Ultrastructure of the tracheal muscle in developing, adult and ageing guinea-pigs.

The musculature of the trachea of adult guinea-pigs is ultrastructurally similar to other visceral muscles. However, tracheal muscle cells have irregular outlines, large accumulations of glycogen, a small number of gap junctions, and many small elastic fibres and collagen fibrils (50 nm in diameter). Fibroblasts, mast cells, Schwann cells and axons are found within the muscle, but no interstitial cells. Capillaries run in the connective tissue septa. The tracheal muscle is well differentiated at the end of the fetal life. At this time, muscle cells have an appearance similar to that of a mature muscle. However, the cells are smaller, especially in length, and their orientation is less regular. There are no undifferentiated cells; a few muscle cells are seen in mitosis. There is a limited amount of stroma, mainly small collagen fibrils not exceeding 30 nm in diameter. Within the muscle there are many nerves, but no fibroblasts or mast cells. Many structural contacts link together the muscle cells, but there are no gap junctions. Gap junctions develop in the first few days after birth and are distinct in 5-day-old animals. In ageing guinea-pigs (30-36 months old) the muscle cells are slightly larger than in the young adult. They display deep invaginations of the cell membrane and a very irregular profile. There is an increase in the number of glycogen granule clusters, and the basal lamina is particularly prominent. The amount of stroma has increased, and is made of large elastic fibres and collagen fibrils up to 105 nm in diameter.