Uterine collagen during involution in cattle.

Post-partum involution of the bovine uterus was assessed by clinical examination and monitored biochemically by measuring the total urinary excretion of hydroxyproline and the collagen cross-link, pyridinoline. Uterine tissue contained 0.13 +/- 0.04 residues of pyridinoline per mole of collagen and negligible (less than 0.005 residues/mole) deoxypyridinoline. The pyridinoline/creatinine ratio was found to be a clearer and more reliable indicator of uterine resorption than the hydroxyproline/creatinine ratio. The amount of collagen cross-link excreted above baseline was equivalent to the amount of cross-link estimated to have been accumulated during pregnancy. Maximal excretion of pyridinoline occurred at day 6 and returned to baseline values in 2-3 weeks after parturition. An electron microscope study during the period of peak resorption revealed macrophage-like cells apparently engulfing or containing collagenous material, indicating phagocytic removal of the connective tissue.