The
yolk sac is a membranous sac attached to an
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...
, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the
developmental circulatory systemErythropoiesis is the process by which red blood cells are produced and it is caused by iron released when a red blood cell dies. In human adults, this usually occurs within the bone marrow. In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac. By the third or...
of the human embryo, before internal circulation begins.
In the mouse, the yolk sac is the first site of blood formation, generating primitive macrophages and erythrocytes.
It is the first element seen in the
gestational sacThe gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy exists, until the embryo is identified....
during
pregnancyPregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the surgical field...
, usually at 5 weeks
gestationGestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals duringpregnancy can have one or more gestations at the same time ....
.
It is a critical landmark, identifying a true gestation sac.
It is reliably seen early in human pregnancy using
ultrasoundUltrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound...
.
The yolk-sac is situated on the ventral aspect of 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...
; it is lined by extra-embyronic
endodermEndoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm....
, outside of which is a layer of extra-embryonic mesenchyme, derived from the
mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
.
Blood is conveyed to the wall of the sac by the primitive
aortaThe aorta is the largest artery in the body, originating from the left ventricle of the heart and bringing oxygenated blood to all parts of the body in the systemic circulation....
, and after circulating through a wide-meshed capillary plexus, is returned by the
vitelline veins-Path:They run upward at first in front, and subsequently on either side of the intestinal canal.They unite on the ventral aspect of the canal, and beyond this are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of...
to the tubular heart of the embryo.
The
yolk sac is a membranous sac attached to an
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...
, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the
developmental circulatory systemErythropoiesis is the process by which red blood cells are produced and it is caused by iron released when a red blood cell dies. In human adults, this usually occurs within the bone marrow. In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac. By the third or...
of the human embryo, before internal circulation begins.
In mice
In the mouse, the yolk sac is the first site of blood formation, generating primitive macrophages and erythrocytes.
In humans
It is the first element seen in the
gestational sacThe gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy exists, until the embryo is identified....
during
pregnancyPregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the surgical field...
, usually at 5 weeks
gestationGestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals duringpregnancy can have one or more gestations at the same time ....
.
It is a critical landmark, identifying a true gestation sac.
It is reliably seen early in human pregnancy using
ultrasoundUltrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound...
.
The yolk-sac is situated on the ventral aspect of 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...
; it is lined by extra-embyronic
endodermEndoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm....
, outside of which is a layer of extra-embryonic mesenchyme, derived from the
mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
.
Blood is conveyed to the wall of the sac by the primitive
aortaThe aorta is the largest artery in the body, originating from the left ventricle of the heart and bringing oxygenated blood to all parts of the body in the systemic circulation....
, and after circulating through a wide-meshed capillary plexus, is returned by the
vitelline veins-Path:They run upward at first in front, and subsequently on either side of the intestinal canal.They unite on the ventral aspect of the canal, and beyond this are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of...
to the tubular heart of the embryo. This constitutes the
vitelline circulationVitelline circulation refers to the system of blood flowing from the embryo to the yolk sac and back again.The yolk-sac is situated on the ventral aspect of the embryo; it is lined by entoderm, outside of which is a layer of mesoderm...
, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo.
At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the
vitelline ductIn the human embryo, the vitelline duct, also known as the omphalomesenteric duct, is a long narrow tube that joins the yolk-sac to the digestive tube...
.
The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between 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 function of this is the protection of...
and the
chorionThe chorion is one of the membranes that exists 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...
and may lie on or at a varying distance from the
placentaThe placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed of via the maternal kidneys...
.
As a rule the duct undergoes complete obliteration during the seventh week, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine,
Meckel's diverticulumA Meckel's diverticulum, a true congenital diverticulum, is a small bulge in the small intestine present at birth. It is a vestigial remnant of the omphalomesenteric duct , and is the most frequent malformation of the gastrointestinal tract...
, which is situated about 60cm proximal to the
ileocecal valveThe ileocecal valve is a sphincter muscle situated at the junction of the small intestine and the large intestine. Its critical function is to limit the reflux of colonic contents into the ileum....
, and may be attached by a fibrous cord to the abdominal wall at the
umbilicusUmbilicus may refer to:*Umbilicus , a feature of gastropod, Nautilus and Ammonite shell anatomy*umbilicus, a synonym for the navel or belly button....
.
Sometimes a narrowing of the lumen of the
ileumThe ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
is seen opposite the site of attachment of the duct.
Histogenesis
The Yolk sac starts forming itself during the second week of the embryonic development, at the same time of the shaping of the amniotic sac. The
hypoblastThe hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...
starts proliferating laterally and descending.
In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.
Modifications
- Primary yolk sac/primitive yolk sac: it is the vesicle constituted in the second week, its floor is represented by the Heuser membrane and its ceiling by the hypoblast
The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...
. It is also known as the exocoelomic cavity.
- Secondary yolk sac: this first transformation is determined by the modification of its cover, in the connection zone between the ipoblast and the Heuser membrane. We can observe a structure. The two parts detach and the inferior one, which is smaller, forms a cyst destined to be eliminated. The upper one is now covered only by the ipoblast.
- The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas. A little portion of the sac, in the upper part, constitutes the intestinal tube. On the other side, the distal part forms a little vesicle that is what remains of the yolk sac.