Apoptosis opens urodeal membrane, cavitates coprodeum, and removes chick urogenital ducts

American Zoologist 37:162a (1997)

Morphogenesis of chick cloaca involves cavitation of occluded cords of cells and removal of membranes and certain urogential ducts. Chick cloaca is described as three successive chambers that are distinct around midgestation, but become indistinct in adults. At 9 days of incubation two of these chambers, urodeum and proctodeum, are separated by urodeal membrane, whereas the coprodeum has not yet cavitated from rectal cord. Between day 9 and 10 the membrane ruptures, and occluded cord begins to cavitate. To test the possibility that programmed cell death (PCD) known as apoptosis is involved, we applied the immunomarker ApopTag™ (Oncor®) to sections of chick embryos representing 12 hour intervals of development between days 8 and 10. Abundant and dense collections of apoptotic cells in day 9 membrane and in rectal cord suggest that PCD is important in morphogenesis of cloaca and caudal gut. Apoptosis was also heavily marked in adjacent urogenital ducts. Apoptosis removes tail gut and remnant of the primitive streak (Miller and Briglin, 1996) in earlier stages. Programmed cell death is clearly a major contributor to morphogenesis in the caudal axis of chick embryos.