(A) Ovarian tissue sections obtained from Dicer^+/+ and Dicer^d/d mice on day 1.5 of pregnancy were stained with H&E (original magnification, ×100). CLs in Dicer^+/+ and Dicer^d/d mice ovaries is denoted by the boxes and are shown at higher original magnification (×400, bottom). The cells had less cytoplasm and less vascularity in the CLs of Dicer^d/d mice. (B) Immunofluorescence analyses of ovaries from Dicer^d/d and Dicer^+/+ mice on day 1.5 of pregnancy show the localization of type IV collagen — a marker of the basal lamina of endothelial cells. Close examination of the vascular network in the CLs denoted by the boxes (top) is shown at higher magnification (×400; bottom). Dicer^+/+ CLs showed more filamentous and punctuated collagen IV staining, which indicated higher vessel densities than those in Dicer^d/d mice. (C) PECAM (CD31) immunofluorescence staining of CLs in Dicer^d/d and Dicer^+/+ mice on day 1.5 of pregnancy. The number and the length of the vessels were greater and longer in Dicer^+/+ CLs. Original magnification, ×400. (D) Cumulative vessel length in CLs was determined as the average of the number of vessels per 100 × 100 μm² multiplied by the vessel length in 3 random fields. The results of type IV collagen immunofluorescence staining from 4 different Dicer^+/+ and Dicer^d/d pairs were used. The data are expressed as mean ± SD. *P = 0.0012.