An ultrastructural study of implantation in the golden hamster. III. Initial formation and differentiation of decidual cells.

The transformation of fibroblasts into decidual cells was studied ultrastructurally in golden hamsters pregnant for 3, 3 1/2, 4, 4 1/2, 5, or 5 1/2 days (post-ovulation). Cells within the endometrial stroma were generally separated from one another and spindle-shapted before blastocyst contact with the uterine epithelium. Once the epithelium enclosed the blastocyst (3 1/2 days), stromal cells adjacent to the blastocyst antimesometrially began to exhibit more extensive Golgi complexes with increased secretory activity. As differentiation proceeded the amount of cellular contact increased and various types of junctions formed between the cells. Throughout the period examined, intercellular space progressively decreased, while the cisternal width of the granular endoplasmic reticulum continually increased. Special organelles, whose functions remain unknown, first appeared in differentiating cells at 4 days (fibrils) and 5 1/2 days (crystalloid and 'dumb-bell' structures).