Morphometric analysis of the cellular composition of the ovine corpus luteum

Abstract

An electron microscopical, morphometric study has been made of the cellular and non-cellular composition of the ovine corpus luteum fixed by perfusion, together with an estimate of total cell numbers in the corpus luteum based on measurement of cell and tissue DNA content. Normal cyclical corpora lutea from six ewes were studied by each method. Total cell numbers per corpus luteum were estimated at 258 X 10(6) by morphometry and 238 X 10(6) by DNA measurement. The existence of four structurally distinct major cell populations in ovine luteal tissue was confirmed. Percentages of the total cell number were as follows: large luteal cells - 4.2%; small luteal cells - 18.5%; fibrocytes - 16.8%; endothelial cells and pericytes - 52.6%. The remaining 7.9% comprised other miscellaneous cell types. Of the four major cell types, cell volume (X 10(3) micron 3 +/- standard error of the mean) was greatest for large luteal cells (13.08 +/- 2.10) and least for endothelial cells and pericytes (0.52 +/- 0.05), with small luteal cells (2.08 +/- 0.19) and fibrocytes (1.61 +/- 0.27) intermediate in size. Large luteal cells occupied 25.4% of luteal volume density, small luteal cells 17.5%, fibrocytes 11.2% and endothelial cells and pericytes 12.7%. Other cell types occupied 5.4%, with the remaining volume occupied by vascular lumina (10.8%) and intercellular space (17.1%). The data on cell volume support morphological findings suggesting that large and small luteal cells of the ovine corpus luteum are distinct populations rather than the upper and lower limits of a single population whose sizes are normally distributed. Of these two types of luteal cell, small luteal cells outnumbered large luteal cells by more than 4:1, while the individual large luteal cells were more than six times the mean volume of the small luteal cells. These facts need to be considered when assessing the relative contributions of the two types of luteal cell to overall luteal steroidogenic function.