Gestational sac

On obstetric ultrasound, the gestational sac is a dark (anechoic) space surrounded by a white (hyperechoic) rim.

Structure

The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus. By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer between the parietal somatopleuric and visceral splanchnopleuric layer of extraembryonic mesoderm.

Development

During embryogenesis, the extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac is a portion of the conceptus consisting of a cavity between Heuser's membrane and the trophoblast.

During formation of the primary yolk sac, some of the migrating hypoblast cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the trophoblast, forming the extraembryonic mesoderm. As development progresses, small lacunae begin to form within the extraembryonic mesoderm which enlarges to become the extraembryonic coelom.

The Heuser's membrane cells (hypoblast cells) that migrated along the inner cytotrophoblast lining of the blastocoel, secrete an extracellular matrix along the way. Cells of the hypoblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm; this disrupts the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the extraembryonic coelom.

The extraembryonic coelom divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primary yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.