In cross-section , the fibers run in opposite directions in alternating layers, much like in plywood , assisting in the bone's ability to resist torsion forces. The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones. They are located on the surface of osteon seams and make a protein mixture known as osteoid , which mineralizes to become bone. The primary anatomical and functional unit of cancellous bone is the trabecula. After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution. Osteoblasts also manufacture hormones , such as prostaglandins , to act on the bone itself. If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture".
Cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. Osteoid is primarily composed of Type I collagen. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur. Intramembranous ossification mainly occurs during formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones, but is later replaced by more resilient lamellar bone. The bones of the wrist and ankle are short bones. Each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal. Some examples of terms used to describe bones include the term "foramen" to describe a hole through which something passes, and a "canal" or "meatus" to describe a tunnel-like structure. Lamellar bone, which makes its first appearance in humans in the fetus during the third trimester,  is stronger and filled with many collagen fibers parallel to other fibers in the same layer these parallel columns are called osteons. These synthesise collagen within the cell, and then secrete collagen fibrils. These lacunae are the result of surrounding bone tissue that has been reabsorbed. Short bones are roughly cube -shaped, and have only a thin layer of compact bone surrounding a spongy interior. They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. They consist of thin layers of compact bone surrounding a spongy interior. Cancellous bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. The primary anatomical and functional unit of cancellous bone is the trabecula. Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones. They are made up mostly of compact bone , with lesser amounts of marrow , located within the medullary cavity , and areas of spongy, cancellous bone at the ends of the bones. The epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous. Cortical bone[ edit ] Cross-section details of a long bone The hard outer layer of bones is composed of cortical bone also called compact bone being much denser than cancellous bone. Osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes. Cancellous bone[ edit ] Micrograph of cancellous bone Cancellous bone , also called trabecular or spongy bone,  is the internal tissue of the skeletal bone and is an open cell porous network. Osteocyte[ edit ] Osteocytes are mostly inactive osteoblasts. Some anatomists still use Latin to refer to bones. These collagen fibers are used as a framework for the osteoblasts' work.
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