Studies of compensatory growth in sheep

Abstract

SUMMARYA number of observations have been made on 5- to 6-month old wether Hampshire heep undergoing uninterrupted growth (36·2% gain in shrunk wool-free live weight over 58 days), undernutrition (a 21·7% fall in body weight over 27 days) and then followed by compensatory growth (a gain of 62·2% in live weight over 52 days).Sheep being underfed had increased, and compensating sheep had decreased, apparent digestibility coefficients of feed dry matter compared to control sheep. Sheep undergoing compensatory growth drank more water, ingested more food per unit body weight, laid down less body fat, more protein and retained more water in their bodies than did control sheep. Tritiated water allowed estimation of total body water with an accuracy comparable to other reports in the literature.Changes in the weight and in RNA-P and DNA-P content of anterior pituitary, thyroid and adrenal glands were measured. Sheep thyroid glands were examined histologically to assess relative TSH outputs and no significant differences detected.Rat bio-assay of cavernous sinus plasma for somatotrophin and ACTH activity was made on these sheep. Somatotrophin was detected, but experimental groups did not statistically significantly differ in relative potency (TJ.S.P. units/ml fresh plasma). Compensating sheep had. significantly lower plasma somatotrophin potency per unit empty body-weight values than did the underfed sheep.No ACTH activity was detected in the sheep plasma of the several groups.During severe undernutrition the anterior pituitary gland decreased in size, but not in cell numbers, and still elaborated somatotrophin. Decrease in body size tends to increase the ratio of circulating somatotrophin per unit body size and thus may facilitate, in a permissive manner, the enhanced growth rate of animals undergoing compensatory growth.Restitution and gain of body size by compensatory growth is accompanied by hypertrophy of the anterior pituitary gland. This gland, after compensatory growth, showed evidence of enhanced synthesizing capacity.