Bone Cells and Bone Structure

Types of Bone Cells

Bone tissue is maintained and remodeled by several specialized cell types, each with distinct functions and morphologies.

• Osteoprogenitor Cells: Stem cells of bone tissue; the only bone cells capable of division. They differentiate into osteoblasts.

• Osteoblasts: Bone-building cells responsible for synthesizing and secreting the bone matrix (osteoid). "B" is for Builder.

• Osteocytes: Mature bone cells derived from osteoblasts. They reside in small spaces called lacunae and maintain bone tissue.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone matrix. "C" is for Crusher.

Bone Matrix and Microstructure

• Osteon (Haversian System): The fundamental structural unit of compact bone, resembling a tree trunk in cross-section.

• Concentric Lamellae: Circular layers of bone matrix within an osteon, similar to tree rings.

• Lacunae: Small cavities between lamellae that house osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing nutrients and signals to pass between osteocytes.

• Central Canal: The central passage of each osteon, containing blood vessels and nerves.

• Perforating (Volkmann's) Canals: Transverse channels connecting central canals of adjacent osteons.

• Circumferential Lamellae: Large rings of bone matrix just beneath the periosteum.

• Interstitial Lamellae: Remnants of old osteons filling spaces between current osteons.

Bone Coverings and Cavities

• Periosteum: Tough outer membrane covering bone; contains nerves and blood vessels, aids in repair.

• Endosteum: Thin membrane lining the internal marrow cavity.

• Marrow Cavity: Central cavity in long bones, containing yellow (fatty) marrow.

• Sharpey's (Perforating) Fibers: Collagen fibers anchoring periosteum to bone.

Types of Bone Tissue

• Compact Bone: Dense outer layer forming the shaft of long bones; composed of tightly packed osteons.

• Spongy Bone (Cancellous Bone): Lightweight, porous inner bone containing trabeculae; found at bone ends and inside flat bones.

• Trabeculae: Lattice-like struts in spongy bone, providing structural support and housing red marrow.

Axial Skeleton

Overview

The axial skeleton forms the central axis of the body, including the skull, vertebral column, and rib cage. It provides protection for the brain, spinal cord, and thoracic organs.

• Skull: Protects the brain and forms the structure of the face.

• Vertebral Column: Supports the trunk and head, protects the spinal cord.

• Rib Cage: Protects the heart and lungs.

Skull

• Cranial Bones: Encase and protect the brain (e.g., frontal, parietal, occipital, temporal, sphenoid, ethmoid).

• Facial Bones: Form the face and jaw (e.g., maxillae, mandible, zygomatic, nasal, lacrimal, vomer, palatine, inferior nasal conchae).

• Key Features:

  • Foramina: Openings for nerves and blood vessels (e.g., foramen magnum, infraorbital foramen, mental foramen).

  • Sutures: Immovable joints between skull bones (e.g., coronal, sagittal, lambdoid, squamous).

  • Sinuses: Air-filled cavities that reduce skull weight (e.g., frontal, maxillary, sphenoidal, ethmoidal).

  • Fontanels (in infants): Soft spots allowing skull growth (e.g., anterior fontanel).

Vertebral Column

• Regions:

  • Cervical (7): Neck vertebrae; C1 (atlas) and C2 (axis) specialized for head movement.

  • Thoracic (12): Upper back; each articulates with a rib.

  • Lumbar (5): Lower back; largest and strongest vertebrae.

  • Sacrum: Five fused vertebrae forming the posterior pelvic wall.

  • Coccyx: Three to five fused vertebrae forming the tailbone.

• Vertebra Structure:

  • Body: Weight-bearing anterior portion.

  • Vertebral Arch: Formed by pedicles and laminae; encloses the vertebral foramen.

  • Processes: Spinous, transverse, and articular processes for muscle and ligament attachment.

  • Intervertebral Discs: Pads of fibrocartilage (annulus fibrosus and nucleus pulposus) between vertebral bodies; absorb shock.

  • Curvatures: Cervical and lumbar lordosis (inward), thoracic and sacral kyphosis (outward).

  • Abnormal Curvatures: Scoliosis (lateral), hyperlordosis, hyperkyphosis.

Thoracic Cage (Rib Cage)

• Ribs: 12 pairs; protect thoracic organs.

  • True Ribs (1–7): Attach directly to sternum via costal cartilage.

  • False Ribs (8–12): Attach indirectly or not at all to sternum.

  • Floating Ribs (11–12): No anterior attachment.

• Sternum: Flat bone at the anterior midline; consists of manubrium, body, and xiphoid process.

• Key Rib Features: Head, neck, tubercle, angle, body, costal groove.

Appendicular Skeleton

Pectoral Girdle and Upper Limb

• Clavicle: S-shaped collarbone; connects sternum to scapula, acts as a brace.

• Scapula: Triangular shoulder blade; features include spine, acromion, coracoid process, glenoid cavity.

• Humerus: Upper arm bone; articulates with scapula at shoulder and with radius/ulna at elbow.

• Radius and Ulna: Forearm bones; radius is lateral (thumb side), ulna is medial (pinky side).

• Carpals: Eight wrist bones arranged in two rows (proximal and distal).

• Metacarpals: Five bones forming the palm, numbered I (thumb) to V (pinky).

• Phalanges: Fourteen finger bones; three per finger, two in the thumb (pollex).

Pelvic Girdle and Lower Limb

• Coxal (Hip) Bones: Each formed by fusion of ilium, ischium, and pubis; together with sacrum and coccyx form the pelvis.

• Pelvic Features: Pelvic brim, inlet, outlet, pubic symphysis, obturator foramen, acetabulum.

• Sexual Dimorphism:

  • Male Pelvis: Narrow, deep, heavy, subpubic angle < 90°.

  • Female Pelvis: Wide, shallow, light, subpubic angle > 90° (adapted for childbirth).

• Femur: Thigh bone; largest and strongest bone in the body. Features include head, neck, greater/lesser trochanters, condyles, epicondyles.

• Patella: Kneecap; largest sesamoid bone, protects knee joint.

• Tibia: Medial, weight-bearing shin bone; features include condyles, tibial tuberosity, medial malleolus.

• Fibula: Lateral, slender bone; stabilizes ankle, does not bear weight.

• Tarsals: Seven ankle bones (e.g., calcaneus, talus, navicular, cuboid, three cuneiforms).

• Metatarsals: Five bones forming the foot arch, numbered I (big toe) to V (pinky toe).

• Phalanges: Fourteen toe bones; three per toe, two in the big toe (hallux).

• Foot Arches: Longitudinal (medial and lateral) and transverse arches distribute body weight and absorb shock.

Articulations (Joints)

Classification and Structure

Joints are sites where two or more bones meet, allowing for movement and providing stability. They are classified by structure and function.

• Structural Types:

  • Fibrous: Bones joined by dense connective tissue (e.g., sutures of the skull).

  • Cartilaginous: Bones joined by cartilage (e.g., intervertebral discs, pubic symphysis).

  • Synovial: Bones separated by a fluid-filled cavity; most movable type (e.g., knee, shoulder, hip).

• Functional Types:

  • Synarthrosis: Immovable (e.g., skull sutures).

  • Amphiarthrosis: Slightly movable (e.g., intervertebral discs, pubic symphysis).

  • Diarthrosis: Freely movable (all synovial joints).

Major Synovial Joints

• Shoulder (Glenohumeral Joint): Ball-and-socket; greatest range of motion, less stability.

• Hip (Coxal Joint): Deep ball-and-socket; high stability, supports body weight.

• Elbow: Hinge joint; allows flexion and extension.

• Knee: Largest hinge joint; includes menisci (fibrocartilage pads) for shock absorption and fit.

• Facet Joints (Spine): Plane synovial joints between articular processes; allow gliding movements.

Joint Structures

• Articular Cartilage: Hyaline cartilage covering bone ends in synovial joints.

• Joint Capsule: Encloses the joint cavity; lined by synovial membrane.

• Synovial Fluid: Lubricates and nourishes articular cartilage.

• Meniscus: C-shaped fibrocartilage pads in the knee; improve fit and absorb shock.

• Bursa: Fluid-filled sacs reducing friction between moving structures.

Selected Tables

Table: Comparison of Male and Female Pelvis

Table: Types of Ribs

Table: Types of Synovial Joints (Examples)

Key Terms and Definitions

• Osteon: Structural unit of compact bone.

• Trabeculae: Lattice-like structures in spongy bone.

• Fontanel: Soft spot in infant skull.

• Foramen: Opening in bone for nerves/vessels.

• Condyle: Rounded articular projection.

• Epicondyle: Raised area above a condyle.

• Process: Any bony prominence.

• Fossa: Shallow depression in bone.

• Facet: Smooth, flat articular surface.

• Meniscus: Fibrocartilage pad in knee joint.

• Bursa: Fluid-filled sac reducing friction.

Additional info:

• Bone matrix is composed of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components, providing both flexibility and strength.

• Red marrow (hematopoietic tissue) is found in spongy bone of flat bones and epiphyses of long bones in adults.

• Sesamoid bones (e.g., patella) develop within tendons to reduce friction and modify pressure.