The cellular pattern of corpus luteal (CL) growth was studied in rats a t Days 6, 10, 12, 14, 16, 17 and 22 of pregnancy: term is Day 23. Measurements were taken of the percentage of the CL occupied by luteal cells, connective tissue and vascular space, luteal cell and nuclear volumes, and the number of luteal and endothelial cells in each of three CL in both ovaries of five rats a t each stage of pregnancy. Total CL volume increased from 1.08-3.23 pl over Days 10-17. This was mainly due to an increase in luteal cell volume from 3.72 pl to 9.30 pl. Neither the number of luteal cells per CL (range 212,000-287,000) nor the percentage of the CL occupied by luteal cells (range 85-90%) had much influence on growth. Nuclear volume increased roughly in proportion to cytoplasmic volume but near term it decreased despite little change in cytoplasmic volume. The number of endothelial cells per CL increased steadily from 398,000 a t Day 6 to 1,545,000 at Day 22. There was a strong negative correlation (r = -0.78) between the number of luteal cells per CL and mean luteal cell volume that was evident at all stages of pregnancy. There was a positive, but weaker correlation (r = 0.35) between number of luteal cells and CL volume. Thus, CL volume seems to be partly determined by the number of luteal cells at Day 6 but this effect is moderated by local control of luteal cell volume.The corpora lutea are the major source of progesterone during pregnancy in the rat. The rate of progesterone production assessed from the collection of ovarian venous effluent (Fajer and Barraclough, '67; Hashimoto et al., '68; Uchida et al., '70) and indirectly from peripheral plasma progesterone levels (Wiest et al., '68; Morishige et al., '73; Bartholomeusz et al., '76) rises during the first five days of pregnancy, plateaus, or falls slightly until about Day 10, increases to a peak at about Day 16 then falls to very low levels about 24 hours prior to parturition on Day 23. Although this temporal pattern of secretion has now been well-established, little is known of its control or indeed of the limitation to progesterone production in any one rat.We are interested in morphological factors which may be of significance and whether morphology can be used as an index of function in the corpus luteum. Changes in the total mass of secreting tissue (Uchida et al., '70) and in some ultrastructural features of luteal cells (Long, '73) have already been reported. However, other than some relatively qualitative studies (Bassett, '49; Long, '73) there is little information on changes in the number and size of luteal cells, particularly during mid-gestation when corpus luteal volume and rate of progesterone synthesis reach maximum levels. This forms the major subject of the present study. We also carried out statistical analyses to identify cellular differences between corpora lutea and whether these might be related to the number of corpora lutea per rat.
MATERIALS A N D METHODSThirty-five nulliparous Albino Wistar rats with a mean weight at mating of...
