Bovine preantral follicles and activin: immunohistochemistry for activin and activin receptor and the effect of bovine activin A in vitro

Activin was originally isolated from follicular fluid as a factor stimulating FSH from the pituitary. Recent studies also suggest a local role for activin in the development of preantral and early antral follicles. In the present study, activin and activin receptor immunoreactivity are shown in oocyte and granulosa cells of bovine preantral follicles. In addition, activin immunoreactivity was observed in the theca of secondary follicles. During culture of isolated preantral follicles, activin increased follicular growth and granulosa cell proliferation in a dose-dependent manner. This increase was further stimulated by addition of FSH. In conclusion, activin and its receptor are present on bovine preantral follicles, and additional activin stimulates development of those follicles.

Oocyte ultrastructure in bovine primordial to early tertiary follicles

The aim of the present study was to describe in detail the changes occurring in the cytoplasmic ultrastructure of the bovine oocyte from the onset of growth in the primordial follicle until the completion of growth in the tertiary follicle. Bovine oocytes from primordial, primary, secondary and early to mid-antral follicles were processed and analysed by light and transmission electron microscopy. The primordial follicular oocyte was characterized by numerous coated pits on the oolemma and the accumulation of free and organelle-related smooth (SER) and rough (RER) endoplasmic reticulum, round mitochondria and Golgi complexes around the nucleus, which was located slightly off centre. Up to the secondary follicular stage the oocyte displayed an increase in the number of microvilli, elongated mitochondria and Golgi complexes. During the secondary follicular stage, formation of the zona pellucida, development of gap junctions between the oocyte and the granulosa cells, formation of the cortical granules in the oocyte and reduction in the number of coated pits on the oolemma were seen. In the tertiary follicular oocyte up to 100 microm in diameter, the number of Golgi complexes and lipid droplets increased and the organelles were dislocated to the deep cortical region. During the final growth of the oocyte up to >120 microm, the organelles were dislocated further to the peripheral region, the extent of the free SER and RER compartments were reduced, the number of individual cortical granules increased, hooded mitochondria became abundant and the perivitelline space developed. In conclusion, the growth of the bovine oocyte is associated with the relocation and modulation of a number of cytoplasmic organelles as well as the development of oocyte specific structures such as the zona pellucida and cortical granules.

Nucleus ultrastructure and transcriptional activity of bovine oocytes in preantral and early antral follicles

An understanding of the recruitment and growth of follicles within the bovine ovary is crucial to their successful exploitation in vitro. The aim of the present study was to describe the nuclear ultrastructure and transcriptional activity of primordial to early tertiary follicular oocytes from bovine adult ovaries. Small blocks of ovarian cortex were incubated in medium enriched with 3H-uridine for 30 min. Subsequently, the tissue blocks were fixed in Karnowsky's fixative, dehydrated, epon embedded, sectioned (2 microns), processed for autoradiography, and examined under light microscopy. Sections showing preantral follicles with presumptive oocyte nucleoli were reembedded for transmission electron microscopy. The follicles were divided into five categories: 1) resting primordial, with a single layer of flattened granulosa cells, 2) activated primordial, with a single layer of flattened and some cuboidal granulosa cells, 3) primary, with a single layer of cuboidal granulosa cells, 4) secondary, with a complete or incomplete bilayer of cuboidal cells, and 5) tertiary, with more than two layers of granulosa cells delineating one or more intercellular cavities. The granulosa cells of all follicle classes were transcriptionally active. However, the oocytes did not display transcriptional activity, as measured by the present means, until the secondary and tertiary follicular stages. The oocyte nucleolus was granular in the primordial follicles. Following follicular activation, fibrillar centres invaded the nucleolus and, in the early tertiary follicle, numerous fibrillar centres were distributed throughout the nucleolus. In conclusion, the oocyte nucleolar function is gradually activated at follicle activation, and oocyte transcription is initiated at approximately the time of the secondary follicle stage.

Distribution of the intermediate filament proteins vimentin, keratin, and desmin in the bovine ovary

The distribution of the intermediate filament (IF) proteins desmin, keratin, and vimentin was studied immunohistochemically in bovine ovaries. Special attention was paid to granulosa cells to examine possible marked changes of IF distribution in relation to folliculogenesis during ovarian development. Therefore, ovaries were used from fetuses from 3 months of gestation onward, calves, heifers, and cows. In all ovaries, desmin immunoreactivity was restricted to smooth muscle cells in blood vessel walls. Keratin appeared a characteristic of the ovarian surface epithelium. Co-localization of keratin and vimentin was observed in the epithelium of rete ovarii tubules in fetuses and calves, and in cortical cord epithelium and pregranulosa cells of primordial follicles in fetuses at 3-7 months of gestation. Vimentin was demonstrated in endothelium and in fibroblasts. In addition, vimentin immunoreactivity was present in granulosa cells of primary, secondary, and antral follicles. In antral follicles, these granulosa cells mainly had an elongated appearance and either contained an oblong or a round nucleus. Those with an oblong nucleus were characteristic for atretic antral follicles. In nonatretic follicles, numerous vimentin immunoreactive, elongated granulosa cells with a round nucleus were observed, especially in the peripheral granulosa layer and in small ( < 3 mm in diameter) antral follicles. Additionally, in antral follicles, protrusions of vimentin-positive corona radiata cells were observed, that penetrated the zona pellucida to contact the oocyte. The data show that the distribution of vimentin containing IFs is associated with various aspects of granulosa cell activity, as mitosis, atresia, and intercellular transport.

Effects of fetal bovine serum, FSH and 17β-estradiol on the culture of bovine preantral follicles

We describe a 7-d culture in droplets of collagen gel of isolated small bovine preantral follicles in medium with or without 10% fetal bovine serum (FBS). In addition, the effect of human recombinant FSH and 17beta-estradiol on the morphology and growth of the preantral follicles was investigated in medium without FBS. After culture in medium with 10% FBS, the increase in follicle diameter was 13.1 +/- 8.4 microm, the percentage of BrdU-labeled cells was 49.9 +/- 11.3 and the number of cells per area granulosa was 11.1 +/- 1.8. Omission of serum from the culture medium had no effect on the percentage of labeled cells, but the diameter increase was lower and the cells were smaller. Apparently, serum affects the size of the granulosa cells from small preantral follicles rather than the stimulation of cell proliferation. Addition of human recombinant FSH and/or 17beta-estradiol to serum-free medium resulted in a larger diameter increase during culture compared with that of the control. With FSH, this was due to an increase in cell proliferation, while with estradiol this was caused by an increase in granulosa cell size. The effects of simultaneous treatment with FSH and estradiol was simply the combination of their individual effects. In conclusion, small bovine preantral follicles can be cultured for 7 d in the absence of serum and hormones. The follicles increase in diameter and react to FSH with enhanced cell proliferation and to estradiol with an increase in cell size.

Extracellular matrix proteins and basement membrane: Their identification in bovine ovaries and significance for the attachment of cultured preantral follicles

Described in the present paper is the immunolocalization of the extracellular matrix proteins (e.g., fibronectin, collagen Types I and III) in the bovine ovary, with special attention to preantral follicles. In addition, we have shown, histochemically and ultrastructurally, that mechanically isolated bovine preantral follicles are surrounded by an intact basement membrane. After 24 h of culture in serum-free medium, only 20.4% of these follicles attached to a plastic substrate. We showed that covering the plastic with extracellular matrix proteins (i.e., fibronectin, collagen Type I and matrigel) significantly increased the percentage of attached follicles to 76.0, 65.2 and 80.4%, respectively, while laminin had no effect (18.6%). When preantral follicles were embedded within three-dimensional collagen gels, no loss of follicles was observed. Restoring surface interactions between preantral follicles and the extracellular matrix in vitro, either in a two- or a three-dimensional system, might be important for maintaining follicular viability and growth in the future.

Isolation and characterization of preantral follicles from foetal bovine ovaries

A simple, mechanical method is described for the isolation of preantral follicles from bovine foetuses of 220-280 days of gestation. On average, 2918 + 621 (s.d.) preantral follicles were isolated per ovary. The isolated preantral follicles were characterized on the basis of the morphological appearance of the surrounding granulosa cells, the number of granulosa cell layers, and their diameter. The results show that primordial, primary, and secondary follicles differ morphologically and that they can be classified by their diameter.

Immunocytochemical localization of vasoactive intestinal peptide and neuropeptide Y in the bovine ovary

The distribution of the neuropeptides vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) was studied immunocytochemically in bovine ovaries from 3 mo of gestation up to and including puberty, and from adult cows at three stages of the estrous cycle. The appearance of VIP and NPY immunoreactivity of 4.5-6 mo of gestation coincided with the onset of follicular development. In contrast to NPY, VIP was first found in the cortex. Both VIP and NPY immunoreactivity increased with age. From 9 mo of gestation onwards, VIP and NPY were found around blood vessels and non-vascular smooth muscle cells, in the stroma near preantral follicles, and in the theca externa of antral follicles. In addition, VIP-positive cells were observed exclusively in the granulosa layer of the preovulatory follicle at the time of the LH surge. The distribution of VIP- and NPY-immunoreactive fibers in the ovary may point to an effect of these neuropeptides on various physiological processes, including follicle development and ovarian blood flow. In addition, the presence of VIP-positive cells in the granulosa layer of the preovulatory follicle is indicative of a role for VIP in ovulation.

Preservation of oocyte and granulosa cell morphology in bovine preantral follicles cultured in vitro

Described in the present paper is a culture system that preserves oocyte and granulosa cell morphology in bovine preantral follicles during 5 d in vitro. The effects of additional hypoxanthine and energy substrata (i.e., pyruvate and glutamine) on the morphology of cultured preantral follicles were investigated. It was shown that addition of a mixture of pyruvate, glutamine and hypoxantine to the culture medium increased the percentage of follicles with an intact oocyte from 29.4 to 78.6%. Morphological criteria are described to discriminate between normal and degenerated preantral follicles during culture by inverted microscopy. In addition, the importance of histological evaluation to judge the quality of oocyte and granulosa cells is demonstrated.

Micromorphology of antral follicles in cattle after prostaglandin-induced luteolysis, with particular reference to atypical granulosa cells

Dutch Friesian heifers (n = 13) and cows (n = 13) were used to obtain information about the number, size and micromorphology of antral follicles (> or = 3 mm in diameter) in cattle after induction of luteolysis with the PGF2 alpha analogue luprostiol. Special attention was paid to the presence of atypical granulosa cells in these follicles to obtain additional data to help evaluate the hypothesis that these cells are markers of follicular atresia. Animals were injected i.m. with 15 mg of the synthetic prostaglandin on day 10 or day 11 of the oestrous cycle. The ovaries were collected on day 12, that is 48 and 24 h after injection of luprostiol, respectively. After prostaglandin-induced luteolysis, the mean number of medium-sized and large nonatretic follicles and of medium-sized atretic follicles had not changed in heifers and in cows, compared with those of untreated animals. However, in heifers, contrary to cows, the development of a preovulatory-sized follicle was initially accompanied by an increase in the number of large definitely atretic follicles. Atypical granulosa cells can be considered as markers for a lower quality follicle, on the basis of their absence in preovulatory-sized follicles and their presence in large numbers in a high proportion of definitely atretic follicles. If it is assumed that only a nonatretic follicle without atypical granulosa cells will grow to preovulatory size, growth of this follicle within 2 days after prostaglandin treatment was almost 9 mm and over 10 mm in heifers and cows, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a combined new mechanical and enzymatic method for the isolation of intact preantral follicles from fetal, calf and adult bovine ovaries

The isolation of preantral follicles from the ovaries of bovine fetuses, calves and adult cows was performed using a simple, rapid mechanical and enzyme method. The ovaries were cut into small pieces with a tissue chopper. Then, the suspension was filtered successively through 500 and 100 mum nylon mesh filters. This simple mechanical procedure resulted in large numbers of isolated preantral follicles: 2,142 +/- 254; 512 +/- 92 and 298 +/- 54 from the ovaries of bovine fetuses, calves and cows, respectively. In addition, the ovarian fragments between 100 and 500 mum were suspended in 10 ml of M199 Hepes medium plus 5% FCS and divided into 2 equal parts: one portion was used for collagenase treatment (200 U/ml) for 20 minutes, while the other served as a control. Collagenase treatment resulted in 841 +/- 161; 216 +/- 51 and 52 +/- 17 preantral follicles from fetuses, calves and cows, respectively, compared with 312 +/- 86; 52 +/- 15 and 10 +/- 2 in the control group. The use of collagenase with ovarian fragments selected by filtration as a method for increasing the rate of recovery of preantral follicles is described here.

Effect of pregnant mares' serum gonadotrophin on the micromorphology of antral follicles in heifers, with special reference to atypical granulosa cells

Five Dutch-Friesian heifers were injected i.m. with 3000 iu pregnant mares' serum gonadotrophin (PMSG) on day 10 of the oestrous cycle, to study the effects on the number and micromorphological quality of antral follicles (> or = 0.3 mm in diameter). The ovaries were collected 48 h after PMSG injection. As well as the presence of mitotic figures and the absence of pyknotic nuclei in the granulosa, atypical granulosa cells were found in nonatretic follicles. These cells had an oblong nucleus and stained with toluidine blue. They were characterized by their dark cell matrix, and the presence of numerous free ribosomes and intermediate filaments of varying quantity. Atypical granulosa cells were micromorphologically similar to fibroblast-like cells in the theca. Their presence coincided with the occurrence of degenerative changes in the cytoplasm of nearby granulosa cells and they were more frequent in atretic follicles. The presence of atypical granulosa cells in follicles hitherto called nonatretic is therefore probably associated with the onset of follicular atresia. In the PMSG-treated heifers, the mean number of large (> or = 6.0 mm in diameter) antral follicles was greater than in the control group (18.4 +/- 4.0 versus 3.0 +/- 1.0), because of an increase in the number of large nonatretic follicles (11.8 +/- 4.4 versus 0.4 +/- 0.2). After hormone treatment, the mean number of medium-sized (3.0-5.9 mm) nonatretic follicles also increased (6.4 +/- 1.3 versus 1.8 +/- 1.0). PMSG did not change the mean number of nonatretic follicles < 3.0 mm or that of atretic follicles in the different size categories. However, when follicles hitherto called nonatretic, with atypical granulosa cells, were taken together with the group of atretic follicles, PMSG appeared to increase the mean number of large atretic follicles (13.6 +/- 2.4 versus 3.0 +/- 1.0). The mean number of medium-sized and large nonatretic follicles without atypical granulosa cells was markedly increased (3.8 +/- 1.0 versus 0.2 +/- 0.2 and 4.6 +/- 1.9 versus 0.0, respectively). The data demonstrate that PMSG stimulates the formation not only of nonatretic follicles > or = 3.0 mm, but also of atretic follicles > or = 6.0 mm.

The effects of alpha-amanitin and cycloheximide on nuclear progression, protein synthesis, and phosphorylation during bovine oocyte maturation in vitro

Bovine cumulus oocyte complexes were cultured for various periods and either denuded and orcein stained or radiolabeled with 35S-methionine or 32P-orthophosphate. Specific inhibitors were added to the culture medium to investigate mRNA and protein synthesis requirements for both nuclear and cytoplasmic changes during maturation in vitro. Inhibition of mRNA synthesis by alpha-amanitin during the first 2 h of culture prevented the phosphorylation of some specific proteins preceding GVBD and decreased the occurrence of GVBD from 97% to 27%. In addition, in oocytes that had undergone GVBD, only part of the changes in protein synthesis after GVBD were observed. Addition of alpha-amanitin after 3 h of culture had no effect on meiotic maturation. When cumulus oocyte complexes were cultured in the presence of cycloheximide, the phosphorylation of specific proteins was also blocked and only 5% of the oocytes underwent GVBD. Addition of cycloheximide after 4, 6, or 8 h of culture resulted in an increasing percentage of GVBD, but the oocytes became arrested in metaphase I. When cycloheximide was added from 12 h of culture onwards, nuclear progression to metaphase II was increasingly restored. It is concluded that after the onset of culture, both mRNA and protein synthesis are necessary for the phosphorylation of specific proteins and for GVBD. Furthermore, transcription during the first hours of culture is needed for the synthesis of new proteins after GVBD.