Comparative cytochemistry of the developing ovarian follicles of the dog, rabbit, and mouse: Origin of the zona pellucida

In vitro growth and steroidogenesis of dog follicles are influenced by the physical and hormonal microenvironment

The present study examined the influences of the physical and hormonal microenvironment on in vitro growth and steroidogenesis of dog follicles. Follicles were enzymatically isolated and individually encapsulated in 0.5% (w/v; n=17) or 1.5% (n=10) alginate and cultured with 0.5 IU/ml equine chorionic gonadotropin for 192 h. In a separate experiment, follicles were encapsulated in 0.5% alginate and cultured with 0 (n=22), 1 (n=23), 10 (n=20) or 100 (n=21) μg/ml FSH for 240 h. Follicle diameter and steroid production were assessed every 48 h in both studies. Follicles encapsulated in the 0.5% alginate grew faster (P<0.05) than those cultured in the 1.5% concentration. Oestradiol (E(2)) and progesterone (P(4)) increased consistently (P<0.05) over time, and follicles in the 1.5% alginate produced more (P<0.05) P(4) than those in the 0.5% solution. Follicles cultured in the highest FSH concentration (100 μg/ml) increased 100% in size after 240 h compared with 50 to 70% in lower dosages. E(2) concentration remained unchanged over time (P>0.05) across FSH dosages. However, P(4) increased (P<0.05) as culture progressed and with increasing FSH concentration. Results demonstrate that dog follicles cultured in alginate retain structural integrity, grow in size and are hormonally active. Lower alginate and increasing FSH concentrations promote in vitro follicle growth. However, the absence of an E(2) rise in follicles cultured in FSH alone suggests the need for LH supplementation to support theca cell differentiation and granulosa cell function.

Biosynthesis of the Canine Zona Pellucida Requires the Integrated Participation of Both Oocytes and Granulosa Cells1

In the dog, attempts to localize the expression of zona pellucida (ZP) proteins during folliculogenesis have failed to demonstrate conclusively whether any or all of the zona proteins are synthesized in the oocyte or the granulosa cells. Probing of paraformaldehyde-fixed prepubertal canine ovarian tissue sections with a panel of fluorescently conjugated lectins localized the expression of glycoproteins during folliculogenesis. We confirm that six lectins (PSA, s-WGA, ECL, GSL-II, LEL, and STL) consistently labeled the ZP and adjacent granulosa cells of the developing follicle and that canine ZP expresses beta-gal(1,4)glcNAc, beta-gal(1,3)galNac, alpha-mannose, and terminal sialic acid residues in a developmentally specific manner. Riboprobes for canine ZPA and ZPC genes were produced and used for in situ hybridization studies of mRNA expression in canine folliculogenesis. In addition, we isolated a partial cDNA transcript from total ovarian RNA for the canine ZPB gene having a high degree of sequence identity with the felid and porcine ZPB homologues. Subsequently, the ZPA gene transcripts were localized to the cytoplasm of oocytes in primordial, primary, and early secondary follicles. We then localized expression of ZPB and ZPC gene transcripts to the granulosa cells of growing follicles, but not in squamous granulosa cells of primordial follicles or oocytes. These observations indicate that in the juvenile canine ovary, the oocyte is responsible for synthesis of the ZPA protein and directing synthesis of the ZPB and ZPC proteins by the granulosa cells and that ZP gene transcription occurs in a sequential manner during folliculogenesis.

Glycohistochemistry of the zona pellucida of developing oocytes in the rabbit and hare

Lagomorpha are often used as animal models in reproductive experiments. The aim of the present study was to examine the glycoconjugate modifications occurring mainly in the zona pellucida during oocyte growth in the rabbit and hare, using a battery of lectins combined with sialidase digestion and chemical treatments. This histochemical approach made it possible to identify sulpho- and asulpho-carbohydrates in the terminal and/or subterminal position linked to sialic acid residues. The lectins that stained the zona pellucida of both species most effectively were SBA, PNA and WGA, indicating the presence of beta-D-N-acetylgalactosamine, beta-D-galactosamine and N-acetylglucosamine residues. The differences in the glucidic residue content and in their spatial distribution that depended on the species and stage of follicle development were also detected.

Pyriform cell differentiation in Podarcis sicula is accompanied by the appearance of surface glycoproteins bearing alpha-galNAc terminated chains

The present histochemical and cytochemical study using a lectin panel (WGA, GSI-A4, GSI-B4, PSA UEA-I, PNA, LCA, Con-A, DBA, MPA, BPA) has demonstrated that, in Podarcis sicula, the differentiation of small follicle cells into pyriform cells by means of intermediate cells is accompanied by the appearance of glycoproteins bearing alpha-GalNAc terminated O-linked side chains on the cell surface. The distribution of DBA- and MPA-binding sites over the follicular epithelium changed during the different stages of oocyte growth. DBA- and MPA-binding sites first appeared at the beginning of folliculogenesis within the zona pellucida (ZP) and on the surface of small cells, i.e., the stem cells of pyriform cells. Afterward, labeling was evident on the cell surfaces of intermediate cells and, later on, also of pyriform cells. On the other hand, no labeling was detected on the small cells located under the basal lamina, which, reportedly, do not differentiate into pyriform cells (Filosa et al. J. Embryol. Exp. Morphol., 1979; 15:297-316). Once pyriform cells were differentiated, the distribution of DBA- and MPA-binding sites over the follicular epithelium remained unchanged until intermediate and pyriform cells underwent apoptosis (Motta et al. J. Exp. Zool., 1996; 276:233-241) and the follicular epithelium transformed into a monolayer composed of small follicle cells only (Filosa Mon. Zool. Ital., 1973; 7:151-165). During this stage of oocyte growth, DBA and MPA labeling gradually decreased to completely disappear in the follicular epithelium of vitellogenic follicles. It is noteworthy that the observed changes in the distribution of DBA- and MPA-binding sites represent the first evidence recognized by lectins of a gradual modification of surface glycoprotein distribution over the follicular epithelium in the ovarian follicles of nonmammalian vertebrates so far studied. Finally, the zona pellucida (ZP), characterized by the presence of GalNAc, GluNAc, Man, and Gal, was demonstrated to be first synthetized by the oocyte and later on by the follicle cells.

Glycohistochemical investigation of canine and feline zonae pellucidae of preantral and antral oocytes

Glycoconjugate modifications were analysed in the zona pellucida during development of oocytes in dog and cat using conventional histochemical staining methods with or without previous carbohydrate digestion. A series of lectins combined with desulphation and sialic acid degradation were applied. No differences were observed between dog and cat follicles using conventional histochemical staining methods. In both species, the zona pellucida and follicular fluid/intercellular matrix strongly reacted with PAS and high iron diamine stain (HID) and reacted moderately with low iron diamine stain (LID). Treatment with testicular hyaluronidase, chondroitinase ABC, chondroitinase AC and chondroitinase B treatment diminished HID and LID positivity of follicular fluid and intercellular matrix. Lectins that gave the most intense staining of the zona pellucida of both species were SBA, PNA, RCA-I, GSA-IB4 and WGA, indicating the presence of beta-D-GalNAc, D-Gal and GlcNAc residues. Sulpho- and asulpho-carbohydrates were identified in terminal and/or subterminal positions linked to sialic acid residues. In conclusion, the results indicate that glycosaminoglycans are not present in the zona pellucida of both species. Differences were observed in carbohydrate residues and in their spatial distribution, depending on species and developmental stage of the follicles. The similarity in lectin affinity between ooplasm and zona pellucida of oocytes present in follicles at different stages of development confirm the involvement of oocytes in zona pellucida production.

Differential expression of ZPC in the bovine ovary, oocyte, and embryo

Using nonradioactive in situ hybridization (ISH), the mRNA encoding the zona glycoprotein bZPC was localized in bovine ovaries, oocytes, and embryos. In the ovary, the distribution of the mRNA was correlated with the developmental stage of the follicle. Whereas in primordial and primary follicles the mRNA was predominantly seen in the oocyte, it was found in both the oocyte and the follicle cells of secondary and tertiary follicles. In 2-day-old embryos produced by in vitro fertilization (IVF), no mRNA encoding ZPC could be demonstrated. Immunoblotting using monospecific polyclonal antibodies against porcine ZPC revealed a distinct band at a molecular weight of 47 kD in the ovarian cortex of cows, calves, and fetuses as well as in bovine follicle cells. Immunohistochemistry using the ZPC antibody displayed a strong signal in the zona pellucida of bovine oocytes and 2- to 6-day-old embryos as well as in the follicle cells. Our results show that during follicular development bovine ZPC is synthesized by the oocyte of the primary follicle and by both the oocyte and the follicle cells of the secondary and tertiary follicle. After fertilization, the synthesis of the zona protein is finished.

Localization of the mRNA encoding the zona protein ZP3 alpha in the porcine ovary, oocyte and embryo by non-radioactive in situ hybridization

The mRNA of the zona pellucida glycoprotein ZP3 alpha was localized in frozen sections of pig ovaries, isolated oocytes and early embryos by in situ hybridization using biotinylated oligonucleotide probes. In follicles, the distribution of mRNA for ZP3 alpha was correlated with the developmental stage: in primordial and primary follicles, the mRNA was shown to be predominantly localized in the oocyte. In secondary follicles, mRNA was found in both the oocyte and follicle cells. In tertiary and preovulatory follicles, the follicle cells showed distinct staining, whereas the oocyte was labelled weakly. In the early embryo, i.e. 2 days after fertilization, mRNA for ZP3 alpha could not be demonstrated. Our results suggest that, in the pig, the zona pellucida protein ZP3 alpha is synthesized by the oocyte and the follicle cells in sequence. After fertilization, synthesis of ZP3 alpha is terminated.

Oocyte and zygote zona pellucida permeability to macromolecules

Zona pellucida intact mouse pre- and post-ovulatory oocytes and zygotes were investigated for permeability to macromolecules using graded neutral fluorescein isothiocyanate (FITC) dextrans (molecular weight range 3.84-170 kDa) and a range of galactose-binding lectins (molecular weight range 40-247 kDa). The use of both FITC neutral dextrans and FITC galactose-binding lectins gave good agreement in the zona pellucida permeability studies. Zona pellucida intact pre- and post-ovulatory oocytes were permeable to both markers up to a molecular weight range of 170 kDa although movement across the zona pellucida was diminished between 100 to 150 kDa with no detectable permeability at molecular weight range of 170 kDa. Zygotes demonstrated a decreased zona pellucida permeability at around 110 kDa. Two galactose-containing oligosaccharide-rich zones in the zona pellucida were identified, and changes in zygote vitelline membrane galactose-binding lectin affinity were detected.

Cytochemical characterization of oligosaccharide side chains of the glycoproteins of rat zona pellucida: an ultrastructural study

BACKGROUND

The zona pellucida (ZP), an extracellular matrix which surrounds mammalian oocytes, is formed by different glycoproteins. Several studies have revealed that carbohydrate residues present in glycoproteins of ZP play a key role in the sperm-egg recognition. However, the origin and the biochemical composition of ZP remain to be completely resolved.

METHODS

ZP glycoproteins from rat ovarian follicles were investigated at light and electron microscopic level by the application of lectins conjugated to peroxidase, digoxigenin, and colloidal gold in combination with enzyme and chemical treatment. A quantitative analysis was also performed.

RESULTS

ZP shows reactivity to WGA, DSA, LFA, AAA, RCA I, and MAA. SBA and PNA showed a variable reactivity ranging from negative to strongly positive. A uniform pattern of binding throughout ZP was observed with DSA, Con A, AAA, MAA, and LFA. However, labeling by RCA I and SBA was higher in the outer ZP while PNA and WGA showed a higher binding in the inner ZP. Lectin reactivity was detected in cortical granules, endoplasmic reticulum, Golgi apparatus, vesicles, and multivesicular bodies of oocytes.

CONCLUSIONS

ZP contained the terminal disaccharides Gal beta 1,4GlcNAc, Gal beta 1,3GalNAc, and GalNAc beta 1,3Gal and the trisaccharides Neu5Ac alpha 2, 3Gal beta 1,4GlcNAc, Neu5Ac-Gal beta 1,3GalNAc, and Neu5Ac-GalNAc beta 1,3Gal sequences. The occurrence of Fucose residues alpha 1,6 linked to the inner core region of N-linked glycoproteins of ZP was demonstrated by the use of several fucose-specific lectins. Methylation-saponification treatment in combination with lectin cytochemistry reveals that Gal, GalNAc, and polyllactosamine residues of rat ZP glycoproteins contain sulphated groups. The reactivity observed in ooplasmic vesicles was similar to that of ZP, thus suggesting that the oocyte is the site of synthesis of ZP glycoproteins.

Regulation of ovarian follicular development: a review of microscopic studies

Follicular development in the mammalian ovary is a complex process regulated by an orchestrated action of the pituitary gonadotropins, e.g., follicle stimulating hormone (FSH) and luteinizing hormone (LH), and local ovarian factors, such as peptide growth factors and steroids. The mechanism of endocrine/paracrine/autocrine regulation of folliculogenesis (i.e., cell proliferation and functions) has been addressed largely by biochemical means. However, the availability of immunological and molecular tools now enables us to undertake critical microscopic studies revealing the ovarian cell-type specific synthesis and/or accumulation of many of these local peptide modulators, their roles in the proliferation and differentiation of follicular cells, and their regulation by gonadotropins and local factors. The objective of this review is to provide a comprehensive yet complete picture of the endocrine/autocrine regulation of mammalian folliculogenesis as revealed by microscopic studies. Efforts have been made to include adequate research information relevant to update our understanding of the process of follicular development; however, to maintain the brevity, many equally important studies could not be included. This review confirms that FSH and LH are still the primary stimuli for follicular development. However, it is clear that the actions of these hormones at the cell level involve a host of peptide factors which are produced locally by different follicular cell types and which are powerful modulators of gonadotropin actions. The temporal and spatial expression of the genes of these modulators, the synthesis of active factors, their interactions, and the dynamics of their receptors on the follicular cell surface may be the ultimate determinants of cellular events which are crucial to coordinated growth and differentiation of follicular cells leading to folliculogenesis and ovulation.

The three-dimensional structure of the zona pellucida in growing and atretic ovarian follicles of the mouse. Scanning and transmission electron-microscopic observations using ruthenium red and detergents

The present study provides further details on the fine-structural three-dimensional architecture of the zona pellucida (ZP) in growing and atretic follicles of mice by use of ruthenium red in combination with the detergents Triton X100 and saponin. These detergents were used for extraction of the "soluble" fraction of the zonal proteins in an attempt to expose the "structural" zonal glycoproteins, which in turn can be viewed as minute three-dimensional networks upon transmission- and scanning electron-microscopic examination. By use of these methods, the ZP of growing follicles appeared to be formed by interconnected filaments which also bind to globular structures building up a three-dimensional lattice. In contrast, the ZP of stage I as well as other (II and III) stages of atretic follicles showed a structure characterized by the presence of closely packed granules connected with short filaments to form a close-mesh reticulum. This structural change of the ZP, which in the present study is also associated with the disappearance of "gap junctions" within the granulosa and cumulus cell population, might represent one of the early events involved in the onset of atresia. These changes, most probably depending on an altered secretory activity of both oocytes and follicle cells, might lead to a degradation of the ZP network structure and to its subsequent increased density (condensation). All these morphodynamic events eventually contribute to a sequestration of the oocyte in the early stage of atresia.