A hypothesis on the origin of the subcylindrical reserve cells of the endocervix.

The structural features of subcylindrical reserve cells of the endocervix are presented. Reserve cells varied in shape. They were randomly oriented and most had a bean-like or spindle shape. Some were loosely apposed with no desmosomes while others were separated by an intercellular matrix. Their nuclei were oval or fusiform, with a prominent nucleolus. Coarse chromatin plaques are distributed irregularly within the nuclei, and surface membrane showed cytoplasmic projections. Within the connective tissue, close to the basal membrane, could be found cells that showed the same structural features; some of them were partially enveloped by amorphous ground substance and collagen, and had small processes extending from the cell surface in the direction of the epithelial layer. Ultrastructural analysis of these cells disclosed some similarities with fibroblasts in the surrounding stroma, and suggested that they were primitive mesenchymal cells originating from Müllerian mesenchyme.