As gastrulation and germ band extension occur, the amnioserosa cells differentiate into flat cells (in particular evident during germ band retraction), connecting the dorsal and ventral halves of the segmented germ band.
During dorsal closure, the two lateral epidermal sheets move dorsally to cover the amnioserosa and ultimately fuse at the dorsal midline (see also: Processes -> dorsal closure for additional information). Amnioserosa cells are thereby internalized, coming to lie ventrally to the cardioblasts. When dorsal closure is close to completion, the amnioserosa cells exhibit a columnar shape. In stage 17 the internalised amnioserosa cells undergo cell death and are phagocytosed. The amnioserosa is called an "extraembryonic epithelium", because it does not contribute to any embryonic tissue. In contrast to all other embryonic tissues, the cells of the prospective amnioserosa do not undergo further cell division from the blastoderm stage onward.
Amnioserosa, lateral (0.3 MB)
Amnioserosa, dorsal (0.3 MB)