This report describes in detail the fine structure of a unique granular cell, termed “granulocyte,” which populates the endometrium of the monkey uterus under certain physiological conditions. Morphological evidence is presented which establishes that this cell is similar, if not identical, to granular cells found in the uteri of a wide variety of vertebrate species, including the human. Evidence supporting the hypothesis that these cells secrete relaxin is reviewed.

The granulocyte is nearly spherical, approximately 10 μ in diameter, and contains an eccentrically located, kidney-shaped nucleus approximately 6 μ × 8 μ in size. Opposite the concave surface of the nucleus is located a cluster of specific granules. The granules vary in diameter, but they rarely exceed 0.5 μ. Each granule is surrounded by a unit membrane, and often a space is present between the enveloping membrane and the granule proper. Smaller vesicles and dense bodies (approximately 0.1 μ in diameter) are found within this space. The morphology of these small vesicles and dense bodies suggests that they have their origin in the Golgi complex and are involved in transporting some material to the specific granules.

Similarities of the granulocyte to other cells types found in uterine endometrium, particularly eosinophilic leucocytes, are pointed out, but it is argued that the fine structure of the granulocyte provides good evidence that this is a distinct uterine cell type.