Bone Function, Development, and Growth

Microscopic Anatomy of Bone

Bone tissue is constantly being remodeled to better suit the body's needs due to the actions of osteoblasts, which are bone-building cells, and osteoclasts, which break down existing bone.
Bone is living tissue. It is constantly growing and changing, being built up and broken down. Old bone cells get destroyed and replaced with newly formed cells, such as when a person breaks an arm or a leg. The way in which the bone breaks is often irregular, so the two broken ends of the bones need to be set, or aligned, so they can properly heal back together. Two of the three main types of bone cells, osteoblasts and osteocytes, originate from the connective tissues when in the embryonic stage of development. An osteoprogenitor cell is a stem cell that can produce other cells that develop into osteoblasts through differentiation. Differentiation is a process by which a cell matures and develops a specialized function. Osteoprogenitor cells are typically found in the periosteum and endosteum. Some of these osteoprogenitor cells remain as stem cells and keep dividing so that when the situation occurs, they can differentiate into the needed type of cell. The other type of bone cell, osteoclasts, is actually a modified form of a white blood cell, which serves to break down and digest worn-out cell parts. The constant breaking down and building up of bone cells is called remodeling.

Bone Cells

There are three primary types of bone cells—osteoblasts, osteoclasts, and osteocytes—all of which arise from generic stem cells.

An osteoblast is a cell that forms new bone tissue by secreting a matrix made of collagen (the protein found in bone) and calcium-binding proteins. These cube-shaped cells are constantly undergoing mitosis, creating new cells from old ones. The collagen proteins are needed because the new bone cells formed lack the hardened minerals of more mature cells. Once the matrix has been completely deposited, osteoblasts turn into another type of bone cell called osteocytes.

An osteocyte is a mature bone cell that is found in the spaces of the bone matrix (which is an intercellular substance made of hydroxyapatite, calcium carbonate salts, and collagen). They monitor and maintain the bone matrix and signal to osteoblasts to increase bone deposition. Osteocytes also are sensitive to changes in stress (for example, the lack of stress present in a zero-gravity environment or the increased stress of a heavy workload). When a change in stress occurs, the osteocytes send signals to the other bone cells (osteoblasts and osteoclasts), alerting them that changes are needed. These cells then either build new bone or destroy existing bone to compensate.

An osteoclast is a cell that destroys existing bone, making room for the deposition of new bone cells. Osteoclasts are derived from the same originators as macrophages, the white blood cells that destroy invaders to the bloodstream. Osteoclasts have a rough border, which allows their protrusions to make direct contact with the bone. These protrusions increase the surface area, which increases the enzyme activity used to break down the bone. Osteoclasts work by reabsorbing the calcium and other hardening minerals found within the bone tissue.

Types of Bone Cells

Different bone cell types function to maintain, build up, or break down bone.

Anatomy of Compact Bone

Compact bone provides rigid strength but also serves as a pathway for blood and nervous signal transmission.

Compact bone provides strength and rigidity to the skeleton. Although it may look solid, it actually has a series of channels and networks going through it. These networks carry blood and nerve signals to different areas of the bones. For example, the signals produced by osteocytes travel to the areas of bone that are in need of attention by osteoblasts or osteoclasts when stress on the bone has changed.

The basic functional unit of mature bone is the osteon (also called the Haversian system, named after Clopton Havers, an English physician who extensively studied bones), a long tube that runs parallel to the axis of bone. It serves as a support beam to hold the weight of the bone. Collectively, osteons create rings (like those seen in the trunk of a tree) that are concentric (share the same center). A lamella (plural, lamellae) is a layer (often concentric) of bone connective tissue and a structural component of the osteon. It forms a strong mesh along with collagen that adds significant strength to bone. Each lamella contains collagen fibers in parallel. The direction of the collagen fibers in each lamella alternates between layers. The fibers are all oriented in the same direction in one lamella but in opposite directions in adjacent lamellae.

An opening called a Haversian canal through the center of each osteon contains nerves and blood vessels that signal and nourish the osteon. Bones have another opening, called a Volkmann (perforating) canal, that runs at a 90° angle to the osteon and connects the medullary canal of the bone with the blood supply and nerve signals. Volkmann canals are found within the endosteum tissue layer.

At the juncture of each lamella is a space called a lacuna (plural, lacunae) where osteocytes can be found. There are also many tiny passages, each called a canaliculus (plural, canaliculi), that join the lacunae to the Haversian canal and to each other. The canaliculi connect the entire osteon together, allowing for the transport of blood and nutrients and the removal of waste products. They also allow each osteon to communicate with others, a process that is important to maintaining homeostasis, or a stable state in the health of the bone.
Within the central canal of compact bone lies an intricate network of blood vessels and nerves that carries nutrients and signals to the living bone tissue.