Mouse uterine glands, obtained during the peri‐implantation period of pregnancy, were investigated using light and electron microscopy. From day 4 to day 6 of pregnancy, there was a progressive luminal dilation and an accumulation of dense homogeneous material in the gland lumina. Although numerous large electron‐lucent vesicles were present in the apical portion of the glandular cells on day 4, their number decreased by days 5 and 6 of pregnancy. Dense granules were present along the apical border of many glandular cells on day 6. In addition, there was an increase in the number and more orderly arrangement of RER cisternae by day 6 of pregnancy. Cytochemical studies on days 4, 5, and 6 of pregnancy, using the periodic acid‐thiocarbohydrazide‐silver proteinate method for the ultrastructural localization of carbohydrate material, showed specific staining of the multivesicular bodies and the saccules of the concave surface of the Golgi complex, but not the dilated saccules of the convex surface. Specific staining was also observed over both the luminal material and apical granules present on day 6 of pregnancy. The cytochemical evidence suggests that the secretory product of the uterine glands has carbohydrate components and that carbohydrate material accumulates in the Golgi complex. In addition, the morphological changes observed imply increased secretory activity of the uterine glands during the peri‐implantation period. Thus, the uterine glands must be considered an important source of uterine fluid components during the peri‐implantation period.
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