Increase in fibre numbers of the rat pterygoid muscles during postnatal growth.

The total numbers of fibres in transverse histological sections of the pterygoid muscles of Lister rats of various ages were counted on enlarged photomicrographic prints. The number in the medial pterygoid of the male doubled between birth and six weeks. This was followed by a decrease of rather more than 10 percent in the numbers between six weeks and the adult stage in both males and females. The number of fibres in the lateral pterygoid of the male increased by about 45 percent between birth and the adult stage. There were about 15 percent fewer fibres in the medial pterygoid of the adult female than in the male; no such sex difference occurred in the lateral pterygoid. Early precursors of muscle fibres were seen in histological sections of these muscles in the newborn, confirming new fibre formation after birth. Such precursors gradually disappeared, starting with the most primitive, so that by the age of four weeks the muscles had a mature appearance.