Osseous (bone) tissue has a rigid extracellular matrix formed by collagen fibers and inorganic salt crystals with a relatively small number of cells found within it. The calcium salt crystals found in bone are called hydroxyapatite. Its structure incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, within a mix of collagen fibers. The hydroxyapatite crystals give bones their hardness and strength while the collagen fibers give them flexibility so that they are not brittle.
Although bone cells compose a small amount of bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue:
Osteogenic cells
Osteoblasts
Osteocytes
Osteoclasts
Bone Cells - Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function change and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells.
Osteogenic cells are undifferentiated stem cells that undergo mitosis at high rates, generating new bone cells. They are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. Both of these groups differentiate and develop into osteoblasts on their respective sides of the bone.
When osteogenic cells divide, they form osteoblasts. The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it; as a result, it changes in structure and becomes an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna (plural, lacunae) and is surrounded by bone tissue. Recall that chondrocytes in cartilage are also located in spaces called lacunae. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes do not perform mitosis. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular, canaliculus), channels within the bone matrix.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. This process along with changes in physical stress over time is responsible for the remodeling of bones over time, causing certain bones—or even parts of bones—to become stronger and weaker than others. The following table reviews the bone cells, their functions, and their locations.
Bone Cells
Cell type
Function
Location
Osteogenic cells
Develop into osteoblasts
Deep layers of the periosteum and the marrow
Osteoblasts
Bone formation
Growing portions of bone, including periosteum and endosteum
Osteocytes
Maintain mineral concentration of matrix
Entrapped in matrix
Osteoclasts
Bone resorption
Bone surfaces and at sites of old, injured, or unneeded bone
terms to know
Osteogenic Cell
The bone stem cell responsible for production of osteoblast.
Osteoblast
The bone cell responsible for formation of bone matrix.
Osteocyte
The bone cell responsible for maintenance of bone matrix; mature osteoblast.
Lacuna
A small space where chondrocytes are located in a cartilage tissue and osteocytes are located in bone tissue (plural, lacunae).
Canaliculi
Channels in the bone matrix extending between lacunae.
Osteoclast
The bone cell responsible for breakdown of bone matrix.
2. Compact and Spongy Bone
recall
Previously you learned that osseous tissue is organized into two forms, compact bone and spongy bone.
Most bones contain both, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.
2a. Compact Bone
recall
Compact bone is a dense osseous tissue able to sustain compressive forces.
It is the denser, stronger of the two types of bone tissue. It can be found under the periosteum and in the diaphyses of long bones where it provides support and protection.
The repeating microscopic structural unit of compact bone is called an osteon, or Haversian system which often is compared to a target sign with many rings surrounding the bullseye at the center. The many rings are calcified bone matrix called concentric lamellae (singular, lamella). The bullseye at the center is called the central canal, or Haversian canal, and contains blood vessels, nerves, and lymphatic vessels. In order to connect adjacent central canals, the vessels and nerves branch off at right angles through a perforating canal, also known as Volkmann’s canals, eventually extending to the periosteum and endosteum.
The osteocytes are located inside lacunae found at the borders of adjacent lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported from the blood vessels in the central canal out to the osteocytes and wastes to be moved back in from osteocytes to the central canal.
Over time, compact bone is naturally turned over—broken down and replaced. Evidence of this can be seen in the spaces between osteons where older bone tissue not part of any visible osteon is located. These portions of older concentric lamellae are called interstitial lamellae. Additionally, just deep to the periosteum, compact bone forms multiple layers of bone matrix called circumferential lamellae which plays a role in bone growth and overall strength.
2b. Spongy Bone
recall
Spongy bone, also known as cancellous bone, is a porous type of osseous tissue.
Spongy bone contains lamellae, osteocytes, and lacunae but they are not arranged in tightly packed formations as compact bone is. Instead, they form a lattice-like network of matrix spikes called trabeculae (singular, trabecula) (see image below). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. Furthermore, the center of each trabeculae does not contain a central canal. Instead, all nutrients are brought in through canaliculi from the superficial surface and all waste is removed by moving back out. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where blood cells are created.
Diagram of Spongy Bone - Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones.
watch
View the following video for more information on this topic.
IN CONTEXT Aging and the Skeletal System: Paget’s Disease
Paget’s disease is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed (broken down) than is laid created. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and, therefore, prone to fracture.
Paget’s disease usually occurs in adults over age 40. While some people have no symptoms, others experience pain, bone fractures, and bone deformities. Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss.
Paget's Disease - Normal leg bones are relatively straight, but those affected by Paget’s disease are porous and curved.
What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.
Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.
Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.
terms to know
Osteon
The repeating microscopic structural unit of compact bone.
Concentric Lamellae
The rings of calcified bone matrix within an osteon.
Central Canal
The center of an osteon containing blood vessels, lymphatic vessels, and a nerve.
Perforating Canal
A passageway through compact bone that connects central canals of adjacent osteons to the periosteum and endosteum.
Interstitial Lamellae
Portions of old bone matrix located between osteons.
Circumferential Lamellae
Layers of bone matrix that surround compact bone just deep to the periosteum.
Trabeculae
The spikes forming the lattice-like network of spongy bone (singular, trabecula).
summary
In this lesson, you learned about the microscopic anatomy of bone. You learned to identify the multiple types of bone cells and tissue. You also learned the individual structures and functions of compact bone and spongy bone.
