Uterine lining modification and trophoblastic development in the cow, at days 21, 23, 28 and 36 of gestation

Abstract

In order to evaluate possible morphological modifications during implantation in the cow, histological studies of the uterine lining epithelium and the trophoblast were carried out in 8 heifers (2 per gestational stage) at 21, 23, 28 and 36 days of gestation. Ten samples were taken from each uterus, and stained for light microscopical evaluation. Embryo location was assessed by direct observation on days 23, 28 and 36 of pregnancy.

All embryos were located caudal to the mid uterine region. The more relevant changes were: 1) modification of the uterine lining epithelium (from a columnar or seudostratified columnar epithelium, to a cuboidal and later to a flattened simple epithelium with sincitia). 2) The development of the trophoblast (change from a simple cuboidal epithelium to a two-layered cuboidal or columnar one). These modifications are initiated near the embryo to spread peripherally afterwards, but they do not appear to be the same in all regions of the uterus or the conceptus. Likewise, they are not seen in a similar extent in conceptuses of the same age. A developed trophoblast is typical of a viable conceptus, but a more precise change of implantation is the presence of a flattened uterine lining epithelium. Implantation in the cow occurs similar to sheep, although somewhat slower. The modulation of trophoblast development could be due genetic programming and/or to paracrine or autocrine secretions such as growth factors.