Bone Tissue Physiology

Physiology

• Bones support muscle and soft tissue attachment.
• Bones protect internal organs, like the brain and heart.
• Muscle attachment to bones enables movement.
• Mineral homeostasis is maintained by calcium and phosphorus.
• Red marrow is the site of hematopoiesis, producing red blood cells (RBCs), white blood cells (WBCs), and platelets.
• Fat is stored as triglycerides in yellow marrow.
• Osteocalcin, a hormone secreted by bone, is related to fat and glucose homeostasis.

Physiology of Bone Tissue

• Compact and spongy bone are two types of bone.
• Compact bone is arranged into osteons with few spaces.
• Strength in compact bone is derived from calcified concentric lamellae.
• Spongy bone is arranged into trabeculae with many spaces.
• Red bone marrow is housed in the spaces of spongy bone.

Four Bone Cell Types

• Osteogenic cells (stem cells) are mitotic and form osteoblasts.
• Osteoblasts secrete collagen; calcification occurs due to osteoblast activity.
• Osteocytes are "trapped" osteoblasts and are the most common cells; they can easily be seen in osteons.
• Osteoclasts, derived from white blood cells (WBCs), facilitate bone resorption.

Bone as Endocrine Tissue

• Bone produces the hormone osteocalcin, giving it an endocrine function.
• Osteocalcin signals the pancreas to make more insulin.
• Osteocalcin signals adipose tissue to secrete adiponectin, which stimulates the production of more insulin receptors by most cells.
• Decreased osteocalcin and adiponectin secretion may cause Type II diabetes and obesity.
• Adiponectin levels are inversely related to body weight.

Mineral Homeostasis

• The balance between calcification and resorption maintains mineral homeostasis.
• Collagen provides a framework for hydroxyapatite crystals, specifically Ca3(PO4)2 and CaCO3.
• Osteoblasts secrete collagen and mineralize this framework, forming the bone matrix.
• Resorption involves breaking down the calcified matrix by lysosomal enzymes and acids secreted by osteoclasts.
• Osteoclasts are fusions of monocytes (WBCs) with 15-20 nuclei.
• The ruffled border of osteoclasts increases surface area for resorption.

Hormones in Calcium Homeostasis

• The parathyroid gland monitors calcium [Ca2+] levels; a decrease stimulates the gland to secrete Parathyroid Hormone (PTH).
• PTH stimulates osteoclasts to increase resorption, the kidney to retain Ca2+, and the intestine to absorb Ca2+.
• The thyroid gland also monitors [Ca2+]; an increase stimulates the gland to secrete Calcitonin (CT).
• Calcitonin inhibits osteoclasts, increases Ca2+ excretion by the kidney, and inhibits Ca2+ absorption by the intestine.

Mineral Homeostasis (continued)

• Bone density and mineral content are affected by gravity, exercise, leptin, estrogen, and testosterone.
• In women over 30, decreasing estrogen levels result in 80% of osteoporosis cases being in women.
• Testosterone levels in men remain constant until around 45 years of age.
• Bone stores phosphorus (as phosphate) and zinc.

Gross Anatomy

• Diaphysis is the shaft of the bone.
• Epiphyses are the ends of the bone.
• Metaphyses contain the epiphyseal line or plate.
• Articular cartilage covers joint surfaces.
• Periosteum is the outer covering of bone.
• Medullary cavity is the hollow space within the diaphysis.
• Endosteum lines the medullary cavity.

Anatomy of Compact Bone

• Compact bone is arranged into osteons.
• Haversian canals vascularize and innervate compact bone.
• Osteocytes reside in lacunae.
• Lamellae compose the calcified matrix.
• Canaliculi connect osteoblasts and blood vessels laterally.
• Blood vessels and nerves branch laterally through Volkmann's canals.

Anatomy of Spongy Bone

• Trabeculae are the structural units of spongy bone.
• Spongy bone lacks Haversian canals.
• Osteocytes are present in lacunae.
• Lamellae is not circular and referred to as interstitial.
• The overall structure is reticulated (3-D).
• Spongy bone is 30-90% porous, provides space for marrow, and its surface area is 10x more than compact bone.

Blood and Nerve Supply

• Periosteal arteries supply the periosteum.
• Nutrient arteries enter through the nutrient foramen and supply compact bone of the diaphysis and red marrow.
• Metaphyseal and epiphyseal arteries supply red marrow and bone tissue of epiphyses.

Bone Formation

• Mesenchyme is embryonic connective tissue.
• Bone formation (osteogenesis or ossification) begins when mesenchymal cells undergo differentiation.
• Intramembranous ossification forms bone directly from or within fibrous mesenchyme.
• Endochondral ossification forms bone from hyaline cartilage models in the sequence mesenchyme > hyaline cartilage > bone.

Growth in Length

• Linear bone growth details the epiphyseal growth.
• Interstitial is the bone growth type.
• Four zones make up the epiphyseal plate.
• "Stacks" of coins are formed by these zones.
• The epiphyseal plate is replaced by bone.
• The epiphyseal line appearing indicates completion of bone growth in length.

Zones of Resting Cartilage

• This zone anchors the growth plate to the bone.

Zone of Proliferating Cartilage

• This zone demonstrates rapid cell division of stacked coins.

Zone of Hypertrophic Cartilage

• This zone exhibits enlarged cells remaining in columns.

Zone of Calcified Cartilage

• Thin zone has mostly dead cells due to a calcified matrix.
• Osteoclasts remove the matrix.
• Bone is created over calcified cartilage as osteoblasts and capillaries move into the area.

Growth in Thickness and Width

• Bones grow in thickness or diameter through appositional growth
• Collagen fibers and hydroxyapatite crystals are secreted to form the matrix by periosteal stem cells that differentiate into osteoblasts,
• New lamellae are formed on the newly formed margin.
• The bone growth mechanism is more complex due to occurrences in 3-D.

Interstitial vs. Appositional Growth

• Interstitial growth is the growth pattern that lengthens the bone
• Appositional growth is the growth pattern that increases bone width.
• Interstitial growth is linear and each stack grows until about 22 years.
• Ongoing appositional patterns ebb and flow based on calcium, gravity, exercise, and age.

Fracture and Repair

• Any break in a bone counts as a fracture.
• First, a fracture hematoma (clot) forms.
• Second, the fracture hematoma is organized into granulated tissue(procallus) ,
• Third, the procallus is transformed into a fibrocartilaginous soft callus.
• Fourth, the soft callus is converted into a bony hard callus.
• Fifth, the callus is remodeled to nearly its original form.

Fracture Types

• Open or compound fracture: Skin is broken by the ends of the bones.
• Comminuted fracture: Fragments exist between two larger pieces of broken bone.
• Impacted fracture: Fragments are driven into each other.
• Greenstick fracture: Partial or hairline fracture.