Number of germ cells and somatic cells in human fetal ovaries during the first weeks after sex differentiation

=BACKGROUND: This study presents the number of germ cells and somatic cells in human fetal ovaries during week 6 to week 9 post conception, i. e. the first weeks following sex differentiation of the gonads. METHODS: One ovary with attached mesonephros from each of 11 individual legal abortions was used for estimation of cell numbers. After recovery of the fetus, the ovary - mesonephric complexes were immediately isolated, fixed and processed for histology. A stereological method was utilized to estimate the total number of oogonia in all ovaries and somatic cells in seven of them. RESULTS: The number of oogonia per ovary increased from similar to 26 000 in week 6 to similar to 250 000 in week 9 and somatic cells from similar to 240 000 to similar to 1.4 x 10(6). The ratio of oogonia to somatic cells tended to increase throughout the period. The concentration of oogonia was similar in the cranial ( mesonephric connected) part and the caudal part of the ovaries. CONCLUSIONS: This is the first stereological estimation of the number of oogonia and somatic cells in human fetal ovaries, and the first estimation of germ cells and somatic cells in ovaries aged < 9 weeks. The number of oogonia in week 9 is comparable to the numbers previously published based on non-stereological estimations. We found early stages of meiosis in fetal ovaries from week 9.