Das Dotterorgan der Wirbelthiere

Embryos of oviparous reptiles develop on the surface of a large mass of yolk, which they metabolize to become relatively large hatchlings. Access to the yolk is provided by tissues growing outward from the embryo to cover the surface of the yolk. A key feature of yolk sac development is a dedicated blood vascular system to communicate with the embryo. The best known model for yolk sac development and function of oviparous amniotes is based on numerous studies of birds, primarily domestic chickens. In this model, the vascular yolk sac forms the perimeter of the large yolk mass and is lined by a specialized epithelium, which takes up, processes and transports yolk nutrients to the yolk sac blood vessels. Studies of lizard yolk sac development, dating to more than 100 years ago, report characteristics inconsistent with this model. We compared development of the yolk sac from oviposition to near hatching in embryonic series of three species of oviparous scincid lizards to consider congruence with the pattern described for birds. Our findings reinforce results of prior studies indicating that squamate reptiles mobilize and metabolize the large yolk reserves in their eggs through a process unknown in other amniotes. Development of the yolk sac of lizards differs from birds in four primary characteristics, migration of mesoderm, proliferation of endoderm, vascular development and cellular diversity within the yolk sac cavity. Notably, all of the yolk is incorporated into cells relatively early in development and endodermal cells within the yolk sac cavity align along blood vessels which course throughout the yolk sac cavity. The pattern of uptake of yolk by endodermal cells indicates that the mechanism of yolk metabolism differs between lizards and birds and that the evolution of a fundamental characteristic of embryonic nutrition diverged in these two lineages. Attributes of the yolk sac of squamates reveal the existence of phylogenetic diversity among amniote lineages and raise new questions concerning the evolution of the amniotic egg. J. Morphol. 278:574-591, 2017. © 2017 Wiley Periodicals, Inc.

Section: Discussionmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Yolk sac development in lizards (Lacertilia: Scincidae): New perspectives on the egg of amniotes

Stewart

Thompson

2017

“…These cells then formed a syncytium, the germ ring, which migrated ahead of the other blastomeres, eventually playing a role in formation of the yolk navel. The vitellocytes seemed to be homologous with a cell type found in reptiles and birds and described by Virchow as "merocytes" (Virchow, 1892a(Virchow, , 1892b. They found no equivalent of the germ-ring either in the literature or in their own observations on the house sparrow (Passer domesticus; Flynn & Hill, 1947).…”

Section: Vitellocytesmentioning

confidence: 84%

T. Thomson Flynn and the monotreme egg from oocyte maturation to germ layer formation

Carter

2021

Knowledge of oocyte development and the early differentiation of the germ layers in monotremes stems largely from two articles by J. P. Hill and T. Thomson Flynn. The completeness of their account was due to the large series of echidna ovaries and eggs collected on Tasmania by Flynn, an Australian biologist of whom a brief account is given. A striking finding in the oocyte and early embryo of monotremes was the presence of a latebra connected to the yolk bed beneath the germinal disc as described in birds and several reptiles. A further resemblance was the presence early in vitellogenesis of a striate zone beneath the zona pellucida. Cleavage resulted first in a lens-shaped blastodisc encircled by cells called vitellocytes that later fused to form a germ-ring. The blastodisc gave rise to a blastoderm that eventually became unilaminar but comprised two cell types identified as ectoderm and primitive endoderm. Eventually these segregated into two layers and formed a blastula. This sequence resembled that in marsupials except there was no obvious distinction between future embryonic and extra-embryonic regions. As the blastoderm extended over the surface of the yolk, it was preceded by the germ-ring that eventually played a role in forming the yolk navel. This was a unique feature of monotreme development. It is shown that Flynn played an important role in analysis of the material as well as in its collection.

“…Lacertidae —European lacertid lizards were the source of some of the earliest studies of embryonic development of squamates (Peter, 1904; Strahl, 1887; Virchow, 1892a, 1892b). The first comparative study of extraembryonic membrane development in squamates featured oviparous L. agilis and viviparous Z. vivipara (Hrabowski, 1926; Stewart & Blackburn, 2020).…”

Section: Developmental Morphologymentioning

confidence: 99%

Developmental morphology and evolution of extraembryonic membranes of lizards and snakes (Reptilia, Squamata)

Stewart

2020

Amniote embryos are supported and nourished by a suite of tissues, the extraembryonic membranes, that provide vascular connections to the egg contents. Oviparous reptiles share a basic pattern of development inherited from a common ancestor; a vascular chorioallantoic membrane, functioning as a respiratory organ, contacts the eggshell and a vascular yolk sac membrane conveys nutrients to the embryo. Squamates (lizards, snakes) have evolved a novel variation in morphogenesis of the yolk sac that results in a unique structure, the yolk cleft/isolated yolk mass complex. This structure is a source of phylogenetic variation in architecture of the extraembryonic membranes among oviparous squamates. The yolk cleft/isolated yolk mass complex is retained in viviparous species and influences placental architecture. The aim of this paper is to review extraembryonic membrane development and morphology in oviparous and related viviparous squamates to explore patterns of variation. The survey includes all oviparous species for which data are available (11 species; 4 families).Comparisons with viviparous species encompass six independent origins of viviparity.