Growth and sexual differentiation in the gonads of chick and duck embryos.

The sex-dependent morphological asymmetry between gonads of bird embryos is generally considered to represent the critical point at which the definitive pattern of sexual differentiation is established. A quantitative study of growth in gonads of chick embryos (6to 11-day-old) and of duck embryos (7to 11-day-old) has been carried out in order to clarify in what manner growth of gonads is involved in sexual differentiation. Growth has been estimated by the determination of total protein and DNA content, and of DNA synthesis [H]Tdr incorporation in vitro). Increase of protein and DNA content follow different growth curves according to the sex and the laterality of gonads. As early as day 6 (chick) or day 7 (duck) significant differences of protein content exist between gonads: left > right in both sex embryos, and male > female on both sides. Differences in DNA content of gonads are large even at the earliest stages. In the 6-day-old chick embryo both testes contain less DNA than the left ovary and more DNA than the right ovary. In the 7-day-old duck embryo both testes contain the same amount of DNA as the left ovary and twice as much DNA as the right ovary. Consequently, the protein/ DNA ratio has a different value according to the sex of the gonad. In the 6-day-old chick embryo and in the 7-day-old duck embryo absolute values of DNA synthesis (cpm/gonad) are higher in left gonads than in right gonads, and higher in male than in female gonads. When calculated as a ratio to the protein content (cpm/protein) DNA synthesis is lower in the left ovary than in the three other types of gonads, both in chick and duck embryos. When calculated as a ratio to the DNA content of gonads (cpm/DNA), DNA synthesis is lower in the left ovary than in the three other gonads in the chick, and lower in the left ovary and in testes than in the right ovary in the duck. The results show that growth of gonads is sex-dependent at a very early stage. The meaning of this sex character is discussed with special reference to the role of steroid hormone secretion.