Quantitative ultrastructure of the luteal cell of the 16-day-pregnant rat and its relation to progesterone secretion

Abstract

The ultrastructure of luteal cells of five Day-16 pregnant rats were examined morphometrically to determine the relationship between the quantity of steroidogenic organelles and membranes and reported rates of progesterone secretion (2.3 micrograms/h). Each rat had 11.8 +/- 1.0 corpora lutea (mean +/- s.e.m.) with an average volume of 4.5 +/- 0.1 microliter. There were 210 000 +/- 10 000 luteal cells per CL and the luteal cell cytoplasm was composed of smooth endoplasmic reticulum (18%), mitochondria (10.6%), lipid droplets (8.9%) and granules (0.6%). The surface area of the smooth endoplasmic reticulum was 192 cm2 per CL, and that of the outer and inner mitochondrial membranes was 20 and 34 cm2, respectively. For each square micrometre of these membranes, respectively, 62, 590 and 355 molecules of progesterone would have been secreted per second. The luteal cell appears to secrete its major steroid hormone at a rate 50 times greater than that reported for the Leydig cell of the testis when secretion is expressed in terms of molecules per unit mass of steroidogenic cell or area of steroidogenic membrane.

Quantitative ultrastructural characteristics relating to transport between luteal cell cytoplasm and blood in the corpus luteum of the pregnant rat

“…The relative densities determined by flotation technique (Meyer and Bruce, 1979) on four perfusion-fixed (PF) CLs and four immersion-fixed (IF) CLs were 1.046 and 1.038, All the values were adjusted back to CL volume by applying appropriate linear shrinkage factors and assuming shrinkage to be isotropic. The factor 0.935 was used for this study (Meyer and Bruce, 1984). Two CLs were selected at random from each of the eight PF ovaries and one CL from each of the eight IF ovaries.…”

“…Functional capacity can depend on intracellular factors, including enzyme concentrations and stores of precursors, and on extracellular or physiological factors including transport mechanisms which govern the environment of the cell. Although a number of quantitative studies have related intracellular structures to rates of hormone secretion in steroidsecreting organs, including the testis (Mori and Christensen, 1980; Zirken et al, 1980) and ovary (Meyer and Bruce, 19841, there have been no quantitative reports on the structural characteristics of the extracellular transport system of endocrine organs.Transport systems could have an important role in the overall control of organ activity, particularly in the corpus luteum (CL), which has a high rate of secretory activity (Meyer and Bruce, 1984). The rate of blood flow to this organ is among the highest in Address reprint requests to Dr.…”

Quantitative changes in steroidogenic organelles in the corpus luteum of the pregnant rat in relation to progestin secretion on day 16 and in the morning and afternoon of day 22

“…The stereological methods used followed the principles outlined by Weibel(1979) as adapted for use in the rat ovary in our laboratory (Dharmarajan et al, 1983;Meyer and Bruce, 1984). The volume of the corpus luteum was calculated from its weight and density (1.038 and 1.030 at days 16 and 22, respectively).…”

“…They were weighed and sliced into small pieces and processed for EM sectioning as previously described (Meyer and Bruce, 1984;Dharmarajan et al, 1985). Briefly, one grid from one block, selected at random, was examined for each CL.…”

“…Thus these data do not support the existence of a consistent structural-functional relationship over the normal range of secretory activity in the rat corpus luteum. It is likely that a number of factors, including a variable reserve capacity for steroidogenesis and secretion of nonsteroid hormones, contribute to this inconsistency.ganelles (Tamaoki, 1973;Miller, 1988), it has been postulated that a quantitative relationship may exist between their volume or surface area and the rate of steroid secretion (Zirkin et al, 1980;Bronstein et al, 1984;Meyer and Bruce, 1984). The development of accurate techniques for quantitating both ultrastructure (Weibel, 1979;Mori and Christensen, 1980) and rates of steroid secretion (Bruce et al, 1980) has made it possible to test such structural-functional relationships.…”