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Animal Reproduction Science 60–61 2000 145–160 www.elsevier.comrlocateranireprosci
The bovine placenta before and after birth: placental development and function in health and disease
D.H. Schlafer , P.J. Fisher, C.J. Davies
Department of Biomedical Sciences, College of Veterinary Medicine, Cornell UniÕersity, T6-020, VRT, Ithaca, NY 14853 USA
Abstract
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This paper reviews bovine placental development, anatomy microscopic and gross , nomencla- ture and classification. The paper focuses on the biology of those specialized cells that arise from
the outermost layer of very early embryos, the trophoblast cells, and on placental macrophages, cells that play a key role in fetalrplacental defense. Data is presented from an immunohistochemi-
cal quantitative study that characterizes the ontogeny of placental macrophages using placental
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tissues from 21 cows sampled from 4 months of pregnancy through the post partum period . Understanding of bovine placental development is essential for veterinarians, pathologists, diag-
nosticians and researchers. Lesions of diagnostic significance can be recognized for many economically important infectious abortifacient diseases, and there is growing evidence that
pregnancy failure of cloned calves is due in part to unexplained placental failure. Placentology and placental pathology are becoming of increasing importance. q 2000 Elsevier Science B.V. All
rights reserved.
Keywords: Bovine; Placenta; Development; Anatomy; Histology; Macrophage; Disease; Review
1. Introduction
The cow is one of the ‘‘eutherian’’ species, i.e. those that have placentas. The word ‘‘placenta’’ is taken from latin meaning ‘‘flat cake’’ from the apparent gross similarity
of the human discoid placenta to round flat loaves of unleavened bread commonly made in ancient times. The gross appearance of placentas varies greatly between species.
Corresponding author. Tel.: q1-607-253-3352; fax: q1-607-253-3317. Ž
. E-mail address: dhs2cornell.edu D.H. Schlafer .
0378-4320r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
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There are very few similarities between the appearance of a placenta from a woman and one from a cow; but during very early embryonic development, before they enter the
uterus, they appear nearly identical. Once placental development begins, dramatic differences become apparent.
When this great variation in anatomy of the definitive placenta from animal species to animal species is combined with the seemingly complex medical terms applied to
developmental and histologic features, placentology can become quickly frustrating. The purpose of this paper is to present bovine placental development from a simplified
perspective, to discuss and illustrate gross and microscopic features and to present preliminary quantitative data on the ontogeny of bovine placental macrophages, cells
that are important in utero-fetal defense systems.
Fig. 1. Essential steps in early placental development. The outer layer of the expanded blastocyst, the trophectoderm, is composed of trophoblast cells. This layer is
Ž also referred to as ‘‘the trophoblast’’. Cells on the inner aspect of the blastocoel cavity which becomes the
. yolk sac cavity will develop into the embryo. The outer trophoblast layer, combined with the subjacent
somatic mesoderm, is called the chorion and it becomes vascularized by apposition to a bulging fluid-filled allantoic sac that develops from the embryo’s hindgut. This layer of allantois and chorion becomes the
chorioallantois of the fetal membranes. The amnion is formed around the embryo by folding of the chorion to reach over the embryo and fuse, thereby, enclosing the embryo in a membrane lined by amniotic epithelial
cells.
2. Placental development
