The endometrium of 40 cycling bitches was studied using cytological, cytochemical, and morphometric techniques. Two principal phases of growth and differentiation can be discerned. Phase one begins at the end of anestrus as serum estrogen levels begin to rise and is completed just prior to estrus. It is characterized by growth of the crypts and differentiation of the glandular epithelial cells into well-developed, mucus-secreting cells. Growth, initially rapid, gradually slows. The second growth phase does not begin until the middle of estrus as serum progestin levels rise and lasts nearly a week. Both hypertrophy and hyperplasia of the glandular epithelium and growth of the basal glands characterize this stage. The gland cells develop many well-defined characteristics of absorptive and secretory cells. Another phase of growth occurs in pregnant animals at the onset of implantation. During the third week of metestrus in non-pregnant bitches, the uterus begins to involute. Acid phosphatase and the number of lysosomes increase dramatically in the epithelial cells particularly in the basal glands. Cells lining the lumen and crypts accumulate numerous large lipid droplets. The data are discussed in relation to the clear separation of two distinct uterine functions: (1) sperm transport and maintenance and (2) production and secretion of nutritive uterine milk. Extended periods of follicular development, breeding, and preimplantation in the bitch probably impose this separation.

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Uteri taken from 25 bitches at various times during the early stages of pregnancy were studies cytologically to determine how the implantation chamber developed and how fetal-maternal relations were established. On day 13 after the end of estrus, knobs of trophoblastic syncytium formed and became wedged between cells of the uterine luminal epithelium. The syncytium quickly spread along the uterine lumen and into the mouths of the glands, dislodging and surrounding maternal cells. As invasion continued trophoblastic villi, consisting of cores of cytotrophoblast covered by a continuous layer of syncytium, penetrated deeper into the endometrium. The syncytium spread to surround maternal vessels and decidual cells. By day 26 the trophoblast had extended down to the large lacunae. Here syncytial trophoblast covering tips of the villi degenerated, leaving cytotrophoblast exposed to the necrotic zone. These cells possessed characteristics of absorbing cells. Hematomas were formed by focal necrosis of fetal and endometrial tissue at the poles of the implantation sites. Large pools of extravasated blood accumulated and red blood cells were phagocytized by surrounding trophoblastic cells. Therefore, the endotheliochorial relationship in the canine placenta appeared to be established by syncytial trophoblast invading a cellular endometrium. In the necrotic zone and hematomas, cellular trophoblast may have lost its syncytial covering, but elsewhere maternal vessels and decidual cells in the placenta were in direct contact only with syncytial trophoblast.

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