Skeletal System - Detailed Comprehensive Notes

Overview

The skeletal system includes bones, joints, and all supporting tissues. Bones are the main organs and are composed of osseous (bone) tissue plus dense collagenous connective tissues (tendons and ligaments) and bone marrow.
Functions of the skeletal system span protection, mineral storage, acid-base homeostasis, hematopoiesis, fat storage, movement as part of a lever system with muscles, and structural support.
Most calcium and phosphorus are stored in bones; red bone marrow produces red blood cells; yellow bone marrow stores fat.
The system works in concert with muscles to enable movement and with nerves and blood vessels to maintain homeostasis and repair.

Bone Composition and Tissue Types

Bone tissue (osseous tissue) is the primary tissue; it is a connective tissue with an extracellular matrix (ECM).
The ECM is split into two major components:
- Inorganic matrix (~65 ext{%} of bone weight) consisting mainly of minerals such as calcium and phosphate salts, including hydroxyapatite crystals $\mathrm{Ca{10}(PO{4}){6}(OH){2}}$ .
- Organic matrix (~35 ext{%}) consisting of collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and other bone-specific proteins.
Hydroxyapatite crystals provide hardness and resistance to compression; collagen provides tensile strength and resistance to torsion.
If collagen is absent, bones become brittle; if calcium is insufficient, bones become soft and cannot resist compression.
Bone is dynamic and constantly remodeled: old bone is broken down and new bone is formed.

Major Functions of the Skeletal System

Protection of vital organs (e.g., brain protected by skull; heart and lungs protected by thoracic cage).
Mineral storage and acid-base homeostasis: bones store ~85 ext{%} of the body’s calcium and a large amount of phosphorus.
Red bone marrow: hematopoiesis (blood cell formation).
Yellow bone marrow: fat storage.
Movement: bones act with muscles to form a lever system; support and shape the body.

Bone Shape Classes (Based on Shape, Not Size)

Long bones: longer than wide; include most arm and leg bones; examples include femur, tibia, humerus.
Short bones: roughly cube-shaped; lengths approximately equal to widths; examples include carpal (wrist) bones and tarsal (ankle) bones.
Flat bones: thin and broad; include ribs, pelvis, sternum; many skull bones are flat.
Irregular bones: do not fit other classes due to irregular shapes (e.g., vertebrae, some skull bones).
Incisemoid (sesamoid) bones: specialized bones within tendons; small, flat or oval; provide mechanical advantage for improved leverage; example: patella (kneecap).

Long Bone Anatomy and Growth Features

Long bones are covered by periosteum, a dense irregular collagenous connective tissue rich in blood vessels and nerves; Sharpey’s fibers (perforating fibers) anchor the periosteum to the bone.
Diaphysis: the shaft of a long bone.
Epiphyses: the ends of a long bone.
Articular cartilage: a thin layer of hyaline cartilage covering the epiphyses to reduce friction at joints.
Medullary (marrow) cavity: hollow cavity within the diaphysis, containing either red or yellow bone marrow depending on age.
Epiphyseal line (and growth plate): separation between proximal and distal epiphyses from the diaphysis; remnants of the epiphyseal (growth) plate in developing bones.
Endosteum: a thin connective tissue layer lining the marrow cavities and inner surfaces; contains bone cells involved in maintenance.
Spongy (cancellous) bone inside epiphyses features bony struts and a honeycomb-like framework; houses red marrow.
Compact bone forms the dense outer layer; provides resistance to linear compression and twisting forces.
In flat or irregular bones, two thin layers of compact bone surround a middle layer of spongy bone (diploë in the skull) and may contain sinuses.
Blood supply: about one-third of long bone length via vessels in the periosteum; the remaining two-thirds via nutrient arteries that enter through a nutrient foramen in the diaphysis.
Blood vessel networks support bone cells and marrow; bones have a rich supply of nerves.
Sinuses: hollow air-filled spaces in some skull bones; reduce bone weight.

Bone Marrow and Hematopoiesis

Red bone marrow: hematopoietic tissue producing red blood cells; primarily in epiphyses of long bones and in spongy bone.
Yellow bone marrow: adipose-rich tissue stored in the medullary cavity; serves as fat reserve.
With age, red marrow decreases and is replaced by yellow marrow in many bones.
Bone marrow transplants: used for certain hematologic diseases; procedure involves harvesting red bone marrow from a matched donor (often via pelvic bone) and transplanting into recipient; approximately two quarts can be removed; donor marrow replaces recipient marrow and begins producing new blood cells in 2–4 weeks; potential complications include flu-like symptoms during initial weeks.

Bone Cells and Cellular Homeostasis

Four major bone cell types:
- Osteogenic (osteoprogenitor) cells: stem-like cells that differentiate into osteoblasts.
- Osteoblasts: deposit the bone matrix (osteoid) and initiate mineralization; eventually become trapped in lacunae as osteocytes.
- Osteocytes: mature bone cells maintaining the extracellular matrix.
- Osteoclasts: perform bone resorption, secreting acids to dissolve calcium salts and release calcium and phosphates back into the bloodstream.
Osteoid: the organic matrix portion of bone; contains collagen, proteoglycans, glycosaminoglycans, glycoproteins, and other matrix proteins; collagen helps resist torsion and aligns with hydroxyapatite crystals to enhance hardness.
If osteoblasts deposit excessive matrix, they can trap themselves in lacunae and become osteocytes.
Bone remodeling involves continuous cycles of formation (osteoblasts) and resorption (osteoclasts); whole-system turnover is roughly on the scale of a decade in humans.

Histology of Bone Tissue

Compact bone: hard, dense outer layer that resists stress; organized into osteons (Haversian systems) – the functional units.
Each osteon consists of concentric lamellae surrounding a central (Haversian) canal lined by endosteum containing blood vessels and nerves.
Osteocytes reside in lacunae between lamellae and communicate via canaliculi (cytoplasmic extensions) forming a network.
The collagen fibers in adjacent lamellae run in opposite directions to resist twisting and bending forces.
Interstitial lamellae: remnants of osteons that have been partly resorbed during remodeling.
Circumferential lamellae: outer ring lamellae surrounding the bone, connected to the periosteum.
Perforating (Volkmann’s) canals: connect adjacent osteons and facilitate blood supply between them (like a water supply between buildings).
Spongy bone: porous, with a lattice of trabeculae; stress-resisting framework with less weight-bearing capacity but housing bone marrow.
Trabeculae consist of concentric lamellae with lacunae and osteocytes connected by canaliculi.
In disease: osteopetrosis is a primary defect where osteoclasts fail to resorb bone, leading to increased bone mass and brittleness.
- Infantile form: severe, genetic; skull openings and marrow cavities enlarge, can compress nerves and reduce hematopoiesis; may require drugs to stimulate osteoclasts.
- Adult form: adolescence or later; bone pain, recurrent fractures, nerve compression, joint pain; treatment is supportive.

Ossification and Growth of Bone

Ossification (osteogenesis) begins early in embryonic development and continues through childhood; most bones complete by age ~7.
Two main frameworks for bone formation:
- Intramembranous ossification: bone forms directly from mesenchymal tissue without a cartilage model; skull bones and clavicles predominantly form this way.
- Endochondral ossification: bones form by replacing a hyaline cartilage model; all bones below the head except the clavicles form via endochondral ossification.
Intramembranous ossification steps (skull bones):
- Mesenchymal cells differentiate into osteoblasts forming primary ossification centers.
- Osteoblasts secrete organic matrix that calcifies.
- Early trabeculae (woven bone) form; larger bones develop additional ossification centers.
- Spongy bone develops and is subsequently surrounded by compact bone; fontanels (soft spots) are areas where ossification is incomplete in newborns.
Endochondral ossification steps: hyaline cartilage model serves as scaffold; bone develops in a fetal stage and continues after birth; most bones ossify by age ~7.
- Formation starts with cartilage model growth and periosteum seeding osteogenic cells; chondrocytes die and leave hollows that calcify.
- Primary ossification center forms in the diaphysis with invasion of blood vessels; osteoblasts lay down bone matrix, replacing calcified cartilage.
- Medullary (marrow) cavity forms as osteoclasts dissolve center.
- Secondary ossification centers form in the epiphyses; articular cartilage and epiphyseal plates remain as cartilage for growth.
Epiphyseal plate (growth plate): hyaline cartilage plate between diaphysis and epiphysis responsible for longitudinal growth; contains zones of cartilage with distinct maturation processes (see Growth Zone details).
Epiphyseal lines mark the remnants after plate closure and bone growth ends.
Summary of endochondral growth steps: cartilage model → primary ossification center → medullary cavity formation → secondary ossification centers → ossification of epiphyses → articular cartilage and epiphyseal plate formation.
Notable terms: chondroblasts secrete cartilage matrix; perichondrium becomes periosteum that forms a bony collar; dying chondrocytes create hollow spaces that calcify; blood vessels invade to supply osteogenic cells.

Growth and Hormonal Regulation of the Skeleton

Osteoporosis: a common bone disease in the United States characterized by weakened bones due to inadequate inorganic matrix (calcium/phosphate) leading to higher fracture risk and slower healing.
- Diagnosis: bone density measurements (DEXA scans).
- Causes: dietary calcium/vitamin D deficiency, gender (female), aging, lack of exercise, hormonal factors (especially estrogen depletion postmenopause), genetics, and other diseases.
- Prevention and treatment: balanced diet, calcium and vitamin D supplementation, weight-bearing exercise to stimulate osteoblast activity, estrogen replacement when appropriate, and drugs that inhibit osteoclasts or stimulate osteoblasts.
Growth in length (longitudinal growth) occurs via the epiphyseal plate with zones of cartilage:
- Zone of proliferation: chondrocytes divide and push growth outward.
- Zone of hypertrophy and maturation: chondrocytes enlarge and lacunae expand.
- Zone of calcification: matrix calcifies.
- Zone of ossification: cartilage replaced by bone.
Growth in width (appositional growth): osteoblasts between periosteum and bone surface lay down new bone forming circumferential lamellae; this does not immediately produce new osteons but forms new outer layers.
Achondroplasia: most common form of dwarfism due to a gene defect (often inherited) that produces an abnormal growth factor receptor (FGFR3) affecting endochondral ossification and possibly articular cartilage and epiphyseal plates; results in shorter limbs with a relatively long trunk and facial abnormalities; complications include joint disorders and spinal issues.
Growth hormone (GH) and sex hormones regulate bone growth:
- GH (from anterior pituitary) enhances protein synthesis and cell division in many tissues, including bone; effects on both longitudinal and appositional growth include:
- Increases chondrocyte proliferation at the epiphyseal plate.
- Increases activity of osteogenic cells and stimulates osteoblasts for appositional growth.
- Testosterone: promotes appositional growth, leading to thicker bones with more calcium salt deposition in males.
- Estrogen: promotes longitudinal growth but accelerates epiphyseal plate closure, leading to height differences between sexes; higher estrogen in teens leads to a growth spurt but faster plate closure.
Growth hormone disorders:
- Gigantism: excess GH during childhood when growth plates are still open, causing excessive longitudinal growth and very tall stature.
- Acromegaly: GH excess after plate closure in adulthood; results in enlarged bones of the skull, face, hands, feet, and tongue; can cause heart and kidney problems and hormonal/metabolic complications; treated by tumor removal when possible.

Bone Remodeling and Calcium Homeostasis

Bone remodeling is a continuous process of bone formation and resorption that maintains calcium ion homeostasis, replaces primary bone with secondary bone, and aids in bone repair and adaptation to mechanical stress.
Remodeling cycle: osteoblasts deposit bone while osteoclasts resorb bone; turnover occurs gradually and the entire skeleton may be renewed over roughly ten years.
Factors influencing remodeling:
- Mechanical: compressive loads and tension from exercise stimulate osteoblast activity and bone formation.
- Hormonal: calcium-regulating hormones modulate osteoclast and osteoblast activity.
- Nutritional: adequate dietary calcium and vitamins (notably vitamins D, C, and K) support remodeling.
Parathyroid hormone (PTH) and calcitonin regulate calcium levels:
- Parathyroid hormone (PTH): released when blood calcium is low; promotes osteoclast-mediated bone resorption to release calcium; increases renal reabsorption of calcium; enhances intestinal calcium absorption; stops once calcium returns to normal.
- Calcitonin: released when blood calcium is high; inhibits osteoclast activity; promotes calcium excretion via kidneys and reduced intestinal absorption; supports bone formation by osteoblasts and lowers blood calcium.

Fractures and Bone Repair

Fracture types:
- Simple fracture: skin and surrounding tissue intact; bone not protruding through skin.
- Compound (open) fracture: skin/tissues damaged; bone may protrude through skin.
- Spiral fracture: twisting forces produce an oblique fracture line.
- Comminuted fracture: bone broken into multiple fragments.
- Greenstick fracture: incomplete break; common in children with more flexible bones.
- Compression fracture: bones crushed under heavy load; common in elderly (e.g., vertebrae).
- Avulsion (avulsed) fracture: tendon or ligament pulls off a fragment of bone.
- Epiphysial plate fracture: involves the growth plate and can interfere with bone growth in children.
Stages of bone repair (four main stages):
1) Hematoma formation: blood vessels bleed, forming a hematoma that fills the gap between bone fragments.
2) Fibrocartilaginous (soft) callus formation: fibroblasts and chondroblasts create a fibrous connective tissue callus bridging the fracture.
3) Bony (hard) callus formation: osteoblasts replace the soft callus with a hard callus of new bone.
4) Remodeling: osteoclasts and osteoblasts remodel the repair tissue to restore the bone's original shape and strength.
This repair process restores stability and function over time; complications can arise if healing is impaired.

Clinical Considerations and Additional Notes

Fontanels in infants: incomplete ossification of skull fontanels allows skull to mold during birth; they typically ossify as growth proceeds.
Endochondral vs intramembranous ossification underscores why some bones (e.g., most long bones) form via endochondral processes while skull bones form intramembranously.
Weight-bearing activity and resistance training promote bone density and remodeling through increased osteoblast activity, while aging can shift the balance toward osteoclast activity, reducing bone mass if intake and activity are inadequate.
Understanding osteoporosis and remodeling informs treatment strategies including lifestyle, nutrition, and pharmacologic interventions to manage bone density and fracture risk.
The skeletal system’s interaction with endocrine signals (GH, estrogens, androgens, PTH, calcitonin) illustrates the integration of metabolism, growth, and aging.

Quick Reference Figures and Concepts (to memorize)

Osteon structure: central canal (blood vessels and nerves) → concentric lamellae → osteocytes in lacunae → canaliculi; interstitial and circumferential lamellae; periosteum and endosteum.
Bone matrix composition: $65\%\text{ inorganic (Ca and PO4 salts, hydroxyapatite)}$ , $35\%\text{ organic (type I collagen, proteoglycans, glycoproteins)}$ .
Hydroxyapatite: $\mathrm{Ca{10}(PO{4}){6}(OH){2}}$ crystals embedded in collagen to provide hardness.
Growth plate zones: proliferation, hypertrophy/maturation, calcification, ossification.
Hormones and effects: Growth hormone enhances chondrocyte proliferation and osteoblast activity; estrogen closes growth plates faster; testosterone stimulates appositional growth; PTH increases serum calcium via bone resorption; calcitonin lowers serum calcium via inhibition of osteoclasts.
Remodeling cycle: osteoblasts vs osteoclasts balance to maintain calcium homeostasis and bone integrity over time.