Nevertheless, it is far from clear to what extent oxygen demand or accumulation of waste products regu lates capillary growth.There are numerous accounts of capillary growth in developing heart or skeletal muscles and some concern ing growth under pathological conditions such as regen eration or hypertrophy, but relatively little is known about growth in adult tissue.This is to be expected, since endothelial cells, not only in capillaries but also in large arteries, represent a very stable population of cells with very low mitotic activity. Capillary growth in adult skeletal or cardiac muscle has been described dur ing endurance training; in skeletal muscle during long term electrical stimulation; in chronically bradycardia lly paced hearts; during long term administration of vasodilating drugs as well as during hypoxia, although this has recently been dis puted muscle of animals treated with thyroid hormones.De spite this mass of data it is still unclear what induces development and adaptation of the capillary bed, which differs within one muscle with respect to different types. This review attempts to summarize some conditions under which capillary growth occurs and to discuss the most likely factors involved in capillary growth. It also supplements more extensive reviews on angiogene sis. Clearly, if capillary growth is to be assessed, these in dexes must be based on the estimation of all anatomi cally present capillaries, using either histochemical staining for various enzymes present in the capillary endothelium, staining of components of base ment membrane, use of various lectins that are bound to the endothelial cell glycoproteins, or direct counts from electron micrographs.Stereological analy sis has contributed significantly to the appraisal of the size of the capillary bed in various tissues and under different conditions, particularly in the last de cade, and this Fluphenazine hydrochloride approach is becoming increasingly impor tant.It can describe the maximal capacity of a system, providing variations in orientation that underlie the apparent order of tissue components are also assessed.In addition, there are cross connections, or Naftifine hydrochloride anastomoses, between adjacent capillar ies.Although both these factors are clearly important when defining physical limits to the capillary network and its connectivity, incorporation into indexes of capil lary supply has caused some confusion.The functional implication of this tortuosity is clear: the effective capil lary length and surface area per unit volume of tissue are greater than those estimated from simple counts in transverse sections.The length density, which varies according to the angle of section ing relative to the longitudinal axis of muscle fibers.If an assumption is made about the distribution of capil lary orientation around the fiber axis, a rather simple method has been devised to overcome this problem, whereby a dimensionless concentration parameter, K, may be used to denote the degree to which capillaries follow a preferred axis of orientation.The ratio of CD from two sections at to each other may be used to calculate this parameter.Capillary length density is probably the fundamental parameter that best de scribes the extent of capillary supply, but it can also be used to derive two indexes of more obvious value, capil lary volume is the mean capillary crosssectional area andb is the mean capillary circumference.