Changes in histochemical distribution of cell surface heparan sulfate proteoglycan in mouse uterus during the estrous cycle and early pregnancy

Heparin for assisted reproduction

BACKGROUND

Heparin as an adjunct in assisted reproduction (peri-implantation heparin) is given at or after egg collection or at embryo transfer during assisted reproduction. Heparin has been advocated to improve embryo implantation and clinical outcomes.  It has been proposed that heparin enhances the intra-uterine environment by improving decidualisation with an associated activation of growth factors and a cytokine expression profile in the endometrium that is favourable to pregnancy.

OBJECTIVES

To investigate whether the administration of heparin around the time of implantation (peri-implantation heparin) improves clinical outcomes in subfertile women undergoing assisted reproduction.

SEARCH METHODS

A comprehensive and exhaustive search strategy was developed in consultation with the Trials Search Co-ordinator of the Cochrane Menstrual Disorders and Subfertility Group (MDSG). The strategy was used in an attempt to identify all relevant studies regardless of language or publication status (published, unpublished, in press, and in progress). Relevant trials were identified from both electronic databases and other resources (last search 6 May 2013).

SELECTION CRITERIA

All randomised controlled trials (RCTs) were included where peri-implantation heparin was given during assisted reproduction. Peri-implantation low molecular weight heparin (LMWH) during IVF/ICSI was given at or after egg collection or at embryo transfer in the included studies. Live birth rate was the primary outcome.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed the eligibility and quality of trials and extracted relevant data. The quality of the evidence was evaluated using GRADE methods.

MAIN RESULTS

Three RCTs (involving 386 women) were included in the review.Peri-implantation LMWH administration during assisted reproduction was associated with a significant improvement in live birth rate compared with placebo or no LMWH (odds ratio (OR) 1.77, 95% confidence interval (CI) 1.07 to 2.90, three studies, 386 women, I(2) = 51%, very low quality evidence with high heterogeneity). There was also a significant improvement in the clinical pregnancy rate with use of LMWH (OR 1.61, 95% CI 1.03 to 2.53, three studies, 386 women, I(2) = 29%, very low quality evidence with low heterogeneity).However these findings should be interpreted with extreme caution as they were dependent upon the choice of statistical method: they were no longer statistically significant when a random-effects model was used.Adverse events were poorly reported in all included studies, with no comparative data available. However, LMWH did cause adverse effects including bruising, ecchymosis, bleeding, thrombocytopenia and allergic reactions. It appeared that these adverse effects were increased if heparin therapy was used over a longer duration.

AUTHORS' CONCLUSIONS

The results of this Cochrane review of three randomised controlled trials with a total of 386 women suggested that peri-implantation LMWH in assisted reproduction treatment (ART) cycles may improve the live birth rate in women undergoing assisted reproduction. However, these results were dependent on small low quality studies with substantial heterogeneity, and were sensitive to the choice of statistical model. There were side effects reported with use of heparin, including bruising and bleeding, and no reliable data on long-term effects. The results do not justify this use of heparin outside well-conducted research trials.These findings need to be further investigated with well-designed, adequately powered, double-blind, randomised, placebo-controlled, multicentre trials. Further investigations could also focus on the effects of the local (uterine) and not systemic application of heparin during ART.

Targeting of syndecan-1 by micro-ribonucleic acid miR-10b modulates invasiveness of endometriotic cells via dysregulation of the proteolytic milieu and interleukin-6 secretion

OBJECTIVE

To study the function of syndecan-1 (SDC1) and its potential regulator miR-10b in endometriosis.

DESIGN

Experimental laboratory study.

SETTING

University medical center.

PATIENT(S)

Not applicable.

INTERVENTION(S)

The human endometriotic cell line 12Z was transiently transfected with SDC1 small interfering RNA or miR-10b precursors and investigated for changes in cell behavior and gene expression. 12Z and primary eutopic endometrial stroma cells of two American Society for Reproductive Medicine class III endometriosis patients were transfected with miR-10b precursors to investigate posttranscriptional regulation of SDC1.

MAIN OUTCOME MEASURE(S)

Quantitative polymerase chain reaction, Western blotting, flow cytometry, 3' untranslated region luciferase assays, and zymography were used to measure miR-10b-dependent targeting of SDC1 and SDC1-dependent expression changes of proteases and interleukin-6. Altered cell behavior was monitored by Matrigel invasion assays, cell viability assays, and mitogen-activated protein kinase activation blots.

RESULT(S)

Knockdown of SDC1 inhibited Matrigel invasiveness by >60% but did not affect cell viability. Interleukin-6 secretion, matrix metalloproteinase-9 expression, and matrix metalloproteinase-2 activity were reduced, whereas plasminogen activator inhibitor-1 protein expression was up-regulated. miR-10b overexpression significantly down-regulated SDC1, reduced Matrigel invasiveness by 20% and cell viability by 14%, and decreased mitogen-activated protein kinase activation in response to hepatocyte growth factor.

CONCLUSION(S)

Syndecan-1, a target of miR-10b, inhibits epithelial endometriotic cell invasiveness through down-regulation of metalloproteinase activity and interleukin-6.

The estrous cycle modulates small leucine-rich proteoglycans expression in mouse uterine tissues

In the pregnant mouse uterus, small leucine-rich proteoglycans (SLRPs) are drastically remodeled within a few hours after fertilization, suggesting that ovarian hormone levels modulate their synthesis and degradation. In this study, we followed by immunoperoxidase approach, the presence of four members of the SLRP family (decorin, lumican, biglycan, and fibromodulin) in the uterine tissues along the estrous cycle of the mouse. All molecules except fibromodulin, which predominates in the myometrium, showed a striking modulation in their distribution in the endometrial stroma, following the rise in the level of estrogen. Moreover, notable differences in the distribution of SLRPs were observed between superficial and deep stroma, as well as between the internal and external layers of the myometrium. Only biglycan and fibromodulin were expressed in the luminal and glandular epithelia. All four SLRPs were found in cytoplasmic granules of mononucleated cells. The pattern of distribution of the immunoreaction for these molecules in the uterine tissues was found to be estrous cycle-stage dependent, suggesting that these molecules undergo ovarian hormonal control and probably participate in the preparation of the uterus for decidualization and embryo implantation. In addition, this and previous results from our laboratory suggest the existence of two subpopulations of endometrial fibroblasts that may be related to the centrifugal development of the decidua. Anat Rec, 2008. (c) 2008 Wiley-Liss, Inc.

Estradiol induces heparanase-1 expression and heparan sulphate proteoglycan degradation in human endometrium

BACKGROUND

This study seeks to determine whether estrogen is able to regulate the expression of heparanase-1 (HPR1) in human endometrium.

METHODS

HPR1 expression and heparan sulphate (HS) deposition in the endometrium collected in various menstrual phases were analysed by immunohistochemical and immunofluorescence staining, respectively. HPR1 expression in the endometrial cells unexposed or exposed to estradiol was analysed by using RT-PCR and luciferase reporter assay. HPR1 activity was analysed by using a novel enzyme-linked immunosorbent assay (ELISA). Cell surface HS levels were analysed by flow cytometry. Serum HPR1 activity in women receiving follicle-stimulating hormone (FSH) for IVF was measured by ELISA.

RESULTS

HPR1 expression was rarely detected in the endometrium in the early and mid-proliferative phases but was increased in the late proliferative phase and in the secretory phases. HPR1 expression was negatively associated with HS in the basement membrane (BM) of the endometrial glands. HPR1 gene expression, HPR1 promoter activity and HPR1 enzymatic activity were increased in the endometrial cells when exposed to 17beta-estradiol (E(2)), whereas cell surface HS levels showed a decrease which could be blocked by PI-88, an HPR1 inhibitor. Serum HPR1 levels were increased in women with moderately elevated blood estrogen levels after receiving FSH.

CONCLUSIONS

HPR1 is differentially expressed in the endometrium in different menstrual phases. Estrogen plays an important role in inducing HPR1 expression, subsequently leading to HS degradation on the endometrial cell surface and in the BM of the endometrium.

Immunological localization of syndecan-1 in human endometrium throughout the menstrual cycle

OBJECTIVE

To examine the expression of syndecan-1 in the endometrium during the menstrual cycle.

DESIGN

The expression of syndecan-1 was determined by tissue microarray and immunohistochemistry.

SETTING

Academic clinical and research laboratories.

PATIENT(S)

Seventy-one regularly cycling women who underwent endometrial biopsy.

INTERVENTION(S)

Endometrial samples representing five stages of the menstrual cycle were used for the study.

MAIN OUTCOME MEASURE(S)

Semiquantitative analysis by evaluating the intensity of immunohistochemical reactivity of syndecan-1 by using a modified HSCORE.

RESULT(S)

Endometrial syndecan-1 was expressed in the luminal and glandular epithelium as well as in the stroma throughout the menstrual cycle in a nonsynchronized fashion. In the luminal epithelium, the expression of syndecan-1 was statistically significantly higher in the mid-secretory phase when compared with the proliferative phase. In the stroma, however, the expression of syndecan-1 was down-regulated after ovulation and remained at a low level through the secretory phases. Differences between the proliferative and mid secretory, as well as between the ovulatory and the early, mid, and late secretory phases, all were statistically significant.

CONCLUSION(S)

Syndecan-1 is up-regulated in luminal epithelial cells during the mid-secretory phase and is down-regulated in the stroma during the early to late secretory phases. The differential expression of syndecan-1 coincides with the endometrial remodeling throughout the menstrual cycle.

Expression of syndecans, cell-cell interaction regulating heparan sulfate proteoglycans, within the human endometrium and their regulation throughout the menstrual cycle

OBJECTIVE

To evaluate the expression of syndecan-1, -2, -3, and -4 in different phases of eutopic endometrium of normal cycling women.

DESIGN

Prospective observational study.

SETTING

University-based research center for reproductive medicine.

PATIENT(S)

Twenty-nine healthy ovulatory volunteers.

INTERVENTION(S)

mRNA and protein expression of syndecan-1 to -4 in human endometrium.

MAIN OUTCOME MEASURE(S)

Real-time polymerase chain reaction of syndecan members and further characterization of mRNA expression of syndecan-1 and -4 with multiprobe RNase protection assays of epithelial and stromal cells after purification with antibody-coated magnetic beads. For confirmation of results, protein expression and localization using immunohistochemistry for syndecan-1 and -4 was performed.

RESULT(S)

All syndecans were expressed within human endometrium. Syndecan-1 and -4 proved to be significantly upregulated in whole endometrium during the secretory phase (2.73-fold and 2.85-fold, respectively). Using multiprobe RNase protection assays, a significant upregulation of mRNA was noted in epithelial cells during the secretory phase for both syndecan-1 and -4 (7.46-fold and 2.52-fold, respectively) and confirmed by immunohistochemistry.

CONCLUSION(S)

Cycle-dependent expression of syndecan-1 and -4 suggests that these adhesion proteins are involved in the regulation of the cycling endometrium.

Sulphated glycosaminoglycans (S-GAGs) and syndecans in the bovine oviduct

In vivo, bull sperm capacitation seems to occur mainly in the oviduct. Capacitation of bull spermatozoa can be triggered in vitro by exposure to heparin, a heavily sulphated glycosaminoglycan (S-GAG). We determined the concentration of S-GAGs in oviductal fluid from dairy heifers, collected over the course of several oestrous cycles via surgically implanted intraluminal catheters. We also investigated the presence of syndecans, i.e. heparan sulphate proteoglycans, in the bovine oviductal epithelium of Swedish dairy cattle during standing oestrus and the luteal phase of the oestrous cycle, using immunohistochemistry for three different polyclonal antibodies raised against human syndecan-2 and rat syndecan-1 and syndecan-2, respectively. The concentration of S-GAGs in oviductal fluid obtained from the ampullar segment of the oviduct was significantly higher (P=0.0026) than it was in fluid from the isthmic segment during the functional period, i.e. from prooestrus to metaoestrus (73.5+/-10.49 mg/L in ampullar ODF, compared to 43.2+/-10.74 mg/L in isthmic ODF); least square mean (L.S.M.)+/-standard error of the mean (S.E.M.). There was also a significantly higher concentration of S-GAGs in the fluid from the oviduct ipsilateral to the ovulation side 73.5+/-10.54 mg/L on the ovulation side, compared to 43.1+/-10.71 mg/L in the oviduct on the contralateral side (L.S.M.+/-S.E.M., P=0.0026) during this period. Both syndecan-1 and syndecan-2 were present in the epithelial cells lining all studied segments of the bovine oviduct, i.e. the UTJ, isthmus and ampulla, during both standing oestrus and dioestrus. The syndecans and S-GAGs found may influence the gametes, while they reside in the oviduct; the amounts of S-GAGs found in the bovine oviduct seem sufficient to act as capacitating factors in vivo.

Hyposulfatemia, growth retardation, reduced fertility, and seizures in mice lacking a functional NaSi-1 gene

Inorganic sulfate is required for numerous functions in mammalian physiology, and its circulating levels are proposed to be maintained by the Na+-SO42- cotransporter, (NaSi-1). To determine the role of NaSi-1 in sulfate homeostasis and the physiological consequences in its absence, we have generated a mouse lacking a functional NaSi-1 gene, Nas1. Serum sulfate concentration was reduced by >75% in Nas1-/- mice when compared with Nas1+/+ mice. Nas1-/- mice exhibit increased urinary sulfate excretion, reduced renal and intestinal Na+-SO42- cotransport, and a general growth retardation. Nas1-/- mouse body weight was reduced by >20% when compared with Nas1+/+ and Nas1+/- littermates at 2 weeks of age and remained so throughout adulthood. Nas1-/- females had a lowered fertility, with a 60% reduction in litter size. Spontaneous clonic seizures were observed in Nas1-/- mice from 8 months of age. These data demonstrate NaSi-1 is essential for maintaining sulfate homeostasis, and its expression is necessary for a wide range of physiological functions.

Gene expression profiling of the effects of castration and estrogen treatment in the rat uterus

The development and functions of female reproductive tissues are regulated by the actions of two major sex steroid hormones, estrogen and progesterone. To investigate estrogen-dependent gene expression in the rat uterus, we studied the effect of ovariectomy with or without estrogen treatment on the uterine expression of 3000 genes using cDNA microarrays. Many genes were regulated by either treatment, but only few were reciprocally regulated by these contrasting treatments. The present study confirms previous findings and identifies several genes with expressions not previously known to be influenced by estrogen. These genes include follistatin-related protein, Thy-1 glycoprotein, alpha-fodrin, CD24, immediate early response 5, insulin-like growth factor-binding protein 2, growth response protein CL-6 (INSIG-1), ladinin1, class I major histocompatibility complex heavy chain, lactadherin, ezrin, and Fas-activated serine/threonine kinase. Because of their function as regulators of proliferation and apoptosis, CD24, insulin-like growth factor-binding protein 2, and Fas/Fas ligand were examined further by immunohistochemical expression and tissue localization analysis. Our analysis confirms a contrasting regulation of these gene products by ovariectomy and estrogen treatment. The present study identifies novel mediators of estrogen actions in the uterus and provides genome-wide expression data from which novel hypotheses regarding uterine function can be generated.

In vivo regulation of syndecan-3 expression in the rat uterus by 17 beta-estradiol

The immature rat uterus has been extensively used as an in vivo model system to study the molecular mechanisms of steroid hormone actions. In this study, we demonstrated the regulated expression of syndecan-3 in the rat uterus by the steroid hormone 17 beta-estradiol. Administration of a single physiological dose of 17 beta-estradiol (40 microg/kg) to ovariectomized immature animals induced a rapid and transient increase in uterine syndecan-3 mRNA. Transcript levels reached a peak elevation of 3-fold above saline control tissues 4 h after hormone administration. Inhibition of message up-regulation by actinomycin D but not cycloheximide indicated a hormone response dependent on RNA transcription but not new protein synthesis. The estrogenic ligands estriol and tamoxifen were also effective at raising syndecan-3 mRNA levels; however, nonestrogenic ligands, including progesterone, 5 alpha-dihydrotestosterone, and dexamethasone, failed to stimulate a change in mRNA levels. Hormone-induced changes in mRNA led to transient changes in syndecan-3 protein content and significant alteration in the temporal and spatial expression in endometrial epithelial cells. Collectively, these data show that the steroid hormone 17 beta-estradiol, regulates transcription of the syndecan-3 gene in the uterus via an estrogen receptor-dependent mechanism. This estrogen-regulated expression of syndecan-3 may play an important role in changes in tissue ultrastructure crucial for proper uterine growth.

Blastocyst implantation: the adhesion cascade

This review covers the sequence of cell adhesion events occurring during implantation of the mammalian embryo, concentrating on data from mouse and human. The analogy is explored between initial attachment of trophoblast to the uterine lining epithelium and that of neutrophils to the endothelial lining of blood vessels at sites of inflammation. The possible role of various carbohydrate ligands in initial attachment of the blastocyst is reviewed. The evidence for subsequent stabilization of cell adhesion via integrins or the trophinin-tastin complex is discussed.

Heparan sulfate proteoglycans as extracellular docking molecules for matrilysin (matrix metalloproteinase 7)

Many matrix metalloproteinases (MMPs) are tightly bound to tissues; matrilysin (MMP-7), although the smallest of the MMPs, is one of the most tightly bound. The most likely docking molecules for MMP-7 are heparan sulfate proteoglycans on or around epithelial cells and in the underlying basement membrane. This is established by extraction experiments and confocal microscopy. The enzyme is extracted from homogenates of postpartum rat uterus by heparin/heparan sulfate and by heparinase III treatment. The enzyme is colocalized with heparan sulfate in the apical region of uterine glandular epithelial cells and can be released by heparinase digestion. Heparan sulfate and MMP-7 are expressed at similar stages of the rat estrous cycle. The strength of heparin binding by recombinant rat proMMP-7 was examined by affinity chromatography, affinity coelectrophoresis, and homogeneous enzyme-based binding assay; the K(D) is 5-10 nM. Zymographic measurement of MMP-7 activity is greatly enhanced by heparin. Two putative heparin-binding peptides have been identified near the C- and N-terminal regions of proMMP-7; however, molecular modeling suggests a more extensive binding track or cradle crossing multiple peptide strands. Evidence is also found for the binding of MMP-2, -9, and -13. Binding of MMP-7 and other MMPs to heparan sulfate in the extracellular space could prevent loss of secreted enzyme, provide a reservoir of latent enzyme, and facilitate cellular sensing and regulation of enzyme levels. Binding to the cell surface could position the enzyme for directed proteolytic attack, for activation of or by other MMPs and for regulation of other cell surface proteins. Dislodging MMPs by treatment with compounds such as heparin might be beneficial in attenuating excessive tissue breakdown such as occurs in cancer metastasis, arthritis, and angiogenesis.

Estradiol induces E-cadherin degradation in mouse uterine epithelium during the estrous cycle and early pregnancy

Mouse uterine epithelium is a tissue that undergoes cyclic endocrine-regulated cell dissociation and regeneration. It shows a dramatic cell loss following normal estrus. If pregnancy ensues, cell loss is averted during the first 2.5-3.5 days. However, this is followed by a precipitous loss of basal-lateral cell adhesion and apoptosis in preparation for blastocyst invasion. By comparing epithelia isolated by protease treatment, we show that a reduction of lateral cell adhesion is a primary event in these instances of normal tissue loss. It was readily induced in ovariectomized adult and immature mice by injections of estradiol (E2), and to some extent also by progesterone (P4). The reduction of lateral adhesion induced by including ethylene glycol-bis (beta-aminoethyl ether) -N,N,N',N'-tetraacetic acid (EGTA) in the isolation medium mimicked and was additive to the effect of E2 injection. However, the E2 effect was different in not being prevented by adding Ca2+. The E2 effect also was mimicked by the action on isolated epithelium of monoclonal antibody against the calcium-dependent cell adhesion molecule, E-cadherin, suggesting that inactivation of E-cadherin was induced by E2. In detergent extracts of estrous and metestrous epithelium there was an increase in 80-kDa extracellular domain of E-cadherin relative to the intact 120-kDa molecule. The loss of adhesion between 3.5 and 4.5 days of pregnancy was associated with a loss of both intact membrane-associated 120-kDa E-cadherin and cleavage products. Cleavage of 80-kDa E-cadherin was uniquely induced by E2 in ovariectomized adult and immature mice; P4 was without effect. The cleavage of E-cadherin correlated with increased basal accumulation of E-cadherin antigen in estrous and E2-injected mice and a loss of both basal and lateral antigen at 4.5 days of pregnancy. Only the E-cadherin antigen within junctional complexes appeared unaffected. The data are consistent with the hypothesis that the cyclic and pregnancy-dependent disruption of uterine epithelial integrity are promoted by E2-dependent modification of E-cadherin, including its extracellular cleavage.

Estrous cycle-related alterations in the expression of glycoconjugates and lectins in the mouse endometrium shown histochemically

Alterations of the hormonal status may influence diverse cell features relevant to intra- and intercellular communication. We studied histochemically the expression of glycans and endogenous sugar-binding proteins (endolectins) in the mouse endometrium during the estrous cycle. The avidin-biotin-peroxidase complex (ABC) technique was used on paraffin sections with a panel of biotinylated lectins and neoglycoproteins. Stage-specific changes were observed with Galanthus nivalis agglutinin (GNA), Ulex europaeus-I agglutinin (UEA-1), Sambucus nigra agglutinin (SNA), Ricinus communis agglutinin (RCA-I) and Glycine max agglutinin (SBA) in both the luminal and the glandular epithelium, but to a lesser extent in the latter. The stromal constituents also exhibited modifications in lectin-binding profiles in cyclic mice. In addition to the analysis of cell glycans endogenous carbohydrate-binding sites were studied using synthetic probes, biotinylated neoglycoproteins. These tools made it possible to conclude that hormonal status can affect the expression of endolectins. We have attempted to correlate the glycohistochemical findings with recently published data obtained biochemically. The results indicate that phase-specific alterations in lectin-binding glycoproteins and endolectins occur in the endometrium of mice associated with their short reproductive cycle.

Specific stimulation of basal lamina heparan sulfate proteoglycan in mouse uterine epithelium by Matrigel and by transforming growth factor-beta 1

The basal lamina of differentiated epithelium normally turns over only slowly unless stimulated by tissue repair and growth. We show here that one mechanism of this stimulation, as modeled by basal lamina proteoglycan synthesis, may be the release of basal lamina-bound transforming growth factor (TGF-beta). A large heparan sulfate proteoglycan (HSPG, 0.2 Kav on Sepharose CL-4B) that was extractable from mouse uterine epithelium with 4 M guanidine-HCl or 1 M KCl was recognized by a specific monoclonal antibody to the basal lamina HSPG, perlecan. This HSPG was metabolically inactive with respect to [35S]-sulfate labeling in pieces of whole uterus during 4 h of culture, but it was labeled in isolated cells under the same conditions, provided that the cells had been cultured at least 6 to 12 h before labeling. The rate of labeling was then constant during at least 4 days in culture in serum-containing medium. Cultures on Matrigel showed an enhanced [35S]-sulfate labeling specifically in the 0.2 Kav HSPG fraction. Partial stimulation was obtained with a serum-free medium extract of Matrigel, which fractionated on Sephadex G-50 in two components; a major one > 30 kDa and the other at about 15 to 25 kDa. The specific stimulation was mimicked by the addition of 10 ng/ml of TGF-beta 1, but there was no specific stimulation by basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), insulinlike growth factor-1 (IGF-1), or interleukin-1 (IL-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of progesterone on hydrophobic cell-associated proteoglycans bound to cholesterol sulfate in glandular epithelial cells of guinea-pig endometrium

Sulfate incorporation was studied in subcultured glandular epithelial cells of guinea-pig endometrium untreated or treated with 10(-8) M 17 beta-estradiol alone or associated with various concentrations of progesterone. In the cells treated with progesterone in association with 17 beta-estradiol, the maximum of the 35S-labelled cell-associated macromolecules failed to bind with an anion-exchange resin (53% of total radioactivity) and had a hydrophobic character. This fraction was separated as an aggregate when the cells were extracted with 4 M guanidine-HCl, and separated as a single component in the presence of Triton X-100, suggesting that it aggregates with cellular lipid. The guanidine-extracted material contained 23.5% proteoglycans. However, the bulk of the radioactivity was in the sulfated lipids (68-75%), essentially represented by cholesterol sulfate. In the progesterone-treated cells, the amount of cholesterol sulfate was significantly higher than in 17 beta-estradiol-treated or untreated cells (1.35-1.5-fold). Thus, the effect of progesterone is located on a lipophilic proteoglycan associated with cholesterol sulfate. These results are discussed in relation to the preparation of the endometrium for embryo implantation.

Proteoglycans and the modulation of cell adhesion by steric exclusion

The hypothesis that cell aggregation may be driven by linear polymers in the matrix, particularly glycosaminoglycans, is revisited in light of more recent evidence. A model is proposed that extends the concept of steric exclusion to include a role in determining the directionality of cell migration and neurite extension. Recent literature is reviewed to support the conclusion that in living tissues the theoretical conditions for driving aggregation and migration by steric exclusion are met. The ability of a linear polymer to exclude cells is a function of its viscosity, which is optimum with glycosaminoglycans similar to chondroitin sulfate. It is ineffective with low viscosity glycosaminoglycans such as most heparin or heparan sulfate. Hyaluronic acid, a massive polymer, excludes cells poorly when present as an open matrix gel but forms an effective exclusion barrier when attached to the cell surface. According to a model for steric exclusion in organogenesis, when cells have a glycocalyx of linear polymer, they should disperse and migrate down a viscosity gradient of excluding matrix polymer; when they shed or internalize their surface coat in the continued presence of matrix, they should be excluded into a smaller volume and thus stimulated to aggregate.