LIN28 expression is germ cell specific and becomes restricted to a subpopulation of germ cells with increasing gestation. At 9 wga (A), all germ cells in the gonad (G) express LIN28 (brown staining) in both the nucleus and cytoplasm. Somatic cells in the gonad (G) and mesonephros (M) display no staining. By 14 wga (B), LIN28-positive cells are concentrated at the periphery of the ovary, although some LIN28-positive germ cells are distributed toward the center of the ovary. At 18 wga (C), LIN28-positive germ cells occupy a thin margin at the edge of the ovary; no staining is detected in more mature germ cells toward the center of the ovary. (D) Magnified image of area identified in (C); meiotic germ cells (identified by their characteristic chromatin arrangement) are LIN28 negative (arrowheads). Somatic cells (SC) in streams and interspersed between LIN28-positive germ cells are also immunonegative. (E) No staining is detected in negative controls. Scale bars in A, B, C, and E: 50 μm; D: 20 μm.