The histology and histochemistry of the developing chorioallantoic membrane in the chick (Gallus domesticus)

Abstract

The chorioallantoic membrane is composed of three layers. (1) An outer chorionic epithelium consisting of a thin outer layer of cells, an inner stratified cell layer, and a central blood sinus lined with flattened ectodermal cells. (2) A middle connective tissue which provides support and a pathway for the distribution of blood and lymphatic vessels. Numerous capillaries pass from this layer to the chorionic sinuses. (3) An inner allantoic epithelium composed of secretory cells. The components of all three layers increase in number and concentration during the first part of the incubation period. After day 17, however, there is a rapid decrease in the composition of each layer. The allantoic epithelium is rich in secretory granules. These granules include a sulfated mucopolysaccharide component. It is suggested that the liberation of this secretion provides a surface coating which protects the underlying tissues against the increasing toxicity of the allantoic contents.