Three-dimensional studies of Prop1-expressing cells in the rat pituitary just before birth

Recently, we demonstrated that differentiation was underway as early as embryonic day (E) 13.5 in the lateral region of the rat pituitary primordium. In this study, we analyze the heterogeneous property of cells in the pituitary at E21.5 (just before birth) leading to its biological function with the differentiation and expansion of tissue. The three-dimensional structure of the pituitary at E21.5 was built up from measurements taken from many DAPI-stained sections and cell populations positive to the stem/progenitor marker SOX2, pituitary-specific transcription factor PROP1 and paired-related homeodomain transcription factor PRX. At E21.5, the pituitary, composed of anterior and intermediate lobes, showed a flattened chestnut shape with dimensions of about 500 μm (dorsoventral axis) by 2500 μm (left-right axis) by 850 μm (rostrocaudal axis) and consisted in approximately 113,500, 16,000 and 14,800 cells in the anterior, intermediate and posterior lobes, respectively. Five cell types were observed expressing Sox2, Prop1 and Prx; these were heterogeneously distributed in the mediolateral and dorsoventral axes. In the anterior lobe, the marginal cell layer (MCL) was mostly occupied by stem/progenitor cells positive for SOX2, with the co-expression of Prop1 and/or Prx, whereas more SOX2-single-positive cells than those for PROP1 and PRX were scattered in the parenchyma. PRX-positive cells of mesenchymal origin invaded the parenchyma, together with PECAM- and NESTIN-positive cells, indicating the advance of vasculogenesis. Thus, marked developmental progress occurs regarding the transition of stem/progenitor cells in the MCL and regarding vasculogenesis in the parenchyma during the prenatal pituitary growth wave.