Development of the early human ovary and role of the mesonephros in the differentiation of the cortex.

The morphogenesis of the primary gonadal differentiation, of the sexual differentiation and the growth of the ovarian cortex during its early period have been studied on 10 human embryos between 12 and 95 mm CR-length. Semithin sections of glutaraldehyde-OsO4-fixed and plastic-embedded material were used to demonstrate the structural events on a cellular level. The primary gonadal blastema within the genital ridge is formed by two types of somatic cells; cells segregated from the mesonephros and cells of the proliferating coelomic epithelium. The two types of cells show a tendency to intermingle and they enclose the immigrating primordial germ cells. In the female gonad the indifferent period terminates between day 40 and 42 of ovulation age (20 to 23 mm CR-length). Between day 40 and 50 the blastemal content of the indifferent gonad is remodelled and an ovarian cortex differentiates. Cellular strands extending from the primary blastema and strands from the superficial blastemal layer contribute to the formation of the cortex. Within the newly formed medulla, remnants of the disintegrating primary blastema differentiate into medullary cords. Cells of mesonephric origin which invade the growing cortex via the rete blastema interact with cells deriving from the superficial epithelium, and both exert their opposite influence on the germ cells. While female sexual differentiation is characterized by failure of the dark mesonephric cells to completely penetrate the gonadal blastema, the morphogenetic process resulting in the formation of the ovarian cortex shows a strong invasion of the cortex by the dark mesonephric cells. Dark cells advance at the most superficial layer of the cortex and increase in number at the deeper level of the cortex. Onset of oogonial proliferation and meiotic prophase seems to depend on the numerical proportion between the activating dark and the inhibiting light supporting cells.