A&P Exam 3 - Bone Physiology

Functions of the skeletal system

1. Support

2. Protection

3. Movement

4. Storage

5. Blood cell production

Function of the skeletal system - support

Bone is hard and rigid, cartilage is flexible yet strong

Function of the skeletal system - protection

Examples: Skull around brain; ribs; sternum, vertebrae protect organs of thoracic cavity (vital organs)

Function of the skeletal system - movement

• Produced by muscles on bones, via tendons (muscle to bone)

• Ligaments (bone to bone) allow some movement between bones but prevent excessive movement

Function of the skeleton system - storage

• Calcium (Ca²⁺) and Phosphorus (PO₄^-)

• Stored then released as needed

• Adipose tissue stored in marrow cavities

Function of the skeletal system - blood cell production

• Bone marrow gives rise to

  • Red Blood Cells (Transport O₂)

  • White Blood Cells (Immune defense)

  • Platelets (Clotting, Scab)

Components of skeletal system

1. Bone (spongy bone and compact bone)

2. 3 types of cartilage: hyaline, fibrocartilage, elastic

3. Tendons and ligaments

Cartilage structure - specialized cells that produce matrix

Chondroblasts, chondrocytes

Chondroblasts

Form matrix

Chondrocytes

Surrounded by matrix, within lacunae

Matrix structure

• Collagen fibers - strength

• Proteoglycans - resiliency

Perichondrium

Double-layered C.T. sheath, covers cartilage except at articulations

Perichondrium - inner layer

More delicate, fewer fibers, has chondroblasts

Perichondrium - outer layer

Blood vessels and nerves penetrate, no blood vessels in cartilage itself

Articular cartilage

• Covers bones at joints, has no perichondrium

• Thin layer of hyaline cartilage

• Reduce friction

Cartilage growth - appositional

• Location - inner layer of the perichondrium

• Growth - chondroblasts and new matrix at the periphery (surface). At the edges

Cartilage growth - interstitial

• Location - within the tissue

• Growth - chondrocytes within the tissue divide. Add more matrix between the cells

Bone matrix

Composed of organic and inorganic substances

Bone matrix - organic

• Fibers

• Composition: collagen and proteoglycans - Rebar

• Function: flexibility and resistance

• When removed: strength, but brittle because no flexibility

Bone matrix - inorganic

• Matrix

• Composition: hydroxyapatite. Composed of CaPO₄ crystals - cement

• Function: rigidity and resistance; withstand bending and weight-bearing forces

• When removed: collagen bone, flexible

Osteoblasts - function

• BUILD

• Form bone by producing bone matrix

  • Collagen produced by ER and golgi. Released by exocytosis

  • Precuroses of hydroxyapetite stored in vesicles, then released by exocytosis

Osteoblasts - origin

• Arises from an osteogenic cell - stem cells that develop from embryonic mesenchyme

• From endosteum and inner layer of the periosteum

• Stem cells - starts as mesenchyme and becomes osteochondral progenitor cells which become chondroblasts or osteoblasts

Osteoblasts - location

Endosteum and inner layer of the periosteum

Osteoblasts - misc.

• Ossification - formation of bone by osteoblasts

  • Communicate through gap junctions

  • Cells surround themselves by matrix

  • Become osteocytes afterwards

Osteocytes - function

• MAINTAIN

• Mature bone cells, stellate (star-shaped)

• Maintains the matrix - surrounded by matrix, but can make small amounts of matrix to maintain it

Osteocytes - origin

• Formed when osteoblast becomes surrounded by its own matrix and entrapped in a lacuna

• Basically from osteoblasts

Osteocytes - location

• Reside in lacunae - a small cavity or depression in a tissue such as bone or cartilage

  • Within bone matrix

Osteocytes - misc.

• Lacunae - spaces occupied by osteocyte cell body

• Canaliculi - canals occupied by osteocyte cell processes

• Nutrients diffuse in liquid surrounding cell and filling lacunae and canaliculi

  • Then can transfer nutrients from one cell to the next through gap junctions

Osteoclasts - function

• BREAK

• Reabsorption of bone - break down bone matrix

• Release acid and enzymes

  • H ions pumped across membrane, forming acid

  • Release enzymes that digest the bone

  • Derived from monocytes in red bone marrow

Osteoclasts - origin

• Monocytes (white blood cells)

  • Form into osteoclast

• Develop from same bone marrow stem cells as blood cells

Osteoclasts - location

Bone surfaces that are being broken down, in shallow depressions called Howship’s lacunae

Osteoclasts - misc.

Ruffled border - where cell membrane borders bone and reabsorption is taking place

Lacuna

Spaces occupied by osteocyte cell body, spaces that contain osteocytes

Canaliculi

Canals occupied by osteocyte cell processes, thin tubular connections between lacunae

Bone remodeling

Removing old bone and adding new bone

Woven bone

• Collagen fibers randomly oriented

• Formed during fetal development and fracture repair

• Remodeled into lamellar bone

Lamellar bone

• Mature bone in sheets called lamellae

• Fibers are oriented in one direction in each layer, but in different directions in different layers for strength

Differences between spongy bone and compact bone

• Spongy bone

  • Appears porous (lots of spaces filled with marrow)

  • Less bone matrix

  • More space

• Compact bone

  • More dense

  • More bone matrix

  • Less space (very compact together)

Spongy bone - location

Within the ends of long bones, pelvic bones, ribs, skull bones, and vertebrae in the spine

Trabeculae

• Interconnecting rods or plates of bone, like scaffolding

• Spaces filled with marrow

• Covered with endosteum (outer covering - layer within and lines spongy bone)

• Oriented along stress lines

Spongy bone - structure

• Most trabeculae are thin

• Several lamellae

• Blood vessels do not penetrate trabeculae

• Surfaces covered with single layer of cells

  • Osteoblasts

  • Few osteoclasts

Compact bone - structure

Arranged in osteons

Osteon

Cylindrical unit of bone that is the functional unit of compact bone

Central (Haversian) Canal

• Parallel to long axis, contains blood-vessels

• Vessels of the central canal, receive blood from perforating canals

• Surrounded by lamellae

Lamella

Tree rings

Concentric lamellae

Surrounds central canal

Circumferential lamellae

Around circumference, surrounds periphery of bone

Interstitial lamellae

Between osteons, remnants from bone remodeling

Perforating (Volkmann’s) Canal

• Perpendicular to long axis, contains blood vessels

• Blood vessels from periosteum penetrate bone

• Not surrounded by lamellae

Nutrients and wastes travel to and from osteocytes via

• Interstitial fluid of lacunae and canaliculi

• From osteocyte to osteocyte by gap junctions

Epiphysis

End of the bone, spongy bone, develops from a center of ossification distinct from the diaphysis

Diaphysis

Shaft, compact bone

Epiphyseal lines (adult)

Bone stops growing in length, site of old growth plate

Epiphyseal plate (youth)

• Plate of hyaline cartilage between epiphysis and diaphysis of long bone

• Growth zone for bone elongation

• Still capable of growing

Medullary Cavity

• Hollow part of bone that contains red marrow in juveniles and yellow marrow in adults

• Located within the diaphysis

Periosteum

• Double-layered connective tissue membrane covering the outer surface of bone except where articular cartilage is present

• Continous with ligaments and tendons

Periosteum - outer layer

Fibrous, dense irregular collagenous tissue

Periosteum - inner layer

Single layer of bone cells including osteoblasts, osteoclasts, and osteochondral progenitors

Periosteum - Sharpey’s Fibers

Tendon and ligament fibers penetrate through the periosteum into the bone, strengthen attachment of tendon to bone

Endosteum

Lines all internal spaces including spaces in spongy bone, such as trabeculae

Long bone

• Longer than wide and serves as a lever

• Anything that looks long

• Ex: femur, humerus, ulna

Flat bone

• Sandwich of spongy bone between compact bone

• No diaphyses, small epiphyses

• Ex: parietal bone from roof of skull, sternum, ribs

Irregular bone

• Irregular shape

• Ex: sphenoid bone from skull, sacrum, coccyx

Short bone

• Short shape

• Ex: carpal/wrist bone, tarsal/ankle bone, patella

Short and irregular bones - structure

• Compact bone that surrounds spongy bone center, similar to structure of epiphyses of long bones

• No diaphyses and not elongated

• Some flat and irregular bones of skull have sinuses lined by mucous membranes

Sinus

Hollowed out spaces in skull lined with mucous membranes