THE AMERICAN JOURNAL OF ANATOMY 191:208-214 (1991)
Changes in Ultrastructure of Transport Systems and in Rates of Progestin Secretion in the Corpus Luteum During Late Pregnancy in the Rat A.M. DHARMARAJAN, NEVILLE W. BRUCE, AND BRENDAN J . WADDELL Department of Anatomy and Human Biology, The University of Western Australia, Nedlands, Western Australia 6009, Australia
rajan et al., 1985).The luteal cells themselves have numerous microvilli (Enders, 1962)and in perfused tissue are generally seen to be adjacent to a t least one capillary (Dharmarajan et al., 1985). Collectively, these features suggest that function and maintenance of the CL could be dependent on adequate and extensive transport structures for interchanging materials between luteal cell cytoplasm and the capillary lumen. We previously used stereological techniques to quantitate such structures in the day 16 pregnant r a t and found that the vascular space occupied 10-20% of the CL, depending on the method of fixation (Dharmarajan et al., 1983).The microvilli of the luteal cell increased its effective surface area by three-fold (Dharmarajan et al., 1985),and more than 90% of the luteal cell surface was adjacent either to perivascular space or to intercellular space that presumably was confluent with the perivascular space. These findings lend quantitative support for the view that a n extensive development of transport structures is required to support normal rates of progestin secretion. Only one stage of gestation was examined in that study, however, and whether altered secretion rates would elicit or necessitate a quantitative change in the ultrastructure of the transport systems was not determined. We have recently shown that total progestin (progesterone plus 20a-hydroxypregn-4-en-3-one)secretion rates in the r a t fall from 32 pglhr per ovary on day 16 to 10 pglhr per ovary on the morning (1100hr) of day 22 (day 22 AM) and rise again to 23 pglhr per ovary in the afternoon (1500 hr) of the same day (day 22 PM). These changes are not accompanied by any comparable changes in CL volume (Waddell e t al., 1989)or in the amount and surface areas of steroidogenic organelles (Dharmarajan et al., 1991).The present work therefore was carried out to test whether changes in transport structures would more closely reflect progestin secretion rates and the increase in total ovarian and CL INTRODUCTION blood flow observed over days 16 to 22 (Bruce et al., Humoral and intracellular mechanisms controlling 1984). luteal steroidogenesis have been studied extensively (see review by Gibori et al., 1988);however, the structural support for steroidogenesis has received less attention. The corpus luteum (CL) is richly supplied with blood vessels and has one of the highest rates of blood Received August 21, 1990. Accepted January 3, 1991. flow, per unit mass of tissue, in the body (Enders, 1973; Address reprint requests to Dr. N.W. Bruce, Department of AnatBruce and Moor, 1975).There is a n extensive network and Human Biology, The University of Western Australia, Nedof capillaries within the CL (Reynolds, 1973;Dharma- omy lands, Western Australia 6009, Australia. rajan et al., 1983),and the capillaries have a characA.M. Dharmarajan’s present address is Department of Obstetrics teristic ultrastructure with numerous fenestrations and Gynecology, The Johns Hopkins Medical Institutions, 600 N. and a sinusoidal appearance (Enders, 1962;Dharma- Wolfe Street, Park Building, Room B2-240, Baltimore, MD 21205.
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 secretion replus 20 a-hydroxypregn-4-en-3-one) ported to be 32,10, and 23 pg/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.
0 1991 WILEY-LISS, INC
209
TRANSPORT SYSTEMS I N THE CORPUS LUTEUM
TABLE 1. Volumes and surface areas of major structures within the corpus luteum of rats at days 16 and 22 of gestation
Day 22 Dav 16
Mean corpus 3.79 t 0.121 luteum volume ( ~ 1 ) Composition of CL (%) Luteal cell Cytoplasm 77.11 t 0.91 Nucleus 3.12 t 0.49 Endothelial cell Cytoplasm**2 3.82 t 0.22" Nucleus 1.44 t 0.43 Pericyte cytoplasm + nucleus* 0.34 t 0.09" Other cell cytoplasm + nucleus 0.22 i 0.05 Interstitial space Cell-cell* 0.51 t 0.06" Cell-capillary** 1.84 2 0.09" Vascular space* 11.60 t 0.60a Surface area (mm2/CL) 108 t 18(6%) LL3 LIL* 706 2 54(38%)a.b 1,064 t 76(57%Ia LIC** Capillary 251 i 11" (outer membrane)* Contribution of surface specializationsto surface area (BJB,) LL 1.04 t 0.01 2.22 2 0.10-b LIL* 3.91 i 0.15" LIC** CaDillarv (0uter"membrane) 1.03 2 0.004
Morning
Afternoon
0.04
3.92 t 0.03
79.25 ? 0.94 4.43 t 0.56
80.79 2 1.23 3.30 t 0.91
3.99
2
2.21 t 0.30b 0.81 t 0.22
3.26 t 0.36" 0.74 t 0.09
0.71 t 0.12b
0.50 t 0.05a,b
0.38 2 0.03
0.79 i 0.25
0.88 t O.lOb 1.38 t O.Ogb 9.96 i 0.75a,b
0.64 t 0.08a,b 1.45 i 0.04b 8.54 i 0.65b
168 t 12(lO%) 811 t 93(48%)" 707 i 63(42%Ib
169 t 23(11%) 537 55(33%)b 922 2 61(56%)a
208 i 7b
214
1.06 t 0.02 2.46 ? 0.10" 2.90 i 0.14b
1.07 t 0.01 1.85 2 0.16b 3.13 i O.lgb
1.05 ? 0.02
1.05 i 0.02
*
?
12b
'Values are means -t SEM (n = 5 for each group) except for those in parentheses, which refer to the rercent of the luteal cell surface area represented by each interface. *, P
Changes in Ultrastructure of Transport Systems and in Rates of Progestin Secretion in the Corpus Luteum During Late Pregnancy in the Rat A.M. DHARMARAJAN, NEVILLE W. BRUCE, AND BRENDAN J . WADDELL Department of Anatomy and Human Biology, The University of Western Australia, Nedlands, Western Australia 6009, Australia
rajan et al., 1985).The luteal cells themselves have numerous microvilli (Enders, 1962)and in perfused tissue are generally seen to be adjacent to a t least one capillary (Dharmarajan et al., 1985). Collectively, these features suggest that function and maintenance of the CL could be dependent on adequate and extensive transport structures for interchanging materials between luteal cell cytoplasm and the capillary lumen. We previously used stereological techniques to quantitate such structures in the day 16 pregnant r a t and found that the vascular space occupied 10-20% of the CL, depending on the method of fixation (Dharmarajan et al., 1983).The microvilli of the luteal cell increased its effective surface area by three-fold (Dharmarajan et al., 1985),and more than 90% of the luteal cell surface was adjacent either to perivascular space or to intercellular space that presumably was confluent with the perivascular space. These findings lend quantitative support for the view that a n extensive development of transport structures is required to support normal rates of progestin secretion. Only one stage of gestation was examined in that study, however, and whether altered secretion rates would elicit or necessitate a quantitative change in the ultrastructure of the transport systems was not determined. We have recently shown that total progestin (progesterone plus 20a-hydroxypregn-4-en-3-one)secretion rates in the r a t fall from 32 pglhr per ovary on day 16 to 10 pglhr per ovary on the morning (1100hr) of day 22 (day 22 AM) and rise again to 23 pglhr per ovary in the afternoon (1500 hr) of the same day (day 22 PM). These changes are not accompanied by any comparable changes in CL volume (Waddell e t al., 1989)or in the amount and surface areas of steroidogenic organelles (Dharmarajan et al., 1991).The present work therefore was carried out to test whether changes in transport structures would more closely reflect progestin secretion rates and the increase in total ovarian and CL INTRODUCTION blood flow observed over days 16 to 22 (Bruce et al., Humoral and intracellular mechanisms controlling 1984). luteal steroidogenesis have been studied extensively (see review by Gibori et al., 1988);however, the structural support for steroidogenesis has received less attention. The corpus luteum (CL) is richly supplied with blood vessels and has one of the highest rates of blood Received August 21, 1990. Accepted January 3, 1991. flow, per unit mass of tissue, in the body (Enders, 1973; Address reprint requests to Dr. N.W. Bruce, Department of AnatBruce and Moor, 1975).There is a n extensive network and Human Biology, The University of Western Australia, Nedof capillaries within the CL (Reynolds, 1973;Dharma- omy lands, Western Australia 6009, Australia. rajan et al., 1983),and the capillaries have a characA.M. Dharmarajan’s present address is Department of Obstetrics teristic ultrastructure with numerous fenestrations and Gynecology, The Johns Hopkins Medical Institutions, 600 N. and a sinusoidal appearance (Enders, 1962;Dharma- Wolfe Street, Park Building, Room B2-240, Baltimore, MD 21205.
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 secretion replus 20 a-hydroxypregn-4-en-3-one) ported to be 32,10, and 23 pg/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.
0 1991 WILEY-LISS, INC
209
TRANSPORT SYSTEMS I N THE CORPUS LUTEUM
TABLE 1. Volumes and surface areas of major structures within the corpus luteum of rats at days 16 and 22 of gestation
Day 22 Dav 16
Mean corpus 3.79 t 0.121 luteum volume ( ~ 1 ) Composition of CL (%) Luteal cell Cytoplasm 77.11 t 0.91 Nucleus 3.12 t 0.49 Endothelial cell Cytoplasm**2 3.82 t 0.22" Nucleus 1.44 t 0.43 Pericyte cytoplasm + nucleus* 0.34 t 0.09" Other cell cytoplasm + nucleus 0.22 i 0.05 Interstitial space Cell-cell* 0.51 t 0.06" Cell-capillary** 1.84 2 0.09" Vascular space* 11.60 t 0.60a Surface area (mm2/CL) 108 t 18(6%) LL3 LIL* 706 2 54(38%)a.b 1,064 t 76(57%Ia LIC** Capillary 251 i 11" (outer membrane)* Contribution of surface specializationsto surface area (BJB,) LL 1.04 t 0.01 2.22 2 0.10-b LIL* 3.91 i 0.15" LIC** CaDillarv (0uter"membrane) 1.03 2 0.004
Morning
Afternoon
0.04
3.92 t 0.03
79.25 ? 0.94 4.43 t 0.56
80.79 2 1.23 3.30 t 0.91
3.99
2
2.21 t 0.30b 0.81 t 0.22
3.26 t 0.36" 0.74 t 0.09
0.71 t 0.12b
0.50 t 0.05a,b
0.38 2 0.03
0.79 i 0.25
0.88 t O.lOb 1.38 t O.Ogb 9.96 i 0.75a,b
0.64 t 0.08a,b 1.45 i 0.04b 8.54 i 0.65b
168 t 12(lO%) 811 t 93(48%)" 707 i 63(42%Ib
169 t 23(11%) 537 55(33%)b 922 2 61(56%)a
208 i 7b
214
1.06 t 0.02 2.46 ? 0.10" 2.90 i 0.14b
1.07 t 0.01 1.85 2 0.16b 3.13 i O.lgb
1.05 ? 0.02
1.05 i 0.02
*
?
12b
'Values are means -t SEM (n = 5 for each group) except for those in parentheses, which refer to the rercent of the luteal cell surface area represented by each interface. *, P
