MC recruitment in postnatal development of the retinal vasculature. (a and b) Counterstaining of P5 retina with hematoxylin and PECAM (a) or PDGFR-β (b). In the retina, PDGFR-β is expressed only around developing vessels (arrowheads) arising from the optic nerve (ON). (c–n) Double immunofluorescent staining using PECAM, PDGFR-β, desmin, and SMA. From the beginning of vascular development, PDGFR-β is expressed around the entire retinal vascular network (c), except for sprouting tips (arrowheads in d). PDGFR-β is expressed in MCs located in the outer linings of endothelial surfaces (e). Desmin is expressed in all types of retinal vessels (f), including the growing vascular edges (g), with filamentous patterns in PDGFR-β⁺ MCs (arrowheads in h). SMA expression over the primary vascular plexus (i and j) is maintained in arterial walls and fades in venous and capillary walls as vascular remodeling proceeds (k). Finally, SMA expression in MCs is confined to arteries (l and m) and newly formed vessels in the advancing regions (n). (o–q) Scanning electron microscopy demonstrating morphological heterogeneity of MCs in accordance with the types of vessels: multiconcentric MCs around arteries (o), single-layer MCs around veins (p), and capillary pericytes with long processes oriented longitudinally along the axis of vessel walls (arrowhead in q).