In the second week, the hypoblast extends into the blastocyst cavity to form?

UPUMS Nsg officer-2023

Applied Physiology

Easy

Appeared in: UPUMS Nsg officer-2023

Explanation

During the second week, the inner cell mass forms a two-layered disc: the epiblast and the hypoblast.
Cells from the hypoblast layer migrate and spread out to line the inner surface of the blastocyst cavity.
This lining of hypoblast cells transforms the pre-existing blastocyst cavity into a new structure, the primary yolk sac.
The yolk sac is essential for early embryonic nutrition and blood cell formation before the placenta becomes fully functional.

Option A: The peritoneal cavity is a major body cavity that forms much later in development from the intraembryonic coelom, not from the hypoblast in the second week.
Option B: The amniotic cavity forms from the other layer of the bilaminar disc, the epiblast. A space appears within the epiblast to create the amniotic cavity.
Option C: The blastocyst cavity is the pre-existing space that the hypoblast cells grow into. It is the container, not the product of the hypoblast's extension.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Early embryonic development and the formation of extraembryonic membranes during the second week of gestation as background academic context rather than a clinical decision trigger.
The yolk sac is the first source of nutrients and blood cells for the developing embryo. Its proper formation is critical for survival.
On early ultrasound scans, the presence and size of the yolk sac are important indicators of a viable intrauterine pregnancy.
What if? If the hypoblast fails to proliferate and form the yolk sac, the embryo would not receive necessary nutrients and would not be able to produce blood cells, leading to early pregnancy loss before the placenta can take over these functions.

How to Approach the Question

First, identify the key terms in the question: 'second week', 'hypoblast', and 'blastocyst cavity'. This places the question in a specific timeframe of early embryology.
Recall the major events of the second week of development, known as the 'week of twos': formation of the bilaminar disc (two layers: epiblast and hypoblast), and two cavities (amniotic cavity and yolk sac).
Mentally picture or sketch the bilaminar disc. The epiblast is the 'upper' layer, and the hypoblast is the 'lower' layer, adjacent to the blastocyst cavity.
Associate the epiblast with the formation of the amniotic cavity above it.
Associate the hypoblast with lining the blastocyst cavity below it to form the yolk sac.
Evaluate the options based on this knowledge, eliminating those that form from different tissues (amniotic cavity from epiblast) or at different times (peritoneal cavity).

Concept Tested & Keywords

Concept Tested: Early embryonic development and the formation of extraembryonic membranes during the second week of gestation.
Stem keywords: second week, hypoblast, blastocyst cavity
Lead-in keywords: to form

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E6 Anatomy Grays 43e Vol 1 Part 2 p. 2