8.22:
Bone as Supporting Connective Tissue
Bones are rigid connective tissues whose functions include protecting and supporting parts of the body.
Bone tissue is classified into compact or cortical bone, which is hard and dense, and trabecular or spongy bone, which is porous.
Compact bone forms the outer layer and consists of a matrix arranged in concentric rings called lamellae. Spaces between lamellae, called lacunae, contain osteocytes.
Spongy bone, located in the interior of the bone, contains trabeculae, thin branching structures made up of lamellae. Spaces between trabeculae contain red or yellow bone marrow.
The bone matrix is rich in collagen fibrils that offer flexibility and mineral salts that add stiffness.
Bone matrix is secreted by cells called osteoblasts, which mature into osteocytes.
Osteoclasts are large, multinucleate cells involved in bone resorption.
Osteocytes regulate the cycle of bone loss and formation, known as bone remodeling. During this process, osteoclasts remove the mineral deposits of old bone tissue, and osteoblasts deposit the circulating minerals to create new bone.
8.22:
Bone as Supporting Connective Tissue
Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the body.
Bone Matrix
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts— mostly calcium salts— that give the tissue hardness. Inadequacy of organic material in the matrix can result in bone breakage while low amounts of inorganic material can make bones bend.
Types of Bone Cells
There are three types of cells in bones: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in bone growth and remodeling. They deposit bone matrices and can differentiate into resident bone cells, or osteocytes. Osteocytes can communicate with each other through their cellular processes. They regulate bone mineral deposition by regulating activity of other bone cells. Osteoclasts are active in breaking down the bone tissue during remodeling and thus, aid in maintaining mineral homeostasis.
Types of Bone Tissue
Bone can be divided into two types: compact (or cortical) bone and spongy (or trabecular) bone. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or metaphysis and epiphysis) of a long bone. Compact bone is organized into subunits called osteons. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the osteocytes. Spongy bone is made of tiny plates called trabeculae; these plates serve as struts to give the spongy bone strength. With age, these plates become thinner and less resilient, making the bone prone to fractures.
This text is adapted from Openstax, Biology 2e, Section 33.2: Animal Primary Tissues