The diaphysis, or shaft, of a long bone features a shell of |compact bone| surrounded by the |periosteum|, a cartilaginous membrane which contains the ~blood~ vessels which provide nutrients to the bone. The diaphysis is separated from the epiphyses (ends) of the bone by the |epiphyseal line|. When bones form, calcium salts are first deposited within the shaft (diaphysis), and this calcification spreads outward to the ends of the bone (epiphyses). As this progresses, the periosteal membrane produces a network of fibrils (osteoblasts) in front of this advancing line of calcification, which provides a mesh framework for the ensuing calcification. Once this calcified ~cartilage~ has developed, the periosteal membrane sends ~blood~ vessels into the bone which carry nutrients as well as regulatory cells known as osteoclasts. The ~osteoblasts~ and osteoclasts work together to replace the calcified ~cartilage~ with true, osseous material. This process is carried out on the ends (epiphyses) of the bones as well, though a layer of uncalcified ~cartilage~ demarcates the epiphyses from the diaphysis until later in the life of the bone. Once the true bone has developed, the center becomes hollowed out, which allows for the development of the |marrow| and the spongy tissue layer (trabeculae). The growth and development of the bone is regulated by a ~hormone~ secreted by the |pituitary gland|, with new growth taking place at the |epiphyseal line|. At some stage, however, ~hormones~, secreted by the |testes| in the male and |ovaries| in the female, cause this bone growth to cease, whereupon the epiphysis fuses to the diaphysis. Beyond this point, which usually occurs earlier in females, the bone undergoes simultanous resorption (where it breaks down and re-absorbs osseous material) and reconstruction. Healthy bone is constantly undergoing resorption and reconstruction, though, in the elderly, the reconstruction of bone is somewhat diminished, making healing of |fractures| slower.
