Effects of the co-culture of follicles and interstitial cells on follicular growth. (a, b) HE-stained images of follicles. Follicles were cultured for 5 days with (a) and without (b) interstitial cells, paraffin-sectioned, and HE-stained. The arrows in (a) show a follicular antrum-like cavity, while the arrowheads in (a) and (b) show cell layers in the outermost part of follicles during culture. O, oocyte; G, granulosa cells. Scale bars = 25 μm. (c) Changes in the size of follicles during culture. Follicle growth is expressed as follicle diameter relative to the size on day 0 (mean ± SE). The actual size of follicles co-cultured with interstitial cells is 123.6 ± 1.42 μm and without interstitial cells is 118.0 ± 2.33 μm. Black square show follicles co-cultured with interstitial cells (n = 84) and white square show follicles cultured without interstitial cells (n = 28). * significance (P < 0.025, Mann-Whitney U test).