The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...

develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before

birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...

. The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitell ...

, contains the amnion and is part of the

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

. On the outer side, the amniotic sac is connected to the

yolk sac The yolk sac is a membranous 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'' is ...

, the allantois, and via the

umbilical cord In placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologi ...

, the

. The yolk sac, amnion, chorion, and allantois are the four

extraembryonic membrane The extraembryonic membranes are four membranes which assist in the development of an animal's embryo. Such membranes occur in a range of animals from humans to insects. They originate from the embryo, but are not considered part of it. They typi ...

s that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo. They form from the inner cell mass; the first to form is the yolk sac followed by the amnion which grows over the developing embryo. The amnion remains an important extraembryonic membrane throughout prenatal development. The third membrane is the allantois, and the fourth is the chorion which surrounds the embryo after about a month and eventually fuses with the amnion.

Amniocentesis Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is n ...

is a medical procedure where fluid from the sac is sampled during

fetal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...

, between 15 and 20 weeks of pregnancy, to be used in prenatal diagnosis of chromosomal abnormalities and fetal infections.

Structure

The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid. The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the

cephalic A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals m ...

extremity and subsequently at the caudal end and sides of the embryo. As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity is formed.

Development

Bilaminar-embryonic-disc-at-14-days-in-the-implantation-site-in-endometrium

At the beginning of the second week, a cavity appears within the inner cell mass, and when it enlarges, it becomes the amniotic cavity. The floor of the amniotic cavity is formed by the epiblast. Epiblast migrates between the epiblastic disc and trophoblast. In this way the epiblastic cells migrate between the embryoblast and

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

. The floor is formed by the epiblast which later on transforms to ectoderm while the remaining cells which are present between the

embryoblast The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of th ...

and trophoblast are called amnioblasts (flattened cells). These cells are also derived from epiblast which is transformed into ectoderm. The amniotic cavity is surrounded by an

, called the amnion. As the implantation of the blastocyst progresses, a small space appears in the embryoblast, which is the primordium of the amniotic cavity. Soon, amniogenic (amnion-forming cells) called amnioblasts separate from the epiblast and line the amnion, which encloses the amniotic cavity. The epiblast forms the floor of the amniotic cavity and is continuous peripherally with the amnion. The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic membrane. This membrane along with hypoblast forms the primary yolk sac. The embryonic disc now lies between the amniotic cavity and the primary yolk sac. Cells from the yolk sac endoderm form a layer of connective tissue, the extraembryonic mesoderm, which surrounds the amnion and yolk sac.

Birth

If, after birth, the complete amniotic sac or big parts of the membrane remain coating the newborn, this is called a

caul A caul or cowl ( la, Caput galeatum, literally, "helmeted head") is a piece of membrane that can cover a newborn's head and face. Birth with a caul is rare, occurring in fewer than 1 in 80,000 births. The caul is harmless and is immediately remov ...

. When seen in the light, the amniotic sac is shiny and very smooth, but tough. Once the baby is pushed out of the mother's

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

, the

, and amniotic sac are pushed out in the after birth. File:Amnion.jpg, The amniotic sac opened during afterbirth examination File:Placenta with fetal membranes.jpg,

Placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

with attached

fetal membranes The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic me ...

(ruptured at the margin at the left in the image).

Function

The amniotic sac and its filling provide a liquid that surrounds and cushions the fetus. It is a site of exchange of essential substances, such as oxygen, between the umbilical cord and the fetus. It allows the fetus to move freely within the walls of the

Buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

is also provided.

Clinical significance

Chorioamnionitis is inflammation of the amniotic sac ('' chorio-'' + ''amnion'' + '' -itis''), usually because of

infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...

. It is a risk factor for neonatal sepsis. During

labor Labour or labor may refer to: * Childbirth, the delivery of a baby * Labour (human activity), or work ** Manual labour, physical work ** Wage labour, a socioeconomic relationship between a worker and an employer ** Organized labour and the la ...

, the amniotic sac must break so that the child can be born. This is known as

rupture of membranes Rupture of membranes (ROM) or amniorrhexis is a term used during pregnancy to describe a rupture of the amniotic sac. Normally, it occurs spontaneously at full term either during or at the beginning of labor. Rupture of the membranes is known col ...

(ROM). Normally, it occurs spontaneously at full term either during or at the beginning of labor. A premature rupture of membranes (PROM) is a rupture of the amnion that occurs prior to the onset of labor. An

artificial rupture of membranes Artificial rupture of membranes (AROM), also known as an amniotomy, is performed by a midwife or obstetrician and was once thought to be an effective means to induce or accelerate labor. The membranes can be ruptured using a specialized tool, su ...

(AROM), also known as an amniotomy, may be clinically performed using an amnihook or amnicot in order to induce or to accelerate labour. The amniotic sac has to be punctured to perform

amniocentesis Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is n ...

. This is fairly routine procedure, but can lead to infection of the amniotic sac in a very small number of cases. Infection more commonly arises vaginally.

References

External links

* https://www.mun.ca/biology/scarr/Amniotic_egg.html * http://staff.um.edu.mt/acus1/IMPLANTATION.htm {{DEFAULTSORT:Amniotic Sac Body fluids Embryology Obstetrics Human anatomy Midwifery