A quantitative description of components of in vitro morphometric change in the rat osteoclast model: relationships with cellular function

Abstract

We describe the in vitro morphometric changes shown by rat osteoclasts that accompany their functional responses to the application of a range of regulatory agents of known physiological importance. We introduce a cellular motility parameter, µ, which was defined through a quantification of retraction-protrusion behaviour. This was used in conjunction with a net cell retraction, ϱ, which is derived from the change in total cell area following the application of an agent. These terms were used together for the description of cellular motility changes in response to specific cellular regulatory agents. The definition of retraction-protrusion was normalised against control cell area, to give a dimension-less variable independent of the net cell retraction. Thus, mutual terms present in either descriptor cancelled when the complementary parameter was held constant. Furthermore, the descriptor, µ remained time-invariant for extended intervals (around 20 min) even when ϱ was varying following cell introduction into culture. Interventions also with substances known to modify osteoclast function, were capable of altering each descriptor, to different extents. Thus elevation of the extracellular Ca²⁺ concentration ([Ca²⁺]_e) at the osteoclast calcium “receptor” altered ϱ without changes in µ. In contrast, the polypeptide amylin (250 nM), within 20 minutes of application, elicited a marked change in µ, but only a relatively small change in ϱ. Finally, human calcitonin treatment (300 pM) influenced both descriptors. When combined together, these morphometric findings accordingly offer complementary descriptions of visible cellular changes in response to added agents of physiological relevance. Such an approach may be useful in the analysis of structure-function relationships in osteoclasts or other cell systems particularly in correlations between quantitative structure and functional responsiveness.