Expression of MECP2 during development and terminal cell differentiation. (A) The onset of MECP2 expression (green) in different cell types of mouse retina. Time lines are shown for pigment epithelial cells (PEC), ganglion cells (GC), amacrine cells (AC), horizontal cells (HC), bipolar cells (BC), cone photoreceptor (CP), and rod photoreceptor (RP). On the left, postnatal age points are shown; numbers below the time lines show the cell birthdays (the day of the last cell division;). Grey horizontal lines mark age points when the outer and inner plexiform layers (OPL and IPL, respectively) become detectable (see also[57–59]). Light green marks a low MECP2 level. The onset of MECP2 expression in neurons coincides with massive formation of synapses and, consequently, IPL and OPL plexi. (B) Arrangement of the nuclear and plexiform layers in mouse retina revealed in a paraffin section after hemalaun-eosin staining and in a cryosection after nuclear counterstain with DAPI. The perikarya of GCs are located in the GCL; those of BCs, ACs, and HCs are in the INL; and those of the photoreceptors are in the ONL. (C) Examples of retinal cells (marked by arrows) with initiated MECP2 expression at three age stages. Single and double asterisks mark OPL and IPL, respectively; the abbreviations are the same as in (A). For comparison with adult mouse retina, see Figure 1A. (D) In the fibroblasts of the dermal papilla (arrowheads) of the hair follicle, MECP2 expression is initiated postnatally and becomes detectable at P2; later, the MECP2 expression in these cells remains stably high (see also Figure 4A,D). (C, D) Single confocal sections. Scale bars: (B) 10 μm; (C) overviews 50 μm, close-ups 10 μm; (D) 25 μm.