Pulmonary arteries of the normal rat: the thick walled oblique muscle segment.

The structure and certain metrical features of the pulmonary arterial tree in the normal rat have been studied in detail by both light and electron microscopy after its distension by injection. In the parts of the tree where the muscle wall is complete a segment with oblique muscle has been identified. The wall of this segment is thick, and is composed largely of obliquely orientated, closely packed smooth muscle cells with little intervening connective tissue, while it lacks a complete external elastic lamina. Both the axial pathway and its side branches have such oblique segments. Proximal and distal to an oblique segment there are transitional regions where obliquely arranged muscle fibres spiral external to the circularly arranged fibres usually described in muscular arteries. The existence of thick walled oblique muscle segments in the pulmonary arterial tree of the normal rat is important since this animal is frequently used in experimental studies, and the thick segments could be taken as evidence of induced hypertrophy.