The amniotic sac, also called the bag of waters
or the membranes,
is the sac in which the
embryo
An embryo is an initial stage of development of 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 spe ...
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 deve ...
develops in
amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
s. It is a thin but tough transparent pair of
membranes
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
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
The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...
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 mater ...
. 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
The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...
, 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 physiologic ...
, 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 mater ...
.
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 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 ...
; 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 ne ...
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
Prenatal testing consists of prenatal screening and prenatal diagnosis, which are aspects of prenatal care that focus on detecting problems with the pregnancy as early as possible. These may be anatomic and physiologic problems with the health of ...
of
chromosomal abnormalities
A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where ther ...
and fetal infections.
Structure
![Gray31](https://upload.wikimedia.org/wikipedia/commons/5/55/Gray31.png)
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
An embryo is an initial stage of development of 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 spe ...
. 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](https://upload.wikimedia.org/wikipedia/commons/8/88/Bilaminar-embryonic-disc-at-14-days-in-the-implantation-site-in-endometrium.png)
At the beginning of the second week, a cavity appears within the
inner cell mass
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 ...
, and when it enlarges, it becomes the amniotic cavity. The floor of the amniotic cavity is formed by the
epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other laye ...
. 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
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
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
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 ...
, 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
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
form a layer of
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
, 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 E ...
, 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 uter ...
, 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 physiologic ...
,
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 mater ...
, 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 mater ...
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
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 uter ...
.
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 p ...
is also provided.
Clinical significance
Chorioamnionitis
Chorioamnionitis, also known as intra-amniotic infection (IAI), is inflammation of the fetal membranes ( amnion and chorion), usually due to bacterial infection. In 2015, a National Institute of Child Health and Human Development Workshop expert ...
is inflammation of the amniotic sac (''
chorio-'' + ''amnion'' + ''
-itis
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
''), 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 dise ...
. It is a risk factor for
neonatal sepsis
Neonatal sepsis is a type of neonatal infection and specifically refers to the presence in a newborn baby of a bacterial blood stream infection (BSI) (such as meningitis, pneumonia, pyelonephritis, or gastroenteritis) in the setting of fever. ...
.
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
''Marvin's Marvelous Mechanical Museum'' is the debut studio album by American rock band Tally Hall, originally released on October 24, 2005. Their previous recordings were all independently produced and distributed demos. All of the tracks on ...
. 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
Prelabor rupture of membranes (PROM), previously known as premature rupture of membranes, is breakage of the amniotic sac before the onset of labor. Women usually experience a painless gush or a steady leakage of fluid from the vagina. Complicati ...
(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 ne ...
. 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