P4.2 gene expression in the mouse spleen and bone marrow. Shown are in situ hybridization analysis of serial spleen sections from a 15-day-old mouse (A through D) using the P4.2 antisense riboprobe. A representative region of red pulp (RP) or white pulp (WP) in the spleen is indicated in (A) and (B). Higher-power views of a red pulp region are shown in (C) and (D). The lightly stained areas (labeled with “E”) enrich more mature erythroid cells and show substantial P4.2 hybridization. Arrowheads indicate isolated megakaryocytes. Sections containing bone marrows from a 7-day-old mouse (E and F) were also hybridized with the P4.2 antisense riboprobe. A representative erythroid colony in the bone marrow is labeled with “E.” Micrographs were taken with a 20× (A and B) or 40× (C through F) objective. (A, C, and E) Bright-field illumination; (B, D, and F) dark-field illumination.