Changes in ultrastructure of transport systems and in rates of progestin secretion in the corpus luteum during late pregnancy in the rat

Abstract

Morphometric studies have confirmed that the corpus luteum (CL) of the pregnant rat contains luteal cells with numerous microvilli which directly face an extensive network of sinusoidal capillaries. From this it has been suggested that extensive development of transport structures is necessary to support progesterone synthesis and secretion. The present study was carried out to determine whether these transport structures could be related quantitatively to different rates of total progestin (progesterone plus 20 alpha-hydroxypregn-4-en-3-one) secretion reported to be 32, 10, and 23 micrograms/hr per ovary on day 16 and the morning (AM) and afternoon (PM) of day 22, respectively. Histological analysis was carried out on two CL, fixed by immersion, from each of five rats, at each stage of gestation. The important findings to emerge were that when the progestin secretion rate was greater, there was a significant increase in surface specializations on the luteal cell and a thickening of the capillary walls. There was also a greater volume of interstitial space between luteal cells and capillaries. However, due to the development of microvilli and unevenness in the capillary wall, the physiological diffusion distance (harmonic distance) between luteal cell cytoplasm and blood was not increased. Collectively, these results show that changes in the rate of progestin secretion are accompanied by significant, although disproportionate, changes in transport structures and suggest that the latter are important in supporting luteal function.

Effect of exogenous oestrogen on blood flow and quantitative histology of the corpora lutea of pseudopregnant rabbits

“…The physiological parameters at the time of the blood flow measurements are summarized in Table 4. No significant differences between groups were found in any of the measured physiological parameters ( (Dharmarajan et al, 1991 (Wiltbank et al, 1990). Because of this lack of autoregulation, there is a positive correlation between mean arterial pressure and luteal blood flow in anaesthetized rabbits (Janson et al, 1981 (Abdul-Karim and Bruce, 1973) and this is presumed to be associated with structural regression of the corpus luteum.…”