Histology and ultrastructure of the chorioallantoic membrane of the mallard duck (Anas platyrhynchos).

The histology and fine structure of the chorioallantoic membrane of the mallard duck (Anas platyrhynchos), and the density of vessels per millimeter of membrane were assessed between days 12 and 24 of incubation. Light and transmission electron microscopy of the chorioallantoic membrane of the mallard duck after various days of incubation was carried out. Blood vessels within the mesoderm were counted per millimeter of membrane by light microscopy (40x). The chorioallantoic membrane had three distinct layers from day 12 to 24 of incubation, the chorionic epithelium, the mesoderm, and the allantoic epithelium. After day 12, chorionic epithelium consisted of two layers of flattened, elongated epithelial cells interfaced by numerous desmosomes, and separated from the underlying mesoderm by a basement membrane. At this stage, the allantoic epithelium consisted of a single layer of flattened, overlapping cells. Blood capillaries were observed in the mesoderm close to the chorionic epithelium on days 12 and 13; by day 14, these capillaries were located within the chorionic epithelium, forming a capillary sinus. Between days 14 and 16, the chorion underwent cellular and cytological differentiation into three cell types: capillary covering cells, villus cavity cells, and less differentiated basal cells. The mesoderm was composed of a loose matrix of mesenchymal cells and collagen fibrils through which coursed blood and lymphatic vessels. The vascular density in the mesoderm increased rapidly from 4.2+/-0.6 vessels per mm (n = 12) on day 12 to a maximum of 9.4+/-0.3 vessels per mm (n = 15) by day 16. From day 16, the allantoic epithelium had two to three layers of elongated and overlapping cells. The luminal layer of allantoic epithelial cells had microvillus projections and varying numbers of membrane-bound dense vesicles at all stages from day 12 onward. The histologic and ultrastructural features of mallard duck chorioallantoic membrane from day 12 to 24 of incubation were very similar to those described in the chorioallantoic membrane of the chicken (Gallus gallus) from day 8 to 20 of incubation. Much of the information available concerning the CAM of the chicken also may apply to the CAM of the mallard, with timing adjusted to match the developmental time-frame recorded here.