102. Fetal membranes – amnion and chorion. Yolk sac. Alantois.

fetal membranes

Fetal Membranes are membranous structures that surround the developing fetus, providing essential functions such as nutrition, respiration, excretion, and protection. They include the amnion, chorion, yolk sac, and allantois.

amnion

• Origin: Derives from the epiblast layer.

• Function: The amnion surrounds the amniotic cavity, which is filled with amniotic fluid. This fluid serves several purposes:

  • Protection: Cushions the fetus, absorbing shocks and preventing physical trauma.

  • Temperature Regulation: Helps maintain a constant temperature.

  • Prevents Adhesions: Keeps the fetus from sticking to the uterine wall.

• Volume: At birth, the amniotic fluid volume is approximately 600 mL.

chorion

• Origin: Formed by the extraembryonic mesoderm and trophoblasts.

• Composition: The chorion consists of two layers:

  • Inner Layer: Formed by the somatic mesoderm.

  • Outer Layer: Formed by the trophoblasts (primitive ectoderm).

• Parts:

  • Chorion Laeve: The smooth part of the chorion that surrounds the developing embryo.

  • Chorion Frondosum (also known as bushy chorion): The part of the chorion that contributes to the formation of the placenta.

yolk sac

• Origin: Derives from the hypoblast layer.

• Appearance: Visible in the gestational sac from approximately the 5th week of development.

• Function:

  • Hematopoiesis: Responsible for the early formation of blood cells (embryonic blood supply).

  • Source of Gonoblasts: Contributes to the development of the gonads.

  • Formation of Primitive Gut: Plays a role in the development of the primitive gut.

• Disappearance: The yolk sac gradually diminishes and is largely inconspicuous by around the 7th week of gestation

allantois

• Origin: Appears as a small membranous sac that forms from the wall of the yolk sac.

• Appearance: Develops during the 3rd week of gestation.

• Function:

  • Embryonic Development: Contributes to the formation of the umbilical cord and the blood vessels within it.

  • Role: In humans, it becomes rudimentary and is less prominent by the end of the first trimester.