This study was conducted to evaluate the ontogeny and cellular site of interstitial collagen formation in amnion, the tissue that provides the tensile strength of the human fetal membranes. The levels of procollagen alpha 1(I), alpha 2(I), and alpha 1(III) subunit mRNAs and the specific activities of the enzymes prolyl 4-hydroxylase and lysyl hydroxylase were greatest in human amnion tissue early in gestation, decreasing rather abruptly after 12-14 wk gestation to a nadir that persisted to term. To evaluate these findings further, amnion epithelial and mesenchymal cells were separated by differential proteinase treatment. The freshly separated cells as well as epithelial and mesenchymal cells maintained in culture were evaluated to assess the cellular site of interstitial collagen synthesis. By Northern analysis of total RNA, large amounts of procollagens alpha 1(I), alpha 2(I), and alpha 1(III) mRNAs were found in mesenchymal cells (freshly separated and in culture), but only negligible amounts of these transcripts were detected in RNA of freshly separated or cultured epithelial cells. The level of prolyl 4-hydroxylase alpha-subunit mRNA in mesenchymal cells was somewhat greater than that in epithelial cells. Radiolabeled collagen I synthesis from [3H]proline and [3H]glycine was not detected in amnion epithelial cells, whereas [3H]collagen alpha 1(I) and alpha 2(I) subunits were synthesized in large amounts by mesenchymal cells. A very small amount of [3H]collagen alpha 1(III) was synthesized in epithelial cells, but large amounts were formed in mesenchymal cells. These findings indicate that the mesenchymal cells are the site of interstitial collagen synthesis and processing in amnion. The density of mesenchymal cells in human amnion declines after the first trimester of pregnancy, and a significant decline in DNA content per unit of amnion tissue dry weight as pregnancy progressed was demonstrated in this study. Since the epithelial cells form an uninterrupted epithelium throughout gestation, the decline in DNA content probably corresponds to the decrease in the density of mesenchymal cells that commences after the first trimester pregnancy. Thus, the increase in the ratio of epithelial to mesenchymal cells as a function of gestational age may explain, at least in part, the decline in the levels of collagen mRNAs and the specific activities of lysyl and prolyl 4-hydroxylase in amnion tissue during human pregnancy.