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Functional morphology, diversity, and evolution of yolk processing specializations in embryonic reptiles and birds.

Authors
  • Blackburn, Daniel G1
  • 1 Department of Biology, Electron Microscopy Center, Trinity College, Hartford, Connecticut, USA.
Type
Published Article
Journal
Journal of morphology
Publication Date
Jul 01, 2021
Volume
282
Issue
7
Pages
995–1014
Identifiers
DOI: 10.1002/jmor.21267
PMID: 32960458
Source
Medline
Keywords
Language
English
License
Unknown

Abstract

Evolution of the terrestrial, amniotic egg of vertebrates required new mechanisms by which yolk material could be processed for embryonic use. Recent studies on each of the major extant reptile groups have revealed elaborate morphological specializations for yolk processing, features that differ dramatically from those of birds. In the avian pattern, liquid yolk is housed in a yolk sac whose endodermal lining absorbs and digests yolk material and sends resultant nutrients into the blood circulation. In snakes, lizards, turtles, and crocodilians, as documented herein, the yolk sac becomes invaded by endodermal cells that proliferate and phagocytose yolk material. Blood vessels then invade, and the endodermal cells become arranged around them, forming elongated "spaghetti-like" strands that fill the yolk sac cavity. This pattern provides an effective means by which yolk material is cellularized, digested, and transported by vitelline vessels to the developing embryo. Phylogenetically, the (non-avian) "reptilian" pattern was ancestral for sauropsids and was modified or replaced in ancestors to birds. This review postulates that evolution of the "avian" pattern involved increased reliance on extracellular digestion of yolk, allowing embryonic development to occur more rapidly than in typical reptiles. Comparative studies of yolk processing that draw on morphological, biochemical, molecular approaches are needed to explain how and why the "reptilian" pattern was replaced in birds or their archosaurian ancestors. © 2020 Wiley Periodicals LLC.

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