Activation of nuclear chromatin in stretch-dependent growth of tissues.

The growth of connective tissue cells can be controlled both by mechanical extension and by diffusion gradients. We have used nuclear fluorescence with acridine orange (AO) as a measure of nuclear activation, to study growth control in vivo in rat mesometrium and mouse spine. Frozen sections of spinal muscle taken from animals after two h exercise, revealed nuclear activation in peripheral cells. The number of active muscle nuclei decreased drastically with age. During mesometrial growth in pregnancy, AO fluorescence showed that activated cell nuclei occurred mainly near the capillaries. The width of the zone of activated nuclei was within 20 microns from the capillary walls. AO appears to be a sensitive stain for tracing gradients of growth in intact connective tissue.