Question

What are extra embryonic membranes? What is their role in embryonic development?

Answer 1

Extra embryonic membranes are: Amnion, Chorion, Yolk sac (vestigial in placental mammals) and Allantois.

Explanation:

All extraembryonic membranes in mammals, develop from outer trophoblast layer of blastocyst, while embryo itself arises from inner cell mass.Extra embryonic membranes develop not only in mammals but also in shelled eggs of reptiles and birds.

YOLK SAC
In amniotic non mammals yolk sac is large (as the egg from which the embryo grows is highly yolky) which supplies food to growing embryo. In mammals, egg is non yolky, hence a very small yolk sac develops in embryo.

AMNION and CHORION
In all of these organisms, actual embryonic mass will soon become enclosed within sacs by appearance of two membranar folds which later fuse (to give rise to amnion and chorion).

Inner amniotic sac will contain fluid to provide the embryo an aquatic environment to develop, and the fluid also acts as a good shock absorber.

Outer chorionic membrane will allow a mammalian embryo to attach to uterine wall by giving rise to finger like structures called chorionic villi. Placenta is the term used for such an intricate fetal and maternal tissue interface.

ALLANTOIS
Allantois is another sac like structure, that stores excretory materials during embryonic life.

Amnion and chorion develop from embryonic ectoderm (in green colour in the following diagram) while yolk san and allantois develop from embryonic endoderm. Both are supported by mesoderm (drawn in red).

In mammals other than marsupials, chorionic villi appear in the area where chorion and allantoic membranes are closely associated, giving rise to chorioallantoic placenta .