Skeletal System Overview

• The skeletal system consists of bones, associated connective tissues (cartilages, tendons, and ligaments), and joints.
• "Skeleton" refers to something "dried."
• The skeletal system provides support, protection, movement, storage, and blood cell production.

Extracellular Matrix

• Collagen provides tough, rope-like protein.
• Proteoglycans are polysaccharides attached to a core protein.
• Large amounts of collagen fibers are located in tendons and ligaments.
• Collagen and proteoglycans are both located in cartilage.
• Bones contain collagen and the mineral calcium phosphate in crystal form (hydroxyapatite).

Bone Categories:

• Long bones
• Short bones
• Flat bones
• Irregular bones

Long Bones

• The main shaft of a long bone is called the diaphysis.
• Each long bone has two epiphyses (ends).

Bone Structure

• Osteons (Haversian systems) are the basic structural units of compact bone.
• Endosteum is the inner membrane lining the medullary cavity of a bone.
• Periosteum is the outer fibrous layer covering the bone.
• Compact bone is the dense outer layer of bone.
• Spongy bone with trabeculae is the inner, less dense layer of bone.
• The medullary cavity contains red marrow in juveniles and yellow marrow in adults.

Bone Cells

• Osteoclasts are bone-resorbing cells that break down bone tissue.

Compact Bone: Haversian System

• Concentric rings of lamellae surround a central canal; these are part of the osteon structure.
• Canaliculi are small channels connecting lacunae.
• Lacunae are small spaces containing osteocytes.
• Osteocytes lie within lacunae.
• Blood vessels connect to central canals between osteons.

Spongy Bone

• Trabeculae are the irregular latticework of thin bony plates in spongy bone.
• Spaces within spongy bone contain bone marrow and blood vessels.
• Osteoblasts are bone-forming cells.
• Osteocytes are mature bone cells embedded in the matrix.

Ossification

• Intramembranous ossification occurs when osteoblasts produce bone within connective tissue membranes.
• Endochondral ossification happens within cartilage models.

Intramembranous Ossification

• Bones like the parietal, frontal, ethmoid, nasal, maxilla, zygomatic, and mandible are formed through this process.

Endochondral Ossification

• A cartilage model is formed by chondrocytes, surrounded by a perichondrium.
• Chondrocytes enlarge, cartilage calcifies, and a bone collar forms.
• A primary ossification center develops as blood vessels and osteoblasts invade the calcified cartilage, forming trabeculae
• Secondary ossification centers develop in the epiphyses of long bones.

Bone Anatomy

• Axial Skeleton*
• Skull: Composed of 22 bones in the adult human skeleton
  • Braincase (8 bones) includes:
    • Paired: Parietal (2), Temporal (2)
    • Unpaired: Frontal (1), Occipital (1), Sphenoid (1), Ethmoid (1)
  • Face (14 bones):
    • Paired: Maxilla (2), Zygomatic (2), Palatine (2), Nasal (2), Lacrimal (2), Inferior Nasal Concha (2)
    • Unpaired: Mandible (1), Vomer (1)
• Auditory Ossicles: Malleus (2), Incus (2), Stapes (2); Total of 6
• Hyoid: 1
• Vertebral Column: 26 bones
  • Cervical Vertebrae: 7
  • Thoracic Vertebrae: 12
  • Lumbar Vertebrae: 5
  • Sacrum: 1
  • Coccyx: 1
• Thoracic Cage
  • Ribs: 24
  • Sternum (3 parts, sometimes considered as 3 bones): 1; Total Thoracic Cage: 25
• Appendicular Skeleton*
• Pectoral Girdle: Scapula (2), Clavicle (2)
• Upper Limb: Humerus (2), Ulna (2), Radius (2), Carpal Bones (16), Metacarpal Bones (10), Phalanges (28)
• Pelvic Girdle: Coxal Bone (2)
• Lower Limb: Femur (2), Tibia (2), Fibula (2), Patella (2), Tarsal Bones (14), Metatarsal Bones (10), Phalanges (28)

Divisions of the Skeletal System:

• Includes the shoulder blades (scapulae), pelvis (ilium, pubis, ischium), and limbs

Axial Skeleton: Skull

• Contains 28 bones, including braincase, face, and auditory ossicles.

Axial Skeleton: Vertebral Column

• Consists of 26 individual bones in 5 regions:
  • Cervical Vertebrae: 7
  • Thoracic Vertebrae: 12
  • Lumbar Vertebrae: 5
  • Sacral Bone: 1
  • Coccyx Bone: 1

Axial Skeleton: Vertebral Column Disorders

• Scoliosis: Lateral curvature of the spine
• Kyphosis: Excessive outward curvature of the spine, resulting in a hunchback
• Lordosis: Excessive inward curvature of the lower spine

Axial Skeleton: Rib Cage

• Includes the ribs, sternum, and thoracic vertebrae.

Appendicular Skeleton: Pectoral Girdle and the Upper Limb

• Features include the clavicle, pectoral girdle, scapula, humerus, ulna, radius, carpal bones, metacarpal bones, and phalanges.

Appendicular Skeleton: Pectoral Girdle

• Composed of the scapula

Appendicular Skeleton: Clavicle

• Commonly known as the collarbone; part of the pectoral girdle.

Appendicular Skeleton: Humerus

• The bone of the upper arm.

Appendicular Skeleton: Radius and Ulna

• Bones of the forearm; radius is lateral, ulna is medial.

Appendicular Skeleton: Wrist and Hand

• Includes the carpal bones, metacarpal bones, and phalanges.

Appendicular Skeleton: Pelvic Girdle

• Consists of the coxal bones (hip bones), which articulate with the sacrum.

Appendicular Skeleton: Femur

• Thigh bone; the longest bone in the human body.

Appendicular Skeleton: Patella

• Kneecap; a sesamoid bone within the quadriceps tendon.

Appendicular Skeleton: Tibia and Fibula

• Bones of the lower leg; tibia is the shin bone, fibula is parallel and lateral to it.

Appendicular Skeleton: Foot

• Consists of tarsal bones, metatarsal bones, and phalanges.

Comparative Anatomy

• Endoskeleton*
• Definition: Internal skeleton, develops within the skin or deeper body tissues.
• Composition: Bones and cartilages
• Representative Organisms: Fish, birds, and mammals
• Exoskeleton*
• Definition: External protective and supporting structure.
• Composition: Proteins (sclerotin) and Chitin (polysaccharide molecules)
• Representative Organisms: Insects, crustaceans, and mollusks