Q.86 During the development of a mammalian embryo, “yolk sac” is formed by which
one of the following?
(A) Syncytiotrophoblast
(B) Primitive endoderm (hypoblast)
(C) Amniotic ectoderm
(D) Embryonic epiblast

Yolk Sac Formation in Mammalian Embryo Development

The yolk sac in mammalian embryos forms from the primitive endoderm (hypoblast), making option (B) the correct answer in this CSIR NET-style question.

Option Analysis

• (A) Syncytiotrophoblast: This outer trophoblast layer invades the uterine endometrium to form the placenta but does not contribute to the yolk sac; it supports implantation and nutrient exchange instead.

• (B) Primitive endoderm (hypoblast): Correct. Hypoblast cells from the inner cell mass delaminate and line the blastocyst cavity, forming the primary yolk sac’s endodermal layer during the second week of development.

• (C) Amniotic ectoderm: Derives from epiblast and forms the amnion roof, enclosing the amniotic cavity; it remains separate from yolk sac structures.

• (D) Embryonic epiblast: Gives rise to the embryo proper (three germ layers via gastrulation), amnion, and allantois, but not the yolk sac endoderm.

Development Stages

Hypoblast specification occurs at the late blastocyst stage, proliferating to create visceral endoderm over the epiblast and parietal endoderm peripherally, completing the primary yolk sac by Carnegie stage 5. A secondary yolk sac follows, aiding hematopoiesis and nutrition before placental dominance.

In mammalian embryo development, yolk sac formation begins post-implantation as the blastocyst’s inner cell mass differentiates into epiblast and hypoblast (primitive endoderm). This structure provides early nutrition, blood cells, and germ precursors before regressing.

Key Structures Involved

• Primitive endoderm lines the exocoelomic cavity, forming the yolk sac’s inner membrane.

• Extraembryonic mesoderm from hypoblast adds vascular layers.

• Primary yolk sac pinches off, replaced by secondary for sustained function.

Exam Relevance for CSIR NET

Understanding yolk sac origins tests knowledge of extraembryonic membranes, distinguishing hypoblast roles from trophoblast or epiblast derivatives.