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Skeletal changes during catch-up growth - suggestions from a rat modelThe removal of a growth limiting influence, such as undernutrition, is frequently followed by a period of growth at a rate greater than normally expected for chronological age. This, so called 'catch-up growth', tends to return affected individuals to their original growth trajectory. How catch-up growth is controlled or regulated remains unclear. Histological studies of the proximal tibial growth plate in rats which had been undernourished by half-feeding between the 56th and 70th days (post partum) showed them to be thinner than those of controls. Upon inspection, the chondrocytes also appeared to be flatter in profile and less numerous. However, during the catch-up period, when food was again allowed ad libitum, previously undernourished rats showed wider growth plates with rounder and more numerous chondrocytes than controls. It was noted that such changes were similar to those that accompany sectioning of the periosteum and the release of growth restraint that results. A study of the flexibility of the sacro-iliac joint in the same animals suggested that it was less flexible following undernutrition but more so during the catch-up period, by comparison with controls. This was in contrast to a progressive loss of flexibility shown by controls during the same period and would appear to result from changes in the characteristics of the fibrous connective tissues associated with the joint. If this reflects a more generalized change in such connective tissues, but particularly the periosteum, ligaments and joint capsules, this may represent a means whereby skeletal growth rate during the catch-up period may be influenced in a co-ordinated manner.