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RESEARCH |
1 Research Centre for Reproductive Health, Department of Obstetrics and Gynaecology, The University of Adelaide, Adelaide, SA 5005, Australia 2 Pest Animal Control Cooperative Research Centre, CSIRO Sustainable Ecosystems, Canberra, ACT, Australia 3 Cardiovascular Research Centre, Monash University, Clayton, Victoria, Australia 4 Department of Chemical Pathology, Adelaide Women’s and Children’s Hospital, Adelaide, SA, Australia and 5 Department of Paediatrics, The University of Adelaide, Adelaide, SA 5005, Australia
Correspondence should be addressed to R J Rodgers; Email: ray.rodgers{at}adelaide.edu.au
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Introduction |
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This would not be an issue except that in vitro studies of follicular growth have focused almost entirely on the replication of granulosa cells. When those species are in vivo with physically large follicles, it is not the number of granulosa cells per se that is an indicator of follicular growth but rather the rapid expansion of the follicular antrum which heralds the development of a dominant follicle to the preovulatory stage. This growth is monitored in vivo by ultrasonography.
Despite the importance of the formation of follicular fluid and expansion of the follicular antrum, there is a dearth of literature on these topics. We and other researchers consider that the fluid component of follicular fluid is originally derived from the vasculature in the surrounding thecal layers. However, the means by which the fluid accumulates at the centre of the follicle is not known. The cells of the membrana granulosa constitute a stratified epithelium; however, they lack a network of tight junctions between cells that occur in some other epithelia. Thus, it would not be possible to establish an osmotic gradient across the membrana granulosa using small molecules like sodium, such as that which occurs, for example, in the renal tubules. The composition of follicular fluid is similar to serum with respect to low molecular weight components with most electrolytes being at the same concentrations in fluid and serum (Shalgi et al. 1972). However, for increasing sizes above 100 kDa, plasma proteins are found at progressively lower concentrations than in plasma (Perloff et al. 1955, Keikhoffer et al. 1962, Manarang-Pangan & Menge 1971, Shalgi et al. 1973, Andersen et al. 1976). This suggests that there is a diffuse nominal "blood-follicle barrier" at sizes >100 kDa. This barrier probably exists at the level of the follicular basal lamina.
Thus, a potential osmotic gradient could be established across the membrana granulosa if the granulosa cells directionally secreted molecules of large molecular weight towards the centre of the follicle. This osmotic gradient could then be responsible for recruiting fluid to the centre of the follicle. The regulation of such large molecules could be the means by which follicular fluid and antrum formation and follicular growth are regulated. Some of these ideas have been suggested a few times in the earlier literature, but they have never been tested nor have the molecules involved been identified. Here, we show that follicular fluid can exert osmotic potential using molecules larger than 300 kDa. We use the term ‘potential’ rather than ‘pressure’. Pressure is the net result of the production of osmotic molecules and their concentration. Thus, if their production is counter balanced by net movement of fluid into the follicle, no net gain in pressure can be measured, even if the production of such molecules was responsible for the recruitment of the fluid. In the current experiments, we are not monitoring pressure but rather identifying molecules whose production could be expected to generate osmotic forces during follicular growth.
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Antibodies
The primary monoclonal antibodies used in proteoglycan identification by ELISA and Western blots are described below. Primary antibody directed toward 4-sulphated chondroitin sulphate/dermatan sulphate (2B6) (Caterson et al. 1985), 6-sulphated chondroitin sulphate/dermatan sulphate (3B3) (Caterson et al. 1985), were purchased from ICN and aggrecan (1C6) (Caterson et al. 1985), was purchased from Developmental Studies, Hybridoma Bank, The University of Iowa, Iowa City, IA, USA. Antibodies recognising bovine decorin (LF94) and bovine versican (GAGß, recognising the GAGß domain present in splice variants V0 and V1) (Schmalfeldt et al. 1998) were obtained from Dr Larry Fisher, Bone Research Branch, National Institutes of Health, Bethesda, MD and Dr Dieter Zimmermann, Molecular Biology Laboratory, Department of Pathology, University of Zurich, Switzerland, respectively. The antibody recognising the heparan sulphate containing the proteoglycan perlecan (A76) was kindly donated by Dr Anne Underwood, formerly of CSIRO Molecular Science, North Ryde, NSW, Australia. The antibody recognising human inter-
-trypsin inhibitor (#A0301) (Salier et al. 1987) was from Dako, Glostrup, Denmark. Silenus laboratories, Victoria, Australia, supplied secondary antibodies, sheep anti-mouse IgG conjugated to horseradish peroxidase and goat anti-rabbit IgG conjugated to horseradish peroxidase were from Chemicon Australia, Victoria, Australia.
Tissues
Ovaries were collected from cycling animals of mixed breeds of Bos taurus from the local abattoir within 20 min of slaughter and placed into HEPES-buffered EBSS, pH 7.5, containing 10 mM N-ethylmalemide, 5 mM benzamidine, 0.5 mM phenylmethylsulfonylfluoride, 1 µg/ml pepstatin A, 1 µg/ml leupeptin, 0.01 M EDTA and 0.1% sodium azide. The animals were deemed to be cycling by observing healthy or regressing corpora lutea and were visually assessed as being non pregnant. The ovaries were then transported on ice to the laboratory.
Follicular fluids
Antral follicles (n = 171) 11–16 mm in diameter were isolated for the determination of osmotic pressure and proteoglycan characterisation. Follicular fluid (440–1200 µl) was aspirated from follicles with a 23G hypodermic needle and a 1 or 2 ml syringe within 2 h of slaughter. The ovaries and follicles remained on ice throughout the procedure. The fluid samples were centrifuged at 2000 r.p.m. for 30 min, 4 °C to remove cellular debris and protease inhibitors (5 mM benzamidine, 0.5 mM phenylmethylsulphonylfluoride, 1 µg/ml pepstatin A, 1 µg/ml leupeptin, 0.01 M EDTA and 0.1% sodium azide) were added before storage at –20 °C. The remaining follicle walls were then fixed in Bouin’s solution and embedded in paraffin. Histological assessment of follicular health (Irving-Rodgers et al. 2001) was undertaken by light microscopy analysis of haematoxylin and eosin-stained 5 µm-thick sections using an Olympus BX50 microscope. On completion of the assessment of follicle health, fluid samples were pooled based on the health status of the follicle from which they were collected. Fluids collected from healthy follicles were termed healthy fluids and those from atretic follicles termed as atretic fluids. Each healthy pool comprised 10 follicles and each atretic pool 8 follicles, with 4 pools used for each health status (see below). Follicles in a range of sizes from 2 to 15 mm were isolated for the determination of inter--trypsin inhibitor levels and their fluid harvested as above. In addition, a large pool of fluid from unclassified antral follicles of all sizes was collected as above for chromatography.
Size exclusion by differential dialysis
In order to determine the contribution of the macromolecular components in the aspirated fluids, the fluids were diluted 1 in 10 with EBSS, pH 7.5, and dialysed against distilled water in membranes of differing MW cut offs (10, 100, 300, 500 kDa) for 24 h at 4 °C. Following dialysis, samples were freeze-dried before reconstitution to their pre-dialysis volumes with EBSS. An aliquot of this material was assessed by 10% SDS-PAGE according to the method of Laemmli (1970) to confirm the removal of appropriately sized molecules. The experiments were performed with EBSS in order to maintain physiological ionic composition.
Enzyme digestions of follicular fluids
To quantify the contribution of classes of molecules to the osmotic pressure of follicular fluid, samples were diluted 1 in 10 with EBSS, pH 7.5, and then treated with a range of enzymes (see Table 1
for details). Control pools of undigested fluid were held at 37 °C or 25 °C for 4 h whilst enzyme treatments were underway. Following this, digestion dialysis (100 kDa or 300 kDa) was carried out against 2 M NaCl solution, 24 h, 37 °C. Dialysis at high salt concentrations and temperature was necessary in this experiment to ensure removal of digested but potentially-aggregating molecules. All samples were then dialysed against distilled water for 24 h at 4 °C. The samples were stored, freeze-dried and reconstituted to their original volumes in EBSS prior to measurement of osmotic pressure as described below. Once again, an aliquot of this material was assessed by SDS-PAGE to confirm the removal of appropriate molecules.
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Ion-exchange and size exclusion chromatography
Proteoglycans were isolated from a pool of follicular fluid from unclassified follicles by anion exchange chromatography on DEAE-Sephacel. Follicular fluid (40 ml) was diluted 1 to 5 with 4 M urea; 0.05 M sodium acetate buffer, pH 7.5, and applied to 5 ml of DEAE-Sephacel at 4 °C. The column was washed with 10 volumes of 4 M urea in 0.05 M sodium acetate buffer, pH 6.0. Proteoglycans were eluted from the column in the same buffer containing 2 M NaCl. The fractions containing proteoglycans, as determined by uronic acid measurement, were pooled, dialysed against water and lyophilised. This sample was resuspended in 500 µl of 2 M guanidine–HCl, 0.1 M sodium acetate, 0.05 M Tris, pH 7.5, and applied to a Sepharose CL-2B (6.5 mm x 1000 mm) size exclusion column. Five hundred microliter fractions were collected, dialysed against MQ water and lyophilised for analysis of protein, glycosaminoglycans and proteoglycans.
Protein and glycosaminoglycan measurements
Protein concentrations were assayed by the Bradford method (Bradford, 1976). Gycosaminoglycan concentrations were determined by assaying the uronic acid concentration according to the method of Blumenkreuntz and Asboe-Hanson (Blumenkrantz & Asboe-Hansen 1973, 1974).
Proteoglycan ELISA
The proteoglycan composition of the column fractions was determined by ELISA using a panel of antibodies specific to versican, decorin, aggrecan, perlecan and inter--trypsin inhibitor and a general antibody to 4-sulphated chondroitin sulphate/dermatan sulphate. Samples analysed for 4-sulphated chondroitin sulphate/dermatan sulphate or 6-sulphated chondroitin sulphate/dermatan sulphate required pre-digestion with chondroitinase ABC. Briefly, an aliquot of follicular fluid extracted from proteoglycans was treated with chond roitinase ABC (0.5 units per 10 ml in 0.1 M Tris acetate buffer containing 0.1% bovine serum albumin, pH 8.0) lyase to expose the antigen. Samples analysed for the presence of aggrecan using the 1C6 antibody required a reduction treatment with dL-dithiothrietol (DTT) prior to analysis. Following attachment to the wells, the samples were treated with 10 mM DTT (100 °C 10 min) and then alkylated with 20 mM iodoacetic acid in the dark (2 h). Samples were treated in the same 96 well to which they had been previously attached and incubated for 1 h at 37 °C. Samples (10 µl) to be tested were diluted in 140 µl of phosphate-buffered saline (PBS, 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 2 mM KH2PO4) and incubated for 1 h at 37 °C in 96-well plates (Nunc, #430341). The wells were then rinsed in PBS/Tween 20 (0.05%) and incubated with 0.1 ml PBS containing 1% skimmed cow’s milk powder and the appropriate d ilution of primary antibody [1:1000- for inter--trypsin inhibitor (A0301), versican (GAGß), 4-sulphated chond roitin sulphate/dermatan sulphate (2B6), 6-sulphated chondroitin sulphate/dermatan sulphate (3B3) and aggrecan (1C6)] and incubated for 1 h at 37 °C. The secondary antibodies were sheep anti-mouse IgG conjugated to horse-radish peroxidase for the mouse monoclonal antibodies, and goat anti-rabbit conjugated to horse-radish peroxidase for the rabbit polyclonal antibodies, diluted at 1/2000–1/5000 in PBS containing 1% skim milk powder and incubated for 1 h at 37 °C. Horse-radish peroxidase was measured as a color reaction using 100 µl of 2,2'-azino-di-[3-ethylbenzthioazoline-6-sulfonic acid] as substrate with 0.01% hydrogen peroxide. The optical density was recorded at 415 nm.
Western and ligand blotting of proteoglycan components
The proteoglycan component of follicular fluid was further characterised by electrophoresis on 0.8% agarose gels or 10% SDS-PAGE. The gels were then either stained with Stains-All or transferred to Hybond C+ by capillary transfer in the case of the agarose gels or by electro transfer to PVDF for poly acrylamide gels (Towbin et al. 1979). Additionally, follicular fluids from follicles of increasing size of 2 mm-15 mm were examined for the presence of inter--trypsin inhibitor by Western blotting. In all cases, the membranes were blocked in 5% skim milk (1 h at 37 °C), washed (3 x PBS/Tween 20 for 5 min at room temperature), and incubated with an appropriate dilution of primary antibody directed against specific proteoglycan species as described above or with biotinylated hyaluronan binding protein (0.5 µg/ml), in 1% skim milk and incubated for 1 h at 37 °C. The membrane was washed (3 x PBS/Tween 20 for 5 min at room temperature) before incubation with an appropriate dilution of secondary antibody in 1% skim milk (1 h at 37 °C) or streptavidin-horseradish peroxidase. The membrane was then rinsed (3 x PBS/Tween 20 for 5 min at room temperature) and the result visualised using either DAB or Super Signal Western blotting detection system according to the manufacturers protocol. Hyaluronan of sizes 220 kDa, 860 kDa and 1.6 x 106 Da were used as size indicators on agarose gels.
DNA assay
DNA components of the follicular fluid were resolved on 1% agarose gels in TAE buffer (0.02 M Tris, 0.5 mM EDTA) and stained with either Stains-All or ethidium bromide. Prior digestion with proteinase K or RNase-free DNase (1 mg/ml for 1 h at 37 °C) was carried out to identify DNA and protein observed in these gels (Mandel-Gutfreund et al. 1995, Jones et al. 2001).
Identification of collagenase sensitive molecules
Following the digestion of the fluid samples with the enzymes mentioned above and subsequent PAGE analysis, it became apparent that there were significant differences in band profiles between the collagenase-digested fluid and undigested samples. In order to identify the components seen following the digestion step, we repeated the digest, isolated the bands of interest from the gel and extracted the proteins contained within the gel and then separated these on a 7% Tris/acetate polyacrylamide gel. Excised bands were washed with 50% methanol in water (v/v) and then dried in vacuo. They were subsequently rehydrated in a minimal volume of 2% SDS, 0.2 M Tris–HCl, pH 8.5. Ten volumes of water were added to the rehydrated gel pieces and PVDF membrane soaked with methanol was added to adsorb the diffusing proteins from the gel. Following complete transfer of the proteins to the PVDF, the membrane was washed five times in 0.5 ml 10% methanol in water (v/v), dried and loaded into a reaction cartridge of an ABI PROCISE CLC protein sequencer followed by the identification of amino acids via HPLC. Peptide sequences were compared with mammalian databases using PROWL (http://prowl.rock-efeller.edu) and those sequences with >90% homology to a known protein were recorded in Table 2.
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). The osmotic pressures of the healthy (1:10 dilution, 2.45 ± 0.45 mm water, n = 4) and atretic follicles (2.35 ± 0.40, n = 4) were similar. A non-significant reduction in osmotic pressure of approximately 35% and 29% was observed following dialysis of healthy and atretic fluids at 300 kDa respectively. Dialysis against a 500 kDa MW cut-off membrane resulted in a significant 60% reduction in osmotic pressure in fluid from healthy follicles and 80% reduction in fluid from atretic follicles. This clearly indicates that large molecular weight molecules or aggregates of molecules contribute significantly to the overall osmotic potential of follicular fluid of both healthy and atretic follicles.
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, and the digested fluids dialysed against 100 kDa or 300 kDa MW cut-offs and the resultant osmotic pressure determined (Fig. 2). All digestion conditions resulted in some reduction of osmotic pressure of follicular fluid compared to the controls. Following dialysis against 100 kDa, results indicated that in fluid from healthy follicles, 43% ± 6% of the pressure was significantly attributed to DNA, and 43% ± 2% and 53% ± 14% to hyaluronan and chondroitin sulphate/dermatan sulphate. In fluid from atretic follicles, 34% ± 2% of the fluid pressure was contributed by DNA and 38% ± 5% by collagen I sensitive molecules. Removal of heparan sulphate and keratan sulphate had no significant effect on osmotic pressure of either healthy or atretic fluid. The major difference between follicular fluid from healthy and atretic follicles was that hyaluronan and chondroitin sulphate/dermatan sulphate did not make a significant contribution to the osmotic pressure in atretic follicles.
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). In fluids from healthy follicles, 58% ± 9% of the pressure was significantly attributed to hyaluronan. In fluid from atretic follicles, DNA contributed 51% ± 2% of the pressure and hyaluronan contributed 57% with protein contributing 32%. Keratan sulphate removal had no significant effect on osmotic pressure of either healthy or atretic fluids.
Glycosaminoglycans and proteoglycans in follicular fluid
Proteoglycans in follicular fluid were isolated by anion exchange chromatography followed by size exclusion chromatography on Sepharose CL-2B. The concentration of protein, glycosaminoglycan, 4-sulphated chondroitin sulphate/dermatan sulphate and versican is shown in Fig. 3. No reactivity was detected against 6-sulphated chondroitin sulphate/dermatan sulphate (data not shown). The glycosaminoglycan poly-disperse peak overlapped that of the protein and the 4-sulphated chondroitin sulphate/dermatan sulphate overlapped that of the glycosaminoglycan peak (Fig. 3). Versican, a chondroitin sulphate proteoglycan, was detected in poly-disperse peak that overlapped the peak containing 4-sulphated chondroitin sulphate/dermatan sulphate. Confirmation of the presence of versican in follicular fluid samples was provided by Western immunoblot (Fig. 4) in which two immunoreactive bands were observed, probably corresponding to V0 and V1. Unpublished observations from RNA analyses confirm that both V0 and V1 are expressed in bovine follicles. Fractions from the size exclusion chromatography were treated with chondroitinase ABC and subjected to Western immunoblotting using an antibody to inter--trypsin inhibitor (Fig 5A). Heavy chains and bikunin were identified, though the intensity of the bikunin band was weak as observed previously by others in other samples (Rouet et al. 1992). Since inter--trypsin inhibitor was unexpected based upon reports in rodents (Chen et al. 1994), follicular fluid from a range of different sized follicles (2 mm to 15 mm) was examined and also found to contain inter--trypsin inhibitor (Fig. 5B). Inter--trypsin inhibitor and or related molecules, pre- inhibitor and inter- like inhibitor, were identified by Western blot analysis to be present in the fluid from all sizes of follicles examined. ELISA and Western blotting of the chromatography fractions did not detect perlecan, decorin or aggrecan (data not shown).
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) of this region showed that the majority of this smear was derived from versican, inter--trypsin inhibitor and or its components and hyaluronan. The sizes of hyaluronan ranged from approximately 400 kDa to 2 x 106 Da.
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) demonstrated the presence of DNA in the fluid as well as the proteoglycans. The DNA of healthy follicular fluid ranged in size from 2799 bases to >9000 bases with some DNA remaining in the well, suggesting it is of a very large size >2 x 106 bases. The DNA component of atretic follicular fluid, ranged from 500 bases to 9000 bases (data not shown).
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). Three major bands were seen following collagenase I digestion. To further resolve what clearly was a mixture of bands, the digest was separated on a 7% Tris-acetate gel after which nine bands were identified and taken for N-terminal amino acid sequencing. Proteins identified in these bands and known to be collagenase sensitive are shown in Table 2. Several other sequences were identified as being present in the fluid samples; however, these were not known to be collagenase sensitive but are known constituents of serum and might therefore be expected in the follicular fluid.
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Hyaluronan has a variety of physiological functions in the extracellular matrix, such as rheological properties, flow resistance, osmotic pressure, exclusion properties and filter effects. Its osmotic effects provide a sensitive mechanism for volume buffering in the interstitium via the formation of highly entangled networks of flexible polysaccharide molecules (Rosengren et al. 2001). Hyaluronan is synthesised at the surface of cells by one of the three hyaluronan synthases (HAS1, 2 or 3) producing a repeating glycosaminoglycan chain of hyaluronic acid polymer, which extrudes into the extracellular space. Unlike proteoglycans, it does not have a core protein. However, there are a number of cell surface proteins (CD44, RRHAM and lyve-1) and other proteins (link protein, versican, inter--trypsin inhibitor), which can bind to hyaluronan to form organised three-dimensional matrices (Jackson 2003). In the ovary, hyaluronan production by cumulus cells has been extensively studied (Chen et al. 1993, 1996, Hirashima et al. 1997, Hess et al. 1999, Kobayashi et al. 1999). However, this production is tailored to release the oocyte and cumulus cell complex from the wall of the follicle into the fluid in preparation for ovulating this complex. For hyaluronan to be involved in follicular fluid formation, hyaluronan would need to be produced much earlier than that produced at ovulation by the cumulus-oocyte complex. In that regard, hyaluronan levels of human follicular fluid have been measured at 50.0 ± 2.6 ng/ml and it has been previously localised adjacent to and including the spaces between antral granulosa cells (Saito et al. 2000). In addition, we observed hyaluronan in follicular fluids at sufficiently large sizes for it to be retained in the follicular antrum, and recently low levels of HAS 2 expression and hyaluronan production were observed in granulosa cells in culture (Schoenfelder & Einspanier 2003). Thus clearly, hyaluronan has all the credentials to be a key player in the formation of follicular fluid.
Earlier research in a number of species has identified glycosaminoglycans, dermatan sulphate and chondroitin sulphate to be present in follicular fluids, and shown that glycosaminoglycans and proteoglycans are synthesised by the granulosa cells in vitro (Ax & Ryan 1979, Bellin et al. 1983, Yanagishita et al. 1981, Eriksen et al. 1997). Here, we identified chondroitin 4-sulphated proteoglycans as significant contributors to the osmotic potential of follicular fluid. Using a combination of techniques, the proteoglycans identified in follicular fluid were versican and inter--trypsin inhibitor. Decorin, perlecan and aggrecan were not detectable. Perlecan has been identified in extracts of whole bovine follicles (McArthur et al. 2000) and its RNA detected in granulosa cells (Princivalle et al. 2001, Irving-Rodgers et al. 2004). By immunohistochemistry, it was localised in the follicular basal lamina and focimatrix in large follicles (McArthur et al. 2000, Irving-Rodgers et al. 2004). It is not a component of follicular fluid. However, its presence in human follicular fluid has been reported (Eriksen et al. 1999), but in that study the fluid was aspirated at ovulation, a time when the follicular wall was being remodeled, allowing easy contamination of follicular fluid with the contents of the follicular wall. Hence, we do not consider that perlecan is a component of follicular fluid during follicle growth. Decorin has also been identified in extracts of whole follicles by us (McArthur et al. 2000), but we did not observe it here in follicular fluid. Localisation of decorin (unpublished observations) found it to reside outside the follicular basal lamina only. Hence, it is not a component of follicular fluid.
Versican is a chondriotin sulphate proteoglycan hyalectan with a broad tissue expression profile (Lemire et al. 2002). It has been shown to be present in extracts of bovine follicles (McArthur et al. 2000), follicular fluid of ovulating follicles (Eriksen et al. 1999), and in the follicular membrana granulosa (McArthur et al. 2000, Irving-Rodgers et al. 2004), and expressed by granulosa cells (Russell et al. 2003). It is involved in the expansion of the cumulus-oocyte complex (Camaioni et al. 1996, Carrette et al. 2001), whereby the heavy chains of inter--trypsin inhibitor become covalently cross-linked to hyaluronan, thereby stabilising the expanding cumulus matrix (Laurent et al. 1995, Eriksen et al. 1997, Saito et al. 2000, Carrette et al. 2001, Kimura et al. 2002). In addition to an N-terminal link-module hyaluronan-binding domain, versican possesses a C-terminal lectin-like domain that may bind cell surface or matrix molecules, and dual epidermal growth factor (EGF)-like modules. Many functional properties of versican are determined by two glycosaminoglycan attachment domains, which are modified to allow for attachment of long chondroitin sulphate side chains. These chains are responsible for versican’s large molecular size and strong charge negativity and hence its osmotic properties. Versican may directly contribute to the osmotic potential of follicular fluid directly by virtue of the high sulphation status of chondroitin sulphate side chains attached to its core protein. However, versican may also contribute to the osmotic potential of follicular fluid by cross-linking other components like hyaluronan to form larger molecular weight components.
Inter--trypsin inhibitor consists of two heavy chains linked by a chondroitin sulphate chain to bikunin and it is produced by the liver and is found abundantly in serum. In mice, inter--trypsin inhibitor appears to be sequestered from the blood stream as it appears within the follicle fluid within minutes of the LH surge (Chen et al. 1992). On entering the fluid, it associates with hyaluronan being synthesised by the cumulus cells, liberating free bikunin and producing a covalent bond between the heavy chains and hyaluronan (Rugg et al. 2005). However, in bovine follicular fluid, we observed inter--trypsin inhibitor (bikunin and its heavy chains) in normal follicular fluids. This is unlikely to be a contaminant from serum on collection of the follicular fluid, as we did not observe any contaminating perlecan or decorin in the samples. We detected inter--trypsin inhibitor and related molecules in samples of fluid from follicles of 2 mm to 15 mm by Western analysis. These data are contrary to that previously published for the mouse ovary and the presence of inter--trypsin inhibitor in follicles of a non-ovulatory size indicates that inter--trypsin inhibitor may have additional roles in the bovine ovary. The recent discovery of inter--trypsin inhibitor in follicular fluid from medium-sized porcine follicles indicates that the bovine is not the only species in which inter--trypsin inhibitor is found in fluid prior to the LH surge (Nagyova et al. 2004). Its role in follicular fluid may be to bind to hyaluronan, cross-linking it to form larger aggregates, and thereby ensuring that hyaluronan does not escape the follicular antrum.
DNA was also found to exert osmotic potential, particularly in follicular fluids of atretic follicles. DNA in follicular fluid is likely to be derived from granulosa cells that align with the follicular antrum, which upon their death release DNA into the follicular fluid (Van Wezel et al. 1999a). These cells do not appear to die by classic apoptotic mechanisms but rather by a process with more in common to terminal differentiation (Van Wezel et al. 1999a). DNA may be entangled with larger molecules such as hyaluronan as suggested by several authors (Turner et al. 1988). This may increase the osmotic effect of the DNA by minimising the loss of DNA or hyaluronan from the follicular fluid. However, the DNA content of follicular fluid is probably not regulated and potentially it could easily be degraded by the release of cellular DNase. Thus DNA, whilst contributing to the osmotic potential of follicular fluid as observed here and possibly in vivo, is probably not a regulated component that contributes to the osmotic potential offollicular fluid.
Another general means of formation of a cavity or lumen has not been discussed thus far. Cell death is important for formation of cavities or lumens as seen in blastocysts where the inner ectodermal cells undergo apoptosis to create a cavity (Coucouvanis & Martin 1995, 1999) and for in vitro ‘tube’ formation (Meyer et al. 1997) or in vivo lumen formation by endothelial cells (Tertemiz et al. 2005). A role for any released DNA as means of providing osmotic pressure does not appear to be have been considered in these situations. Whilst it is unlikely in species with proportionally large follicular antra that cell death is a major cause of antrum formation, it is possible that such death plays a more substantive role in species with proportionally smaller follicular antra such as mice or rats. Certainly the difference in inter--trypsin inhibitor between such species indicates potential differences between species in the way antra are formed.
Reduction in osmotic pressure by collagenase was interesting. Collagenase I used in the digestion of the fluid samples cleaves the helical strands of intact native collagen and degrades a number of other fibrillar proteins. We identified the following molecules: secretin, serum albumin, involucrin, agglutinin like protein precursor 3, coagulation factor X, prothrombin precursor, collagen IV 2, collagen XIII 1, and 2-macroglobulin from digested fluid. However, from this screen, we did not identify any molecules that we believe would contribute to the regulated production of osmotic potential.
It is interesting to note that the overall reduction in osmotic potential caused by removal of hyaluronan, chondroitin sulphate/dermatan sulphate and DNA appears to be >100% of the total pressure. This phenomenon can be explained by research conducted by Dick (Dick 1966) and Laurent and Ogston (Laurent & Ogston 1963). The former showed that in non-ideal solutions such as follicular fluid, solution – solvent interactions occur and result in osmotic potentials greater or less than that predicted by the Van’t Hoff Equation (Van’t-Hoff 1887). The result of this is that the contributions of hyaluronan, proteoglycans and collagens to the follicular fluid osmotic potential cannot be considered as mutually exclusive contributions, since each is known to interact with the other.
In summary, we have identified glycosaminoglycans and proteoglycans in follicular fluid that can exert osmotic pressure. These glycosaminoglycans and proteoglycans are too large, either individually or collectively, to freely diffuse from the follicles and may therefore contribute to accumulation of follicular fluid by exerting an osmotic potential within the antrum of the follicle.
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Footnotes |
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References |
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