Tissue zone dependence and asymmetric distribution of membrane ruffles of in-situ bovine chondrocytes.

Abstract

Purpose: The surface of chondrocytes is covered with membrane ruffles. Chondrocytes sense mechanical signals through cell membrane stretching and unfolding, a process mediated by the pericellular matrix (PCM). PCM is known to be thicker on the bone-facing (bf) inferior side than the surface-facing (sf) superior side, a feature thought to minimize membrane strain. Previously, we studied chondrocyte membrane morphology and noticed that membrane ruffles of some cells are concentrated at specific surface regions. We hypothesized that PCM asymmetry drives this non-uniform ruffle distribution.

Methods: Fresh cartilage samples (n = 10) were harvested and histologically processed for electron microscopy. Cell membrane outline was tracked from micrographs. Cells were divided into bf inferior and sf superior halves and ruffle density was quantified as waviness and fractal dimension. The density and distribution of membrane ruffles were measured for chondrocytes in the superficial, middle, and deep zones of cartilage.

Results: We found that membrane ruffles almost double the surface area of chondrocytes. Ruffle density is tissue zone-dependent; deep zone cells have ~20% less ruffles than superficial and middle zone cells. Asymmetric membrane ruffles were observed for cells in the top 100 µm of the tissue; the extent of asymmetry decreased with tissue depth.

Conclusions: Chondrocyte membrane morphology is likely adapted to their mechanical and osmotic surroundings. Asymmetric membrane ruffles may be a multi-purpose strategy to increase membrane reservoir size and to deposit thick PCM layers for strain attenuation. This study highlights the importance of studying cells in their native environment to provide insights into in-situ cell mechanobiology.