Forelimb-Hindlimb Developmental Timing Changes across Tetrapod Phylogeny

Background: Tetrapods exhibit great diversity in limb structures among species and also between forelimbs and hindlimbs within species, diversity which frequently correlates with locomotor modes and life history. We aim to examine the potential relation of changes in developmental timing (heterochrony) to the origin of limb morphological diversity in an explicit comparative and quantitative framework. In particular, we studied the relative time sequence of development of the forelimbs versus the hindlimbs in 138 embryos of 14 tetrapod species spanning a diverse taxonomic, ecomorphological and life-history breadth. Whole-mounts and histological sections were used to code the appearance of 10 developmental events comprising landmarks of development from the early bud stage to late chondrogenesis in the forelimb and the corresponding serial homologues in the hindlimb. Results: An overall pattern of change across tetrapods can be discerned and appears to be relatively clade-specific. In the primitive condition, as seen in Chondrichthyes and Osteichthyes, the forelimb/pectoral fin develops earlier than the hindlimb/pelvic fin. This pattern is either retained or re-evolved in eulipotyphlan insectivores (= shrews, moles, hedgehogs, and solenodons) and taken to its extreme in marsupials. Although exceptions are known, the two anurans we examined reversed the pattern and displayed a significant advance in hindlimb development. All other species examined, including a bat with its greatly enlarged forelimbs modified as wings in the adult, showed near synchrony in the development of the fore and hindlimbs. Conclusion: Major heterochronic changes in early limb development and chondrogenesis were absent within major clades except Lissamphibia, and their presence across vertebrate phylogeny are not easily correlated with adaptive phenomena related to morphological differences in the adult fore- and hindlimbs. The apparently conservative nature of this trait means that changes in chondrogenetic patterns may serve as useful phylogenetic characters at higher taxonomic levels in tetrapods. Our results highlight the more important role generally played by allometric heterochrony in this instance to shape adult morphology.Organismic and Evolutionary Biolog

M i n i r e v i e w Vertebrate Extracellular Preovulatory and Postovulatory Egg Coats

ABSTRACT Extracellular egg coats deposited by maternal or embryonic tissues surround all vertebrate conceptuses during early development. In oviparous species, the time of hatching from extracellular coats can be considered equivalent to the time of birth in viviparous species. Extracellular coats must be lost during gestation for implantation and placentation to occur in some viviparous species. In the most recent classification of vertebrate extracellular coats, Boyd and Hamilton (Cleavage, early development and implantation of the egg. In: Parkes AS (ed.), Marshall's Physiology of Reproduction, vol. 2, 3rd ed. London: Longmans, Green & Co; 1961:1-126) defined the coat synthesized by the oocyte during oogenesis as primary and the coat deposited by follicle cells surrounding the oocyte as secondary. Tertiary egg coats are those synthesized and deposited around the primary or secondary coat by the maternal reproductive tract. This classification is difficult to reconcile with recent data collected using modern molecular biological techniques that can accurately establish the site of coat precursor synthesis and secretion. We propose that a modification to the classification by Boyd and Hamilton is required. Vertebrate egg coats should be classed as belonging to the following two broad groups: the preovulatory coat, which is deposited during oogenesis by the oocyte or follicle cells, and the postovulatory coats, which are deposited after fertilization by the reproductive tract or conceptus. This review discusses the origin and classification of vertebrate extracellular preovulatory and postovulatory coats and illustrates what is known about coat homology between the vertebrate groups. conceptus, extracellular coat, female reproductive tract, homology, ovar

Uterine histology of the dasyurid marsupial, Antechinus Stuartii: relationship with differentiation of the embryo

Uterine samples from Antechinus stuartii on days 1, 4, 6, 8, 11, 13, 15, 18, 21 and 23 after ovulation were examined histologically. Animals were pregnant, nonpregnant and unmated, or nonpregnant and mated but found to have only unfertilized eggs on autopsy. The histological parameters used were thickness of the myometrium, endometrial stroma, and endometrial epithelium, and density of uterine stromal glands and of lymphocytes at the endometrial basal lamina. Overall, the fluctuation patterns of these parameters were superficially similar between pregnant and nonpregnant animals (mated or unmated). However, statistically significant differences were detected between pregnant and unmated nonpregnant animals in every parameter examined at nearly every time point except day 13. Comparison of these results with known data on embryonic stages, corpus luteum development and plasma progesterone concentrations revealed that the gravid uterus underwent histological changes co-incident with changes in both progesterone concentration and developmental delay or embryonic arrest. It was concluded that the uterus mediates progesterone-driven changes in embryonic developmental rate. Although determination of number of lymphocytes provided inconclusive evidence of cellular immunity against embryos, the possibility that embryonic signalling to the uterus occurs is discussed

Histological differences between gravid and non-gravid uteri in the dasyurid marsupial, Sminthopsis macroura (Spencer)

Uterine samples from pregnant Sminthopsis macroura representing the first 10 days of its 11 day gestation period and samples from non-pregnant animals were compared histologically and examined for differences in the following characteristics: thickness of the endometrial stroma, luminal epithelium, myometrium and glandular epithelium, and the density of stromal glands and number of lymphocytes at the endometrial basal lamina. A highly significant difference between gravid and non-gravid uteri with respect to thickness of the endometrial epithelium was found on day 3, when lineage divergence occurs between the pluriblast and trophoblast. The endometrial stroma was significantly thicker in pregnant animals on day 8, when the epiblast differentiates into ectoderm, endoderm and mesoderm. Other differences between gravid and non-gravid uteri were detected in myometrial thickness on days 1 and 5. Taken together, these results indicate that despite similar endocrinological profiles of pregnant and non-pregnant marsupials, there are subtle, but significant, differences in uterine histology. The observed concordance between histological differences and differentiative events in embryogenesis is considered as indicative of embryo--maternalsignalling

A staging scheme for assessing development in vitro of organogenesis stage embryos of the stripe-faced dunnart, Sminthopsis macroura (Marsupialia: dasyuridae)

The inaccessibility of mammalian organogenesis stage embryos has precluded their widespread use in embryological and teratological studies. As organogenesis occurs during the last 1.5 days of the 10. 7 days of gestation in the stripe-faced dunnart (Sminthopsis macroura), the aim of the present study was to investigate whether day 9 and day 10 embryos and fetuses could be grown to term in vitro. High glucose Dulbecco\u27s modified Eagle\u27s medium with 10% fetal calf serum (FCS) supported embryonic growth for various periods of time, some to within 5 h of the predicted time of parturition. A roller culture system maintained at 35 degrees C was used to incubate organogenesis stage embryos (n = 43). Nine unincubated (control) embryos were either fixed for microscopic analysis or frozen for microprotein determination. The results of the present study indicate that with some optimization of the culture conditions (increasing oxygen in the gas phase in the culture tubes, replacing FCS with rat serum), it might be possible for organogenesis stage S. macroura embryos to be grown to term. A scoring scheme for assessing morphological development was devised for use as a standard in staging organogenesis stage embryos. This scheme reflects the highly compressed schedule of developmental events that occurs mainly during day 9 of gestation in S. macroura embryos. In comparison, during embryogenesis in Didelphis virginiana these developmental events occur from day 8 to day 10.5 of gestation, and birth occurs on day 13

Founding Editorial: Embryology — An Integrated Approach

We introduce the Embryology domain of TheScientificWorld and outline the scope and aims. We argue for an interdisciplinary approach to problems in develop-mental biology. Three areas are identified as being of particular relevance to this domain: evolutionary developmental biology, teratology, and descriptive or experimental embryology