Gross anatomical and histomorphological observations on the terminal rectum and the cloaca in the Ostrich Struthio camelus

Abstract

In birds, the ability to void urine separate from faeces is unique to ostriches. To further explore this characteristic, the anatomy of the terminal rectum and cloaca of the Ostrich Struthio camelus was studied in four ostriches by gross anatomical dissection and light microscopy. The terminal rectum had an unusual tunica muscularis externa (TME) and was clearly demarcated from the caudal part of the rectum proper by a semilunar fold, an abrupt thickening of the gut wall and an increase in the calibre of the gut. The cloaca had a distinct rectocoprodeal fold at the terminal rectum–cloaca junction with a well-formed sphincter muscle. The cloaca had a proximal coprodeum, a middle urodeum and a caudal proctodeum. The mucosa of the cloaca was folded and lined by simple columnar epithelium except in the urodeum and the floor and ventral walls of the proctodeum. The coprodeal wall had a thick circular muscle layer; however, the other parts of the cloaca mainly had longitudinally/obliquely directed fibres in the TME. This muscle arrangement could contribute to the dynamics of the terminal rectum that allow for separate defecation and micturition. We also propose a schema for this phenomenon that for birds is unique to ostriches.