THE FINE STRUCTURE OF THE FIBROUS MEMBRANE FORMING REGION OF THE ISTHMUS OF THE OVIDUCT OF <i>GALLUS DOMESTICUS</i>

Abstract

The fibrous membrane forming region of the isthmus of the oviduct of Gallus Domesticus forms the upper two thirds of the anatomical isthmus. The secreting surface of the isthmus is arranged in a series of longitudinal ridges. The mucous membrane consists of a surface epithelium of columnar cells beneath which is a layer of tubular glands. In the membrane forming part of the isthmus the tubular gland cells are filled with prominent secretory granules, hence the name granular isthmus. The tubular glands of the lower third of the isthmus have few prominent granules and the region has a characteristic reddish tinge; hence the name red region of the isthmus is appropriate. In the granular isthmus there are three types of surface epithelial cell. About half are ciliated cells. These have nuclei situated towards the apex of the cell and contain scattered electron dense granules, particularly towards the apex. Lysosome‐like bodies are also present. Alternating with the ciliated cells are secretory cells termed granular cells which tend to bulge out of the surface to form blebs which are filled with aggregations of granules which are usually electron dense. This material is PAS positive and alcian blue negative in histological preparations. The nuclei are situated basally and they are usually associated with granular endoplasmic reticular (GER) profiles and cisternae filled with homogeneous material. Golgi substance is not prominent. Some cells contain masses of small granules morphologically resembling glycogen particles. A third type of cell constitutes about 8% of the surface epithelium. It contains columns of mitochondria running from the base to the apex of the cell. There are no cilia or secreting granules present, yet the apex bulges above the mucosal surface. The function of these mitochondrial cells is obscure; there is no movement of water, Na + or K + into the granular isthmus lumen. The tubular gland cells are filled with secretory granules ranging from 0·2 µm to 3 µm. Most of the granules are of uniform electron density but granules with a wide variety of density pattere are seen. Microvilli are prominent as are mitochondria often associated with GER profiles. The GER profiles can be prominent and they have a remarkable beaded appearance due to an altemation of dense and pale areas. Golgi material is prominent and complex when it is visible in cells depleted of secretory material. The granules are not secreted as such. Amorphous material condenses into a membrane fibre in the lumen of a tubular gland and by the time these fibres reach the luminal surface of the oviduct they have the characteristic core and mantle structure of the basic fibre of the shell membrane.