Abstract
Transmission electron microscopy of the trophoblast cells during diapause revealed an abundance of electron-dense, membrane-bound granular inclusions and a marked lack of cytoplasmic organelles. The cells also possessed a well-developed covering of branched microvilli, numerous caveolae, micropinocytotic vesicles and a lamina of fine fibrillae. The progressive enlargement of the blastocyst during diapause was correlated with a decline in the height of the trophoblast cells and a reduction in the density of microvilli and caveolae associated with the outer membrane. The granular inclusions also declined in number and electron density during the delay phase, suggesting the progressive utilisation of energy reserves. Embryonic elongation was associated with the disappearance of the granular inclusions, a reduction in the amount of fibrillar material and a dramatic increase in the development of cytoplasmic organelles. During diapause, clear vesicles, apparently derived from the Golgi apparatus, gradually accumulated in the supranuclear region of each gland and non-ciliated duct cell. Embryonic elongation was associated with the sudden release of these vesicles into the glandular lumen and thence into the uterine lumen. Numerous apical protrusions were also observed projecting from the luminal and ductal epithelia at this time, suggesting the formation of an apocrine secretion. Another type of secretion was produced during the early stages of placental attachment by the hypertrophied granular endoplasmic reticulum of the ductal epithelium.
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