Article Review: The corn snake yolk sac becomes a solid tissue filled with blood vessels and yolk-rich endodermal cells

The evolution of amniotes of reptiles has always been considered a key component in allowing reptiles to exist a sole terrestrial life cycle unlike it's ancestral trait of a complex one. The yolk of many squamates and terrestrial reptiles tend to resemble those of a chicken. A recent article I came across across observed the yolk of the corn snake, Pantherophis guttatus, which did not resemble the typical yolk of squamates and chickens. This suggests different theories of amniotic evolution.

In the study, multiple corn snake eggs were collected and the embryo were removed and studied at different stages of development. Nuclei of cells were stained to be studied, and chicken yolk was removed similarly to be compared to the corn snake. There were 2 major differences noted in the snake embryo: 1. the snake embryo was only at half the length of hatching 36 days after laying and 2. the yolk was more solid and consisted of large mass of endodermal cells and a mesh of blood vessels coated with endodermal cells (figure c.) Typically when the yolk sac is cut it tends to spill out as a liquid.

Through the staining of the nuclei it was noted that the mass also contained large sized cells with unusual appearances which some were attached to the blood vessels and others not. Most amniote eggs tend to be vascularized but are typically not found within the interior of the yolk. Even at an earlier stage of development of corn snake embryo these same meshed of blood vessels were found. After viewing a multitude of stages of corn snake embryo development it was discovered that the final third of development is solid tissue rather than the typical sac found in most other squamates and birds.

This study is important to our understanding of the evolution of amniote eggs for terrestrial reptiles as it raises new hypothesis in the development of amniote eggs. There are questions as to whether this solid tissue is basal or derived compared to the common found yolk sac. The best way to understand it's role in amniote evolution is to study multiple other reptiles to see whether this solid tissue is found in other species as well.

The article suggests that holoblastic cleavage as typically seen, is more primitive than meroblastic cleaving as seen in corn snakes. Thus could also raise questions as to why solid tissue was needed during development or why this supposed derived trait developed.

Article: http://europepmc.org/articles/pmc3917329