Amniotic Cavity Development Movie
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| chorionic cavity is lost by fusion with the expanding amniotic cavity.
Note that as the yolk sac (yellow) is continuous with the midgut you can also follow development of the gastrointestinal tract regions of foregut, midgut and hindgut.
Embryo and placental membranes are shown to the left and the developing placenta is shown to the right. Embryo and developing placenta are also shown in white, maternal decidua would be the surrounding grey region.
- coelom - (Greek, koilma = cavity) Term used to describe a fluid-filled cavity or space. Placental vertebrate development have both extraembryonic (outside the embryo) and intraembryonic (inside the embryo) coeloms. The extraembryonic coeloms include the yolk sac, amniotic cavity and the chorionic cavity. The initial single intraembryonic coelom located in the lateral plate mesoderm will form the 3 major body cavities: pleural, pericardial and peritoneal.
The fluid-filled (amniotic fluid) extraembryonic coelom (cavity) formed initially by epiblast and then lined by ectoderm and surrounding extraembryonic mesoderm. In humans, it forms the innermost fetal membrane, produces amniotic fluid expanding to eventually fuse with the chorionic membrane during week 8 of development. This fluid-filled sac initially lies above the trilaminar embryo disc and with embryonic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membrane led to the description of reptiles, bird, and mammals as "amniotes".
The fluid-filled extraembryonic coelom (cavity) formed initially from trophoblast and extraembryonic mesoderm that forms placenta. chorion and amnion are made by the somatopleure. The chorion becomes incorporated into placental development. The avian and reptilian chorion lies beside the egg shell and allows gas exchange. In humans, this cavity is lost during week 8 when the amniotic cavity expands and fuses with the chorion.
An extraembryonic membrane which is endoderm origin and covered with extraembryonic mesoderm. Yolk sac lies outside the embryo connected initially by a yolk stalk to the midgut with which it is continuous with. The endodermal lining is continuous with the endoderm of the gastrointestinal tract. The extra-embryonic mesoderm differentiates to form both blood and blood vessels of the vitelline system. In reptiles and birds, the yolk sac has a function associated with nutrition. In mammals the yolk sac acts as a source of primordial germ cells and blood cells. Note that in early development (week 2) a structure called the "primitive yolk sac" forms from hypoblast, this is an entirely different structure.
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- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G