The gestational sac is the large cavity of

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

surrounding the

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

. During early

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

, it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

. It is the only available structure that can be used to determine if an intrauterine

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

exists until the

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

Structure

The gestational sac is

spherical A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...

in shape, and is usually located in the upper part (fundus) of the

. By approximately nine weeks of

gestational age In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method, if available. Such metho ...

, due to folding of the trilaminar germ disc, the

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of biological membrane, membranes that hold a developing embryo (and l ...

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 A conceptus (from Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), ...

consisting of a cavity between Heuser's membrane and the

trophoblast The trophoblast (from Greek language, Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after Human fertilization, fertilization in humans. They provide nutrients to the embryo an ...

. During formation of the primary yolk sac, some of the migrating

hypoblast In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac. The hypoblast is a layer of ...

cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the

, forming the extraembryonic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

. As development progresses, small lacunae begin to form within the extraembryonic

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 In biology, the extracellular matrix (ECM), also called intercellular matrix (ICM), is a network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide structural and bio ...

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

into two layers: extraembryonic

splanchnopleuric In the anatomy of an embryo, the splanchnopleuric mesenchyme is a structure created during embryogenesis when the lateral mesodermal germ layer splits into two layers. The inner (or splanchnic) layer adheres to the endoderm, and with it forms the ...

, which lies adjacent to Heuser's membrane around the outside of the primary yolk sac, and extraembryonic

somatopleuric In the anatomy of an embryo, the somatopleure is a structure created during embryogenesis when the lateral plate mesoderm splits into two layers. The outer (or somatic) layer becomes applied to the inner surface of the ectoderm, and with it (parti ...

, 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.

Clinical significance

Ultrasound

Gestational sac diameter by gestational age

The mean sac diameter can effectively estimate the

between 5 and 6 weeks, with an accuracy of about +/- 5 days. The

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

and

should be readily identifiable when the gestational sac reaches a certain size — a yolk sac should be seen when the gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm. Gestational sacs can be identified via ultrasound and are generally identified by the following four characteristics: # The sac has a round or elliptical shape in longitudinal and transverse views # The sac is surrounded by a white echogenic rim (''choriodecidual reaction'') # The sac is located in the uterine fundus # The sac is not implanted on the midline, but eccentrically (to one side of the uterine cavity line).

References

External links