Bone Tissue

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.