Collagen fibrillar skeleton in pregnant rabbit endometrium at term: A SEM study after NaOH maceration

This paper describes the spatial organization of the collagen fibrillar skeleton in pregnant rabbit endometrium at term, employing an alkali/low temperature/maceration technique followed by scanning electron microscopic (SEM) observations. Parallel light microscopic (LM) and transmission electron microscopic (TEM) investigations were made to identify the location and possible changes in the endometrial collagen network. Two different types of NaOH maceration were applied, demonstrating separately: 1) the collagen structures (2N-NaOH maceration removes the cells and basal laminae); and 2) the cellular elements (6N-NaOH maceration removes the collagen fibrils). After 2N-NaOH maceration, the collagen network of the endometrium is seen in a superficial compartment around the glands and a deep compartment situated near the endometrial-myometrial junction. Significant changes are observed only in the superficial compartment. The luminal mucosal surface is characterized by numerous thin projections reducing the uterine cavity which, as a consequence, further appears very irregular and highly convoluted. The subepithelial collagen network is composed of densely packed fibrils with a woven course. It contains many tubular or channel-like invaginations (100- 150 μm in width and 200-300 μm in length) where endometrial glands are located. These invaginations, corresponding to the glandular impressions, are extremely dilated, enlarged and variable in shape. The collagen fibrils are arranged concentrically around the glandular orifices without forming bundles. At the bottom of the spaces between the mucosal projections, small fenestrations (4-8 μm in diameter each) are present. They form small groups of about 10-20 in number and are due to the endometrial blood capillaries. The deep compartment of the endometrial collagen network is little altered, preserving its general lamellar arrangement. The changes in the endometrial collagen skeleton are due to a variety of complex mechanical and hormonal stimuli affecting the uterus during pregnancy. These may be significant for correct implantation, placentation and delivery.