Development of the bovine placentome during the second month of gestation

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Abstract
Summary. Nulliparous heifers were killed, 3–5 each day, at 3-day intervals between Days 30 and 45 after insemination and 4 additional animals were killed between Days 48 and 67. Placentomes were collected from the region near the fetus and prepared for examination by light and electron microscopy. At 30 days the placentomes were visible on the internal surface of the fixative-treated uteri as discrete, oval structures with a lighter appearance than the surrounding tissues. The maternal–trophoblast interface ranged from almost completely flat to slightly undulating but villi and crypts were not evident. Microvilli were present on both the uterine and trophoblast epithelial cells which were very closely apposed and a tenuous attachment had developed between the two layers. Villi and crypts which had formed by 33 days subsequently lengthened and developed secondary branching. The uterine epithelium was composed mainly of lightly staining cuboidal cells which were interspersed with some dark elongated cells, particularly in the earliest stages, and a few multinucleate giant cells with little cytoplasm. The major cell type in the trophoblast was a low columnar, epithelial cell interspersed with darker staining giant cells. Considerable lipid material was present in the uterine and trophoblast cells at the earlier stages examined but became less pronounced as gestation progressed. An extensive cytocavitary network developed in many of the uterine epithelial cells between Days 33 and 42 but this vacuolization was subsequently reduced.