Extraembryonic Membranes

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Extraembryonic Membranes

 

the membranous structures that in some invertebrate animals, sharks, all higher vertebrates, and man provide for the life activities of the embryo and ensure its protection from injury.

As distinguished from the egg membranes, extraembryonic membranes are formed not during the maturation of the ovum but during embryonic development from the cells of the embryo. Among the extraembryonic membranes are the amnion, the chorion, and the allantois. The amnion is formed from the lateral folds of the extraembryonic ectoderm and the external layer of the lateral plates, which rise and close up over the embryo, or by means of the formation of a cavity among the embryo cells that is gradually transformed into the membrane that surrounds the embryo. The amnion is filled with fluid and protects the embryo from desiccation; it shields the embryo from contact with other membranes (sometimes very hard, such as the eggshell) and from mechanical injury.

The outer wall of the amniotic folds forms the chorion. In mammals and man the chorion is in direct contact with the wall of the uterus. In reptiles and birds this extraembryonic membrane is usually called the serosa. In mammals and man the chorion provides for the exchange of substances between the body of the mother and the fetus; it forms during the early stage of development, when the embryo is still a small vesicle (the so-called blastocyst) consisting of an aggregation of small cells, the so-called embryoblast, surrounded by a layer of larger cells, the trophoblast. The chorion arises from the cells of the trophoblast, which are then lined with the cells of the extraembryonic mesoderm. It has characteristic villi that take root in the tissue of the uterus and form the fetal part of the placenta. The villi are supplied with vessels that form in connection with the development of the yolk sac and the allantois. The allantois begins as an outgrowth of the posterior section of the intestinal tube of the embryo. In reptiles and birds it resembles a pouch in shape. It grows intensively and covers the amnion and yolk sac. Supplied with a large number of blood vessels, the allantois serves as the embryonic organ of respiration. The metabolic products of the embryo, principally urine, collect in the allantoic cavity. In mammals and man the allantois is small; the vessels of the umbilical cord, which connect the embryo with the placenta, are formed in its mesenchyme. In the later stages of development the urinary bladder (in mammals and man) and the cloaca (in birds and reptiles) form from the intraembryonic portion of the allantois.

REFERENCE

Ivanov, P. P. Obshchaia i sravnitel’naia embriologiia. Moscow, 1937.

A. P. DYBAN and V. S. BARANOV

References in periodicals archive ?
OSKM Induce Extraembryonic Endoderm Stem Cells in Parallel to Induced Pluripotent Stem Cells," Stem Cell Reports, March 2016 DOI: 10.
2009) Eset partners with Oct 4 to restrict extraembryonic trophoblast lineage potential in embryonic stem cells.
A simplified placenta like system for the transport of extraembryonic nutrients during embryo-genesis of Bugula neritina (bryozoa).
Human chorionic gonadotrophin and cz-fetoprotein levels in matched samples of amniotic fluid, extraembryonic coelomic fluid, and maternal serum in the first trimester of pregnancy.
Developmental plasticity in response to the environment, including nutrient availability, of mammalian embryos indicates the capacity for newly emerging embryonic and extraembryonic cell lineages to initiate compensatory responses which may attune nutrient delivery to the needs
showed, for the first time, that RHAMM is differentially expressed during all stages of preimplantation human embryos and human embryonic stem cells (hESC), and indicated that RHAMM knockdown results in downregulation of several pluripotency markers in hESCs, induction of early extraembryonic lineage, loss of cell viability, and changes in hESC cycle (2).
This de novo methylation silences genes that are involved in maintaining pluripotency and is more accentuated in the inner cell mass of the blastocyst, which generates the adult tissues, than in the outer layer of cells (trophectoderm), which develops into the extraembryonic tissues, such as the placenta.
The pathogenesis of methotexate involves fluid imbalance, resulting perhaps from interference with osmoregulatory cells in extraembryonic capillary beds, which is partially responsible for the malformation.
Expression of the novel basic helix-loop-helix gene eHAND in neural crest derivatives and extraembryonic membranes during mouse development.
Totipotent stem cells can differentiate into embryonic and extraembryonic cell types.
Only the zygote produced when a human sperm and egg merge and very early embryonic cells are truly totipotent in that they can generate any type of cell needed for human life including, importantly, the placenta and extraembryonic membranes (e.
On the other hand, because salamander embryos contain pluripotent cells they had the raw material to evolve completely new structures such as extraembryonic membranes, which are fundamental to the development of reptiles and mammals.