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Somatic development in premature mice from birth to weaning.

This paper reports a statistical and transversal study of growth in about 2400 surviving premature mice during neonatal development. The mice were removed by caesarean section on day 19 of pregnancy and resuscitated for 30 min; surviving mice as well as newborns delivered vaginally on day 20 (control mice) were raised from birth to day 20 by nursing mothers spontaneously delivered at the same time. Only litters from nursing dams which suckled newborns regularly were used in the statistical study. Somatic development in the premature mice and newborn controls was estimated by changes in body weight and in the weight of several organs (liver, kidney, adrenal glands) after the newborns had been killed at five developmental stages: 30 min after reanimation or parturition, 6 h after birth and at 9 h on days 2, 6 and 20. Growth was analysed by growth curves, percentage of weight gain per day, regression lines and allometric lines; weight gain was calculated as a function of the number of newborns per litter. The low birth weight of live premature mice affected body weight and kidney and adrenal weights. After an early, transitory loss of body and organ weights, which occurred d ring the first neonatal day, the weight handicaps of premature mice were overcome before neonatal day 20 as follows: adrenal weight between 6 h and day 2, liver weight between days 2 and 6, kidney and body weights between days 6 and 20. Throughout these stages, the weight gain of body and organs was higher than that of full-term newborns. The weight gains of premature mice, like those of full-term newborns, were in inverse ratio to the number of newborns per litter. The role of some factors involved in the growth of organs of premature mice has been discussed and the somatic development of surviving premature mice has been compared to that of smallest premature human infants.