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Biology of Reproduction 60, 1120-1127 (1999)
©Copyright 1999 Society for the Study of Reproduction, Inc.

Intrafollicular Content of Luteinizing Hormone Receptor, {alpha}-Inhibin, and Aromatase in Relation to Follicular Growth, Estrous Cycle Stage, and Oocyte Competence for In Vitro Maturation in the Mare1

Ghylène Goudeta, François Belina, Jacqueline Bézarda, and Nadine Gérard2,a

a I.N.R.A.-Haras Nationaux, Reproduction Equine, P.R.M.D., F-37380 Nouzilly, France


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The intrafollicular content of LH receptor, {alpha}-inhibin, and aromatase are known good indicators of follicular status. We investigated the amounts of these proteins in granulosa and cumulus cells in relation to oocyte competence for in vitro maturation, follicular growth, and estrous cycle stage in the mare.

Follicular punctures were performed 34 h after an injection of crude equine gonadotropins, either during the follicular phase, at the end of the follicular phase, or during the luteal phase. The cumulus-oocyte complex, granulosa cells, and follicular fluid of follicles larger than 5 mm were collected. The nuclear stage of the oocytes after in vitro culture was determined microscopically. Granulosa and cumulus cell amounts of LH receptor, {alpha}-inhibin, and aromatase were assessed by the semiquantitative Western blot method and image analysis. Follicular fluids were assayed for progesterone (P4) and estradiol-17ß (E2).

The three factors were expressed in mural granulosa and cumulus cells from all follicles from the gonadotropin-independent growth period until the preovulatory stage. Considering all the follicles punctured, the amounts of LH receptor and {alpha}-inhibin in granulosa cells were not different for the three physiological stages studied. The amounts of aromatase in granulosa cells, as well as the E2:P4 ratios, were higher for follicles punctured during the follicular phase than for the two other groups (p < 0.05). Considering the data from the three groups, the E2:P4 ratio and the LH receptor and aromatase contents, but not {alpha}-inhibin, in granulosa cells increased with an increase in follicular diameter (p < 0.01). The E2:P4 ratios and the amounts of LH receptor, {alpha}-inhibin, and aromatase in granulosa cells were lower in follicles 5–9 mm in diameter than in larger ones (p < 0.05). In cumulus cells, the amounts of the three factors were different neither between the three groups nor between the follicular diameters.

Although we could not establish any obvious relationship to oocyte competence for in vitro maturation, the influence of the follicle diameter on the content of LH receptors, {alpha}-inhibin, and aromatase in granulosa cells was similar to the influence of follicle diameter on oocyte competence. Therefore, one can hypothesize that, in the mare, there is a link between the acquisition of oocyte competence and the expression of these factors in the follicular cells.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Typically in the mare, one follicle is selected and becomes dominant 8–10 days before ovulation, while the cohort follicles become atretic. The mare ovary also displays follicular growth during the luteal phase, and large antral follicles can be observed. These generally regress but occasionally reach the preovulatory size and ovulate [1]. Moreover, whereas the in vitro maturation rate of oocytes exceeds 90% in most domestic mammals (goat [2], sow [3], cow [4]), it remains low in the mare. Actually, whatever the culture conditions used, less than 70% of equine oocytes are in metaphase II at the end of the culture period [5] (for review, see [6, 7]). Moreover, the conditions for in vivo oocyte maturation in the mare are somewhat different from those in other domestic mammals: the ovulatory LH surge is a progressive increase lasting many days, with a maximum concentration occurring one day after ovulation [8, 9]. Thus, follicle and oocyte maturation in the mare could involve some species-specific mechanisms.

The reasons for the failure of equine oocytes to resume meiosis during in vitro culture are unknown. However, we showed recently that the oocyte competence for in vitro maturation increases with an increase in the follicular diameter and is lower in follicles 5–9 mm, which are thought to be gonadotropin-independent [6, 10–12]. Moreover, we showed that oocyte competence is influenced by the mare's reproductive status [6]. This allows us to assume that some local factors, in relation to follicular growth and maturation, whose expression varies during the estrous cycle, may play a crucial role in the acquisition of oocyte competence for in vitro maturation. Among them, steroid hormones, LH receptors, {alpha}-inhibin, and aromatase are good candidates.

The purpose of the present study was to investigate in the mare LH receptor, {alpha}-inhibin, and aromatase contents in mural granulosa and cumulus cells, as well as steroid concentrations in follicular fluid, in relation to 1) follicular size and estrous cycle stage, and 2) oocyte competence for in vitro maturation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experimental Design

Ten adult cyclic mares (Selle Français) were used from April to June. They were in good body condition, from 8 to 19 yr old, kept indoors, and fed with concentrates. They received an initial synchronization treatment [13] with an intravaginal sponge containing 0.5 g altrenogest (Regumate; Roussel UCLAF, Romainville, France) plus 50 mg of estradiol benzoate (ß-estradiol 3-benzoate; Sigma, La Verpillière, France) for 1 wk. On the day of sponge removal, mares received a prostaglandin F2{alpha} analogue injection (cloprostenol, 250 µg/mare i.m.; Estrumate; Pitman-Moore, Meaux, France) to induce luteolysis, and all follicles larger than 5 mm were punctured to free the ovaries of atretic follicles. Healthy follicles developed afterwards. Ovarian activity was then assessed by routine rectal ultrasound scanning using an Aloka 210 (Tokyo, Japan) with a 5-mHz linear probe.

Follicular Punctures

To collect oocytes and follicular cells at various stages of the estrous cycle, 7 successive ultrasound-guided follicular punctures were performed for each of the 10 mares, either during the follicular phase (group DF; n = 3; see below), at the end of the follicular phase (group EF; n = 2; see below) or during the luteal phase (group DL; n = 2; see below). Follicles of five mares were punctured in the order EF, DL, EF, DL, DF, DF, DF; and those of five others in the order DF, DF, DF, EF, DL, EF, DL.

For punctures assigned to be performed at the end of the follicular phase (group EF), an i.v. injection of 25 mg of crude equine gonadotropin (CEG; [14]) was given when the largest follicle reached 35 mm, to induce ovulation; all follicles larger than 5 mm were punctured 34 h later, i.e., just before the expected ovulation. For punctures assigned to be done during the luteal phase (group DL), a CEG injection was performed when the largest follicle reached 16 mm, in order to increase the percentage of oocytes that completed meiosis after in vitro culture [6]; all follicles larger than 5 mm were punctured 34 h afterwards. On the day of the puncture in the luteal phase, luteolysis was induced by a prostaglandin F2{alpha} analogue injection. During the follicular phase (group DF), the punctures were performed before the emergence of the dominant follicle: a CEG injection was given when the largest follicle reached 16 mm, and all follicles larger than 5 mm were punctured 34 h afterwards.

During each follicular puncture attempt, mares were sedated with detomidine (0.6 mg/100 kg BW i.v. Domosedan; Smith Kline & French, Courbevoie, France), and the rectum was relaxed with prifinium bromide (45 mg/100 kg BW i.v. Prifinial; Vétoquinol, Lure, France). After puncture, the mares received an antibiotic injection (Mixtencilline: 1 600 000 IU penicillin/100 kg BW and 1.3 g dihydrostreptomycin/100 kg BW i.m.; Rhône Mérieux, Lyon, France).

Collection of Cumulus-Oocyte Complexes (COCs), Follicular Fluids, and Granulosa Cells

The follicles were punctured using a transvaginal ultrasound-guided follicular aspiration technique [15] with a 7.5-mHz sectorial probe (Kretz; Soframed, Truchtersheim, France). To improve the recovery rate of oocytes, two types of aspiration needles were used: a one-way needle (length, 600 mm; outer diameter, 1.8 mm; Thiebaud Frères, Jouvernex Margencel, France) was used for puncturing follicles > 25 mm, and a two-way needle (length, 700 mm; outer diameter, 2.3 mm; internal diameter, 1.35 mm; CASMED, Cheam Surrey, England) was used for follicles <= 25 mm. After follicular fluid aspiration, the follicle was scraped with the needle and flushed five times with PBS (Dulbecco A, Unipath, Dardilly, France) containing heparin (50 IU/ml; LEO S.A., St-Quentin Yvelines, France) at 37°C. The one-way needle allows isolation of pure follicular fluid, while the two-way needle induces the recovery of follicular fluid diluted with PBS. All aspirated fluids were individually examined under a stereomicroscope for oocyte recovery and then centrifuged for 10 min at 1500 x g to obtain a pellet of granulosa cells. Pure and diluted follicular fluids were individually stored at -20°C.

Oocyte Culture and Examination, and Cumulus Cell Preparation

At recovery, oocytes were individually classified according to cumulus aspect, as expanded COCs or compact COCs [6]. Compact COCs were cultured individually in a humidified atmosphere (95% air:5% CO2) at 38.5°C for 30 h in 500 µl of maturation medium, as previously described [6]. After culture, the COCs were stripped of their cumulus cells with small glass pipettes in 500 µl of PBS solution supplemented with 87.5 IU/ml hyaluronidase (type III, 875 IU/mg; Sigma) at 37°C. Totally denuded oocytes were rinsed in PBS with 1% inactivated fetal calf serum (Gibco, Eragny, France) at 37°C, stained with 1 µg/ml bis-benzamide (Hoechst 33342; Sigma) in PBS for 5 min at 37°C for DNA detection, and observed in a drop on a slide under a fluorescence microscope. They were classified according to chromatin configuration as germinal vesicle, dense chromatin, metaphase I, metaphase II, and degenerated [6]. Expanded COCs at recovery were immediately stripped of their cumulus cells, stained, and classified as described above.

The cumulus cell suspension was centrifuged (13 000 x g, 1 min) to obtain a pellet of cells. Pellets were suspended in 10 µl of a hypotonic buffer (KCl 10 mM, Tris 10 mM, EDTA 0.5 mM) containing 348 µg/ml PMSF (Sigma), 33.2 µg/ml N{alpha}-p-tosyl-L-lysine chloromethyl ketone (TLCK; Sigma), and 34.1 µg/ml N-tosyl-L-phenylalanine chloromethyl ketone (TPCK; Sigma). They were stored at -20°C until analysis.

Granulosa Cell Preparation

Follicular cell pellets were suspended in 2 ml of an NH4Cl solution (8.3 g/L in Tris-HCl 10 mM, pH 7.5) and incubated for 5 min before the addition of an equal volume of PBS. After centrifugation (1500 x g, 10 min), granulosa cells were rinsed twice with PBS and were suspended in one volume of the hypotonic buffer containing PMSF, TLCK, and TPCK, as described above. They were stored at -20°C.

Gel Electrophoresis and Immunoblotting

One-dimensional (1D) SDS-PAGE was prepared according to the discontinuous buffer system of Laemmli [16] with a 12% separating gel and a 4% stacking gel. Acrylamide-bisacrylamide solution was purchased from Serva GmbH & Co. (Heidelberg, Germany), and other reagents were purchased from Sigma.

Pellets of cumulus cells and granulosa cells were sonicated for 2–15 min, diluted with an equal volume of electrophoresis buffer (Tris-HCl 160 mM pH 6.8, EDTA 10 mM, SDS 10%, ß-mercaptoethanol 10%, glycerol 20%, and bromophenol blue), and boiled for 4 min before being loaded. In order to load enough proteins, 2–3 cumulus pellets were pooled in accordance with the nuclear stage of the oocyte. Electrophoresis was performed at a constant intensity of 25 mA per gel. At the end of migration, gels were electroblotted on nitrocellulose membranes (Schleicher & Schuell, Dassel, Germany) overnight at 4°C.

Each membrane was used for 5 successive immunological detections (see below): LH receptor, actin, {alpha}-inhibin, aromatase, and plasma membrane proteins of erythrocytes. After each immunological detection, the membranes were stripped of bound antibodies by incubation in stripping buffer (100 mM ß-mercaptoethanol, 2% SDS, 62.5 mM Tris-HCl pH 6.7) at 50°C for 30 min, and two rinses in TBS (10 mM Tris, 150 mM NaCl) containing 0.1% Tween 20, as recommended by the manufacturer (Amersham Life Science, Buckinghamshire, UK).

For LH receptor immunological detection, membranes were washed with TBS-Tween 20 and incubated for 1 h in blocking solution (5% dry milk, 0.2% Nonidet P40 in TBS) supplemented with 10% goat serum. The membranes were then incubated for 3 h with a mouse monoclonal antibody raised against porcine LH receptors (775; provided by Dr. Salesse, INRA, Jouy-en-Josas, France; [17]). After two rinses in TBS-Tween 20 and a 30-min incubation in the blocking solution supplemented with 10% goat serum, membranes were incubated for 1 h with a peroxidase-conjugated rabbit anti-mouse IgG (Vector Laboratories, Burlingame, CA). Immunological detections of actin, {alpha}-inhibin, aromatase, and plasma membrane proteins of erythrocytes were performed using the protocol described above, except that the blocking solution was not supplemented with goat serum. The primary antibodies used were, respectively, a mouse monoclonal antibody raised against chicken actin (Amersham Life Science), a sheep antiserum raised against the 1–26 fragment of porcine {alpha}-inhibin (provided by Dr. Terqui, INRA, Nouzilly, France), a rabbit antiserum raised against equine aromatase (provided by Professor Séralini, IBBA, Caen, France [18]), and a rabbit anti-human erythrocyte membrane antibody (Dako, Glostrup, Denmark). The peroxidase-conjugated secondary antibodies used were either a rabbit anti-mouse IgG (Vector Laboratories), a donkey anti-sheep IgG (Sigma), or a goat anti-rabbit IgG (Institut Pasteur, Paris, France).

The enhanced chemiluminescence detection system (ECL; Amersham Life Science) was used to detect immunoreactive polypeptides. The nitrocellulose membranes were exposed to Hyperfilm MP (Amersham Corp., Arlington Heights, IL) that was afterwards digitalized with an Eikonix 1412 scanner camera (Eastman Kodak, Rochester, NY). Patterns were quantified using the Kepler software (Large Scale Biology Corporation, Rockville, MD). The amounts of LH receptors, {alpha}-inhibin, and aromatase in cumulus and granulosa cells were analyzed as the ratio to the actin amount, after subtraction of the amount of actin due to the erythrocytes within each lane. The latter had been previously determined in a pre-experiment using blood cell pellets.

RIAs of Steroids in Follicular Fluid

Steroid concentrations in follicular fluid were determined by RIA without extraction, according to the methods of Saumande et al. [19] for progesterone (P4) and Terqui et al. [20] for estradiol-17ß (E2), as previously described for the horse [21]. Intraassay coefficients of variation were 10.8% and 15.6%, respectively. For each steroid, all samples were analyzed in the same assay to avoid interassay variability. The results were expressed as E2:P4 ratio, considering that the dilution factor of the follicular fluid in PBS was unknown.

Statistical Analysis

The chi-square test was used to compare oocyte maturation rate in the three groups (DF, EF, and DL). Analysis of oocyte maturation after in vitro culture according to follicle diameter was performed by logistical regression analysis (Statistical Analysis System Software, Cary, NC). A nonparametric test (G-test = 2I-test) was used for comparison of nuclear maturation between the different classes of follicle diameter. Analysis of LH receptors, {alpha}-inhibin, and aromatase amounts was performed by unbalanced variance analysis (SAS Software; GLM procedure).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
At the end of the follicular phase (group EF), follicles considered as preovulatory (larger than 35 mm in diameter), and nonpreovulatory subordinate follicles were punctured. During the follicular phase (group DF) and during the luteal phase (group DL), all follicles were nonpreovulatory.

Oocyte Recovery and Nuclear Stage

From the 20 puncture attempts performed at the end of the follicular phase, 27 preovulatory follicles were flushed, and 14 COCs were collected. All of them had an expanded cumulus; half of the enclosed oocytes were mature (7 of 14 in metaphase II).

From the 70 puncture attempts (20 EF, 20 DL, and 30 DF), 320 COCs were recovered from 647 nonpreovulatory follicles. The 299 oocytes with a compact cumulus were cultured in vitro and analyzed for nuclear stage after culture. The maturation rate increased with an increase in follicular diameter (p < 0.05) and was lower in follicles of 5–9 mm than in follicles of 10–34 mm (p = 0.07) (data not shown). Moreover, the maturation rate was higher for the oocytes recovered in group EF than for the oocytes recovered in group DL (51%, 52 of 101 vs. 34%, 30 of 88; p < 0.05); the maturation rate of oocytes collected in group DF was lower than rates of the other two (15%; 17 of 110; p < 0.01).

Intrafollicular E2:P4 Ratio

The E2:P4 ratio was significantly lower in follicular fluids aspirated during the luteal phase (group DL) than in fluids aspirated at the end of the follicular phase (group EF) (p < 0.05) when all follicles larger than 5 mm present on the ovaries were considered. Moreover, it was lower in the fluids of both group DL and group EF than in fluids collected during the follicular phase (group DF) (p < 0.01). Nevertheless, no significant difference between groups within each follicular size class studied was observed (Fig. 1).



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  		FIG. 1. Ratio of the concentrations of E2:P4 in relation to the group and the follicular diameter. Values with different superscripts within a group differ significantly (p < 0.05).

Considering the data from the three groups, the E2:P4 ratio significantly increased with an increase in follicular diameter (p < 0.01); it was lower in 5- to 9-mm follicles than in 10- to 50-mm follicles (p < 0.01). Within each of the three groups, the E2:P4 ratio also increased with an increase in follicular diameter (p < 0.01 each; Fig. 1), and it was significantly lower in small follicles (5–9 mm in group DF, 5–14 mm in groups EF and DL) than in larger ones (p < 0.05 each).

No relationship was observed between the intrafollicular E2:P4 ratio and oocyte competence for in vitro maturation: this ratio was not significantly different between the nuclear stages of the corresponding oocytes after in vitro culture.

Cellular Content of Aromatase, {alpha}-Inhibin, and LH Receptor

A total of 61 samples of cumulus cells and 204 samples of granulosa cells were analyzed for actin, plasma membrane proteins of erythrocytes, aromatase, {alpha}-inhibin, and LH receptor amounts. The expected band for actin at 42 kDa was visualized in most samples (Fig. 2A); only 14 samples did not contain sufficient amounts of proteins. The antibody raised against plasma membrane proteins of erythrocytes revealed a specific band at 30 kDa in the samples contaminated with blood cells (Fig. 2B). The expected bands for aromatase and {alpha}-inhibin were visualized at 55 kDa (Fig. 2C) and 50 kDa (Fig. 2D), respectively. The LH receptor antibody revealed two bands at 55 and 90 kDa and a doublet at 68–70 kDa (Fig. 2E).



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  		FIG. 2. A) Representative profiles of actin expression in equine follicular cells. Lanes 1–3, samples with increasing amounts of actin; lane 4, one sample with no detectable amount of actin. B) Representative profiles of expression of plasma membrane proteins of erythrocytes in equine follicular cells. Lane 1, one sample without any contamination with erythrocytes; lane 2, one sample contaminated with erythrocytes. C) Representative profiles of aromatase expression in equine follicular cells. Lanes 1–3, samples with increasing amounts of aromatase. D) Representative profiles of {alpha}-inhibin expression in equine follicular cells. Lanes 1–3, samples with various amounts of {alpha}-inhibin. E) Representative profiles of LH receptor expression in equine follicular cells. Lanes 1 and 2, samples with various amounts of LH receptors.

Cellular Content of Aromatase, {alpha}-Inhibin, and LH Receptor in Relation to Estrous Cycle Stage and Follicular Diameter

Within the whole follicular population studied, the amount of aromatase was significantly higher in granulosa cells recovered during the follicular phase (group DF) than in cells recovered at the end of the follicular phase (group EF; p < 0.05) or during the luteal phase (group DL; p < 0.01). The amounts of {alpha}-inhibin and LH receptors (calculated as the sum of all the bands) were not significantly different between the three physiological stages (groups DF, EF, and DL).

As illustrated in Figure 3, the amounts of aromatase increased with an increase in follicular diameter for granulosa cells collected at the end of the follicular phase (group EF, p < 0.001) and during the luteal phase (group DL, p = 0.08). The amounts of LH receptors increased with an increase in follicular diameter in group EF (p < 0.01). The amounts of aromatase and {alpha}-inhibin were lower in 5- to 9-mm follicles than in larger ones in group EF. The amounts of LH receptors were lower in 5- to 9-mm follicles than in larger ones in groups DF, EF, and DL (5–9 mm vs. > 10 mm).



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  		FIG. 3. Quantitative analysis of the expression of aromatase, {alpha}-inhibin, and LH receptors (arbitrary unit) in granulosa cells in relation to the group and the follicular diameter. a,b Values with different superscripts within a group differ significantly (p < 0.05). a,c Values with different superscripts within a group tend to differ (p < 0.10).

Finally, considering the data from the 3 groups, the amounts of aromatase and LH receptor increased with an increase in follicular diameter (p < 0.01). Moreover, the amounts of aromatase, {alpha}-inhibin, and LH receptors were lower in 5- to 9-mm follicles than in larger ones (p < 0.05 each).

In cumulus cells, quantitative analysis of the amount of aromatase, {alpha}-inhibin, and LH receptor was limited by the low number of samples. The amounts of these factors in cumulus cells were not significantly different either between the three groups or between the classes of follicular diameter.

Cellular Content of Aromatase, {alpha}-Inhibin, and LH Receptor in Relation to Oocyte Competence for In Vitro Maturation

Quantitative analysis of the content of aromatase, {alpha}-inhibin, and LH receptor in granulosa and cumulus cells in relation to oocyte competence for in vitro maturation is illustrated in Figure 4. The amounts of these three proteins in granulosa cells and in cumulus cells were not significantly different between the nuclear stages of the corresponding oocytes after in vitro culture. However, the amounts of these three proteins in cumulus cells tended to decrease as the oocyte progressed in nuclear maturation.



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  		FIG. 4. Quantitative analysis of the expression of aromatase, {alpha}-inhibin, and LH receptors (arbitrary unit) in granulosa and cumulus cells in relation to the nuclear stage after in vitro culture of the corresponding oocytes. GV, Oocytes in germinal vesicle stage; DC, oocytes with dense chromatin; MI, oocytes in metaphase I; MII, oocytes in metaphase II. Within granulosa cells and cumulus cells, the amounts of aromatase, {alpha}-inhibin, and LH receptors were not significantly different between the nuclear stages of the corresponding oocytes after in vitro maturation.

Moreover, as shown in Table 1, the amounts of aromatase, {alpha}-inhibin, and LH receptor in cumulus cells tended to be lower for oocytes that reached metaphase II after in vitro maturation than for preovulatory oocytes that reached metaphase II in vivo.


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  		TABLE 1. Expression of aromatase, {alpha}-inhibin, and LH receptor (arbitrary unit; mean ± SEM) in cumulus cells of oocytes that reached metaphase II after in vitro vs. in vivo maturation.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The aim of this study was to analyze the amounts of LH receptor, {alpha}-inhibin, and aromatase in equine mural granulosa and cumulus cells, as well as the E2:P4 ratio in follicular fluid, in relation to 1) follicular size and estrous cycle stage and 2) oocyte competence for in vitro maturation. In order to normalize the various experimental groups, all mares received 35 mg i.v. crude equine gonadotropins 34 h before ovarian punctures. We observed that LH receptor, {alpha}-inhibin, and aromatase were expressed in mural granulosa and cumulus cells from all follicles from the gonadotropin-independent growth period to the preovulatory stage. Their amounts in granulosa cells were influenced by the follicle diameter and the estrous cycle stage. No obvious relationship with the oocyte competence for in vitro maturation was observed.

The LH receptor antibody used in the present study revealed two bands at 55 and 90 kDa, respectively, and a doublet at 68–70 kDa. Similar molecular sizes were observed in the pig [17, 22]. In the pig [22] and the rat [23], the sequence of the receptor cDNA allows calculation of a molecular mass of 75 kDa. The 90-kDa band may thus correspond to the whole glycosylated receptor, whereas the 55-kDa band may correspond to the extracellular domain. The bands at 68–70 kDa may correspond to a partly degraded or nonglycosylated receptor; in the present study, the relative abundance of these bands may reflect proteolysis of the receptor during the collection and preparation of the cells. We detected LH receptors in granulosa cells of all follicles from 5 mm onwards. In other domestic mammals, LH receptors, as revealed by mRNA content or I125-hCG binding, do not appear in granulosa cells before the large antral follicle stage (rat; see [24–26] for review; porcine [27, 28]; ovine [29]; bovine [30, 31]), which corresponds to equine follicles 20–25 mm in diameter. The intracellular LH receptor level in all equine follicles studied could explain, at least in part, the improvement in oocyte competence for in vitro maturation after exogenous LH injection in the mare [6]. We showed that LH receptor content in the granulosa cells increased with an increase in follicular diameter, as previously observed in the mare by using I125-hCG binding [32]. This is in agreement with findings in bovine [33, 34], porcine [27, 35], and ovine [29, 36, 37] species. In cumulus cells, as observed in mural granulosa cells, we detected LH receptors from all equine follicles from 5 mm onwards. LH receptors were detected in porcine cumulus cells from preovulatory follicles [27], but not in corresponding bovine cells [38], whereas conflicting results have been reported in the rat [25, 39, 40]. The presence of LH receptors in equine cumulus cells could be involved in the positive effect of gonadotropins during equine oocyte in vitro culture (see [5] for review).

Equine {alpha}-inhibin subunit was observed in granulosa cells at 50 kDa, as previously shown in equine granulosa-theca cell tumors [41], whereas an 18-kDa band was observed in equine follicular fluid [42]. Until this study, no data were available in the mare on the expression of inhibin in relation to the follicle diameter or the estrous cycle stage. We observed that, at the end of the follicular phase (group EF), the granulosa cells contained lower amounts of {alpha}-inhibin in 5- to 9-mm follicles than in larger ones. Despite the fact that few studies are available on small follicles in other species, our results are consistent with findings in the pig [43, 44] and rat [45]. Moreover, equine cumulus cells showed an immunostaining for {alpha}-inhibin in agreement with findings in the rat [46], the sheep [47, 48], and the human [49].

Aromatase has previously been immunohistochemically localized in the granulosa cells of various mammalian species, including the mare [18, 50]. In the present study, aromatase was observed in granulosa cell lysates at 55 kDa, at the three physiological stages and across the range of diameters studied. During the luteal phase (group DL), the low amount of aromatase in granulosa cells and the low E2:P4 ratio in fluids highlight the fact that granulosa cells are primarily P4 producers during the luteal phase. During the follicular phase (group DF), we observed a higher amount of aromatase in granulosa cells as well as a higher E2:P4 ratio than at the end of the follicular phase (group EF). This is in agreement with estradiol concentrations in equine follicular fluid, which decrease at the preovulatory stage after LH stimulation and are very low in subordinate follicles [51, 52]. On the other hand, we showed that the amount of aromatase in granulosa cells, and consequently the E2:P4 ratio, increased with an increase in follicular diameter, as previously observed by using immunohistochemistry [18]. This result is in agreement with aromatase expression in the rat [53], the pig [28], and the sheep [54]; with estradiol and P4 levels in equine follicular fluids [52]; and with mRNA amounts in the pig and the cow [28, 31]. Such an increase in aromatase mRNA in the equine species has yet to be tested. In equine cumulus cells, aromatase was observed at the three physiological stages and across the range of diameters studied, whereas in humans and in superovulated cattle, pigs, and sheep, the cumulus cells lack aromatase [55–57]. To our knowledge, aromatase has been localized in cumulus cells only from rat preovulatory follicles [58].

The second purpose of the present study was to investigate the amounts of LH receptors, {alpha}-inhibin, and aromatase in mural granulosa and cumulus cells in relation to oocyte competence for in vitro maturation. Actually, these factors play a role in oocyte maturation in several species: the addition of inhibin to the culture medium stimulates primate and bovine oocyte maturation in vitro [59, 60], inhibin levels in porcine follicles were double when the oocytes resumed meiosis in comparison to immature oocytes [61], and an improvement in in vitro maturation of equine oocytes is obtained with the addition of estradiol or gonadotropins in the culture medium ([62]; see [5] for review). Nevertheless, we did not observe any obvious relationship between the amounts of {alpha}-inhibin, LH receptors, and aromatase and the nuclear stages of the corresponding oocytes after in vitro culture. Moreover, our results show that the influence of the follicle diameter on the intracellular levels of LH receptors, {alpha}-inhibin, and aromatase in granulosa cells is similar to the influence of follicle diameter on oocyte competence for in vitro maturation. On the one hand, we observed that the amounts of aromatase and LH receptors increased with an increase in follicular diameter, and that the follicles 5–9 mm in diameter contained lower amounts of {alpha}-inhibin, LH receptors, and aromatase than the larger ones. On the other hand, we showed, in the present study as previously [6, 10], that the oocyte maturation rate increased with the follicle diameter and was lower in follicles 5–9 mm in diameter than in larger ones. This reinforces our hypothesis that the follicles of 5–9 mm represent the basal gonadotropin-independent follicular growth [6, 10] and allows us to imply a relationship between oocyte incompetence and the expression of {alpha}-inhibin, LH receptors, and aromatase in granulosa cells.

Finally, the amounts of aromatase, {alpha}-inhibin, and LH receptors in cumulus cells tended to be lower for oocytes that reached metaphase II after in vitro maturation than for preovulatory oocytes that reached metaphase II in vivo. This could be linked to the higher amounts of these factors in cells from the preovulatory follicle in comparison with the small follicles, or to unsuitable culture conditions.

In conclusion, our results demonstrate that aromatase, {alpha}-inhibin, and LH receptors in the equine species are expressed in mural granulosa and cumulus cells from all follicles from the gonadotropin-independent growth until the preovulatory stage. No obvious relationship between the amounts of these three factors and oocyte competence for in vitro maturation could be established. However, the expression of the three proteins studied, as well as oocyte competence, was influenced by the follicle diameter, showing a link between the acquisition of oocyte competence and the expression of aromatase, {alpha}-inhibin, and LH receptors in follicular cells.


   ACKNOWLEDGMENTS
 
We wish to thank Drs. Gilles Séralini (IBBA, Caen, France), Roland Salesse (INRA, Jouy-en-Josas, France), and Michel Terqui (INRA, Nouzilly, France) for the gifts of the antibodies against aromatase, LH receptor, and {alpha}-inhibin, respectively. We wish to thank Guy Duchamp for critical discussion and technical assistance, and Eric Palmer for being present at the origin of our work. We are grateful to Isabelle Couty, Monique Ottogalli, and the staff of the experimental stud for technical assistance; to Daniel André and the staff of the laboratoire de Dosages Hormonaux (INRA, Nouzilly, France) for RIA of steroids; and to Alain Beguey and Odile Moulin for photographic work.


   FOOTNOTES
 
1 This work was supported by grants from the Institut National de la Recherche Agronomique (INRA, France) and the Haras Nationaux (France). G.G. was supported by a fellowship from INRA (France) and the Région Centre (France). Back

2 Correspondence. FAX: 33 2 47 42 77 43; gerard{at}tours.inra.fr Back

Accepted: December 8, 1998.

Received: April 21, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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