Regeneration of the uterine epithelium in later stages of pseudopregnancy in the rabbit. An ultrastructural study.

Morphological changes of the uterine epithelium in later stages of pseudopregnancy in the rabbit have been studied using different morphological methods. The highly proliferated mucosa with numerous symplasms of a pseudopregnant animal returns to the morphology of a nonpregnant animal by apoptosis, moderate necrosis and lytic transformation of symplasms back to typical endometrial cells without desquamation of cells. The first signs of lytic transformation are observed on Day 8 of pseudopregnancy. Enhanced regeneration with apoptosis and lysis of the symplasmic nuclei is observed between Day 14 and Day 16. Full restoration of the epithelium with reappearance of ciliated cells, typical columnar and partly mucified epithelial cells is not completed earlier than Day 24 p. hCG. This epithelium, however, differs clearly from the epithelium of a virgin rabbit due to several residues of epithelial transformation. Thus, from a morphological point of view, pseudopregnancy in the rabbit lasts up to or even longer than Day 28 p. hCG with persisting ultrastructural remnants of the preceding cycle.