Morphologic changes in annulus fibrosus during aging. An ultrastructural study in rats.

Morphologic studies of annulus fibrosus of newborn, young, and old rats were carried out to determine the tissue changes occurring with aging. Newborn annulus consists of an inner portion containing chondroblast-like cells, a middle portion showing cells with intermediate features between chondroblasts and fibroblasts, and an outer portion containing fibroblast-like cells. The cells, arranged in rows, show a prominent ergastoplasm and are surrounded by pericellular lucunae. In the inner portion, the collagen fibers in the matrix are oriented randomly; in the middle and outer portions, the fibers between adjacent cell rows are arranged parallel, forming fibrous lamellae. The collagen fibers have fairly uniform diameters (average 510 nm), are strongly positive to the PASM reaction, and are surrounded by small proteoglycan granules. Both immature and mature elastic fibers are present. Young annulus consists of a transition zone, composed of large cells with no definite arrangement and bundles of collagen fibers oriented in various directions, and a lamellar zone consisting of fibrous lamellae separated by interlamellar septa containing the cells. These show intermediate features between chondrocytes and fibrocytes and well-developed ergastoplasm. The interlamellar septa consist of collagen fibrils and proteoglycans and contain few aggregates of electron dense material. The collagen fibers have highly variable diameters (average 740 nm). The larger fibers are scarcely periodate-silver-methenamine (PASM) positive. The proteoglycans are similar in size to those in the newborn. Few, mature elastic fibers are present. In old annulus, the transition zone is larger than in the young tissue. The cells are less numerous, and most of them show a sparse ergastoplasm.(ABSTRACT TRUNCATED AT 250 WORDS)