Reelin signaling as a survival factor for the limb undifferentiated mesoderm. We found that Reelin activates canonical survival pathways in the limb mesoderm involving protein kinase B and focal adhesion kinase. Our findings support that Reelin plays a role in interdigital cell death, and suggests that anoikis (apoptosis secondary to loss of cell adhesion) may be involved in this process.

Tumor suppressors: enhancers or suppressors of regeneration?

Tumor suppressors are so named because cancers occur in their absence, but these genes also have important functions in development, metabolism and tissue homeostasis. Here, we discuss known and potential functions of tumor suppressor genes during tissue regeneration, focusing on the evolutionarily conserved tumor suppressors pRb1, p53, Pten and Hippo. We propose that their activity is essential for tissue regeneration. This is in contrast to suggestions that tumor suppression is a trade-off for regenerative capacity. We also hypothesize that certain aspects of tumor suppressor pathways inhibit regenerative processes in mammals, and that transient targeted modification of these pathways could be fruitfully exploited to enhance processes that are important to regenerative medicine.

Shaping organisms with apoptosis

Programmed cell death is an important process during development that serves to remove superfluous cells and tissues, such as larval organs during metamorphosis, supernumerary cells during nervous system development, muscle patterning and cardiac morphogenesis. Different kinds of cell death have been observed and were originally classified based on distinct morphological features: (1) type I programmed cell death (PCD) or apoptosis is recognized by cell rounding, DNA fragmentation, externalization of phosphatidyl serine, caspase activation and the absence of inflammatory reaction, (2) type II PCD or autophagy is characterized by the presence of large vacuoles and the fact that cells can recover until very late in the process and (3) necrosis is associated with an uncontrolled release of the intracellular content after cell swelling and rupture of the membrane, which commonly induces an inflammatory response. In this review, we will focus exclusively on developmental cell death by apoptosis and its role in tissue remodeling.

Cdc42 is required for chondrogenesis and interdigital programmed cell death during limb development

Cdc42, a member of the Rho subfamily of small GTPases, is known to be a regulator of multiple cellular functions, including cytoskeletal organization, cell migration, proliferation, and apoptosis. However, its tissue-specific roles, especially in mammalian limb development, remain unclear. To investigate the physiological function of Cdc42 during limb development, we generated limb bud mesenchyme-specific inactivated Cdc42 (Cdc42(fl/fl); Prx1-Cre) mice. Cdc42(fl/fl); Prx1-Cre mice demonstrated short limbs and body, abnormal calcification of the cranium, cleft palate, disruption of the xiphoid process, and syndactyly. Severe defects were also found in long bone growth plate cartilage, characterized by loss of columnar organization of chondrocytes, and thickening and massive accumulation of hypertrophic chondrocytes, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed that expressions of Col10 and Mmp13 were reduced in non-resorbed hypertrophic cartilage, indicating that deletion of Cdc42 inhibited their terminal differentiation. Syndactyly in Cdc42(fl/fl); Prx1-Cre mice was caused by fusion of metacarpals and a failure of interdigital programmed cell death (ID-PCD). Whole mount in situ hybridization analysis of limb buds showed that the expression patterns of Sox9 were ectopic, while those of Bmp2, Msx1, and Msx2, known to promote apoptosis in the interdigital mesenchyme, were down-regulated. These results demonstrate that Cdc42 is essential for chondrogenesis and ID-PCD during limb development.

Smad1/Smad5 signaling in limb ectoderm functions redundantly and is required for interdigital programmed cell death

Bone morphogenetic proteins (BMPs) are secreted signals that regulate apical ectodermal ridge (AER) functions and interdigital programmed cell death (PCD) of developing limb. However the identities of the intracellular mediators of these signals are unknown. To investigate the role of Smad proteins in BMP-regulated AER functions in limb development, we inactivated Smad1 and Smad5 selectively in AER and ventral ectoderm of developing limb, using Smad1 or/and Smad5 floxed alleles and an En1(Cre/+) knock-in allele. Single inactivation of either Smad1 or Smad5 did not result in limb abnormalities. However, the Smad1/Smad5 double mutants exhibited syndactyly due to a reduction in interdigital PCD and an increase in interdigital cell proliferation. Cell tracing experiments in the Smad1/Smad5 double mutants showed that ventral ectoderm became thicker and the descendents of ventral En1(Cre/+) expressing ectodermal cells were located at dorsal interdigital regions. At the molecular level, Fgf8 expression was prolonged in the interdigital ectoderm of embryonic day (E) 13 Smad1/Smad5 double mutants, suggesting that the ectopic Fgf8 expression may serve as a survival signal for interdigital epithelial and mesenchymal cells. Our result suggests that Smad1 and Smad5 are required and function redundantly as intracellular mediators for BMP signaling in the AER and ventral ectoderm. Smad1/Smad5 signaling in the AER and ventral ectoderm regulates interdigital tissue regression of developing limb. Our mutants with defects in interdigital PCD could also serve as a valuable model for investigation of PCD regulation machinery.

Molecular control of cell differentiation and programmed cell death during digit development

During the hand plate development, the processes of cell differentiation and control of cell death are relevant to ensure a correct shape of the limb. The progenitor cell pool that later will differentiate into cartilage to form the digits arises from undifferentiated mesenchymal cells beneath the apical ectodermal ridge (AER). Once these cells abandon the area of influence of signals from AER and ectoderm, some cells are committed to chondrocyte lineage forming the digital rays. However, if the cells are not committed to chondrocyte lineage, they will form the prospective interdigits that in species with free digits will subsequently die. In this work, we provide the overview of the molecular interactions between different signaling pathways responsible for the formation of digit and interdigit regions. In addition, we briefly describe some experiments concerning the most important signals responsible for promoting cell death. Finally, on the basis that the interdigital tissue has chondrogenic potential, we discuss the hypothesis that apoptotic-promoting signals might also act as antichondrogenic factors and chondrogenic factors might operate as anti-apoptotic factors.

Smad8 is expressed in the anterior necrotic zone: evidence for a role of bone morphogenetic proteins/SMAD signaling in the activation of a molecular cascade that culminates in cell death

Bone morphogenetic proteins (BMPs) play a crucial role in programmed cell death (PCD), a biological process required for the sculpturing of the embryonic limbs. However, it is unknown if BMP signaling directly promotes cell death, or if it induces a molecular cascade that culminates in cell death. Given that Smad8, which encodes one component of BMP signaling, is expressed during the regression of interdigital tissue and responds to BMPs, we presumed that it may be expressed in other cell death areas during chick limb development such as the anterior and posterior necrotic zones (ANZ and PNZ). The present study found that the Smad8 expression pattern in the anterior mesoderm of the hindlimb is very similar to that observed in limbs stained to detect cell death. Also, BMPs and retinoic acid, which act as apoptosis-promoting factors, induced expression of Smad8 before the onset of cell death, while sonic hedgehog protein, acting as a survival factor, inhibited Smad8 expression in the ANZ. However, although there was correlation between Smad8 expression patterns and PCD in the ANZ, phosphorylated forms of SMAD1/5/8 and TUNEL staining did not co-localize in dying cells. Interestingly, a short pulse of BMP was sufficient to trigger cell death. On the other hand, most dying cells were located in the avascular region, while many cells expressing Smad8 were located in the vascular region of the ANZ. These results suggest that BMPs mediated by SMAD signaling activate a molecular cascade that culminates in PCD.