Ossification of tracheal cartilage in aged humans: a histological and immunohistochemical analysis

Tracheal cartilage has been considered as permanent cartilage without endochondral ossification. We examined ossification of the tracheal cartilage in 25 adults (mean age 67.8 years; range 20-87 years; male:female = 17:8). Thirteen cases (52%) demonstrated ossification of the tracheal cartilage, accompanied by the formation of lamellar bones with fatty bone marrow. Ossification was observed at the lateral peripheral region of the tracheal cartilage, while vascular invasion into the cartilage was seen even where no ossification was present. Twenty-two cases (88%) showed marked hyalinization in the central region of the tracheal cartilage. Type II collagen was immunolocalized in the cartilage matrix, except for in the central hyalinized area, while type I collagen was immunolocalized around the perichondrium and ossified areas in the tracheal cartilage. Type X collagen immunolocalized on territorial matrices around the lacunae near the ossified regions. These results suggest that tracheal cartilage in aged humans frequently ossifies physiologically, and that aging promotes osteogenesis, even in permanent cartilage.