Introduction to the Skeletal System

Introduction

SKELETON

Skeletal Framework of Vertebrates

Shape

Support

Protection

Muscle attachment (locomotion)

Exoskeleton (Dermal Skeleton)

Primitive trait

Teeth, membrane bones of skull (higher chordates = dermal)

Endoskeleton

Distinguishing characteristic (chordates)

Appears earlier in ontogeny

GENERAL STRUCTURE

Mineralized connective tissue deposited in collagen (Matrix)

Bone

Cartilage

Enameloids

Dentin

Ligaments vs Tendons

Ligaments—Bones to bones

Tendons—Muscles to bones

FORMATION OF MINERALIZED STRUCTURES

FORMATION OF SKELETAL TISSUES

To memorize the formation of skeletal tissues, you can use the mnemonic "Fabulous Frogs Create Colorful Bundles, Dancing Delightfully". Here’s how it breaks down:

Fabulous - Fibroblasts
Frogs - Fibrils
Create - Collagen Fibers
Colorful Bundles- Collagen Bundles
Dancing - Dense Connective Tissue
Delightfully- Deposition of Minerals

CARTILAGE

Avascular (no blood) — Specialized connective tissue of chondrocytes in lacunae in a matrix (chondromucin)

Perichondrium — Dense connective tissue

Mesodermal — Origin of cartilage; head and gill region (originate from neural crest cells)

*no canaliculi, no blood vessels

FORMATION OF CARTILAGE

To help you memorize the key components in the formation of cartilage, you can use the mnemonic "Charming Mice Form Amazing Fuzzy Chins". Here’s how it breaks down:

Charming - Chondroblast
Mice - Mucopolysaccharides in matrix
Form - Formation of perichondrium
Amazing - Addition of cartilage from perichondrium, fibroblasts, and chondroblasts
Tunnels - Transforming to become Chondrocytes

TYPES OF CARTILAGE

*Hyaline Cartilage*	Most abundant, least differentiated Precursor (replacement bone) Articular surfaces of bone
*Fibrous Cartilage*	Intervertebral discs Attachment of tendons and ligaments
*Elastic Cartilage*	External ear and epiglottis
*Calcified Cartilage*	Calcium salts are deposited Within interstitial substance of hyaline cartilage/fibrocartilage

BONE

Vascular (with blood) — Connective tissue (calcified bone matrix)

Bone matrix — Osteoblasts and osteocytes (water & mucopolysaccharides; cementing substance)

Inorganic component — Calcium hydroxyapatite (Calcium, Phosphate, & Hydroxyl ions)
Organic component — Type I Collagen (amorphous ground substance)

TYPES OF BONE CELLS

*Osteogenic Cells/ Scleroblasts*	Stem cells (mesenchyme) Foundation of connective tissues
*Osteoblasts*	Bone formation Growth regulation Secretes organic and mineral substances (ossification)
*Osteocytes*	Osteoblasts —> own intercellular deposits Maintains cellular activities (bone tissue)
*Osteoclasts*	Contains enzymes for bone resorption (release of ion stores) Contains hormone receptors for regulation

CLASSIFICATION OF BONES

*SHAPE*	Long bone Short bone Sesamoid bone Flat bone Irregular bone
*STRUCTURE*	Compact bone Lamellae around haversian canal *Haversian system* — Canal + Surrounding lamellae (absent in amphibians) Spongy bone Bone trabeculae + bone marrow
*ORIGIN*	Intramembranous Ossification Membrane bones from connective tissue Bones of lower jaw, skull, pectoral girdle, dermal bones, & vertebrae (teleosts, urodeles, apodans) Either: *Compact* or *Spongy, Lamellar* or *Non-Lamellar* Endochondral Ossification Bone from hyaline cartilage
*FUNCTION*	Support (soft tissues, muscles) Locomotion Protect vital organs (skull, ribs, vertebrae) Hematopoiesis (RBC production in bone marrow) Reservoir (Calcium, Phosphate)
*POSITION*	Axial Skeleton Notochord, vertebral Column Ribs + Sternum; Skull & visceral skeleton Appendicular skeleton Pectoral + Pelvic girdles Skeleton — paired fins, paired limbs; median fins of fishes Heterotopic skeleton

HETEROTOPIC BONES

Formation of marrow-containing bone outside of normal skeleton

*OS CORDIS*	Deer, Bovines
*OS PENIS*	Dogs, Basal Primates, Other Mammals
*OS CLITORIDIS*	Female Mammals
*GIZZARD BONE*	Doves
*TONGUE*	Bats
*GULAR POUCH*	South American Lizard
*DIAPHRAGM*	Camel
*UPPER EYELID*	Crocodilians
*OS ROSTRI*	Swine
*CLOACAL BONES*	Lizards

DENTIN

Dentinal Tubules — Canaliculi, coated in enamel

Found in scales of basal ray-finned & elasmobranch fishes + teeth

Skeletal Remodeling

CALCIUM STORAGE + WITHDRAWAL

Calcium — Deposited/withdrawn from storage (amount of calcium in serum)

Withdrawal — Regulation of parathyroid gland & calcitonin

SKELETAL REMODELING

Purpose — Accommodates organs it protects (possibly from stress; THICKER BONES)

Characteristics — Roughened surface areas, bony ridges, prominence in muscle attachments from sustained muscle use

Tendons, Ligaments, and Joints

TENDONS VERSUS LIGAMENTS

TENDONS

MUSCLE-TO-BONES

Maximal resistance to tension when muscle contracts

Continuous with epimysium of muscles + perichondrium (periosteum of cartilages/bones)

LIGAMENTS

BONE-TO-BONE

Less regular than tendon; directly continuous with periosteum

Largest ligament in mammals (nuchal ligament)

TYPES OF JOINTS

Joint — Where two bones/cartilages meet

FIBROUS (synarthroses)

Minimal to no movement

Function — Hold bones together

Sutures — Flat bones of cranium
Syndesmoses — Between two bones
Gomphoses — Specialized joint; holds teeth in plate

CARTILAGINOUS (amphiarthroses)

Limited movement

Function — stretching or compression

Hyaline cartilage joint (synchondroses) — Growing bone; lost in mature animals
Fibrocartilaginous joint (symphyses) — Mature skeleton; ossifies with age

SYNOVIAL (diarthroses/true joints)

Wide range of movement

Single plane or multiple planes

Number (articulating surfaces) — Simple, complex, compound

Components

Two bones — Covered by hyaline cartilage
Joint cavity — Lies between two bones
Joint capsule — Outer, fibrous layer (protection, strengthening), & inner, synovial layer (viscous, slick synovial fluid)

FUNCTION AND MOVEMENTS OF SYNOVIAL JOINTS

*FLEXION & EXTENSION*
*HYPEREXTENSION*
*ABDUCTION & ADDUCTION*
*ROTATION*
*CIRCUMDUCTION*

SHAPE/FORM OF SYNOVIAL JOINTS

*PLANE*	Gliding/sliding movements Multi-axial Example: Carpals of wrist
*HINGE*	Flexion & extension in one plane Example: Elbow
*PIVOT (TROCHOID)*	One bone rotates to another Example: Atlanto-axial joint, proximal radioulnar joint, distal radioulnar joint
*ELLIPSOIDAL (CONDYLAR)*	Two bones fit together (concave, convex) Flexion, extension, abduction, & adduction Example: Wrist joint
*SADDLE*	Similar but better movement than ellipsoidal Example: Carpo-metacarpal joint, sternoclavicular joint
*BALL-&-SOCKET*	Allows all movements except gliding Example: Shoulder, hop joints