Figure 9

Survey of follicular renewal in adult human ovaries. Follicular renewal in adult human ovaries is a two-step process based on mesenchymal-epithelial transition of tunica albuginea (ta) bipotent progenitor cells into OSC. A) Epithelial nests: Segments of the OSC directly associated with the upper ovarian cortex (uc) are overgrown with tunica albuginea, which forms a solid epithelial cord that fragments into small epithelial nests (en) descending into the lower ovarian cortex (lc) and associating with the blood vasculature. Initiation of this process may require cellular and other local signaling (CS & LS), possibly neural [87]. B) Germ cells: Under the influence of cellular signaling (CS) of ovary-committed MDC & T cells (OC-BMC) and hormonal signaling (HS), some OSC covering the tunica albuginea undergo asymmetric division and give rise to new germ cells (gc). The germ cells subsequently divide symmetrically and enter adjacent cortical blood vessels. During vascular transport, they are picked up by epithelial nests associated with vessels. D) The ovary-committed bone marrow cells originate from bone marrow (MDC) and from lymphoid tissues (T cells) carrying "ovarian" memory (om), which diminishes with utilization; when spent, the follicular renewal ceases, in spite of persisting hormonal signaling (Table 1).