FORMATION AND BREAKDOWN OF COLLAGEN AND ELASTIN IN THE HUMAN UTERUS DURING PREGNANCY AND POST-PARTUM INVOLUTION

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Abstract
During pregnancy the human uterus increases at least 11-fold in wet weight, 7-fold in collagen and 5- to 6-fold in elastin content. After parturition there is a rapid involution which is 75% complete by 8-11 days post partum. Collagen and elastin undergo rapid breakdown during this process. When involution is complete, the uterus has not returned to its original condition but is enlarged and contains more collagen and elastin. Successive pregnancies produce further enlargement of the uterus. In non-gravid uteri that have undergone 6 or more preg-nancies, collagen and wet weight are double, and elastin is 5 times, the values found for nulliparous uteri. It was not possible to demonstrate any unusual solubility or digestibility properties of uterine collagen which might explain its rapid breakdown. A cathepsin activity that can digest collagen in vitro at acid pH was shown to be distributed through-out the uterine wall in non-gravid, gravid and involuting uteri. This catheptic activity is depressed in pregnancy and elevated during involution. However, the magnitude of these changes cannot in itself account for the vast changes of uterine protein metabolism in pregnancy and puerperium.