Smooth muscle cells in the walls of ovarian follicles in the Japanese quail

Summary The walls of pre-ovulatory follicles of the Japanese quail were examined at the ultrastructural level for the presence of cells displaying the typical morphological features of smooth muscle cells. These characteristics were found in the cells of the chordae, the tunica albuginea, and the theca externa. Small, elongated cells, containing microfilaments, were observed in the theca of prelampbrush follicles localized in the ovarian cortex. These thecal cells were considered as the putative precursors of the thecal smooth muscle cells of the pre-ovulatory follicle. The difference between the smooth muscle cells of the pre-ovulatory follicle and those in the wall of the most recent post-ovulatory follicle is the contracted state of the latter, which is most evident in the cells of the theca externa. It can be concluded that the cells of the theca externa are smooth muscle cells which are mainly contracted during the ovulatory process. A comparison was made with other vertebrate species.     

Immunohistochemical localization of desmin in the quail ovary

Summary We have localized desmin in the quail ovary, by the unlabelled antibody peroxidase-antiperoxidase technique, using two monoclonal and one polyclonal antisera. Special attention has been paid to the influence of fixation and of proteolytic pretreatment of sections. It appeared that the immunostaining of desmin largely depends on the nature of the fixative. Carnoy fluid, Bouin's fixative, and a paraformaldehyde-acetic acid fixative preserved the histological structure very efficiently. However, trypsin pretreatment proved to be necessary to unmask the antigenic sites in the ovaries fixed in Bouin's fixative and the paraformaldehyde-acetic acid fixative. Desmin immunoreactivity was detected in the tunica albuginea and the chordae, a number of which surrounding the blood vessels, from the hilus to the thecal surface of the follicles. Small branches of chordae connected them with the tunica albuginea, forming a suspensory apparatus. Desmin was also localized in the smooth-muscle cells of the blood vessels. In the theca, immunoreactivity was detected in the wall of arterioles, of venules, and of capillaries. Further experimental and immunohistochemical research have to be performed to establish if the suspensory apparatus is a myoid tissue.     

Immunohistochemical localization of desmin in the quail ovary. Demonstration of a suspensory apparatus

We have localized desmin in the quail ovary, by the unlabelled antibody peroxidase-antiperoxidase technique, using two monoclonal and one polyclonal antisera. Special attention has been paid to the influence of fixation and of proteolytic pretreatment of sections. It appeared that the immunostaining of desmin largely depends on the nature of the fixative. Carnoy fluid, Bouin's fixative, and a paraformaldehyde-acetic acid fixative preserved the histological structure very efficiently. However, trypsin pretreatment proved to be necessary to unmask the antigenic sites in the ovaries fixed in Bouin's fixative and the paraformaldehyde-acetic acid fixative. Desmin immunoreactivity was detected in the tunica albuginea and the chordae, a number of which surrounding the blood vessels, from the hilus to the thecal surface of the follicles. Small branches of chordae connected them with the tunica albuginea, forming a suspensory apparatus. Desmin was also localized in the smooth-muscle cells of the blood vessels. In the theca, immunoreactivity was detected in the wall of arterioles, of venules, and of capillaries. Further experimental and immunohistochemical research have to be performed to establish if the suspensory apparatus is a myoid tissue.     

Functional morphology of the theca interna of the vesicular follicle in human ovary]

A study of histological serial sections of human ovaries and histochemical reactions established that several cell types differentiate in the theca interna during the formation and development of tertiary follicles (2nd growth period). This leads, in turn, to triple-layering of the theca interna in follicles of the 2nd resting period. The inner thecal layer consists of fusiform cells from the basement membrane, which are apposed to the membrana granulosa and show a strong positive reaction to alkaline phosphatases. A tropic function for the follicular epithelium must be assigned to this layer. The middle layer consists of epitheloid thecal cells, which show a strong positive reaction to 3beta-ol-steroid hydrogenase. These represent the estrogen glands of the follicle. The outer layer of the theca interna, whose transition to the theca externa is indistinct, also has fusiform cells, which are available as reserve material for the differentiation of further epitheloid thecal cells to pre-ovulatory follicles in later periods of development.     

Immunohistochemical localization of tissue-type plasminogen activator in ovaries before and after induced and spontaneous ovulation in the rat

Summary The observation that tissue-type plasminogen activator (tPA) activity increased dramatically in preovulatory follicles has led to the hypothesis that plasminogen activation is causally related to follicle rupture. With immunohistochemistry, we have studied the appearance of tPA in ovaries of immature rats induced to ovulate and in adult cycling rats. Treatment of immature female rats with a single dose of pregnant mare serum gonadotropin (PMSG) induced follicular maturation. A subsequent human chorionic gonadotropin (hCG) injection resulted in follicle rupture 12–14 h later. PMSG treatment alone did not induce appearance of tPA-immunoreactive cells in any ovarian compartment. After hCG stimulation, however, theca cells, granulosa cells, and oocytes of pre- and postovulatory follicles displayed distinct tPA immunoreactivity. Fibroblastlike cells in the theca layers and tunica albuginea of the follicle apex also demonstrated localized cytoplasmic tPA reactivity. In addition to tPA synthesis in preovulatory follicles, hCG also induced tPA staining in the theca (but not granulosa) layers of non-ovulatory follicles. At 24 h after hCG treatment, there was a marked tPA staining in developing corpora lutea, ovulated ova, and oviductal epithelium. Ovaries from regularly cycling adult rats displayed a similar ovulation-related pattern of tPA immunostaining. The appearance of tPA in different cell types of the preovulatory follicle and in the fibroblast-like cells at the follicle apex, strengthens the hypothesis of a direct involvement of tPA in follicle rupture. Presence of tPA in postovulatory oocytes, cumulus cells, and surrounding oviductal epithelium may also indicate a role for tPA in the transfer of eggs in the oviduct.This work was supported by NIH Research Grants HD-14084; 12303     

Histological and histochemical studies of post-ovulatory and pre-ovultory atretic follicles in Mustelus canis

The atresia of post-ovulatory and pre-ovulatory follicles of the viviparous smooth dogfish, Mustelus canis, is compared for approximately the first fourth of an 11 month gestation. A thick collagenous sheath and numerous tubules in the theca identify the large, folded stage A post-ovulatory follicle. In stage B the tubules have been filled by cells to form “islands.” In stage C the entire structure is greatly diminished, adjacent islands tend to fuse, the collagenous sheath is virtually gone and the granulosa is degenerating. Preovulatory follicles from large, yolky oocytes pass through four stages beginning with yolk phagocytosis by granulosa cells of the villi (stage I), which are long and granular in stage II; villi fuse, theca cells increase greatly, fill with granules (stage III), encroach on the granulosa and disperse it into small groups of cells which finally disappear (stage IV) leaving a mass of thecal cells. A special type of pre-ovulatory follicle from small non-yolky oocyte atresia exhibits prominent thecal tubules and an unusual arrangement of granulosa cells. This follicle appearrs to enlarge during the summer, becoming multilobed; few granules are present. The distribution of lipid in frozen sections, stained by Oil red O, is described for all types of follicles. Schultz and Lewis and Lobban tests for steroids were made on frozen sections with corresponding results. Positive green tests indicating the presence of steroids or possible steroidogenesis were limited to: (1) one post-ovulatory follicle, in the islands; (2) four stage III and seven late stage IV pre-ovulatory yolky atretic follicles; (3) two special atretic follicles. The special atretic follicle appears to be a unique feature of M. canis and it is suggested tentatively that it may be related to viviparity.     

Evidence for the existence of substance P in the prepubertal rat ovary. II. Immunocytochemical localization

Nerve fibers containing substance P (SP) were localized in ovaries from juvenile and peripubertal rats by immunofluorescence. These fibers were closely associated with the theca externa of antral follicles, as well as being in the interstitial tissue and within the tunica adventitia of small blood vessels, mostly arterioles. Consistently, the greatest amount of SP immunoreactivity was observed surrounding the ovarian vasculature. Substance P was not detected in cells or within the corpora lutea (CL). Additionally, the peripubertal animals seemed to have a greater concentration of ovarian SP than the juvenile animals. Possible functional roles for this peptide in the ovary are discussed.     

Localization of relaxin in the pig follicle during preovulatory development

The avidin-biotin immunoperoxidase method and antisera to purified porcine relaxin were used to localize relaxin in sections of follicles from pregnant mare's serum gonadotropin (PMSG)/human chorionic gonadotropin (hCG)-primed pigs during preovulatory development. Prepubertal pigs were treated i.m. with PMSG (750 IU) and 72 h later with hCG (500 IU) to induce follicular development and ovulation. Follicles were collected from untreated gilts or from gilts 24, 48, 60, 72, 84, 96, or 108 h after PMSG treatment. Light immunostaining in the theca interna was observed early in follicular development, at 48 and 60 h post-PMSG. At 72 h post-PMSG, relaxin immunostaining in the theca interna of the preovulatory follicle was more intense. After hCG treatment, the intense thecal immunostaining persisted and was apparent 84 and 96 h after PMSG. At about 6 h prior to expected ovulation (108 h post-PMSG), there was thinning of the follicle wall and a reduction in relaxin immunostaining in the theca interna. Immunoactive relaxin was not detected in follicles from untreated gilts, follicles 24 h post-PMSG, small healthy or atretic follicles, or in granulosa cells, theca externa or ovarian stroma, at any of the time points studied. These studies support the hypothesis that the theca interna is the primary source of follicular relaxin and provide further evidence for a paracrine role for relaxin in the ovulatory process.     

Effect of indomethacin on ovarian follicle rupture--a morphological observation

The purpose of this investigation was to study the role played by indomethacin in blocking ovulation. Immature Wistar rats induced to maturation by PMSG and HCG and normal mature rats were used. Changes in follicle wall of preovulatory follicles occurred after indomethacin treatment were studied both by light and electron microscopy, and were compared with those in controls. 94% of PMSG and HCG stimulated rats, then followed indomethacin injection (3 mg/rat), were inhibited to ovulate; while rats only given hormonal stimulation ovulated in 100%. Adult females in proestrus were treated with indomethacin in doses either of 5 mg or 7.5 mg, none of them ovulated. Whereas, ova were found in the ampullae of normal controls. Ovarian histological examinations of indomethacin treated rats showed that ovum frequently went through the stratum granulosa, however, the theca or the albuginea failed to rupture. The electron microscopy examinations showed that a large amount of collagen fibers scattered under the albuginea layer and interwove with cells of albuginea and theca externa. These two layers, due to containing abundant collagen fibers, thus became barriers for an ovum escaping from a follicle. Follicle walls near the gap of ovulated follicles in controls only had a small quantity of collagen fibers which were more or less with obscure appearance. Cytolysis in albuginea and theca externa layers was also noted. Theca interna cells and granulosa cells, with well developed Golgi bodies and more smooth endoplasmic reticulum in experimental rats revealed that these two tissue components still had a normal endocrine function in spite of receiving indomethacin treatment. The possible effects of prostaglandins on degradation of collagen fibers and contraction of preovulatory follicles were also discussed.     

Structural alterations of rabbit ovarian follicles after mating with special reference to the overlying surface epithelium

Summary Rabbit ovarian preovulatory follicles and in particular the overlying surface epithelium were studied by morphological and ultrahistochemical means at different times after mating.By light microscopy an increase of cytoplasmic granules was found in the surface epithelium at the follicle apex 4 h after mating. The granules increased in amount and showed maximal accumulation 8–9 h after mating. They then disappeared at the same time as the connective tissue elements in the underlying tunica albuginea and theca externa disintegrated.Transmission electron microscopy showed that the membrane-bounded granules or dense bodies fused with one another and by 8 h after mating they often changed character and appeared more electron lucent. Furthermore, open communications were found between altered granules and vacuoles and between vacuoles and the extracellular space below the epithelium. Acid phosphatase reaction product was localized to the granules and Golgi cisternae. Not all the dense bodies were enzyme positive. At later stages, close to the time of follicle rupture, the epithelial cells were attenuated and thin, with only a few granules.By scanning electron microscopy it was found that the epithelial cells at the follicle apex increased in size approaching the time of follicle rupture and that their microvilli decreased in number and in size. At 8 h and later, the contours of intracellular granules could be visualized.The results of this study were similar to those found when rabbits were induced to ovulate by HCG-stimulation. This further strengthens the hypothesis that the surface epithelium contributes proteolytic enzymes which help to disintegrate the follicle apex prior to rupture.     

消炎痛对大鼠成熟滤泡破裂的影响——形态学观察

本工作的目的是,观察前列腺素合成的抑制物——消炎痛对大鼠排卵的抑制作用,并从形态学的角度对已排卵的以及排卵受阻的滤泡壁成份进行比较,藉此,探讨卵巢自身的前列腺素在滤泡破裂机制中的作用。25—27天龄Wistar大鼠用PMSG与HCG诱导成熟,其中部分动物作为对照,排卵率为100%;另一部分动物皮下注射消炎痛3.0mg/只,94%的动物被抑制排卵。处于动情前期的自然成熟鼠,注射消炎痛5mg/只或7.5 mg/只,排卵抑制率达100%,而对照鼠均排卵。从诱导成熟及自然成熟的大鼠卵巢组织切片所见,对照鼠已排卵滤泡的裂口处的白膜与膜层的结缔组织纤维断裂;被消炎痛抑制排卵的动物,卵母细胞因不能突破膜层或白膜而受阻。于电镜下见到,对照动物滤泡裂口附近泡壁的白膜细胞及外膜细胞出现退化、解体;与此同时,存在于这两种组织巾的胶元蛋白纤维也呈溶解现象。实验组排卵被阻的滤泡,白膜下与外膜细胞中间仍存在大量胶元蛋白纤维,因而使白膜与外膜层成为阻挡卵母细胞排出的屏障。另外,内膜与颗粒细胞的高尔基体发达,粗糙的内质网趋向光滑,说明这两类细胞分泌功能旺盛。关于前列腺素对胶元蛋白纤维降解的可能作用以及动物经消炎痛处理后仍产生类固醇激素的可能性均进行了讨论。     

Spatiotemporal expression of epidermal growth factor receptor messenger RNA and protein in the hamster ovary: follicle stage-specific differential modulation by follicle-stimulating hormone,luteinizing hormone,estradiol, and progesterone

Spatiotemporal expression, endocrine regulation, and activation of epidermal growth factor receptor (EGFR) in the hamster ovary were evaluated by immunofluorescence and in situ hybridization localization. Whereas granulosa cells (GC) of primordial through large preantral (stage 6, 7-8 layers GC) follicles had low immunoreactivity, granulosa cells of antral follicles, theca, and interstitial cells had intense EGFR immunoreactivity. EGFR expression in GC of primordial and small preantral follicles increased progressively from estrous through proestrous, but a significant increase occurred in mural GC of antral follicles following the gonadotropin surge. Interstitial cells around small preantral follicles had strong immunofluorescence, and the intensity increased significantly in fully differentiated thecal cells. Distinct EGFR protein was localized in the nucleus of the oocytes and granulosa cells. FSH significantly stimulated EGFR expression in the GC, especially the mural GC, theca, and interstitial cells in hypophysectomized hamster. Estrogen stimulated EGFR expression in preantral GC as well as in interstitial cells. Progesterone and hCG effect was limited to theca and interstitial cells. EGFR expression correlated well with EGFR activation following endogenous or exogenous gonadotropin exposure. Receptor mRNA expression closely followed the protein expression, with increased mRNA expression in mural GC of antral follicles. These results suggest that low levels of EGF signal as a consequence of low levels of receptors in preantral GC may be critical for cell proliferation, but higher receptor density may evoke increased signal intensity due to activation of other intracellular signal pathways, which activate cellular processes related to granulosa, theca, and interstitial cell differentiation. The spatiotemporal cell type and follicle stage-specific expression of receptor mRNA and protein and EGFR activation is critically regulated by gonadotropins and ovarian steroids, primarily estradiol.     

Cell-specific localization of progesterone receptors in the bovine ovary at different stages of the oestrous cycle

This immunohistochemical study describes the localization of progesterone receptors (PR) in the bovine ovary of 23 cows at different stages of the oestrous cycle. In primordial, primary and secondary follicles the score for PR in the follicle cells increased progressively with the maturation of the follicle. In vital tertiary follicles and cystic atretic follicles a moderate score for PR was found, while in obliterative atretic follicles the score was much lower. Scores were high in corpora hemorrhagica, low in corpora lutea and still lower in corpora albicantia. Low PR scores were also found in the tunica albuginea and surface epithelium. Cyclic variations of PR immunoreactivity were manifest in most ovarian tissues. Follicular scores for PR were high in oestrus and decreased during the following stages, whereas scores in corpora lutea cells varied according to a characteristic pattern with high levels during oestrus and metoestrus. The variations in the scores for PR in the different ovarian cell types suggest a cell-specific and cycle-dependent influence of progesterone. A negative correlation was found between the PR scores and the plasma progesterone concentration.     

Immunohistochemical localization of epidermal growth factor in the ovary of the adult Japanese quail

Summary The present study focuses on the immunohistochemical localization of epidermal growth factor in the ovary of the adult Japanese quail. Immunoreactivity was predominantly found in the smooth muscle cells of blood vessels and of chordae, in granulosa cells of pre-lampbrush follicles, in interstitial cells, in the Balbiani complex of pre-lampbrush oocytes, and in ganglia. In developing follicles, immunoreactivity was also detected in some granulosa and thecal cells, in the zona radiata, and especially in cell clusters localized in the thecal periphery. The number of immunostained cells in the granulosa decreased during folliculogenesis, and increased after ovulation. In the ooplasm of oocytes, immunoreactivity was shifted from the Balbiani complex to the zona radiata during development. These observations support the hypothesis that epidermal growth factor acts primarily on less differentiated follicles. It is also suggested that epidermal growth factor can modulate ovarian contractility. Finally, in one ovary, we detected immunostained bodies in the ooplasm of small developing oocytes. Senior Research Assistant of the Belgian National Fund for Scientific Research.     

Localization of the renin-angiotensin system in the bovine ovary: cyclic variation of the angiotensin II receptor expression.

Angiotensin (Ang) II may modulate reproductive function in the bovine ovary. Therefore, expression and localization of a local ovarian renin-angiotensin system (RAS) were investigated by elucidating the influence of the estrus cycle, pregnancy, and the presence of follicular cysts. Receptor analysis and autoradiography were used to characterize and localize Ang II receptors. Cyclic variations in the density of ovarian Ang II receptors were found with a higher value in estrus than in diestrus. The density in ovaries with follicular cysts was in the same order of magnitude as in estrus. The Ang II receptor type 2 (AT(2)) dominated in all three groups. Autoradiography showed that the majority of antral follicles and follicular cysts had intense AT(2) receptor binding in the theca externa. Binding was less intense in the theca interna, whereas there was no binding in the granulosa layer. In the corpora lutea, the AT(2) receptor was dominant in the capsule and in connective tissue infoldings, whereas no binding was observed in the luteal tissue. The type 1 Ang II receptor (AT(1)) was dominant in the stroma and showed no cyclic changes. Angiotensin-converting enzyme (ACE) activity was detected in all aspirated follicular fluids and homogenates of ovarian tissue. Autoradiography showed that most of the ACE was localized on endothelial cells. Renin immunoreactivity was found in granulosa and thecal cells of antral follicles and in luteal cells. Furthermore, solitary cells in the stroma, presumably macrophages, displayed intense staining. Our finding of cyclic changes support the concept of an active and regulated RAS in the bovine ovary.     

Expression of follicle stimulating hormone and luteinizing hormone receptors during follicular growth in the domestic cat ovary

In order to better understand the pituitary regulation of follicular growth in the domestic cat, follicle stimulating hormone (FSH) and luteinizing hormone (LH) receptors (R) were localized and quantified in relation to follicle diameter and atresia using in situ ligand binding on ovarian sections. Expression of FSHR was homogeneous and restricted to follicle granulosa cells from the early antral stage onwards, whereas expression of LHR was heterogeneous on theca cells of all follicles from the early antral stage onward, and homogeneous on granulosa cells of healthy follicles larger than 800 microm in diameter and in corpora lutea. LHR were also widely expressed as heterogeneous aggregates in the ovarian interstitial tissue. Atretic follicles exhibited significantly reduced levels of both FSHR and LHR on granulosa cells, compared with healthy follicles whatever the follicular diameter, whereas levels of LHR on theca cells were lower only for atretic follicles larger than 1,600 microm in diameter. In healthy follicles, levels of FSHR and LHR in all follicular compartments increased significantly with diameter. Although generally comparable to that observed in other mammals, the expression pattern of gonadotropin receptors in the cat ovary is characterized by an early acquisition of LHR on granulosa cells of growing follicles and islets of LH binding sites in the ovarian interstitial tissue.     

Ultrastructural observations of previtellogenic ovarian follicles of dove

Dove ovarian follicle is a complex structure composed of oocyte surrounded by a somatic compartment consisting of theca externa, theca interna and granulosa. The structure of ovarian follicle (1 and 2 mm) of dove was studied by electron microscopy. The granulosa was pseudostratified in the 1-mm-diameter follicles and stratified with two or three irregular rows of cells in the 2-mm-diameter follicles. In the larger follicle indentations between oocyte and granulosa cells become more numerous and the microvilli of granulosa cell elongated to form a zona radiata with similarly elongated oocyte microvilli. Lining bodies were present at the tips of granulosa microvilli and in the cortical region of the oocyte. In the oocyte cortex were observed coated pits, coated vesicles, dense tubules, multivesicular bodies and primordial yolk spheres. Primordial yolk spheres may contain lining bodies and were observed fused with dense tubules and multivesicular bodies or associated with smooth cisternae.     

Keratinocyte growth factor acts as a mesenchymal factor that promotes ovarian primordial to primary follicle transition

An important but poorly understood process in ovarian biology is the transition of the developmentally arrested primordial follicle to the developing primary follicle. Interactions between the epithelial and mesenchymal cells of the follicle are critical for the coordination of ovarian follicle development. The mesenchymal growth factor keratinocyte growth factor (KGF) (i.e., fibroblast growth factor-7) and the epithelial growth factor kit ligand (KITL) are known to interact to coordinate the growth of later-stage antral follicles. The hypothesis tested in the current study is that KGF acts as a mesenchymal factor to promote the primordial to primary follicle transition. A postnatal 4-day-old rat ovary organ culture system was used to investigate the actions of KGF. KGF treatment promoted 65% of follicles to undergo the primordial to primary follicle transition, but only 45% underwent development in control ovaries. Neutralizing antibody for KGF was found to attenuate the stimulatory action of KITL, but neutralizing antibody for KITL was not able to attenuate the stimulatory action of KGF. Further analysis demonstrated that KGF was found to stimulate the expression of KITL (i.e., mRNA levels) by granulosa cells. KITL in turn was found to stimulate the expression of KGF to create a positive feedback loop. Interestingly, KGF expression was localized to selected mesenchymal cells (i.e., precursor theca cells) surrounding the developing primordial follicle. Observations suggest that developing granulosa cells of the primordial follicles produce KITL, which helps recruit precursor theca cells to the follicle; the thecal cells then produce KGF, which acts on the granulosa to amplify KITL expression and support primordial follicle development. KGF appears to be a mesenchymal factor that promotes the primordial to primary follicle transitions.     

Evolution of prothymosin alpha and proliferating cell nuclear antigen (PCNA) immunoreactivity through the development of rat ovarian follicles

Summary The cellular distribution of prothymosin alpha (ProT) was studied in ovarian follicles of adult cycling rats. We found positive granulosa and theca cells throughout follicular maturation. When both ProT and proliferating cell nuclear antigen (PCNA) immunoreactivity was studied, we observed that both proteins were expressed in the same granulosa and theca cells, although sometimes ProT immunoreactivity was weak or absent in the mitotic (M) phase. Moreover, both peptides share the nuclear distribution, but ProT immunoreactivity was never seen in nucleoli. Therefore, we conclude that in mitotic cells ProT is expressed only in actively proliferating cells, since all ProT-positive cells were also positive for PCNA. ProT and PCNA immunoreactivities during the meiotic division were studied in oocytes. The presence of PCNA was, unlike ProT, constant throughout follicle development (except atretic oocytes). Oocytes expressed ProT from primordial follicles to the eighth generation, but more developed oocytes and atretic oocytes were not immunoreactive. In hypophysectomized rats, all oocytes were immunoreactive. Interestingly, in hypophysectomized rats treated with follicle stimulating hormone (FSH) that promoted follicle development, the more developed oocytes did not show ProT immunoreactivity. Since hypophysectomized rats were not treated with luteinizing hormone we conclude that ProT expression is not required to complete meiotic division I.     

Evidence for a role of the ovarian surface epithelium in the ovulatory mechanism of the sheep: secretion of urokinase-type plasminogen activator

The extent of dissolution of tissues within the apical wall of the preovulatory ovine follicle (formative site of rupture) is greater than that of the counterpart basal hemisphere. It has been hypothesized that proteolytic enzymes released from contiguous ovarian surface epithelial cells contribute to apical follicular weakening and ovulation. Ovulation occurs from the dominant ovarian follicle of proestrous ewes at approximately 24 h after administration of luteinizing hormone-releasing hormone (LHRH). Follicular rupture was inhibited in sheep in which the ovarian surface epithelium was surgically removed at 8 (but not at 16) h following LHRH. Plasminogen activator bioactivity was greater within the follicular apex compared to basal wall at 12 h; this difference was negated by prior removal of epithelium at 8 h after LHRH. A low M_r plasminogen activator of the urokinase-type (uPA) was secreted by epithelial cells recovered from the surface of preovulatory follicles (Western blot analysis). Ovarian epithelium, not associated with a preovulatory follicle, produced very little uPA. Finally, ovulation was suppressed by intrafollicular injection (8 h post-LHRH) of uPA antibodies. It is suggested that secretion of uPA by ovarian surface epithelium and consequent plasmin up-regulation within neighboring tunica albuginea and follicular theca is a contributing factor in the mechanism of ovulation.