The Skeletal System: An Overview

Introduction to the Skeletal System

• Sitting, standing, walking and breathing all require the skeletal system
• Serves as the structural framework that provides body shape
• Offers protection for internal organs and soft tissues

Components of the Skeletal System

• Cartilage
• Bones
• Joints
• Ligaments
• Dynamic and living tissue capable of growth

Functions of the Skeletal System

• Provides support and is well-suited for bearing weight
• Bone is hard and able to protects other organs
• Connective tissue allows skeletal muscles to attach to bones by tendons, facilitating movement.
• Storage: Minerals like calcium and phosphorus are stored
• Fat, or adipose tissue, also gets stored within bone cavities
• Blood cell production occurs in bone marrow-filled cavities, giving rise to blood cells and platelets

Cartilage

• The human skeleton begins primarily with cartilages and fibrous membranes, but bone eventually replaces most of these
• Cartilage remains where flexible skeletal tissue is needed.
• Consists mainly of water
• Contains no blood vessels and no nerves
• Surrounded by perichondrium: a dense irregular connective tissue layer
• Acts as a girdle to resist outward expansion when compressed
• Blood vessels and nerves penetrate the outer layer, but do not enter the cartilage matrix, instead nutrients diffuse through cartilage matrix

Types of Cartilage

• Hyaline provides support with flexibility and resilience, the most abundant skeletal cartilages
• Elastic contains stretchy elastic fibers
• Fibrocartilage is highly compressible with great tensile strength, has thick collagen fibers

Cartilage Growth

• Appositional growth: new matrix secreted against the external face of existing cartilage tissue
• Interstitial growth: chondrocytes divide and secrete new matrix, expanding cartilage from within

Number of Bones

• An average adult skeleton has 206 bones
• Although this is the traditional number, there is some variation between individuals
• The number of bones decreases with age as some bones become fused

Divisions of the Skeleton

• Axial: skull, vertebral column and rib cage
• Appendicular: bones of the upper and lower limbs, and girdles that attach them to the axial skeleton

Bone Tissue

• Compact bone is homogeneous
• Spongy bone has small needle-like pieces of bone and many open spaces

Bone Classification by Shape

• Long: longer than wide with a shaft and two ends. Examples are the femur and humerus
• Short: cube shaped. Examples are carpals and tarsals
• Flat: thin and flattened, often curved, with thin layers of compact bone around a layer of spongy bone. Examples include most skull bones, the sternum, scapulae and ribs
• Irregular: have complicated shapes, such as the vertebrae and coxae

Key Components of a Long Bone

• Diaphysis
• Epiphysis
• Epiphyseal plate

Diaphysis

• The shaft of the bone
• Primarily composed of compact bone
• Consists mostly of bone matrix with small spaces

Epiphysis

• Located at the end of the bone
• Primarily made of spongy bone
• Consists of small spaces/cavities surrounded by bone matrix
• Outer surface contains a layer of compact bone
• The epiphyses within joints are covered by articular cartilage

Epiphyseal Plate

• Is hyaline cartilage located between the epiphysis and diaphysis
• Bone growth in length occurs here
• The plate becomes ossified to the epiphyseal line

Medullary Cavity

• The diaphysis has a large space called the medullary cavity
• Cavities of cancellous bone and the medullary cavity are filled with marrow cells.
• Red bone marrow is the site of blood cell formation
• Yellow bone marrow contains mostly adipose tissue

Periosteum

• A connective tissue membrane covering the outer surface of bones
• The outer fibrous layer is dense, irregular collagenous connective tissue with blood vessels and nerves.
• The inner layer is a single layer of bone cells, which includes osteoblasts, osteoclasts, and osteochondral progenitor cells

Sharpey's Fibers

• They secure the periosteum to underlying bone

Endosteum

• A connective tissue membrane which lines the internal surfaces within bones
• Such as the medullary cavity of the diaphysis and the smaller cavities in cancellous and compact bone
• A single layer of cells, which includes osteoblasts, osteoclasts and osteochondral progenitor cells

Articular Cartilage

• Covers the external surface of epiphyses
• Made of hyaline cartilage
• Decreases friction at joint surfaces

Marrow in Children

• Spaces within bones are filled with red marrow

Marrow Maturation

• As children mature, yellow marrow replaces the red marrow in the skull and limbs

Marrow in Adults

• Bones of the skull and limbs, excluding proximal epiphyses, have yellow marrow
• The rest of the skeleton contains red marrow

Gross Anatomy of Short and Irregular Bones

• Consisting of periosteum-covered compact bone on the outside and endosteum-covered spongy bone within
• Flat bones contain a spongy bone layer called diploe
• Bone marrow between the trabeculae

Microscopic Bone Cell Classification

• Osteogenic (osteoprogenitor) cells
• Osteoblasts
• Osteocytes
• Osteoclasts

Microscopic Anatomy of Spongy Bone

• Interconnecting bone plates called trabeculae
• Spaces between trabeculae are filled with bone marrow and blood vessels
• Most trabeculae consist of lamellae with osteocytes between the lamellae
• Osteocytes are associated with each other through canaliculi
• No blood vessels penetrate the trabeculae
• Osteocytes must obtain nutrients through canaliculi
• Surfaces of trabeculae are covered with mostly osteoblasts and some osteoclasts

Microscopic Anatomy of Compact Bone

• An osteon/haversian system consists of a central canal (containing blood vessels and nerves), its contents, and concentric lamellae and osteocytes

Canaliculi

• Tiny canals radiating from central canals to all lacunae
• They transport nutrients to all osteocytes within the hard matrix

Volkmann's Canals

• Provide communication between bone layers and the bone's center
• These are perforated canals that are perpendicular to the bone length

Bone Markings

• Surface features of bones. Includes sites of attachments for muscles, tendons and ligaments
• Allows passages for nerves and blood vessels
• Projections/processes grow out from the bone surface
• Depressions/cavities are indentations

Bone Composition

• Organic components: include cells (osteoblasts, osteocytes, and osteoclasts) and osteoid, ground substance (proteoglycans and glycoproteins) and collagen fibers, contribute to flexibility and tensile strength
• Inorganic components: 65% of bone mass, mineral salts mainly hydroxyapatite, which accounts for hardness and compression resistance

Types of bone formation

• Cartilage occurs when the tissue is made of cartilage
• Bone occurs when the osseous tissue makes the tissue

Bone Formation & Remodeling

• Except for flat bones, which form on fibrous membranes, most bones develop using hyaline cartilage structures as their "models"

Ossification

• When bone matrix completely covers the bone-forming cells
• These cells are called osteoblasts
• Then the enclosed hyaline cartilage model is digested away, and a medullary cavity forms

Intramembranous Ossification

• Begins with mesenchymal cells which form a collagen membrane containing osteochondral progenitor cells
• Trabeculae just deep to the periosteum thicken, forming a woven bone collar that is later replaced with mature lamellar bone

Endochondral Ossification

• Begins with embryonic mesenchymal cells becoming that produce a cartilage template enclosed by the perichondrium.
• Process: the perichondrium of the diaphysis becomes the periosteum, and a bone collar forms internally as chondrocytes hypertrophy and calcified cartilage is formed

Bone Growth

• Bones increase in size only by appositional growth
• This process consists of the formation of new bone on the surface of older bone or cartilage.

Bone Growth in Length

• Long bones and bony projections increase in length due to growth at the epiphyseal plate
• Involves new cartilage formation by interstitial cartilage growth, followed by appositional bone growth on the cartilage surface

Zones of The Epiphyseal Plate

• Proliferation (growth)
• Hypertrophic
• Calcification
• Ossification (osteogenic)

Factors Affecting Bone Growth

• Determined genetically, but can be affected by factors such as nutrition and hormones

Nutrients Affecting Bone Growth

• Minerals including Calcium, Phosphorus, Magnesium, Boron, and Manganese
• Vitamins: Vitamin D increases intestinal absorption of dietary calcium and retards urine loss. Deficiency may causes rickets in children and osteomalacia in adults
• Deficiency of Vitamin C causes scurvy
• Vitamin A controls the activity, distribution, and coordination of osteoblasts & osteoclasts during development
• Vitamin B12 may play a role in osteoblast activity

Hormones Affecting Bone Growth

• Human Growth Hormone (hGH): secreted by pituitary, to stimulate reproduction of cartilage at epiphyseal plate
• Sex hormones (estrogens and androgens) to aid osteoblast activity . Estrogen effect is greater than androgen effect
• Thyroid hormones (T3 and T4) stimulate replacement of cartilage by bone in the epiphyseal plate
• PTH and calcitonin also play a role

Osteoporosis

• A condition is which there is bone loss
• Risk factors: lack of estrogen or vitamin D, petite body form, immobility, low levels of TSH and diabetes mellitus
• Calcium, vitamin D and fluoride help treat
• Drugs like Fosamax can help treat

Paget's Disease

• Excessive and disorganized bone formation and breakdown in the spine, pelvis, femur or skull
• High ratio of spongy to compact bone, results in in a reduced mineralization

Bone Remodeling

• Bones are remodelled to changes in: bone mass/calcium levels
• Osteoclasts removes old bone by secreting acids which dissolves the matrix
• New matrix is deposited by osteoblasts
• Wolff's law - Bone remodeling also occurs in the event of bone being pulled or stretched from muscles

Bone fractures

• Can be classified in 4 different classifications
• Position: displaced/nondisplaced
• Completion: complete/incomplete
• Axil:linear/transverse
• Skin: compound/simple

Bone Fracture Repair

• Includes four major events: Hematoma formation, fibrocartilage callus formation, bony callus formation, and bone remodeling.

Developmental Aspects

• The timing of ossification allows one to estimate the developmental age of emryos and fetuses
• At birth, most long bones are well ossified (except epiphyses) and nearly all bones are completely ossified by age 25. Decline in bone mass after the 4th decade
• Bone Resorption predominates in old age.