DAT: Skeletal System

Framework that provides structural support and protection for the body

Skeleton

Both cartilage and bone are made up

Connective tissue

• External skeleton made of protein layers

• found in invertebrates and all arthropods

Exoskeleton

• Internal skeleton made of bone and cartilage

• found in vertebrates

Endoskeleton

• Central axis of body

• Protects vital organs

• Includes:

  • Skull

  • Spine

  • Ribs

  • Sternum

Axial Skeleton

• Limbs (femur, radius) and girdles

• Main role = movement

Appendicular Skeleton

• Bone forms directly from mesenchyme

• Forms flat bones (skull, face, mandible, cavlicles)

Intramembranous Ossification

• initial cartilage cell formation that is gradually replaced by bone

• Forms long bones, spine, limbs, hands, feet, femur and phalanges

Endochondral Ossification

• dense Outer layer → strength & rigidity

• Organized into osteons

• surrounds medullary cavity

Compact (Cortical) Bone

• Inner bone, porous & flexible

• Highly vascular

• Contains red bone marrow

Spongy (Cancellous/Trabecular) Bone

Central canal containing blood vessels and nerves

Haversian Canal

structure of cortical bone connects the Haversian canals to the periosteum

Volkmann’s Canal 

concentric rings formed by osteoblasts in compact bone

Lamellae

space between lamellae where osteocytes may reside

Lacunae

• Osteocytes within lacunae can exchange nutrients via small canals

• osteocytes communicate extending parts of the cell outwards

Canaliculi 

• forms the framework of spongy bone

• Branched structures in spongy bone that create open spaces for marrow

Trabeculae

• Produces red blood cells (hematopoiesis)

• found in epiphyses of long bones

Red Bone Marrow

• Stores fat

• adipose cells concentrated in _

• found in the diaphysis of long bones

Yellow Bone Marrow

Mostly cortical bone; longer than wide; support movement (e.g., femur)

Long Bones

Cube-shaped; mostly cancellous bone surrounded by cortical bone; provide support (e.g., carpals)

Short Bones 

Two cortical layers surrounding spongy bone; protect organs (e.g., skull, ribs)

Flat Bones 

Small bones embedded in tendons to improve leverage (e.g., patella)

Sesamoid Bones

Complex shapes that don’t fit other categories (e.g., vertebrae)

Irregular Bones 

furthest End of a long bone; forms joints and contains red marrow

Epiphysis

long shaft with medullary cavity

Diaphysis

• Hollow center of the diaphysis

• contains yellow marrow for fat storage

Medullary Cavity 

Region between the diaphysis and epiphyseal plate; contains red marrow

Metaphysis

• Hyaline cartilage area that allows long bones to grow during childhood

• vertical bone growth in humans

• NO bone marrow

• located @ metaphysis

Epiphyseal Plate (Growth Plate)

Ongoing process of bone resorption and formation to maintain strength and calcium balance

Bone Remodeling

Bone formation by osteoblasts

Ossification

Bone breakdown by osteoclasts

Resorption

• differentiates into osteoblasts

• Used for bone growth, remodeling, and fracture repair

Osteoprogenitor Cells

• BUILD bone

• Make the organic bone matrix (mainly collagen): Collagen and organic compounds

• differentiate into osteocytes

• occurs if stress is put on a human bone

Osteoblasts

• MAINTAIN bone

• Former osteoblasts trapped in lacunae

• Exchange nutrients/waste with blood

• most abundant bone cell type i

• NO periosteum

Osteocytes 

• BREAK bone

• Multinucleated cells

• Release Ca²⁺ into blood

• resorbs bone matrix

Osteoclasts

Organic bone matrix made of collagen providing tensile strength

Osteoid 

• calcium in the bone matrix is stored as

• Inorganic mineral matrix made of calcium phosphate providing hardness

Hydroxyapatite

Increases blood calcium by stimulating osteoclasts and inhibiting osteoblasts

Parathyroid Hormone (PTH)

Increases calcium absorption from intestines; works with PTH in feedback regulation

Vitamin D

Lowers blood calcium by stimulating osteoblasts and inhibiting osteoclasts; secreted by thyroid gland

Calcitonin

• Connect immovable bones

• immovable and connect bones together extremely rigidly

• Example: skull sutures

Fibrous Joints

• Connect muscle → bone

• Allow muscles to move bones

Tendons 

• Connect bone → bone

• Stabilize joints

• Ex) ACL

Ligaments

Cells that produce and repair fibrous tissue

Fibroblasts

• Outer membrane covering bone

• Vascularized

• has fibrous and cambium layers providing nutrients and collagen attachment

• similar to Endosteum

Periosteum

Inner lining between cortical and cancellous bone; lines medullary cavity and bone canals

Endosteum

• Soft, flexible connective tissue

• Avascular (no blood vessels)

• Provides support + cushioning

• appears as triple helix

• receives nutrients via diffusion

Cartilage

Cells that build cartilage and mature into chondrocytes

Chondroblasts

Mature cartilage cells maintaining existing cartilage

Chondrocytes

• filled with fluid for lubrication

• Allow wide range of motion

• allows the skeleton to bend in certain regions

• Examples: elbow, knee, shoulder, wrist

Synovial Cartilage

Slightly flexible cartilage that reduces friction in joints (nose, trachea, ribs)

Hyaline Cartilage

Rigid cartilage that resists compression (intervertebral discs, meniscus)

Fibrous cartilage

Flexible cartilage allowing stretch (ears, epiglottis)

Elastic Cartilage 

Site where two or more bones meet; allows varying movement

Joint (Articulation)

• Immobile joints that hold the skull bones together

• Made of dense connective tissue

• Fibrous joints

• Function: hold bones together & protect the brain

Sutures

• Joints that connect bone to cartilage

• Provide shock absorption

• Example: intervertebral discs

Cartilaginous Joints

• Hard, strong, vascular tissue

• Most of the calcium (Calcium phosphate) in the body is stored in the __

• Provides:

  • Structural support

  • Protection

  • Movement

  • Blood cell production

  • soft tissue support

Bone

• first to form during the repair of the fracture

• Disorganized, weak

• Less mineralized

• both spongy & compact bone are this type

• Forms during:

  • Growth

  • Fracture healing

Woven (immature) Bone

• Organized, strong

• Highly mineralized

• Replaces woven bone over time

• Both spongy and compact bones are types of _

Lamellar (Mature) Bone

bone broken down

Low blood Ca²⁺

stored in bone

High blood Ca²⁺

point of attachment from muscle to stationary bone

Origin

• point of attachment of the muscle to the bone that moves

• Muscle contracts: _ pulls the bone

Insertion

MOST movement (shoulder/hip)

Ball & socket

bend like a door (elbow/knee)

Hinge

ROTATE only (neck)

Pivot

THUMB = unique

Saddle

wrist, oval-in-oval

Condyloid

small sliding bones

Gliding

Axial and appendicular

vertebrate skeleton

connects femur to tibia

ACL

• includes the lamellae and Haversian canals in compact bone

• fundamental unit of compact bone

Osteon

Bone helps to regulate energy metabolism

endocrine function

when cartilage cells have been replaced by bone, the epiphyseal plate becomes the_

Epiphyseal line