The
amniotic sac is the sac in which the
fetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the
ChorionThe chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...
, contains the amnion and is part of the
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. Its wall is the
amnionThe amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the...
, the inner of the two
fetal membranesThe fetal membranes, or chorioamniotic membranes, are the amnion and chorion which surround and protect a developing fetus....
. It encloses the
amniotic cavityThe 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 extremity, and subsequently at the caudal end and sides of the embryo...
and the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. The amniotic cavity contains the
amniotic fluidAmniotic fluid or liquor amnii is the nourishing and protecting liquid contained by the amniotic sac of a pregnant woman.- Development of amniotic fluid :...
. On the outer side, the amniotic sac is connected to the
yolk sacThe yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
, to the
allantoisAllantois is a part of a developing animal conceptus . It helps the embryo exchange gases and handle liquid waste....
and, through the
umbilical cordIn placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
, to the
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
.
The
amniotic sac is the sac in which the
fetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the
ChorionThe chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...
, contains the amnion and is part of the
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. Its wall is the
amnionThe amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the...
, the inner of the two
fetal membranesThe fetal membranes, or chorioamniotic membranes, are the amnion and chorion which surround and protect a developing fetus....
. It encloses the
amniotic cavityThe 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 extremity, and subsequently at the caudal end and sides of the embryo...
and the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. The amniotic cavity contains the
amniotic fluidAmniotic fluid or liquor amnii is the nourishing and protecting liquid contained by the amniotic sac of a pregnant woman.- Development of amniotic fluid :...
. On the outer side, the amniotic sac is connected to the
yolk sacThe yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
, to the
allantoisAllantois is a part of a developing animal conceptus . It helps the embryo exchange gases and handle liquid waste....
and, through the
umbilical cordIn placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
, to the
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
.
In mammals
The amniotic sac and its filling provide a liquid that surrounds and cushions the fetus. It allows the fetus to move freely within the walls of the
uterusThe uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
. Buoyancy is also provided.
In humans
If, after birth, the complete amniotic sac or big parts of the membrane remain coating the newborn, this is called a
caulA caul is a thin, filmy membrane, the amnion, that can cover a newborn's head and face immediately after birth.-Obstetrics:A child "born with the caul" has a portion of the amniotic sac or membrane remaining on the head. There are two types of cauls. The most common caul is adhered to the head...
.
When seen in the light, the amniotic sac is shiny and very smooth, but tough.
Amniotomy
An artificial rupture of membranes (
AROM), also known as an
amniotomyAn artificial rupture of membranes , also known as an amniotomy, may be performed by a midwife or obstetrician. This is usually performed using an amnihook or fingercot. It is intended to induce or accelerate labour.-Indications:...
, may be performed by a midwife or
obstetricianObstetrics is the medical specialty dealing with the care of all women's reproductive tracts and their children during pregnancy , childbirth and the postnatal period...
. This is usually performed using an amnihook or fingercot and is intended to induce or accelerate labour.
Diversity
The presence of the amnion identifies humans and other
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s as
amnioteThe amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s, along with
reptileReptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s,
dinosaurDinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s,
birdBirds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s but neither
amphibianAmphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s nor
fishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
.