ACP5 and osteoporosis: 30, 31. In the 1960s, E.A. Tonna first reported the presence of TRAP+ mononuclear osteoclasts on the surface of cortical bone critical for cortical bone growth and repair 32. Conflicting findings have since emerged regarding osteoclasts in different types of osteoporosis. In estrogen-deficient osteoporosis, the number of osteoclasts on trabecular bone surfaces increases 33, 34. Conversely, primary osteoporosis in the elderly shows a significant reduction in cortical bone osteoclasts, correlating with substantial cortical bone thickness loss 35, 36.