Scanning electron microscopy of the surface epithelium of the bovine endometrium

Abstract

The surface epithelium of the bovine endometrium comprises at least 2 cell types (ciliated cells and secretory cells with microvilli), but their distribution and morphological changes over the estrous cycle are poorly understood. The objective was to quantify the number of ciliated cells and assess morphological changes in secretory cells on the uterine surface epithelium during the estrous cycle. Caruncular endometrium (CAR) and intercaruncular endometrium (ICAR) samples were collected from the uterine body, the horn ipsilateral to the corpus luteum or dominant follicle (H-CL/DF), and the horn contralateral to the corpus luteum or dominant follicle (H-NCL/NDF) from heifers following slaughter on d 0 (estrus; n = 5) or d 14 (mid-luteal phase; n = 5) of the estrous cycle. Samples were prepared for scanning electron microscopy at 1,000× magnification. Four to 10 fields (256 × 225 µm) for each sample were examined (n = 567 images). The number of ciliated cells was counted and the surface was scored for the morphology of the secretory cells (0 = absence of microvilli on surface; 3 = 100% of surface covered with microvilli). Ciliated cells were present in both the CAR and ICAR regions. The number of ciliated cells per field increased from d 0 to 14 in CAR and decreased from d 0 to14 in ICAR. The scanning electron microscopy revealed a general lack of uniformity in the lawn of microvilli on the surface of the endometrium. Based on the scores, approximately 25% of the fields had a surface that was <50% covered by microvilli. Depletion of microvilli may be explained by a normal process where apical protrusions are formed and either regress back into the cell surface or break to release their contents into the uterine lumen. These studies support the hypothesis that the surface of the luminal epithelium changes during the estrous cycle through a process that involves remodeling of the apical surface. The morphology of the apical surface may have a key role in governing pregnancy establishment.

Keywords: cilia; endometrium; microvilli; scanning electron microscopy.

Figure 1.

Scanning electron micrographs of the surface epithelium of the bovine endometrium demonstrating the morphological scoring system: A, score = 0 (devoid of microvilli); B, score = 1 (> 0 but < 50% covered with microvilli); C, score = 2 (≥ 50 but < 100% covered with microvilli); and D, score = 3 (completely covered with microvilli). Ciliated cells (white arrow) were observed in regions that were depleted (E) or replete (F) for microvilli. Cells that lacked microvilli on their surface had a cratered appearance (E, black arrows). The scale bar for A, B, C, and D = 20 μm and for E and F = 10 μm. Samples collected on d 0 (C, D) or d 14 (A, B, E, F) of the estrous cycle.

Figure 2.

Scanning electron micrographs of the surface epithelium of the bovine endometrium. A) Low magnification of caruncular tissue. Note the caruncular folds, lack of glands and widely dispersed ciliated cells (black arrows). B) Low magnification of intercaruncular tissue. Note the glands (white arrows) with folds between individual gland openings. C) Higher magnification of caruncular tissue with individual ciliated cells (black arrow). D) Higher magnification of gland opening (white arrow) showing the large number of ciliated cells at the gland opening. The scale bar for A and B = 200 μm and for C and D = 20 μm. Samples collected on d 0 (B, D) or d 14 (A, C) of the estrous cycle.

Figure 3.

Scanning electron micrographs of the surface epithelium of the bovine endometrium during the formation of apical protrusions (pinopodes). A) Endometrial surface fully covered with microvilli. B) Pinopode formation may begin with expansion of the apical surface to form “sea-anemoe-like” structures (white arrow). C) Further expansion of the cell surface either flattens or pushes aside the microvilli and pinopodes extend into the uterine lumen. Some cells do not appear to have an intact apical surface perhaps because pinopodes have ruptured (white arrow). D) Higher magnification of C. Vacuoles (white arrow) are exposed to the uterine lumen in a cell that does not have an intact apical surface. Scale bars: A and B = 10 μm; C = 20 μm; and D = 5 μm. Samples collected on d 0 (C, D) or d 14 (A, B) of the estrous cycle.