Chapter 12, Problem 1DQ

Summary Introduction

To review:

The fate of blastocyst in the embryos of monotreme mammals (e.g. platypus) and marsupial mammals (e.g. kangaroo) that lay eggs and have extremely short pregnancies respectively.

Introduction:

The blastocyst is defined as a structure which is formed during the early developmental stage of mammals. It is mainly characterized by the presence of an inner cell mass (ICM) which is responsible for the formation of embryo. The outer layer of blastocyst is known as trophoblast. Unlike eutherian mammals, marsupials have short pregnencies, and are born very early in embryonic development. In oviparous monotremes, the offsprings also raised in maternal pouch. Deposition of the egg from the cloaca takes place directly for the incubation.

Explanation of Solution

The blastocyst is responsible to carry the embryo from cleavage to gastrulation. This structure bears a remarkable diversity in its mode of formation, longevity, morphology and association with uterine endometrium in different organisms. In terms of evolution of viviparity, the principal drivers of blastocyst evolution is associated with loss of yolk coupled with evolution of the placenta. In many species the trophoblast segregation is associated with blastocyst formation, but in case of marsupials these processes not coincide. In monotremes and most of the marsupials the zygote undergo polarization. The clevage signal are related to this polarized state. The embryonic-cell lineages are developed at the center of the blastocyst epithelium and extra embryonic lineages are developed at the periphery.

There are a number of similarities and differences are present in eutherians and other mammals like marsupials and monotremes. In marsupials and monotremes, the pluriblast or the inner cell mass is not enveloped by the trophoblast. Here the ICM is part of a continuous epithelium that forms the unilaminar blastocyst, and this pluriblast than segregates into hypoblast and epiblast.

Conclusion

Thus it is concluded that unlike eutherian, marsupials and monotremes bears the inner cell mass, not enveloped by the trophoblast, which later forms the hypoblast and epiblast which give rise to different germ layers.