Micromechanical Analysis of the Hyaluronan-Rich Matrix Surrounding the Oocyte Reveals a Uniquely Soft and Elastic Composition

Abstract

The cumulus cell-oocyte complex (COC) matrix is an extended coat that forms around the oocyte a few hours before ovulation and plays vital roles in oocyte biology. Here, we analyzed the micromechanical response of mouse COC matrix by colloidal-probe atomic force microscopy. We found that the COC matrix is elastic insofar as it does not flow and its original shape is restored after force release. At the same time, the COC matrix is extremely soft. Specifically, the most compliant parts of in vivo and in vitro expanded COC matrices yielded Young's modulus values of 0.5 ± 0.1 Pa and 1.6 ± 0.3 Pa, respectively, suggesting both high porosity and a large mesh size (≥100 nm). In addition, the elastic modulus increased progressively with indentation. Furthermore, using optical microscopy to correlate these mechanical properties with ultrastructure, we discovered that the COC is surrounded by a thick matrix shell that is essentially devoid of cumulus cells and is enhanced upon COC expansion in vivo. We propose that the pronounced nonlinear elastic behavior of the COC matrix is a consequence of structural heterogeneity and serves important functions in biological processes such as oocyte transport in the oviduct and sperm penetration.

Glycosaminoglycans in extracellular matrix organisation: are concepts from soft matter physics key to understanding the formation of perineuronal nets?

“…A striking in vivo example of a soft GAG-rich matrix is the expansion of the cumulus cell-oocyte complex (COC) prior to ovulation. We have recently reported Young's modulus values below 1 Pa for the COC matrix [22], which to our knowledge makes it the softest elastic biological material known.…”

Heavy chain-1 of inter-α-inhibitor has an integrin-like structure with immune regulatory activities

“…However, it seems reasonable to propose that low affinity binding between HCs could mediate the aggregation of HC•HAs seen in synovial fluids from rheumatoid arthritis patients (Yingsung et al, 2003) and that this, combined with more stable interactions between HCs and PTX3 (Baranova et al, 2014), underpin the formation and crosslinking of the cumulus extracellular matrix during COC expansion. Furthermore, dynamic HC-HC interactions could make an important contribution to the mechanical properties of tissues; for example, they might explain the elasticity and extreme softest of the cumulus matrix (Chen et al, 2016). Certainly HC•HAs have different compositions of heavy chains in different tissue contexts (Day and Milner, 2019) and it seems likely that this will engender distinct hydrodynamic and functional properties.…”

“…This is due to the impaired formation of the cumulus extracellular matrix that normally drives the expansion of the cumulus-oocyte-complex (COC). This elastic matrix (Chen et al, 2016) protects the oocyte during the expulsion of the COC from the follicle and also provides a large surface area facilitating sperm capture in vivo (Nagyova, 2015; Russell and Salustri, 2006). The cumulus matrix is rich in the non-sulphated GAG hyaluronan (HA), where this high molecular weight polysaccharide becomes modified by the covalent attachment of HC1, HC2 and HC3 (Mukhopadhyay et al, 2001).…”

“…Either deletion of PTX3, or loss/impairment of TSG-6’s HC transferase activity in mice, also leads to the failure of COC expansion and, hence, infertility (Briggs et al, 2015; Fülöp et al, 2003; Ochsner et al, 2003; Salustri et al, 2004). Here the cooperation between HA, IαI, PTX3 and TSG-6 (Baranova et al, 2014) leads to the formation of an elastic tissue, which is the softest described to date with a Young’s modulus on the order of 1Pa (Chen et al, 2016).…”

Inter-α-inhibitor heavy chain-1 has an integrin-like 3D structure mediating immune regulatory activities and matrix stabilization during ovulation

“…This is due to the impaired formation of the cumulus extracellular matrix that normally drives the expansion of the cumulus-oocyte-complex (COC). This elastic matrix (4) protects the oocyte during the expulsion of the COC from the ovarian follicle and also provides a large surface area facilitating sperm capture in vivo (5,6). The cumulus matrix is rich in the non-sulphated glycosaminoglycan hyaluronan (HA), where this high molecular weight polysaccharide becomes modified by the covalent attachment of HC1 and HC2 from IαI and HC3 from PαI (7).…”