Bone Tissue

Types of Bone Tissue

Bone tissue is classified by the organization of collagen fibers within the bone matrix.

Woven Bone

Collagen fibers are randomly oriented around cells.
First type of bone made during embryonic skeleton development and bone repair.
Quick and fast way of making bone.
Osteoclasts break it down, and osteoblasts rebuild it in a more organized fashion (remodeling).

Lamellar Bone

Bone is organized into sheets or layers called lamellae.
More organized than woven bone.
Found in all bones of the body.

Types of Lamellar Bone

Classified based on the amount of matrix and the organization of the matrix.

Spongy Bone (Cancellous or Trabecular Bone)

Less bone matrix and more space, giving it a porous appearance.
Found on the inside of bones.
Fills spaces of irregularly shaped bones (vertebrae), flat bones (sternum, skull), and ends of long bones.
Forms a thin layer in regions with cavities inside bones.
Makes up 20% of the total skeleton mass.
Provides strength without extra mass.
Protected by a layer of compact bone.

Trabeculae

Bone matrix is organized into connecting rods or plate-like structures.
Spaces between trabeculae are filled with bone marrow and blood vessels.
Red bone marrow is typically found in the ends of long bones and irregularly shaped bones, but yellow bone marrow can also be present.
Organized along lines of stress for strength.
Lamellar bone is organized into sheets within each trabecula.
Osteocytes are sandwiched between sheets in lacunae and connected via canaliculi.
Nutrients and gases diffuse through canaliculi to reach osteocytes.
The surface of each trabecula has a layer of cells, including osteoclasts (for bone breakdown) and osteoblasts (for bone rebuilding), along with osteochondral progenitor cells.

Compact Bone (Cortical Bone)

Dense with very few spaces.
Found on the outside of all bones.
Makes up the majority of bone tissue in the shafts of long bones.
Makes up 80% of the total mass of the skeleton.
Provides strength to support body weight and withstand muscle forces.

Spongy vs. Compact Bone

Spongy bone is found on the inside and appears more porous.
Compact bone is found on the outside.
Periosteum surrounds the outside of all bones, composed of dense fibrous connective tissue and a cell layer (similar to perichondrium).
Medullary cavity is the space within bones like the shafts of long bones.

Periosteum

A double layer surrounding the outsides of bones.
Outer layer: dense fibrous connective tissue with fibroblasts secreting collagen fibers.
Inner layer (osteogenic layer): contains osteoblasts, osteoclasts, and osteochondral progenitor cells.

Endosteum

Lines the inside of bone in the medullary cavity.
Similar to the periosteum but only has the cell component (osteogenic layer).
Contains osteoblasts, osteoclasts, and osteochondral progenitor cells.
Blood vessels enter the bone through perforating canals (Volkmann's canals), which run perpendicular to the length of the bone.
Blood vessels branch off and run parallel to the bone in central canals (Haversian canals).

Structure of Compact Bone

Compact bone has very organized sheets of lamellar bone.
Lamellar bone is organized in three different ways.

Concentric Lamellae

Form a series of circles around blood vessels in the central canal.
Osteocytes are sandwiched between layers of lamellar bone, connected by canaliculi.
Nutrients and gases move outwards from the central canal to the cells.

Osteon (Haversian System)

Organization of bone into concentric lamellae around a central canal.
Repeats throughout compact bone.
Central canal contains blood vessels.
Connected to each other and the outside of the bone via perforating or Volkmann canals.

Circumferential Lamellae

Bundle all the osteons together within the compact bone.
Outer circumferential lamellae: Runs around the entire outside surface of the bone, just underneath the periosteum layer.
Inner circumferential lamellae: Surrounds the inside cavity of the bone where trabecular or spongy bone forms.

Interstitial Lamellae

Fill spaces between osteons.
Remnants of osteons broken down by osteoclast cells.

Blood Vessels in Bone

Run parallel to the length of the bone through central or Haversian canals.
Connected to the outside of the bone and to each other via perforating or Volkmann canals, allowing blood vessels to move perpendicularly through the bone.
Periosteum has an outer layer of dense fibrous connective tissue and an inner osteogenic layer (osteoblasts, osteoclasts, and osteochondral progenitor cells).
Endosteum is connective tissue lining internal surfaces of the bone, consisting of the cellular layer of the periosteum without the fibrous layer.

Long Bone Structure

Long bones are longer in length than in width.

Epiphysis

The region of the bone that's closest to the end of the bone is known as the epiphysis.
Proximal epiphysis: Closer to the trunk of the body.
Distal epiphysis: Farther away from the trunk of the body.
Surrounded by hyaline cartilage (articular cartilage) at joints.
Primarily spongy bone filled with red bone marrow (sometimes yellow marrow).

Diaphysis

The shaft portion of the bone.
Mostly compact bone, surrounding a thin layer of spongy bone on the inside.

Metaphysis

Regions connecting the diaphysis and the epiphysis.

Periosteum (Long Bone)

Double layer surrounding the outside of all bones.

Medullary Cavity

Space inside the shaft region of bones.
Contains red bone marrow in babies and children.
Turns into yellow marrow (adipose tissue/fat) in adults.
Lined by the endosteum, containing osteogenic cells (osteoblasts and osteoclasts).

Epiphyseal Plate/Line

Epiphyseal plate: Layer of cartilage between epiphysis and diaphysis that allows bone to grow in length (present until adulthood).
Epiphyseal line: Ossified cartilage that remains after bone stops growing in length.

Bone Marrow

Medullary cavity contains red marrow in children, which changes to yellow marrow in long bones and the skull as they age.
Some regions of spongy bone retain red marrow (e.g., proximal epiphysis of long bones, pelvic bones), which produce blood cells.

Bone Development (Osteogenesis or Ossification)

Process of forming bone tissue.
Occurs in the embryo at about 8 weeks.
Mesenchyme cells (stem cells for connective tissues) become osteochondral progenitor cells when blood vessels invade.

Intramembranous Ossification

Bone formation in connective tissue membranes.
Occurs in skull bones, mandible (jawbone), and some parts of the clavicle.
Mesenchyme cells create a collagen fiber membrane.
Osteochondral progenitor cells turn into osteoblasts, which ossify the membrane starting at 8 weeks.

Endochondral Ossification

Bone formation starts as cartilage and then turns into bone.
Occurs in the base of the skull, parts of the clavicle, and most other bones in the body.
Mesenchyme cells become osteochondral progenitor cells, which turn into chondroblasts.
Chondroblasts form the hyaline cartilage skeleton by 8 weeks.
Blood vessels invade the perichondrium surrounding the cartilage.
Stimulates osteochondral progenitor cells to become osteoblasts.
Perichondrium becomes the periosteum.
Osteoblasts invade the cartilage area and remodel it into lamellar bone.
Both methods (intramembranous and endochondral) produce woven bone first, which is then remodeled into spongy or compact bone.

Bone Growth

Bone cannot undergo interstitial growth due to the solid matrix.
Only undergoes appositional growth (formation of bone on the surface of old bone).
Utilizes interstitial growth in cartilage at the epiphyseal plate for bone growth in length.

Growth in Bone Length

Occurs at the epiphyseal plate, made of hyaline cartilage, between the diaphysis and epiphysis.

Zones of the Epiphyseal Plate:

Zone of resting cartilage: Anchors the epiphysis to the diaphysis; very slow chondrocyte division (interstitial growth).
Zone of proliferating cartilage: Rapidly dividing chondrocytes in stacks (interstitial growth).
Zone of hypertrophy: Chondrocytes enlarge and mature, start to secrete matrix vesicles that contain hydroxyapatite which calcifies the matrix.
Zone of calcified cartilage: Calcified cartilage layer forms; chondrocytes die off; blood vessels and osteoblasts invade from the endosteum.
Osteoblasts develop bone on the side of the diaphysis.
These zones are maintained as the bone grows in length until the individual reaches their genetic potential for height.
Eventually, cartilage layers ossify, and the epiphyseal plate becomes the epiphyseal line.

Growth in Bone Thickness (Appositional Growth)

Osteoblast cells beneath the periosteum lay down matrix, forming bone.
Bone forms ridges around periosteal blood vessels.
Ridges create a groove, eventually forming a tunnel.
Periosteum gets pinched off, and the inner layer becomes the endosteum.
Osteoblasts in the endosteum fill in concentric lamellae towards the blood vessel, forming a new osteon.
Central/Haversian canal forms with blood vessels running parallel to the bone.
Periosteum continues laying down bone on the outside.
Forms circumferential lamellae surrounding the osteons.

Bone Remodeling

Continuous function of osteoblast and osteoclast activity.
Osteoclasts break down bone, and osteoblasts build it back up.
In children, bone remodeling allows bones to change in size.
The medullary cavity increases in size as osteoclasts destroy the inside of the bone, while osteoblasts form bone on the outside.
Bone thickness stays relatively constant.
Osteoclasts remodel the inside of the bone into trabecular bone with a medullary cavity.
Osteoblasts in the periosteum form compact bone on the outside.
Growth also occurs underneath the cartilage covering the epiphysis.
Articular cartilage remains in adulthood, while the rest of the cartilage ossifies.
The epiphyseal line is calcified from the epiphyseal plate.
Spongy bone fills in the epiphysis and becomes more dense in the adult bone.

Bone Remodeling in Adulthood

Continual process of bone being broken down and rebuilt by osteoblasts and osteoclasts (takes about ten years for the entire skeleton).
Stimulated by stress on the bone, fractures, changes in body minerals, and exercise.

Example

An elite athlete involved in racket sports shows much greater bone density in their dominant arm compared to their non-dominant arm due to muscle pull stimulating bone growth.
Exercise stimulates osteoblast cells to build bone more strongly and densely to withstand greater muscle force.