[Light and electron microscopic studies of the paraplacenta in the cat]

Even in the early stages of pregnancy microscopically demarcated zones were distinguished on the paraplacenta of the cat towards the poles and way from the zonary placenta. Along the zonary placenta there exists a transitional zone; this is characterized by having a uterine epithelium which possesses coral-shaped buddings or sprouts. In the main, the epithelium of the latter structures is decomposed apically and detritus is present. From the open-lying and partially injured capillaries there appears a ring shaped haematoma which is subsequently characterized as a marginal haematoma. In this, likewise, there grows a branched uterine epithelium with protruding buds. The chorion of the paraplacenta becomes immersed in the extravasated blood of the haematoma by means of long cell protrusions which bear clusters of microvilli. Polewards from the marginal haematoma there is a contact zone with adhesions between the chorionic and uterine epithelium. In the position of close contact with the chorion, the uterine epithelium is transformed into a maternal epithelial symplasma. Furthermore, the chorion develops a rosettelike epithelial proliferation. In addition, small and larger vesicles appear which could have developed from both epithelia. Finally there is an area on the dome-shaped poles of the placenta where there is no contact between the chorionic and uterine epithelium. The epithelium of the paraplacental chorion becomes almost entirely dissociated 1...2 days prior to birth. However, the uterine epithelium with the exception of the protruding buds on the margin of the zonary placenta remains intact. The post partum epithelialization of the wound area commences from this intact area. With the exception of a few short, deep lying tubuli the crypts and glandulae uterinae of the paraplacental disappear in the course of pregnancy. Through the stretching of the uterine wall they are widened and imcorporated into the uterine surface epithelium.