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J Reprod Fertil. 1976 Nov;48(2):257-264.

Studies on the equine placenta II. Ultrastructure of the placental barrier.


In early pregnancy the equine placenta consists of a simple apposition of fetal and maternal epithelia, but it becomes more complex with the formation of microcotyledons between 75 and 100 days of gestation. Although the placental barrier maintains an epitheliochorial arrangement throughout the course of pregnancy, a thinning of the maternal epithelium and a progressive indentation of the chorionic epithelium by fetal capillaries shortens the length of the diffusion pathway and reduces the amount of placental tissue between fetal and maternal bloodstreams. These structural modifications may reflect the changing requirements of the fetus for O2 and other metabolites as gestation proceeds. During the first 200 days of pregnancy there is evidence of intense pinocytotic activity by the cells of the trophoblast. From the 100th day of pregnancy there is a pronounced development of smooth endoplasmic reticulum, while rough endoplasmic reticulum and irregular, dense, membrane-bound bodies are a prominent feature of the paranuclear cytoplasm from Day 200. These changes suggest that the cells of the trophoblast become more highly involved in synthetic processes with increasing gestational age.


The ultrastructure of the equine placenta was observed on gestation Days 46, 61, 75, 98, 101, 150, 200, 250, and 300 in Welsh Mountain Ponys mares. In early pregnancy, up to 75 days, when the placenta is epitheliochorial, the trophoblast forms a wide band of columnar epithelium with large nuclei, pinocytotic vesicles, and a defined basal lamina beneath the fetal vessels. Maternal epithelium has branched microvilli, spherical nuclei, droplets of mucopolysaccharide, and a moderately thick capillary endothelium. From Days 100-200 the villi and crypts of the microcotyledons develop. The trophoblast shows larger pin ocytocic vesicles (.5 MCM), extensive smooth endoplasmic reticulum (ER), and the fetal capillaries indent into the trophoblast. Maternal epithelium becomes progressively thinner. In the last trimester the trophoblast becomes thinner with nuclei and mitochondria near the indented capillaries, and dense bodies apical to the nucleus. By this time the maternal epithelium is only 1/3 its original thickness, with few organelles except Golgi, mitochondria, and an irregular and sometimes pyknotic nucleus. The ultrastructure supports the idea that the trophoblast may be synthesizing steroids in the later stages of pregnancy.

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