Skeletal System Anatomy and Physiology

Skeletal System Components

• The skeletal system comprises bones, cartilage, and ligaments.
• Cartilage serves as the embryonic framework for bones.

Bone Functions

• Bones provide support, protection, facilitate movement, and maintain electrolyte balance.
• Bones regulate acid-base balance and perform blood formation.

Bone Classification

• Bones are classified as long, short, flat, irregular, or sesamoid.
• The femur is a long bone
• The sternum is a flat bone
• Vertebrae are irregular bones
• Cuneiforms are short bones
• The Patella is a sesamoid bone.

Bone Categories by Location

• The axial skeleton is one category.
• The appendicular skeleton is the other category.

Bone Tissue Types

• Compact bone constitutes 1/4 of all bone tissue.
• Spongy bone contains the medullary cavity.
• Diploe is a characteristic feature of bone organization.

Bone Features

• Diaphysis is one feature.
• Epiphysis is another.
• Periosteum has an outer fibrous collagen layer and an inner osteogenic layer, with nutrient foramina.
• Endosteum is made of reticular CT, osteoclasts, and osteoblasts.
• Epiphyseal plate is a point of elongation where hyaline cartilage transforms into bone, ultimately becoming the epiphyseal line.

Bone: Composition

• Bone is made of matrix and cells.

Bone Cells

• The four types of bone cells are osteogenic cells, osteoblasts, osteocytes, and osteoclasts.
• Osteogenic cells are found in the endosteum and periosteum.
• Osteoblasts are non-mitotic, found in the endosteum and periosteum, secreting collagen that encrusts with hydroxyapatite.
• Osteocytes are trapped in the matrix and connected via canaliculi
• Osteoclasts originate from bone marrow cells, secrete acid phosphatase to digest collagen and HCl to digest minerals.

Bone Matrix

• Bone matrix is 1/3 organic and 2/3 inorganic matter, by weight 1/3 is organic.
• The organic matter of bone matrix consists of collagen, glycoproteins, and proteoglycans, which resist tension and bending.
• Inorganic matter is composed of 85% hydroxyapatite and resists compression.

Osseous Tissue Parts

• Lamellae are connected to other lamellae via canaliculi.
• Haversian canal is also known as the central canal.
• Osteon is an osseous tissue part.
• Volkmann canal (perforating canals) is another.

Spongy (Cancellous) Bone Parts

• Trabeculae develop along lines of stress and are parts of spongy (cancellous) bone.
• The spaces within spongy bone are filled with bone marrow.
• Providing an advantage of osseous tissue.

Bone Marrow: Types and Locations

• Bone marrow is soft tissue between trabeculae and within medullary cavities.
• Red bone marrow is hemopoietic tissue, and its distribution differs between children and adults.
• Yellow bone marrow consists mostly of adipocytes.
• Can revert to hemopoietic tissue.
• Over time becomes gelatinous bone marrow.

Ossification Types

• Ossification, also known as osteogenesis, has two types: intramembranous and endochondral.

Intramembranous Ossification

• Intramembranous ossification forms flat bones around week 8 of development.
• Mesenchymal cells differentiate into osteoblasts and form ossification centers.
• Osteoblasts create trabeculae, secrete a soft matrix, and deposit calcium phosphate.
• Forming trabeculae become spongy bone (diploe).
• Osteoblasts become osteocytes.
• Outermost trabeculae calcify, forming compact bone and periosteum.

Skull Changes After Birth

• Flat bones in the skull fuse via fibrous CT joints called sutures.
• Fontanels are present, with the frontal fontanel closing last.
• Sphenoidal and mastoidal fontanels close within the first year along with the occipital fontanel.

Endochondral Ossification

• With endochondral ossification, most bones are formed this way from about week 6 of development.
• Mesenchyme transforms into hyaline cartilage, which acts as a template for ossification.

Endochondral Ossification Steps

• A bone collar forms as blood vessels trigger periosteum formation.
• Osteoblasts then differentiate in the periosteum and form the bone collar
• Chondrocytes then swell (hypertrophy), signaling matrix mineralization.
• The collar cuts off circulation, leading to chondrocyte death.
• The primary marrow site then forms.
• Cavity is also invaded by blood vessels, hematopoietic cells, nerves, osteoblasts, osteoclasts
• Periosteal bud forms.
• Osteoblasts form a primary ossification center.
• They deposit osteoid matrix over calcified cartilage.
• Forming spongy bone as ossification spreads towards the epiphysis.
• This forms medullary cavity
• With the help of osteoclasts and secondary ossification centers.
• Spongy bone develops at heads as blood vessels invade the head.
• The development is similar to process in bone shaft.

Bone Growth After Birth

• Bones grow after birth via interstitial and appositional growth.
• Interstitial growth involves cartilage organizing in zones.
• New cartilage is added on the epiphyseal side.
• Osteoblasts invade and produce bone on the diaphysis side.
• Appositional growth occurs as well.

Endochondral Ossification Steps

• Zone 1: reserve cartilage zone.
• Zone 2: proliferation zone where chondrocytes multiply.
• Zone 3: hypertrophy zone.
• Zone 4: calcification zone.
• Zone 5: deposition zone, osteoblasts invade here, secrete osseous matrix.

Appositional Growth

• Appositional growth maintains bone shape through balanced bone remodeling for increased strength while retaining mass.
• Osteoclasts in the endosteum dissolve bone, expanding the medullary cavity, while osteoblasts in the periosteum produce new bone.

Bone Growth Factors

• In childhood, bone growth is regulated by growth and thyroid hormones.
• At puberty, sex hormones like estrogen and testosterone cause bone formation to outpace cartilage growth.
• Leading to the formation of the epiphyseal line, typically around age 18 in women and 21 in men.
• Osteon is ~200 um.

Moment of Inertia

• Is I=mr^2, a measure of mass density from the neutral axis affecting bending resistance.

Bone Remodeling

• Bone remodeling describes bone being resorbed and created.

Bone Growth Types

• Intramembranous and Endochondral ossification.

Skeletal System: Associated Pathologies

• Achondroplasia is an autosomal dominant genetic bone growth disorder, and involves a failure of epiphyseal cartilage to grow.
• Wolf's Law says that bone adapts to loads it's placed under, discovered by German anatomist Julius Wolff (1836–1902)
• Osteocytes respond to stress according to Wolf's Law.
• Bone remodeling is governed by a balance of osteoclast and osteoblast activity.

Bone stress

• Resorption is in response to decreased stress.
• Osteoclasts dominate.
• Ex: Disuse, immobilization, microgravity
• Deposition is in response to increased stress.
• Osteoblasts dominate.
• Ex: Weight-bearing exercise

Spine Shape Change

• Newborns have a C-shaped spine then it develops curvatures as it grows.
• Spinal curvature has a primary and secondary form.

Bone Health

• Bone mass loss of 1% per week can be caused by inactivity.
• Decreased bone size reduces moment of inertia.
• In torsion, a 20% decrease in diameter results in 60% reduction in strength.

Demineralization: Forms And Definition

• Demineralization refers to loss of calcium.
• Resulting in Osteoporosis or Osteomalacia (adults).
• Vit D deficiency or Rickets (children) are conditions affecting bone mineralisation.

Bone Homeostasis

• 5-7% of bone mass is recycled weekly.
• Head of femur recycles every 5-6 months.
• Use determines rate of replacement.
• At both endosteal and periosteal surfaces.

Bone Remodeling Process

• Dissolving of Bone, followed by deposition.
• Osteoclasts then secrete, excavating tunnel, releasing calcium.
• Osteoblasts then enter tunnel and secrete matrix
• Matrix mineralizes

Blood Calcium Regulation

• Calcium levels in blood are monitored.
• Thyroid and parathyroid hormones which include: Thyroid: calcitonin and Parathyroid: PTH, regulate blood calcium levels.

Calcium Imbalance: Low

• Hypocalcemia or too little calcium can cause: Excessive excitation of nerves, Tetanus, Laryngospasm

Calcium Imbalance: High

• Hypercalcemia is too much (rare) and can cause: Less excitable nerves, muscles, Muscle weakness, sluggish reflexes, Cardiac arrest possible

Bone Repair

• Bone repair is initiated.

Fracture Types

• Listed are Closed, Compound, Spiral, Greenstick, and Comminuted fractures but there are many more types
• Each type indicates a different kind of issue.

Fracture Treatment Options

• Reduction including Closed (Manipulation, No surgery) and Open (Surgical exposure, Screws, pins, etc.) reduction.
• Immobilization with a Cast or traction.

Fracture Healing

• Body repairs fractures similar to skin repair process.
• Fibrous callus forms, then Bone Callus forms, finally remodeling of spongy bone into original shape.

Skeletal Fractures

• Salter-Harris Fracture: Epiphysis fractures 5 classifications, Stunt growth in the involved limb, A portion of the damaged growth plate may remain functional (open) and thus the bone and limb becomes twisted
• Torsional Fracture: The radiograph illustrates the typical spiral oblique fractures of the tibia and fibula resulting from a torsional injury such as a skiing accident.
• An Ilizarov fixator: applied externally to stabilize fractures and lengthen limbs.

Osteoporosis

• Osteoporosis results in a loss of bone mass, especially in cancellous tissue, and is most common in elderly women.
• Prevention and drugs can help with this
• Osteoporosis: bone resorption > bone deposits
• Causes increased risk of fracture.
• The bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, altered amount/variety of non-collagenous proteins.
• WHO: bone mineral density 2.5 standard deviations below peak bone mass (20-year-old healthy female average) measured by DXA.
• Postmenopausal: often in women after menopause, can occur in men/presence of hormonal disorders, chronic diseases, meds.
• In the United States, nearly 10 million people already have osteoporosis.
• Another 18 million people have low bone mass.
• Bisphosphonates, Estrogens, SERMS and Calcitonin are treatments.

Bone Implants

• An artificial joint introduces several different materials into a bone and can no longer be modeled as a homogenous beam.

Calcium

• People older that 50 need to have 1200mg while adults 20-49 need 1000mg
• Less than 30% of calcium is actually absorbed

Cola Consumption

• Cola Consumption linked with Bone Mineral Density.

Exercise

• Exercise with impact or weight-bearing is preferred.
• Is a window of opportunity for building BMD with rigourous training before puberty, and with increased daily exercise and physical activity

Explore skeletal system anatomy and physiology. This quiz covers bone remodeling, fracture types, epiphyseal plate zones, bone matrix composition, and skeletal system components. Evaluate your understanding of bone structure and function.