Classify the different forms of bones in the skeleton Exsimple the role of the various cell kinds in bone Exsimple just how bone forms in the time of development

Bone, or osseous tconcern, is a connective tissue that constitutes the endoskeleton. It has specialized cells and a matrix of mineral salts and collagen fibers.

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The mineral salts generally incorporate hydroxyapatite, a mineral formed from calcium phosphate. Calcification is the process of deposition of mineral salts on the collagen fiber matrix that crystallizes and also hardens the tconcern. The process of calcification only occurs in the existence of collagen fibers.

The bones of the human skeleton are classified by their shape: lengthy bones, short bones, level bones, sutural bones, sesamoid bones, and also ircontinual bones (Figure (PageIndex1)).

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Figure (PageIndex1): Shvery own are different kinds of bones: level, irconstant, lengthy, short, and sesamoid.

Long bones are longer than they are wide and also have actually a shaft and 2 ends. The diaphysis, or central shaft, contains bone marrowhead in a marrow cavity. The rounded ends, the epiphyses, are extended through articular cartilage and also are filled with red bone marrow, which produces blood cells (Figure (PageIndex2)). Most of the limb bones are lengthy bones—for instance, the femur, tibia, ulna, and also radius. Exceptions to this encompass the patella and also the bones of the wrist and ankle.

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Figure (PageIndex2): The lengthy bone is covered by articular cartilage at either finish and includes bone marrow (presented in yellow in this illustration) in the marrow cavity.

Brief bones, or cuboidal bones, are bones that are the exact same width and length, giving them a cube-favor shape. For example, the bones of the wrist (carpals) and ankle (tarsals) are brief bones (Figure (PageIndex1)).

Flat bones are thin and relatively broad bones that are discovered wbelow comprehensive defense of organs is compelled or where wide surfaces of muscle attachment are compelled. Examples of level bones are the sternum (breast bone), ribs, scapulae (shoulder blades), and also the roof of the skull (Figure (PageIndex1)).

Irregular bones are bones through facility shapes. These bones may have actually brief, flat, notched, or ridged surencounters. Examples of irconstant bones are the vertebrae, hip bones, and also a number of skull bones.

Sesamoid bones are little, level bones and are shaped similarly to a seexact same seed. The patellae are sesamoid bones (Figure (PageIndex3)). Sesamoid bones build inside tendons and might be discovered close to joints at the knees, hands, and feet.

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Spongy Bone Tissue

Whereas compact bone tworry creates the external layer of all bones, spongy bone or cancellous bone forms the inner layer of all bones. Spongy bone tconcern does not contain osteons that constitute compact bone tworry. Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Red bone marrowhead is uncovered between the trabuculae. Blood vessels within this tproblem supply nutrients to osteocytes and also remove waste. The red bone marrow of the femur and also the inner of other large bones, such as the ileum, develops blood cells.

Spongy bone reduces the thickness of bone and also permits the ends of lengthy bones to compush as the result of stresses used to the bone. Spongy bone is influential in areas of bones that are not heavily stressed or wright here stresses arrive from many kind of directions. The epiphyses of bones, such as the neck of the femur, are subject to anxiety from many directions. Imagine laying a hefty framed picture flat on the floor. You might organize up one side of the image through a toothpick if the toothpick was perpendicular to the floor and also the picture. Now drill a hole and stick the toothpick into the wall to hang up the picture. In this instance, the attribute of the toothpick is to transmit the downward press of the photo to the wall. The pressure on the picture is right down to the floor, but the force on the toothpick is both the photo wire pulling down and the bottom of the hole in the wall pushing up. The toothpick will break off right at the wall.

The neck of the femur is horizontal prefer the toothpick in the wall. The weight of the body pushes it down close to the joint, yet the vertical diaphysis of the femur pushes it up at the other end. The neck of the femur should be strong enough to carry the downward pressure of the body weight horizontally to the vertical shaft of the femur (Figure (PageIndex5)).

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Development of Bone

Ossification, or osteogenesis, is the procedure of bone development by osteoblasts. Ossification is unique from the procedure of calcification; whereas calcification takes location throughout the ossification of bones, it deserve to likewise occur in other tproblems. Ossification begins approximately six weeks after fertilization in an embryo. Before this time, the embryonic skeleton is composed totally of fibrous membranes and also hyaline cartilage. The advance of bone from fibrous membranes is called intramembranous ossification; development from hyaline cartilage is referred to as endochondral ossification. Bone growth continues till approximately age 25. Bones deserve to grow in thickness throughout life, but after age 25, ossification functions mostly in bone remodeling and also repair.


Intramembranous Ossification

Intramembranous ossification is the process of bone development from fibrous membranes. It is connected in the formation of the flat bones of the skull, the mandible, and also the clavicles. Ossification begins as mesenchymal cells form a design template of the future bone. They then differentiate into osteoblasts at the ossification facility. Osteoblasts secrete the extracellular matrix and also deposit calcium, which hardens the matrix. The non-mineralized portion of the bone or osteoid continues to create roughly blood vessels, creating spongy bone. Connective tissue in the matrix differentiates right into red bone marrow in the fetus. The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.


Endochondral Ossification

Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of the body, except for the flat bones of the skull, mandible, and also clavicles, are developed via endochondral ossification.

In lengthy bones, chondrocytes create a design template of the hyaline cartilage diaphysis. Responding to complicated developmental signals, the matrix begins to calcify. This calcification stays clear of diffusion of nutrients right into the matrix, resulting in chondrocytes dying and also the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone. Osteoclasts then break dvery own some of the spongy bone to produce a marrow, or medullary, cavity in the facility of the diaphysis. Dense, irregular connective tconcern develops a sheath (periosteum) roughly the bones. The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and also elongate as the cartilage cells at the epiphyses divide.

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In the last phase of prenatal bone advancement, the centers of the epiphyses start to calcify. Secondary ossification centers create in the epiphyses as blood vessels and also osteoblasts enter these areas and transform hyaline cartilage right into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth plate), which is the area between the diaphysis and epiphysis that is responsible for the lengthwise expansion of lengthy bones (Figure (PageIndex6)).

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