Analysis of human, sheep and mouse ovarian surface epithelial and mouse testicular cell smears: Haematoxylin and Eosin stained smears of human (A), sheep (B) and mice (D) ovary surface epithelial (OSE) cell smears. Human OSE smear image is from our earlier publication . Cysts implying stem cell clusters are clearly evident in ovaries of all species. In mice, number of cysts was markedly increased after PMSG treatment (D inset) compared to normal ovary (D). Immunostaining with anti-PCNA antibody of sheep OSE smears shows that the cysts are positive for PCNA (C) suggesting their proliferative state. H & E stained smears of busulphan (25 mg/Kg) treated testis (E). Due to the treatment, all germ cells are completely depleted however; large Sertoli cells with abundant cytoplasm and small, spherical stem cells are clearly visualized in the smears (E). The somatic epithelial cells in ovary (A, B, D) and Sertoli cells in testicular (E) smears have abundant cytoplasm and pale stained nuclei whereas the stem cells are spherical in shape, with high nucleo-cytoplasmic ratio and are easily visualized by H & E staining as two distinct populations comprising smaller VSELs and slightly larger progenitors which are ovary germ stem cells (OGSCs) in the ovary and spermatogonial stem cells (SSCs) in the testis. The smaller cells marked by arrow in A-D indicate VSELs whereas the dividing slightly bigger progenitors including cysts are marked by asterisks. Confocal imaging of OCT-4 (F inset) and SSEA-4 (F) in the cysts in sheep OSE smears clearly showing presence of cytoplasmic continuity amongst the dividing stem cells. A-E represents composites prepared by capturing 4-5 fields to show the stem cells and somatic cells together in a small field. Stem cells are not very frequent as shown here. Scale bar represents 20 μm (A-E) and 10 μm (F).