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Bones: Structure, Function, Classification, and Hormonal Regulation

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Bones: Overview and Classification

Introduction

The human skeleton consists of 206 bones, which are divided into two major groups: appendicular and axial. These bones provide structure, protection, and support for the body, and play essential roles in movement, mineral storage, and blood cell formation.

  • Appendicular bones: Include bones of the upper and lower limbs, pelvic (hip) bones.

  • Axial bones: Include the skull, vertebral column, and rib cage.

Functions of Bones

  • Protection: The skull protects the brain, and the vertebral column protects the spinal cord.

  • Mineral Storage: Bones store essential minerals such as calcium, magnesium, and phosphorus.

  • Blood Cell Formation: Red bone marrow produces red blood cells through hematopoiesis.

  • Fat Storage: Yellow bone marrow contains adipocytes for fat storage.

  • Movement: Bones serve as attachment sites for muscles, acting as levers for movement.

  • Support: Bones maintain muscle tone and provide a framework for the body.

Classification of Bones

Bones are classified based on their shape:

  • Long bones: e.g., humerus, femur

  • Short bones: e.g., wrist (carpals), ankle (tarsals)

  • Flat bones: e.g., skull bones, rib cage

  • Irregular bones: e.g., vertebrae

  • Sesamoid bones: e.g., patella (kneecap)

Structure of Bones

Gross Anatomy of Long Bones

  • Epiphyses: Ends of long bones; contain red bone marrow.

  • Diaphysis: Shaft of the bone; contains marrow in the medullary cavity.

  • Periosteum: Double-layered membrane with nerves and blood vessels; entry point for vessels is the nutrient foramen.

Structure of Compact Bone

  • Hard, dense outer bone

  • Spongy inner bone: Honeycomb framework

  • Epiphyseal lines/plates: Growth plates; subject to growth hormone (GH) in children and young adults

Microscopic Structure of Compact Bone

  • Lamella: Hollow tubes, like growth rings

  • Central Canal (Haversian Canal): Contains blood vessels and nerves

  • Perforating Canals (Volkmann's Canals): Connect blood and nerves to periosteum

  • Osteocytes: Found in cavities called lacunae; maintain bone

  • Canaliculi: Tiny canals connecting lacunae

Structure of Flat Bones

  • Spongy bone in flat bones is called diploe.

Bone Marrow

  • Red bone marrow: Site of red blood cell formation

  • Yellow bone marrow: Site of fat storage

The Matrix of Bones

Inorganic Matrix

  • Comprises 65% of total bone weight

  • Mainly hydroxyapatite crystals (calcium and phosphate salts)

  • Provides strength and resistance to compression

  • Includes bicarbonate (HCO3-)

Organic Matrix

  • Comprises 35% of bone weight

  • Mainly collagen, glycoproteins, glycosaminoglycans

  • Contains proteins such as osteocalcin (binds calcium ions and hydroxyapatite crystals)

Bone Cells

  • Osteoblasts: Build bone; responsible for bone deposit

  • Osteocytes: Mature bone cells; maintain bone; reside in lacunae

  • Osteoclasts: Break down bone; responsible for bone resorption

Bone Diseases

Osteoporosis

  • Brittle, porous bones; femur is commonly affected

  • Occurs when bone resorption > bone deposit

  • Common in postmenopausal women (ages 60-70)

Osteomalacia

  • "Soft bones"; calcium salts are not deposited

  • Results in pain and bone weakness

Rickets

  • Osteomalacia in children

  • Symptoms: bowed legs, pelvic deformities

  • Caused by insufficient calcium or vitamin D

Paget's Disease

  • Excessive bone deposit and resorption

  • High ratio of spongy bone to compact bone

  • Causes weak bones; can affect any part of the skeleton

Achondroplasia

  • Most common cause of dwarfism

  • Decreased endochondral ossification

  • Average height is 4 ft; chest wall deformities

Bones and Hormones

  • Testosterone: Male sex hormone; increases appositional growth (width); epiphyseal plates close in boys by ages 16-17

  • Estrogen: Female sex hormone; epiphyseal plates close by ages 14-15

  • Calcitonin and PTH (parathyroid hormone):

    • Calcitonin inhibits osteoclasts and increases calcium deposit in bones

    • PTH activates osteoclasts and increases breakdown/release of calcium from bones

  • Growth Hormone (GH): Targets epiphyseal plates during growth; acromegaly occurs when GH is secreted after plate closure, causing abnormal growth of hands, feet, and jaw

Summary Table: Bone Types and Examples

Bone Type

Example

Long Bones

Humerus, Femur

Short Bones

Carpals (wrist), Tarsals (ankle)

Flat Bones

Skull, Ribs

Irregular Bones

Vertebrae

Sesamoid Bones

Patella (kneecap)

Key Equations

  • Bone matrix composition:

  • Hydroxyapatite formula:

Additional info:

  • Bone remodeling is a dynamic process involving osteoblasts and osteoclasts, regulated by hormones and mechanical stress.

  • Proper nutrition (calcium, vitamin D) and hormonal balance are essential for healthy bone development and maintenance.

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