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

In order to evaluate possible morphological modifications during implantati on in the cow, histological studies of the uterine lining epithelium and th e 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 assess ed by direct observation on days 23, 28 and 36 of pregnancy. All embryos were located caudal to the mid uterine region. The more relevan t changes were: 1) modification of the uterine lining epithelium (from a co lumnar or seudostratified columnar epithelium, to a cuboidal and later to a flattened simple epithelium with sincitia). 2) The development of the trop hoblast (change from a simple cuboidal epithelium to a two-layered cuboidal or columnar one). These modifications are initiated near the embryo to spr ead peripherally afterwards, but they do not appear to be the same in all r egions of the uterus or the conceptus. Likewise, they are not seen in a sim ilar extent in conceptuses of the same age. A developed trophoblast is typi cal 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 troph oblast development could be due genetic programming and/or to paracrine or autocrine secretions such as growth factors.