A unique exocelom-like space during early pregnancy in the horse

The free allantois and allantochorion of conceptuses from 17 mares between 20 and 90 days of gestation were examined to determine the manner in which the associated mesodermal derivatives differentiated. It was found that a robust basement membrane developed under the allantoic endoderm, and that this basement membrane was partially isolated from the vascular layer of the allantois by a mesothelial layer and an exocelom-like space. The exocelom-like space persisted until approximately the stage of villous formation, and remnants of the space persisted over larger allantoic vessels even later. It is suggested that originally the presence of the mesothelial layer facilitates type I collagen formation in the thick basement membrane under the basal lamina of the allantoic endoderm. The basement membrane maintains the integrity of the allantois while the underlying space allows some slippage between the allantois and its vascular layer which is more closely associated with the endometrial and exocelomic surfaces, respectively.