Morphogenetic functions of extraembryonic membranes in insects

• Amnioserosa cells are functionally and evolutionarily comparable to amnion cells.
• Amnioserosa cells regulate dorsal closure through controlled osmosis and stiffness.
• The amnion/amniotic cavity determines the site of serosa rupture.
• The serosa drives extraembryonic membrane rupture and amnion relocation.
• The yolk sac is likely an important player in insect morphogenesis.

Morphogenetic functions of the amnioserosa, the serosa, the amnion, and the yolk sac are reviewed on the basis of recent studies in flies (Drosophila, Megaselia), beetles (Tribolium), and hemipteran bugs (Oncopeltus). Three hypotheses are presented. First, it is suggested that the amnioserosa of Drosophila and the dorsal amnion of other fly species function in a similar manner. Second, it is proposed that in many species with an amniotic cavity, the amnion determines the site of serosa rupture, which, through interactions between the serosa and the amnion, enables the embryo to break free from the amniotic cavity and to close its backside. Finally, it is concluded that the yolk sac is likely an important player in insect morphogenesis.