A morphological analysis of the early implantation stages in the rat

Abstract

By making use of the known sequence of ovulation and fertilization in Holtzman rats, the Pontamine Blue reaction, and electron microscopy, the first stages in implantation were studied. Implantation is initiated when the blastocyst becomes clasped by the endometrium and hence assumes a fixed position (evening of day 5 ) . At this stage, the trophoblast cells are in close association with the uterine epithelial cells, with interdigitating microvilli in places, but decidualization of the fibroblasts is just beginning. In the second stage of implantation the decidualized fibroblasts form a cup around the luminal epithelium. Evidence of adhesion of the cell membranes of trophoblast cells with the cell membranes of luminal epithelial cells can be seen at this time. The layer of fibroblasts immediately surrounding the luminal epithelium becomes epithelioid, resulting in a relative isolation of the luminal epithelium from its vascularization. This stage is well-developed by the afternoon of day 6. By the morning of day 7, the luminal epithelium has disappeared from the region of the forming ectoplacental cone down to the level of the abembryonic trophoblast. The trophoblast cells on the lateral aspect of the blastocyst are directly in contact with the residual basement membrane of the luminal epithelial cells, and are separated by this structure and a small connective tissue cleft from the stromal cells. The importance of the relative isolation of the epithelium by the stromal reaction and the adhesion of the cell membrane of the trophoblast cells to the cell membranes of epithelial cells with regard to removal and phagocytosis of epithelial elements are discussed, and many of the cytological features observed during the process are described.The ready availability of the rat and mouse have made these animals favorite objects for the study of implantation. Duval (1891) took advantage of the abundance of rats in the slaughterhouses of Paris for his study materials. However, he used Lataste's collection of mouse reproductive tracts for the comparison of stages and the timing of these stages. His studies, plus those that followed in the next two decades dealing with both the mouse (Burckhard, '01; Sobotta, '03) and the rat and mouse (Melissinos, '07), established many of the essential features of implantation in these myomorph rodents. The mouse was the major subject of these studies, and little distinction was made between the mouse and rat, which were considered to be in the same genus at that time. Although these studies predated a precise knowledge of the reproductive cycle, timed implantation stages were obtained by using Sobotta's method of placing female mice that had given birth to litters 21 days previously with males, then AM. J. ANAT., 120: 185-226.examining them subsequently for copulation plugs.The most comprehensive study of implantation in the rat is that of Huber ('15). He reviewed not only the literature on implantation in the mouse mentioned above, but also the less distinguished literature on th...

Apoptosis at the time of embryo implantation in mouse and rat

“…The observation that luminal epithelial cells facing the mural trophoblasts appear to detach from their basal lamina during the phase of apposition has been reported a long time ago in the rat 90 and in the mouse. 91,92 The immediate consequence of this exfoliation process is that the trophoblasts come in direct contact with the residual basement membrane.…”

“…Early descriptions based on electron microscopy have demonstrated that degenerating epithelial cells located along the lateral margins of the implantation sites round up and show moderate signs of alterations in their cytoplasmic organelles and nucleus. 90 Cellular debris can be seen in the residual uterine lumen but most degenerated cells seem to be promptly engulfed and removed by trophoblasts. Additional observations based on a combination of light and electron microscopy have revealed a three-phase sequence in the anti-mesometrial area of the mouse uterus.…”

Determinants of uterine aging: lessons from rodent models

“…In the uterus, the blastocysts firstly connect with the epithelium to establish intimate relationship with the maternal tissue, and this process is called attachment. During the normal implantation, the uterine lumen and associated epithelial cell morphology are dynamically altered [62]. The uterine lumen changes from being irregular and branched in cross-section to slit-like, with which surface microvilli lose their tall, regular profiles to become shorter and more irregular in shape [63].…”

Melanoma Invasion in the Mouse Uterus

“…The parallel in timing and gross morphology between blastocyst implantation and melanoma invasion in the mouse has been pointed out by Wilson (1963) and the similarity between experimental implan¬ tation in spayed rats maintained on exogenous hormones and the invasion of Walker carcino-sarcoma has been established by Short & Yoshinaga (1967). The hormonal background of nidation in rodents is partly understood and some of the ultrastructural prerequisites of blastocyst attachment and invasion have been defined (Mayer, Nilsson & Reinius, 1967;Enders & Schlafke, 1967). In the spayed rat maintained on progesterone, the attachment of the blastocyst can be delayed until a minute dose of oestrogen (0-1 µg oestradiol-17ß) is given.…”