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A luteal cell is a biological cell belonging to the corpus luteum; in endocrine descriptions, the term usually emphasizes its steroid-producing populations. These cells arise as follicular tissue undergoes luteinization and contribute progesterone. The whole corpus luteum also contains vessels and other supporting cells, so an endocrine-cell count is not a count of every cell in the structure.^[ref]

Functional diversity can persist within the steroid-producing population. In 1982, researchers separated larger and smaller luteal cells from sheep by a flow-based separation procedure. For equal cell counts, unstimulated progesterone release from the larger-cell preparation was about twenty times that from the smaller-cell preparation. The result concerns the tested ewe preparation and conditions, not a universal ratio for all mammals.^[ref]

For an illustrative accounting model, ten cells each secreting one unit per hour and ten cells each secreting twenty units per hour contribute ten and two hundred units per hour respectively. Equal cell numbers need not imply equal endocrine contributions. Actual circulating progesterone additionally depends on distribution and removal from the blood.

T. A. Fitz is the first author of a 1982 study distinguishing two steroid-producing cell populations in the sheep corpus luteum. The researchers separated small and large luteal cells before measuring progesterone secretion and responses to stimulating substances. Comparing separated populations allowed a tissue-level output to be related to its cellular components.^[ref]

Without stimulation, equal numbers of large luteal cells released about 20 times as much progesterone as the small-cell population. The equal-count comparison is essential: a higher total secretion could otherwise arise merely because one preparation contained more cells. It also explains why a change in the mixture of cell types can alter total tissue secretion even when each population's individual behavior stays unchanged.^[ref]