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Bone

Bone tissue is also known as osseous tissue. It is a type of connective tissue. It forms the structural framework of the skeleton in vertebrates. Together with cartilage, they make up the skeletal system.

Microscopic ground compact bone. Image displays multiple osteons in light gray shades. The osteons appear as concentric rings surrounding dark oval-shaped central canals. Fine, radiating lines are observed extending outward from each ring. The overall composition fills the frame with a dense, irregular pattern.
Figure 1:Microscopic view of ground compact bone showing concentric circles (osteons) around dark central canals.

Cells

Osteoblasts are bone-forming cells responsible for synthesizing and secreting osteoid, the organic components of the bone matrix. Osteoblasts are found in the inner cellular layer of the periosteum surrounding a bone.  They play a central role in bone formation during growth and remodeling processes. Osteoblasts deposit new bone matrix in sheets called lamella (plural lamellae). They become trapped between these sheets. Eventually, they differentiate into osteocytes. 

Osteocytes are mature bone cells that are derived from osteoblasts. Once osteoblasts produce the bone matrix and become surrounded by it, they become osteocytes. This process is similar to “painting yourself into a corner.” Osteocytes are embedded within the mineralized matrix of bone tissue. They reside in lacunae (singular lacuna) or little holes within the solid matrix.  Osteocytes can be distanced from the artery in the central canal of their osteon. They have cellular extensions, like big, long arms that give them a spider-like appearance. These arms extend into tiny channels called canaliculi. These canaliculi are all interconnected with each other. They connect with the central canal. This allows nutrients and oxygen to diffuse out from the artery to the cells.

Osteoclasts are large, multinucleated cells responsible for bone resorption, the process by which bone tissue is broken down and remodeled. Osteoclasts secrete acids and enzymes that dissolve the mineral component of bone tissue and degrade the organic matrix. This process is called osteolysis a word that means “bone breaking.”  This process releases calcium and other minerals into the bloodstream. It also allows for the removal of old or damaged bone tissue. Overactivity of osteoclasts can result in osteoporosis. This condition develops significantly for women after the loss of estrogen. Estrogen is a hormone that encourages osteoclasts to do their osteolysis.  These cells are found in the endosteum, that incomplete lining between the bone marrow and the face of the bone.

The picture above is a picture of compact bone.  The large “holes” or black circles are the central canals of an osteon.  In life, this central canal carried an artery, a vein, and a nerve innervating and vascularizing bone tissue.  This is why bones hurt and heal.  Osteoblasts will move outward from the central canal, depositing matrix in lamella (sheets).  You can see these concentric lamellae circling the central canals.  The little tiny holes are the lacunae in which the osteocytes reside.  Little tiny canals called canaliculi connect the lacunae to each other and to the central canal. They weave through the concentric lamellae. This connection helps link all the osteocytes to one another and to the central canal.

Fibers

Bone is primarily composed of collagen fibers, which provide tensile strength and flexibility to the bone. Collagen fibers are made primarily of type I collagen, which forms a mesh-like network within the bone matrix. This organic component gives bone its resilience and ability to withstand bending and stretching forces.

Matrix

The inorganic matrix of bone tissue is primarily composed of hydroxyapatite crystals, or, basically calcium salts.  This is why drinking calcium enriched foods builds strong bones.  Hydroxyapatite crystals are deposited onto the collagen fibers, forming a hardened matrix . Water is also an essential component of the bone matrix, accounting for approximately 25-30% of its total weight. Water keeps the collagen fibers hydrated. This hydration ensures their flexibility. It facilitates the movement and interactions of these fibers within the bone tissue.

The matrix of bone can sometimes be referred to as osteoid.  This is incorrect.  Osteoid refers to only the organic, or carbon containing, structures in bone such as the collagen or elastin fibers.  Osteoid is secreted and deposited first, then calcification creates bone tissue.

Histological slide depicts intramembranous ossification. Pink-stained bone matrix is interspersed with numerous small, ovoid lacunae containing osteocytes. Scattered light areas indicate active bone formation. The overall composition shows an irregular network of bone trabeculae characteristic of early bone development. Image captured via light microscope.
Figure 2: Micrograph of bone formation (intramembranous ossification) showing osteocytes and lacunae in a pink matrix.
Microscopic slide of intramembranous ossification. The image's central area consists of a teal-stained matrix, speckled with small, red-pink cellular elements, and interspersed with several white empty spaces. A prominent cluster of deep red-brown cells appears in the lower central area. The left margin shows another cluster of cells separated by the material stained teal-green. Bone tissue appears granular.
Figure 3: Microscopic view of bone tissue undergoing intramembranous ossification, showing cells and developing matrix.
High-magnification microscopic image of ground compact bone stained red. Circular osteons are centrally positioned within concentric lamellae. Whitish lacunae are irregularly distributed within the red matrix and surrounding the osteons. The overall composition highlights the organized structure of bone tissue.
Figure 4: Microscopic view of ground compact bone, showing osteons and lacunae in a reddish matrix

Functions

One of the primary functions of bone tissue is to provide structural support for the body. Bones form the framework of the skeleton, which supports the body’s weight.  If we did not have spongy bone tissue, your skeleton would be too heavy for your muscles to move. 

Bone tissue serves as a protective shield for delicate internal organs.  The cranial vault, composed of flat bones fused together, protects the brain.  The rib cage has a completely different structure. It protects the heart and lungs in the thoracic cavity. It also allows movement of these organs.

Bones, along with skeletal muscles, tendons, and ligaments, play a role in movement and locomotion. Bone tissue provides attachment sites for muscles and ligaments, allowing muscle contractions to create the force that causes movement.

Locations

The axial skeleton includes the bones along the body’s central axis, including the skull, vertebral column (spine), and rib cage. The appendicular skeleton includes the bones of the limbs (arms and legs) and their associated girdles (shoulder and pelvic girdles).

In long bones such as the femur and humerus, compact bone forms a collar. This collar is around the superficial face of the bone. Spongy bone is found between that collar and the medullary cavity (bone marrow cavity).  In flat bones, like those found in the skull, spongy bone is sandwiched between two layers of compact bone. This structure is called diploe.

Easily Confused With

Hyaline cartilage

Figure 1: Ground section of compact bone, cross section, 40X Slide 093-B
Figure 2: Fetal leg, cross section, H&E, 40X Slide 048
Figure 3: Fetal leg, cross section, Masson stain, 40X Slide 048b
Figure 4: Decalcified bone, spider monkey, H&E, 40X Slide 050
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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Adipose CT
Areolar CT
Reticular CT
Dense Regular CT
Dense Irregular CT
Elastic CT
Bone CT
Hyaline Cartilage CT
Elastic Cartilage CT
Fibrocartilage CT

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