Figure 6.

Schematic showing the graded structural changes in the media of the pulmonary arterial wall, moving from muscular arteries to precapillary non-muscular vessels. The top panel illustrates the gradual loss of musculature. The middle panel shows the transition from vascular smooth muscle cells (M) to intermediate cells (I) and finally to pericytes (P). The bottom panel represents cross-sections, highlighting the partial circumferential coverage of the media. From Reid et al. (252), with permission.