Early during implantation, a space develops between the embryonic cell mass and adjacent trophoblastic cells (see Fig. 5-9). Small cells that line this inner surface of trophoblasts have been called amniogenic cells-precursors of amnionic epithelium. The amnion is irst identiiable on the 7th or 8th day of embryo development. It is initially a minute vesicle, which then develops into a small sac that covers the dorsal embryo surface. As the amnion enlarges, it gradually engulfs the growing embryo, which prolapses into its cavity (Benirschke, 2012).
Distention of the amnionic sac eventually brings it into contact with the interior surface of the chorion laeve. Apposition of the chorion laeve and amnion near the end of the irst trimester then causes an obliteration of the extraembryonic coelom. he amnion and chorion laeve, although slightly adhered, are never intimately connected and can be separated easily. Placental amnion covers the placental surface and thereby is in contact with the adventitial surface of chorionic vessels. Umbilical amnion covers the umbilical cord. With diamnionic-monochorionic placentas, there is no intervening tissue between the fused amnions. In the conjoined portion of membranes of diamnionic-dichorionic twin placentas, amnions are separated by fused chorion laeve.
Amnionic luid ills this amnionic sac. Until about 34 weeks' gestation, the normally clear fluid increases in volume as pregnancy progresses. Mter this, the volume declines. At term, amnionic luid averages 1000 mL, although this may vay widely in normal and especially abnormal conditions. The origin, composition, circulation, and function of amnionic luid are discussed further in Chapter 11 (p. 225).
Epithelial cells of the amnion derive from fetal ectoderm of the embryonic disc. hey do not arise by delamination from trophoblasts. his is an important consideration from both embryological and functional perspectives. For example, HLA class I gene expression in amnion is more akin to that in embyonic cells than to that in trophoblasts.
he ibroblast-like mesenchymal cell layer of the amnion is likely derived from embryonic mesoderm. Early in human embryogenesis, the amnionic mesenchymal cells lie immediately adjacent to the basal surface of the amnion epithelium. At this time, the amnion surface is a two-cell layer with approximately equal numbers of epithelial and mesenchymal cells. Simultaneously with growth and development, interstitial collagens are deposited between these two cell layers. his marks formation of the amnion compact layer, which separates the two layers of amnion cells.
As the amnionic sac expands, the compactness of the mesenchymal cells is progressively reduced, and they become sparsely distributed. Early in pregnancy, amnionic epithelium replicates at a rate appreciably faster than mesenchymal cells. At term, these cells form a continuous uninterrupted epithelium on the fetal amnionic surface. Conversely, mesenchymal cells are widely dispersed, being connected by a ine lattice network of extracellular matrix with the appearance of long slender ibrils.