The interface of cells and their matrices in mineralized tissues: a review

The interface between cells and matrices in mineralized tissues formed in vivo has been studied mainly by looking at the matrix surface, which is easily prepared, and not at the cell surface, which presents problems. Vertebrate calcified tissues range from being acellular to highly cellular, but for all the tissues the formative cells lay down and organise a cell-specific matrix, although this may be deposited initially on a different tissue-type. The formation of hard tissues is a group activity of many cells; resorption is the province of one cell, though it may be controlled by others in the vicinity. Cell-matrix interfaces that develop in vitro have also mainly been studied at the matrix side. The main difficulty with in vitro studies of hard tissue interfaces is that the cells do not have the same activity or even cellular functions as they had in vivo under the complex control of physiological regulation. The question of osteoblastic osteoclasis falls into this category. It is possible to provide new substrata for both formative and resorptive hard tissue cells to test for the interaction between the cells and the 'matrix' on to which they are seeded. The changing cell-matrix interface may also be modelled using computer simulation of osteoclastic movement across a substrate based on known patterns exhibited by other cell types in vitro. Comparison with the shapes of complex resorption pits shows a surprising match. This suggests that the track of the osteoclast due to cell motility and the bone resorptive mechanism resulting in pits along that track are likely to be separately controlled phenomena.