The ultrastructure of the endometrial stroma in rats bearing deciduomata was examined in detail on day 9 (day of vaginal cornification = day 0) of pseudopregnancy, together with the development of deciduomata from day 4 to day 8. Five major regions were recognizable on day 9. 1) In the basal zone, which contributes to endometrial regeneration following decidualization, stromal cells remained fibrocyte-like and were separated by wide bands of collagen fibrils. Capillaries were fenestrated and large. 2) The capsule surrounding the antimesometrial (region of the) deciduoma was composed of flattened cells, showing mitosis, which appeared to provide a source of appositional growth of the deciduoma. 3) The large, tightly packed, polyploid cells of the antimesometrial deciduoma showed morphological evidence of protein synthesis activity, an absence of smooth endoplasmic reticulum, and surface specializations suggestive of adhesion (adherens junctions), communication (gap junctions), and maintenance of an extensive surface area (lamellar processes). These cells came into very close apposition with capillary endothelial cells, which were seldom fenestrated, lacked any continuous basal lamina, and showed evidence of high metabolic activity. 4) The loosely packed mesometrial deciduoma contained a structurally supportive, "fixed" population of spiny cells and a "free" population of granulated cells and their putative precursors, together with a meshwork of large, sinusoidal capillaries whose endothelium was generally supported by a basal lamina. 5) The peripheral parts of the glycogenic area, which was structurally similar to the mesometrial deciduoma, contained many grossly enlarged intercellular spaces of undetermined function. The stromal cells at day 4, which give rise to all non-vascular elements except the granulated cells mentioned above, were structurally simple and fibrocyte-like. Only two distinct regions, subepithelial and deep, were recognizable. Capillaries at day 4 generally possessed a continuous endothelium, with a complete basal lamina and many pericytes. Between days 5 and 8, the stromal cells showed divergent forms of differentiation according to their position in the endometrium. Ultrastructural differentiation of the vasculature also showed divergence according to position, leading to the various types of capillaries seen at day 9. Regional variation, and high levels of structural organization, characterize the "programmed" decidual response in the pseudopregnant rat.
