Human endometrium taken during the early to mid-secretory portion of the menstrual cycle is studied with the electron microscope. A description of the nucleolus is given. In addition, an unusual structure within the endometrial cell nucleolus is described, consisting of amorphous matrix, 150-A dense granules, and a series of tubular channels. This structure is named the nucleolar channel system. A description is given of the geometric variability of the nucleolar channel system, its contents, and its relationship to the cytoplasm. The morphologic basis for a nucleolar-cytoplasmic interrelationship via the nucleolar channel system is described. Some of the implications of this relationship are discussed. The work of previous investigators on the nucleolar channel system is discussed.

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