Anatomy of the long bone (2/2)
Read previous page...potassium. Bone structure can be described based on its overall macroscopic shape and microscopic composition. The end region of the bone is called the epiphysis and the middle region is called the diaphysis or bone shaft, Figure 1 (on the previous page). The region between is called the metaphysis. Between the metaphysis and epiphysis is the epiphyseal disk or plate, which is responsible for longitudinal bone growth in childhood. It is at the epiphysis where one bone contacts another in a joint to allow for movement. Each epiphysis is coated with an articular cartilage. The articular cartilage is simply a coating of hyaline cartilage, which reduces friction and absorbs shocks at freely moveable joints.
All bones are covered by a thin membrane called a periosteum. The periosteum is made of two layers of a dense connective tissue. The outer fibrous layer consists of fibroblasts and collagen fibres. The inside, or osteogenic, layer contains osteoprogenitor cells. Long bones have a hollow region called the medullary cavity in the middle of the diaphysis. The perimeter of the medullary cavity is covered with an endosteum. The cavity itself is filled with marrow. Marrow can be either red or yellow depending on its function and composition. Red marrow is responsible for generation of blood cells and yellow marrow stores fat.
There are two types of bone tissue: spongy and compact, also known as dense. Spongy bone makes up most of the tissue of epiphyses. It consists of lamellae arranged in an irregular latticework of thin plates of bone called trabeculae. The spaces between trabeculae are filled with red bone marrow. Compact bone structure is based on Haversian systems. Haversian systems are located in the diaphysis. They also cover spongy bone in the epiphyses. The functions of Haversian systems are to protect, support, and resist stress.
On the microscopic level, the long bone has five main types of cells found in the skeletal tissue matrix. Osteoprogenitor cells are located in the inner layer of periosteum, endosteum, central and perforating canals. Their function is to divide by mitosis and develop into osteoblasts cells. Osteoblasts are the second type of cells. They spread over the surfaces of bones. Their function is to form bone tissue by secreting a matrix of collagen plus other organic compounds. Mature bone cells, known as osteocytes, are the third type of cells. They are responsible for maintenance of the bone matrix and are located throughout the bone tissue in each lacuna. The last type of cells is the osteoclasts. They are formed by the fusion of 2 to 50 monocytes, a type of white blood cell. Their function is destruction of bone matrix leaving tiny unfilled spaces behind for the osteoblasts, also known as resorption. Resorption allows for the repair and optimisation of the geometry and strength of the bone .
Long bone anatomy references
1. Perren SM. Physical and biological aspects of fracture healing with special reference to internal fixation. Clinical Orthopaedics & Related Research, 1979(138): p. 175-96.
2. Manolagas SC. Editorial: Cell Number Versus Cell Vigor--What Really Matters to a Regenerating Skeleton? Endocrinology, 1999. 140(10): p. 4377-4381.