A morphometric and stereological study of the chorioallantoic membrane of the developing ostrich (Struthio camelus australis) embryo: A light and transmission electron microscopy investigation

The developing chorioallantoic membrane (CAM) of the ostrich (Struthio camelus) was studied between embryonic days 16 (E16) and E37. Egg masses and volumes were estimated prior to harvesting of the CAM. CAM volumes were obtained before the CAM was sampled for histology and transmission electron microscopy analysis. Stereological methods were used to estimate volume densities and absolute volumes of CAM structural components. Growth rate estimates of the CAM and its major components were obtained. At E16, the three layers of the CAM were clearly delineated, but large parts still had not developed the blood-gas barrier (BGB) portions. By E37, chorionic blood capillaries had assumed a superficial position with thin BGB portions covering most of the chorionic surface. On regression analyses, the CAM had two growth phases, namely phase I that occurred between E16 and E25, when the CAM grew rapidly from a volume of 5.55 ± 1.27 to 28.82 ± 5.62 cm 3 to then decrease to 25.18 ± 4.79 cm 3 during phase II (E25-E37). The latter decline was attributed to changes in the chorionic and allantoic layers, while regression in the mesoderm mainly characterized blood and lymphatic vessels. This article is part of the theme issue 'The biology of the avian respiratory system'.