多疣壁虎胚胎发育分期的形态学特征

脊椎动物发育生物学的研究通常依赖于数量有限的模式生物的形态变化,胚胎发育分期表的建立为物种胚胎发育的一系列过程确立了一个统一的标准,成为研究形态演化的重要工具。本研究对多疣壁虎(Gekko japonicus)28℃孵化条件下的胚胎发育过程进行显微观察,并记录了整个胚胎发育历程。基于多疣壁虎胚胎发育过程中头部、咽、四肢等形态变化及皮肤色素沉积和被鳞的情况,将多疣壁虎胚胎发育分为42个时期。刚排出体外的受精卵,其胚胎发育一般已经发生至28期,该期胚胎头部和躯干分化明显,眼泡、咽弓、心和体节可见;29期前、后肢芽均可见;30期肢芽延长并开始出现分区,31期可见明显肢身,32期四肢均出现肢柱和肢杆的分区;33期咽裂消失,指和趾开始显现;35期指和趾间带退化,指和趾完全形成;36期出现爪;37期爪完全形成;38期皮肤色素沉积明显;39期指、趾底部膨大,形成单行攀瓣;40期身体背部和四肢色素沉积且被鳞明显;41期腹部出现色素沉积且被覆鳞片。42期鼻孔开放,体背整体呈灰棕色。对多疣壁虎卵产出后胚胎28~42期发育期形态学变化进行了详细描述,旨在为蜥蜴类胚胎发育研究提供参考。     

Development of limbs in urodeles and the origin of tetrapod limbs

On the basis of studies on serial sections of larval Ranodon sibiricus limbs and published data, the hypothesis of the origin of tetrapod limbs from the biserial archipterygium is proposed. The mesomeres of the central axis of the biserial fin correspond (in proximodistal direction) to the humerus, ulna, ulnare, all carpalia distalia, metacarpale 1, and phalanges of the first digit in the forelimb of caudate amphibians and to the femur, fibula, fibulare, tarsalia distalia, metatarsale 1, and phalanges of the first digit in the hind limb. The preaxial elements of the zygopodium and autopodium, which are positioned proximal to the digital arch, correspond to the preaxial rays of the biserial fin, and digits 2–5 correspond to its postaxial rays. As the fin transformed into the limb, the central axis curved preaxially, forming the digital arch and resulting in partial reduction and fusion of preaxial rays.     

Growth and apoptosis during larval forelimb development and adult forelimb regeneration in the newt (<Emphasis Type="Italic">Notophthalmus viridescens</Emphasis>)

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.     

Morphogenesis of the prehensile autopodium in the common chameleon (Chamaeleo Chamaeleo)

This report documents the development of the autopodium of the common chameleon (Chamaeleo chamaeleo) using light microscopy, scanning electron microscopy, and transmission electron microscopy. Three main periods were distinguished during the morphogenesis of this structure. In the first period (stages 33-35 of chameleon development) the autopodium is paddle-shaped with a prominent apical ectodermal ridge (AER) along the distal margin. During this period the AER has structural features similar to other reptilian and avian vertebrates except for the scarcity or absence of gap junctions. The second period of autopodium morphogenesis (stage 36 of chameleon development) is characterized by the formation of a central cleft which divides this structure into two digital segments. In the forelimb the autopodial cleft occupies the space between digits 3 and 4. In the hindlimb the cleft occupies the space between digits 2 and 3. Mesenchymal cell death constitutes a constant feature during cleft formation. In addition to cell death during this process, we have observed that the AER flattens out in the zone of cleft formation while in the digital portions of the autopodium it takes on a polystratified appearance. In the last period of autopodial morphogenesis (stage 37 of chameleon development) digits become free by means of interdigital mesenchymal cell death.     

Tamoxifen-Dependent,Inducible Bmp2CreER Drives Selective Recombinase Activity in Early Interdigital Mesenchyme and Digit Collateral Ligaments

During limb development, the interdigital mesenchyme has been proposed to play a signaling role instructing morphogenesis of different digit types, as well as undergoing programmed cell death necessary to free digits in animals not adapted for swimming or flying. We have generated a conditional, tamoxifen-dependent Cre line, Bmp2CreER, which drives highly selective recombination restricted to the distal limb mesoderm, largely restricted to the interdigits, and selectively active in digit ligament but not tendon progenitors at later stages. The Bmp2CreER provides a valuable new tool to dissect roles of interdigital mesenchyme and potentially investigate divergence of ligament and tendon lineages.     

THE MODE OF LIFE OF HYPSILOPHODON, THE SUPPOSEDLY ARBOREAL ORNITHOPOD DINOSAUR

Previous discussions of the mode of life of Hypsilophodon (Lower Cretaceous, England) are reviewed, and it is concluded that this primitive ornithopod was cursorial. There are no specific adaptations for an arboreal mode of life, and the hallux was not opposable. Metatarsal I was closely applied along its length to metatarsal II, and the first digit moved parallel to or. slightly away from the other digits. Relative to the trunk, the hind limb is long, with an elongate tibia and metatarsus, so that the proportions of the different regions fall in the range for cursorial mammals.     

The absence of cell death during development of free digits in amphibians

Massive cellular death occurs in the interdigital regions of developing limbs of free-digited birds and mammals. This mesodermal degeneration occurs at the same time that digits become free. The present study of digit formation in amphibians, using vital staining and histological and autoradiographic techniques, demonstrates the absence of zones of interdigital degeneration during the formation of free digits. Furthermore, no other areas of predictable cell death occur during amphibian limb development, a situation quite unlike the case for avian limb development where predictable zones of degeneration occur in the mesoderm along the pre- and postaxial borders of the developing wing and leg. Thus, zones of cell death are not a part of amphibian limb morphogenesis. Analysis of the labeling index of the developing free-digited forelimb of Xenopus laevis reveals that during stage 52 the interdigital and digital labeling indexes are the same. The change in the ratio of interdigital labeling index to the digital labeling index in the forelimb suggests that during subsequent development the interdigital labeling index decreases while the digital labeling index is maintained. In comparison, the same analysis indicates that the interdigital labeling index of the webbed hindlimb increases when compared to the digital labeling index, which stays the same from early to late stages. It is proposed that free digits develop in Xenopus laevis forelimb as a result of a decrease in the proliferation rate of the interdigital region as compared to the digital region, which remains unchanged. Conversely, webbed digits develop in the hindlimb as a result of an interdigital rate at least equal to the digital rate.     

Ectopic dermal ridge configurations on the interdigital webbings and postaxial marginal portion of the hindlimb in Hammertoe mutant mice (Hm)

The effects of the hereditary malformation of Hammertoe mutant mice (gene symbol Hm) on the digital pads and dermal ridge configurations on their hindlimbs were examined. In the wild‐type (+/+) mice with normally separated digits, dermal ridges developed only on the pads. Heterozygous (Hm/+) and homozygous (Hm/Hm) mutant mice, however, had a broad big toe, fused interdigital soft tissues, reduced claws, an extra rudimentary postaxial digit and camptodactyly. The dermal ridges appeared not only on the pads, affected in their number and configurations, but also on the ventral surface of the interdigital webbings and postaxial marginal area exhibiting an extra rudimentary digit and webbing. These aberrant configurations may be related to the abnormal occurrence of programmed cell death (PCD) in the interdigital zones and the postaxial marginal portion in Hm/+ and Hm/Hm mice. That is, the diminished cell death may fail to decrease the cell density in the interdigital zones and postaxial marginal portion and result in the webbing and an extra rudimentary digit and webbing, respectively. Simultaneously, it could also interrupt the migration of surviving cells of these areas toward the neighboring digits and the distal area of the sole and produce the ectopic dermal ridges on the way to the as yet unformed (presumptive) digital and plantar volar pads. The present findings suggest that normal interdigital and pre/postaxial PCD contributes not only to the separation of digits, the initial formation of individual digits of different sizes, and the inhibition of the extra digit but also to the development of the presumptive digital and plantar pads, including dermal ridges. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Tbx Genes Specify Posterior Digit Identity through Shh and BMP Signaling

Despite extensive studies on the anterior-posterior (AP) axis formation of limb buds, mechanisms that specify digit identities along the AP axis remain obscure. Using the four-digit chick leg as a model, we report here that Tbx2 and Tbx3 specify the digit identities of digits IV and III, respectively. Misexpression of Tbx2 and Tbx3 induced posterior homeotic transformation of digit III to digit IV and digit II to digit III, respectively. Conversely, misexpression of their mutants VP16 Delta Tbx2 and VP16 Delta Tbx3 induced anterior transformation. In both cases, alterations in the expression of several markers (e.g., BMP2, Shh, and HoxD genes) were observed. In addition, Tbx2 and Tbx3 rescued Noggin-mediated inhibition of interdigital BMP signaling, signaling which is pivotal in establishing digit identities. Hence, we conclude that Tbx3 specifies digit III, and the combination of Tbx2 and Tbx3 specifies digit IV, acting together with the interdigital BMP signaling cascade.     

Sequential changes of programmed cell death in developing fetal mouse limbs and its possible roles in limb morphogenesis

Apoptotic cell death in the developing limb of mouse fetuses was examined sequentially on days 11–15 of gestation by means of Nile blue (NB) sulfate staining with special reference to its relation to limb morphogenesis. With some exceptions, programmed cell death (PCD) in the hand and foot was observed in the mesenchyme but not in the surface ectoderm. We found that during digital formation PCD begins at the proximal portion of the interdigital mesenchyme and subsequently expands distally. Therefore, the initial PCD that occurs in the interdigital zones may determine the proximal ends of digital separation and also contribute to the demarcation between the palm (sole) and digits (toes). During digital separation, the areas of PCD in the interdigital zones were found to become larger and expand distally on day 13, which may be necessary for the separation of digits and for determining the interdigital area to disappear. PCD in presumptive phalangeal joints was also found to proceed from proximal to more distal joints. The PCD in presumptive joints may be required for the separation of phalanges and metacarpal (metatarsal) bones and for the formation of joint cavities. In addition, intense PCD was observed in the radial (tibial) and ulnar (fibular) margins of the hand and foot plates for 4–5 days. Such PCD at marginal areas seems to prevent the formation of supernumerary digits (preaxial and postaxial polydactyly) and other digital malformations. Therefore, the timing when PCD commences and ends, the sites where PCD occurs, and the intensity, duration, and proximo-distal progress of PCD appear to be genetically determined, and the elimination of unnecessary cells by PCD may be essential for normal limb morphogenesis. The present findings also suggest that the normal progress of PCD in the hand and foot plates of rodent fetuses may prevent the formation of some limb malformations such as webbing fusion of digits, polydactyly, or cleft hand/foot. © 1996 Wiley-Liss, Inc.     

Ontogenetic allometry of the digital rays of the leopard gecko (Gekkota: Eublepharidae; Eublepharis macularius)

The role of allometry in producing the variation in autopodial morphology observed among the lizards is not well understood. Allometry of metapodial and digit lengths in the manus and pes of the primitively padless gekkotan (Eublepharis macularius) is explored using maximum‐likelihood repeated‐measures ANCOVAs with body length as the covariate. Estimated variance–covariance matrices differed significantly within and between autopodia, and integration was stronger among the metapodials than the digits. The first metapodial and the first digit of each autopodium exhibit the strongest covariances with each of the remaining components in each variance–covariance matrix, suggesting that the lengths of the first rays are important for allometric integration of both manus and pes. Metapodials scale isometrically and digits negatively allometrically; both display allometric heterogeneity among themselves in both autopodia. Both autopodia exhibit changes in proportion over the ontogenetic size range, attributable to variation in scaling among the components of the rays. Allometric coefficients do not vary among pedal digits, despite differences in phalanx number, although phalanx number is associated with differences in slope in the manual digits. This is suggestive of heterogeneity in allometry among the manual phalanges, which thus may be associated with variation in phalanx length within gekkotan digits.     

Bmp-4 requires the presence of the digits to initiate programmed cell death in limb interdigital tissues

The effects of Bmp-4 on interdigital cell death were investigated in the mouse. Affi-Gel beads, loaded with recombinant Bmp-4 protein, were transplanted into the interdigital tissues of day 12.5 hindlimb, ex utero. It was established that Bmp-4 could induce precocious interdigital cell death. Using in situ hybridization, the expression patterns of bmp-4 and alk-6 receptor were established. Both genes were found coexpressed in the interdigital region of 12.5- and 13. 5-day hindlimbs. This suggests that Bmp-4 may act in an autocrine fashion. We have also studied the effects of Bmp-4 on 12.5-day interdigital tissue cultures. In all specimens examined, the interdigital tissues produced cartilage instead of participating in cell death. The addition of exogenous Bmp-4 to the interdigital cultures did not induce apoptosis but instead enhanced chondrogenesis. The discrepancy between the effects of Bmp-4 in vitro and ex utero was attributed to the presence of digits. When a flanking digit was left attached to the interdigital tissues, in vitro, Bmp-4 promoted apoptosis instead of chondrogenesis. In sum, the results suggest that Bmp-4 is a multifunctional protein and its effect on the interdigital tissues is dependent on the modulating influence of the digits.     

A histological study of cutaneous glands in the brown brocket deer

We examined the histological structure of 8 skin areas thought to contain cutaneous glands of potential importance in scent communication in 16 brown brocket deerMazama gouazoubira Fisher, 1814, using standard histological techniques. Frontal areas and preorbital sacs had scant glandular development. Sebaceous gland development was prominent in vestibular nasal glands and prepucial glands. Apocrine sudoriferous glands and sebaceous glands were well developed in tarsal glands, the caudal skin area and the interdigital glands of front and hind feet. The tail had a unique arrangement of apocrine sudoriferous glands. Anal glands had moderate glandular development, and metatarsal glands were absent. Several of these glandular areas may be important in the chemical communication among brocket deer.     

Cryptic diversity and morphological convergence in threatened species of fossorial skinks in the genus Scelotes (Squamata: Scincidae) from the Western Cape Coast of South Africa: implications for species boundaries, digit reduction and conservation

We investigated the evolutionary relationships among populations of two threatened Red Data Book fossorial skinks, Scelotes gronovii and Scelotes kasneri, along the Western Cape Coast of South Africa. The genus Scelotes shows considerable variation in limb and digit reduction. We sampled four localities purported to contain S. gronovii and seven of S. kasneri, encompassing all of each species' limited distribution. Each of these species lack forelimbs, and differ by the number of digits on the hind limbs, among other morphological characters; S. gronovii bears a single digit and S. kasneri bears two digits on the hind limbs. Sequence data obtained from three mtDNA (16S ribosomal RNA, cytochrome b, and nicotinamide adenine dinucleotide dehydrogenase 1 unit; 2035 bp ttl.) and two nuclear (dynein axonemal heavy chain 3 and the natural killer tumor recognition; 1848 bp ttl.) gene regions were used to reconstruct the evolutionary relationships among the two focal species and several other co-distributed species (Scelotes bipes, Scelotes montispectus, and Scelotes sexlineatus). Phylogenetic results (Bayesian and parsimony) revealed that several populations previously considered S. kasneri actually belong to other species, and others are paraphyletic with respect to one another. Additionally, populations of S. gronovii were also found to be paraphyletic, with populations south of the Berg River supported as sister to S. bipes, and populations north of the Berg River sister the remaining sampled species. Our results require a redefinition of S. sexlineatus to encompass populations morphologically convergent with S. kasneri and restrict the ranges of the already threatened S. kasneri and S. gronovii even further. The paraphyly of S. gronovii and the placement of each clade as sister to clades of species bearing two digits on the hind limbs suggests that digit loss has occurred at least twice in this group.     

Experimental manipulation of yolk testosterone affects digit length ratios in the ring-necked pheasant (Phasianus colchicus)

In humans, most of the mammals and one bird species studied so far, the relative length of individual digits is sexually dimorphic. Most studies of humans have been concerned with the ratio between second (2D) and fourth digits (4D), whereas some studies of humans and other mammals have also investigated other digit ratios. Inter- and intra-sexual variation in 2D:4D may depend on differential exposure to androgens during embryonic life, and the genetic mechanisms linking 2D:4D to androgens may be mediated by Hox genes. Because Hox genes are conserved in vertebrates, similar patterns of variation in digit ratios might be expected across vertebrate classes. The observation of correlations between digit ratios and physiological, psychological and performance traits in humans has generated interest in exploring the possibility that digit ratios are a marker of embryonic exposure to androgens, which have diverse consequences on several phenotypic traits. However, the hypothesis that digit ratios depend on androgen effects during development has never been tested experimentally. In this study, we increased testosterone concentration in ring-necked pheasant eggs and measured length ratios between the second, third and fourth digits of both feet in fully grown offspring. Females from testosterone-injected eggs had larger 2D:3D in the left foot, whereas this was not the case in males. The other digit ratios were unaffected by hormone treatment in both sexes. However, digit ratios showed no sexual dimorphism among controls. Thus, present results are consistent with the hypothesis that variation in testosterone levels during development affects digit ratios.     

Hind limb myology of the common hippopotamus,Hippopotamus amphibius (Artiodactyla: Hippopotamidae)

Based on morphological traits, hippos have traditionally been classified with pigs and peccaries in the suborder Suiformes. However, molecular data indicate that hippos and cetaceans are sister taxa. This study analyses muscle characters of the common hippo hind limb in order to clarify the phylogenetic relationships and functional anatomy of hippos. Several muscles responsible for propelling the body through water are robust and display extensive fusions, including mm. semimembranosus, semitendinosus, biceps femoris and gluteus superficialis. In addition, common hippos retain long flexor and extensor tendons for each digit, reflecting the fact that all four toes are weight‐bearing. These flexor tendons, together with the well‐developed intrinsic muscles of the pes, serve to adduct the digits, preventing splaying of the toes when walking on soft terrain. Lastly, common hippos retain a number of primitive features, including the presence of m. articularis coxae, a well‐developed m. obturator internus, superficialis and profundus tendons to all digits, mm. flexor digitorum brevis, abductor digiti V, lumbricalis IV, adductores digitorum II and V, and two mm. interossei per digit. Pygmy hippos share these features. Thus, hippopotamids retain muscles that have been lost in the majority of artiodactyls, including other suiforms. These and previously reported findings for the forelimb support the molecular data in indicating an early divergence of the Hippopotamidae from the rest of the Artiodactyla. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 661–682.     

Shape variability in tridactyl dinosaur footprints: the significance of size and function

The functional anatomy of the hindlimb of bipedal dinosaurs has been intensively studied. Yet, surprisingly little work has been done concerning functional adaptation of digits for terrestrial locomotion. While complete and articulated pes skeletons are scarce, pes shape is abundantly recorded by fossil footprints. We elucidate the significance of footprint shape and size for locomotion using a large sample (n = 303) of tridactyl dinosaur footprints from a broad range of geographical localities and time slots. Size and shape variation are characterized separately for theropods and ornithischians, the two principal trackmaker taxa. At smaller sizes, theropod footprints are best discriminated from ornithischian footprints by their smaller interdigital angle and larger projection of digit III; at larger sizes digital widths are effective discriminants. Ornithischian footprints increase in size from the Early Jurassic to the Late Cretaceous, a trend not observed in theropod footprints. Size and function are argued to be important determinants of footprint shape, and an attempt made to infer function from shape. Digit III projection and length-to-width ratio of the footprints are negatively correlated with size in both groups; digit impression width is positively correlated with size only in ornithischians. Digit III projection appears to be positively correlated with cursorial ability. Increased interdigital angles are associated with a decrease in digital width, possibly an adaptation for stability. Weak digit III projection and increased digital width are interpreted as adaptations for graviportality. Footprints yield great potential for the understanding of the functional morphology of dinosaur feet.     

Developmental basis of mammalian digit reduction: a case study in pigs

Digit reduction has occurred in parallel in many mammalian lineages. However, despite this pattern's prevalence, the developmental mechanisms underlying mammalian digit reduction remain controversial. We therefore undertook a study of digit development in the pig (Sus scrofa), a mammal with reduced first, second, and fifth digits. Our results indicate that from its earliest formation, the pig limb bud is significantly narrower than that of the model pentadactyl mammal, mouse. Furthermore, the cartilage condensations of the pig's reduced digits are noticeably smaller than those of their nonreduced counterparts from the time of their formation. In addition, growth rates of pig digits are comparable, as are the patterns of cell death in developing pig and mouse limbs. Taken together, results suggest that pig's first, second, and fifth digits are primarily reduced through evolutionary modifications in the early developmental patterning of their limbs. Results of this study, coupled with those from study of limb development in other mammals, suggest that although major developmental reorganizations (e.g., complete digit or limb loss) during early limb development may be selected against, it may be common for more subtle evolutionary modifications in limb development (e.g., changes in relative digit size) to occur at this time.     

Are there general laws for digit evolution in squamates? The loss and re‐evolution of digits in a clade of fossorial lizards (Brachymeles,Scincinae)

Evolutionary simplification of autopodial structures is a major theme in studies of body‐form evolution. Previous studies on amniotes have supported Morse's law, that is, that the first digit reduced is Digit I, followed by Digit V. Furthermore, the question of reversibility for evolutionary digit loss and its implications for “Dollo's law” remains controversial. Here, we provide an analysis of limb and digit evolution for the skink genus Brachymeles. Employing phylogenetic, morphological, osteological, and myological data, we (a) test the hypothesis that digits have re‐evolved, (b) describe patterns of morphological evolution, and (c) investigate whether patterns of digit loss are generalizable across taxa. We found strong statistical support for digit, but not limb re‐evolution. The feet of pentadactyl species of Brachymeles are very similar to those of outgroup species, while the hands of these lineages are modified (2‐3‐3‐3‐2) and a have a reduced set of intrinsic hand muscles. Digit number variation suggests a more labile Digit V than Digit I, contrary to Morse's law. The observed pattern of digit variation is different from that of other scincid lizards (Lerista, Hemiergis, Carlia). Our results present the first evidence of clade‐specific modes of digit reduction.