Bones: Structure and Function

• Bones are organs comprising bone tissue, blood vessels, lymphatic vessels, and nerves
• Bone tissue is dynamic, constantly remodeling, constructing new bone and breaking down old bone.
• Osteology studies bone and treats bone disorders

Functions of Bones

• Provide support
• Protect organs via structures like the skull, vertebrae, ribs, and pelvic girdle
• Assist in movement by serving as attachment points for muscles
• Store minerals, especially calcium (Ca+2) and phosphorus (P), for homeostasis
• Facilitate hemopoiesis in red bone marrow
• Store triglycerides in adipose tissue within yellow bone marrow

Structure of a Long Bone

• Diaphysis: the shaft of the long bone
• Epiphysis: the ends, both proximal and distal
• Metaphyses: regions in mature bones where the epiphysis and diaphysis meet
• In immature bones, the epiphyseal plate contains hyaline cartilage, which is where bone lengthening occurs.
• Bone growth in length stops between 14-24 years when ossification of the epiphyseal plate occurs, forming the epiphyseal line
• Articular cartilage: hyaline cartilage on the epiphyses that absorbs shock and reduces friction, lacks perichondrium thus limited repair
• Periosteum: the outer surface of the diaphysis that is richly vascularized
• Attaches to bone via thick bundles of collagen that extend into the matrix
• Functions in ligament/tendon attachment, protection, repair, nourishment, and growth in thickness
• Two layers: an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer with osteoprogenitor cells
• Medullary cavity: contains fatty yellow bone marrow and blood vessels in adults
• Endosteum: lines the medullary canal and contains osteoprogenitor cells, osteoclasts, and some connective tissue cells

Histology of Bone Tissue

• Bone matrix is composed of fibers and ground substance
• Composition is approximately 15% water, 30% collagen, and 55% crystallized mineral salts.
• Calcium phosphate is the most abundant salt.
• Inorganic salts known as hydroxyapatites.
• Salts are placed in a framework of collagen
• Matrix is not entirely solid, forming tiny spaces for storage, cell processes, and blood vessels

Organic vs. inorganic matrix components

• Organic: proteins, especially collagen fibers, provide flexibility and tensile strength
• Inorganic: hydroxyapatites that provide hardness

Bone Cells

• Four types of bone cells include osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts

• All connective tissue is derived from mesoderm, and most mature connective tissues develop from mesenchyme

• Osteoprogenitor cells are sometimes called osteogenic cells

• Osteoclasts originate from monocytes

• Osteoprogenitor cells: stem cells from mesenchyme and are the only bone cells capable of mitosis, differentiating into osteoblasts.

• Osteoblasts: bone matrix building cells that secrete collagen and organic components of the matrix and initiate calcification

• Osteoblasts use calcium in the blood to build the matrix

• Active osteoblasts lower blood calcium levels

• Osteocytes: Mature bone cells trapped in the matrix that maintain bone tissue and are involved in the exchange of nutrients and wastes

• Osteoclasts: From fused monocytes, use lysosomal enzymes to digest the matrix and function in resorption, which is critical for calcium homeostasis

• Active osteoclasts raise blood calcium levels by releasing calcium from the matrix into the blood

Compact vs Spongy Bone

• Compact bone has fewer spaces than spongy bone and is the strongest form of bone tissue

• Compact bone is located in the bulk of the diaphysis of long bones, on the external surface of all bones, and on the inner surface at the medullary canal

• Blood vessels and lymphatic vessels enter via perforating canals (Volkmann's canals) that run perpendicular to the longitudinal axis of the bone; they run along the longitudinal axis within central canals and the medullary canal

• Compact Bone: Organized into repeating structural units called osteons, which include:

  • Central canal
  • Concentric lamallae of matrix
  • Lacunae in the matrix where osteocytes are located
  • Canaliculi (tiny canals extending from the lacunae)

• Interstitial lamellae exist between osteons

• Spongy Bone: Located inside of a bone

  • Makes up the bulk of short, flat, irregular, and sesamoid bones
  • Makes up most of the epiphysis of long bones
  • Also found as a narrow rim around the medullary canal
  • Reduces the overall weight of bone

• Spongy bone has no osteons and has trabeculae

• Newborns have virtually all red bone marrow, while adults have red marrow in the pelvis, ribs, sternum, vertebrae, skull, and proximal epiphyses of the humerus and femur

Blood and Nerve Supply of Bone

• Bone has an excellent blood supply and BVs often enter with nerves via the periosteum
• Periosteal arteries enter bone through Volkmann's canals, and the nutrient artery enters in the center of diaphysis via the nutrient foramen.
• Metaphyseal and epiphyseal arteries are also present

Bone Formation

• Also known as ossification or osteogenesis
• Occurs during initial bone formation in an embryo and fetus, growth of bones in infancy, childhood, and adolescence, remodeling of bone throughout life, and repair of fractures throughout life

Interstitial vs. Appositional Bone Growth

• Interstitial: growth from within existing tissue, occurs only in young, pliable cartilage during childhood and adolescence
• Appositional: growth at the surface through the deposition of new tissue
• Initial bone formation in an embryo and fetus involves layers of mesenchyme
• 2 mechanisms of ossification in the embryo and fetus: intramembranous and endochondral ossification

Intramembranous vs. Endochondral Ossification

• Intramembranous ossification: mesenchyme forms a connective tissue membrane directly into bone; occurs in flat skull bones, most facial bones, the mandible, and the medial portion of the clavicle
• Endochondral ossification: mesenchyme forms a hyaline cartilage template into bone; occurs in the long bones and most of the rest of the skeleton

Growth of Bones

• Bones lengthen by interstitial growth at the epiphyseal plate and thicken by appositional growth

Zones of the Epiphyseal Plate

• Zone of resting cartilage: Chondrocytes anchor the plate to epiphysis

• Zone of proliferating cartilage: Chondrocytes are undergoing interstitial growth

• Zone of hypertrophic cartilage: Large, maturing chondrocytes.

• Zone of calcified cartilage: Dead chondrocytes trapped in calcified matrix.

• Growth in Thickness:

  • Appositional growth on the surface
  • Periosteum has osteoprogenitor cells that differentiate into osteoblasts
  • Osteoblasts deposit matrix on the outer surface of bone, and they become trapped in the matrix, turning into osteocytes
  • Osteoclasts of the endosteum enlarge the medullary canal

Bone Remodeling

• Ongoing process to renew/respond to mechanical stress/repair
• Osteoclasts resorb matrix and osteoblasts deposit matrix
• Calcium and phosphorus ions are taken up by osteoclasts via endocytosis and released to interstitial fluid by exocytosis.

Factors Affecting Bone Growth and Bone Remodeling

• Include minerals, vitamins, hormones, exercise, and aging

Bone Fracture Repair

• Reactive Phase (Formation of fracture hematoma): Blood vessels break & phagocytes clean up.
• Reparative Phase
  • Fibrocartilaginous callus formation
  • Osteoprogenitor cells develop into chondroblasts & fibroblasts produce collagen.
• Bony callus Formation: Osteoblasts produce spongy bone trabeculae.
• Bone Remodeling: Osteoclasts resorb dead fragments of broken bone & compact bone replaces spongy bone

Bone's Role in Calcium Homeostasis

• Calcium levels (9-11 mg/100 ml blood), in bone matrix or blood
• Needed for exocytosis, nervous system activity, and muscle contraction
• Bone is the main reservoir for calcium in the body

Hormonal Control

• Parathyroid hormone (PTH): produced by parathyroid gland, targets osteoclasts and kidneys
  • Increases osteoclast activity
  • Causes kidney to reabsorb calcium and produce calcitriol
• Calcitonin: produced by thyroid gland, targets osteoclasts
  • Decreases osteoclast activity
  • Promotes deposition of calcium into the matrix