Comprehensive Study Notes on Human Anatomy: Tissues and Skeletal System

Tissue Level of Organization

• Four Major Tissue Types:

  • Epithelial Tissue: Covers body surfaces, lines cavities, hollow organs, and ducts, and forms glands.

  • Connective Tissue: Supports, protects, and binds organs, stores fat energy, and aids immunity.

  • Muscular Tissue: Generates physical force for movement.

  • Nervous Tissue: Detects environmental changes and initiates nerve impulses to maintain homeostasis.

• Cell Junction Types:

  • Tight Junctions: Web-like strands of transmembrane proteins that seal off passageways between adjacent cells.

  • Adherens Junctions: Contain plaque (dense protein layer) and cadherins (transmembrane glycoproteins) anchored to actin microfilaments.

  • Desmosomes: Plaque and cadherins anchored to keratin intermediate filaments.

  • Hemidesmosomes: Integrins that anchor cells to laminin in the basement membrane rather than to adjacent cells.

  • Gap Junctions: Membrane proteins called connexins forming fluid-filled tunnels (connexons) for chemical/electrical communication.

• Epithelial Tissue Classification:

  • Arrangement Layers: Simple (single layer), Stratified (multiple layers), Pseudostratified (single layer appearing stratified).

  • Cell Shapes: Squamous (flat), Cuboidal (cube-like), Columnar (tall/cylindrical), Transitional (variable shape for stretching).

  • Glands: Exocrine (secrete into ducts; unicellular like goblet cells or multicellular) and Endocrine (secrete directly into bloodstream).

• Connective Tissue Classification:

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  • Extracellular Matrix: Ground substance (water, hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate) and Fibers (Collagen, Elastic, Reticular).

  • Cells: Fibroblasts, Adipocytes, White Blood Cells (Neutrophils, Eosinophils), Macrophages (Fixed/Wandering), Plasma cells, and Mast cells (produce histamine).

  • Loose Connective: Areolar, Adipose, Reticular.

  • Dense Connective: Dense Regular, Dense Irregular, Elastic.

  • Cartilage: Hyaline (most abundant, weakest, reduces friction), Fibrocartilage (strongest, intervertebral discs), Elastic (provides shape/flexibility, external ear).

  • Liquid Connective: Blood (plasma, erythrocytes, leukocytes, thrombocytes) and Lymph.

• Body Membranes:

  • Epithelial Membranes: Mucous (lines cavities opening to exterior), Serous (lines closed cavities; parietal and visceral layers), Cutaneous (skin).

  • Synovial Membranes: Lines freely movable joint cavities and secretes lubricating synovial fluid.

• Nervous & Muscle Tissue Components:

  • Muscular: Skeletal (attached to bone), Cardiac (heart wall), Smooth (walls of hollow organs/vessels).

  • Nervous: Neurons (convert stimuli into action impulses; cell body, axon, dendrites) and Neuroglia (supportive non-conductive cells).

• Medical Terminology:

  • Atrophy: Shrinkage of tissue/organ size due to decreased cell size.

  • Biopsy: Microscopic examination of living tissue samples.

  • Hypertrophy: Increase in tissue size from cell enlargement without cell division.

Overview & Structure of the Skeletal System

• Primary Functions: Structural support, internal organ protection, assistance in movement, mineral storage, blood cell production (hemopoiesis), and triglyceride storage.

• Bone Textures:

  • Compact Bone: Sturdy outer protective layer organized into osteons (Haversian systems), containing concentric lamellae, lacunae with osteocytes, canaliculi, and central (Haversian) canals.

  • Spongy (Trabecular) Bone: Internal network composed of a needle-like or flat lattice called trabeculae; lacks true osteons.

• Anatomy of a Long Bone:

  • Diaphysis: Main tubular shaft (primarily compact bone).

  • Epiphysis: Distal and proximal ends (primarily spongy bone).

  • Metaphysis: Region between diaphysis and epiphysis containing the epiphyseal plate (growth plate), which becomes the epiphyseal line in adults.

  • Periosteum: Tough connective tissue membrane covering the exterior bone surface.

• Microscopic Anatomy:

  • Matrix Composition: $25\%$ water, $25\%$ collagen fibers, and $50\%$ mineral salts.

  • Osteogenic Cells: Stem cells in periosteum and endosteum that divide to produce osteoblasts.

  • Osteoblasts: Bone-building cells that synthesize and secrete matrix.

  • Osteocytes: Mature bone cells that maintain matrix protein and mineral content.

  • Osteoclasts: Large multinucleated cells that resorb and break down bone matrix.

• Bone Surface Markings:

  • Depressions & Openings: Fissure (narrow slit), Foramen (opening for vessels/nerves), Fossa (shallow depression), Sulcus (furrow), Meatus (tube-like passageway).

  • Joint-Forming Processes: Condyle (large, rounded protuberance), Facet (smooth articular surface), Head (rounded projection supported on a neck).

  • Attachment Processes: Crest (prominent ridge), Epicondyle (projection above a condyle), Trochanter (very large projection), Tubercle (small rounded projection), Tuberosity (large rounded roughened projection).

Bone Classification

• Long Bones: Greater in length than width; function as levers for movement (e.g., femur, humerus, clavicle, radius, ulna, tibia, fibula).

• Short Bones: Cube-shaped, roughly equal in length, width, and thickness (e.g., tarsals, carpals).

• Flat Bones: Thin, protective curved plates serving as muscle attachment points (e.g., cranial bones, ribs, sternum, scapula).

• Irregular Bones: Complex, non-standard shapes (e.g., vertebrae, facial bones).

• Sesamoid Bones: Develop inside tendons exposed to high physical stress to protect against wear (e.g., patella).

The Axial Skeleton

• System Overview: Consists of $80$ total bones structured along the central axis of the body.

• The Skull ($22$ Bones Total):

  • Cranial Bones ($8$ Total): Enclose and protect the brain. Includes paired Parietal ($2$) and Temporal ($2$); single Frontal ($1$), Occipital ($1$), Sphenoid ($1$), and Ethmoid ($1$).

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  • Facial Bones ($14$ Total): Form facial structure. Includes paired Maxillae ($2$), Zygomatic ($2$), Nasal ($2$), Lacrimal ($2$), Palatine ($2$), and Inferior Nasal Conchae ($2$); single Vomer ($1$) and Mandible ($1$, the only movable skull bone).

• Associated Bones ($7$ Total):

  • Auditory Ossicles ($6$ Total): Located in middle ear cavities: Malleus ($2$), Incus ($2$), and Stapes ($2$).

  • Hyoid Bone ($1$ Total): U-shaped neck bone suspended by ligaments; supports the tongue and provides attachment for neck/pharyngeal muscles without articulating with other bones.

• Vertebral Column ($26$ Bones Total):

  • Cervical Region ($7$ Vertebrae, C1–C7): Neck segment; identified by transverse foramina. C1 (Atlas) lacks a body and spinous process, articulating with occipital condyles for nodding movements ("yes"). C2 (Axis) features a projection called the dens (odontoid process) serving as a pivot for head rotation ("no"). C4–C6 possess bifid spinous processes; C7 (vertebra prominens) features a long, non-bifid spinous process.

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  • Thoracic Region ($12$ Vertebrae, T1–T12): Mid-back segment; feature heart-shaped bodies, long inferiorly directed spinous processes, and costal facets for rib articulation.

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  • Lumbar Region ($5$ Vertebrae, L1–L5): Lower back segment; built for weight-bearing with massive kidney-shaped bodies and short, blunt spinous processes.

  • Sacrum ($1$ Bone): Composed of $5$ fused sacral vertebrae (S1–S5) forming the pelvic base.

  • Coccyx ($1$ Bone): Composed of $4$ fused coccygeal vertebrae forming the tailbone.

  • Column Curvatures: Four standard curves. Cervical and lumbar curves are convex anteriorly; thoracic and sacral curves are concave anteriorly.

• Thoracic Cage ($25$ Bones Total):

  • Sternum ($1$ Bone): Flat anterior bone comprising the manubrium, body, and xiphoid process.

  • Ribs ($24$ Bones / $12$ Pairs): True Ribs (Pairs 1–7, attach directly to sternum via costal cartilage), False Ribs (Pairs 8–12, attach indirectly or remain unattached), Floating Ribs (Pairs 11–12, subcategory of false ribs with no anterior sternal connection).

The Appendicular Skeleton

• System Overview: Consists of $126$ total bones supporting upper and lower limbs ($206$ total adult bones; infants are born with around $270$ bones that fuse during growth).

• Pectoral (Shoulder) Girdle ($4$ Bones): Bilateral Clavicles ($2$) and Scapulae ($2$) attaching upper limbs to the axial framework.

• Upper Limb Bones ($60$ Bones Total):

  • Arm: Humerus ($2$).

  • Forearm: Radius ($2$, lateral thumb side) and Ulna ($2$, medial pinky side featuring olecranon and trochlear notch).

  • Wrist & Hand ($54$ Bones Total):

    • Carpals ($16$ total, $8$ per wrist): Proximal row (Scaphoid, Lunate, Triquetrum, Pisiform); Distal row (Trapezium, Trapezoid, Capitate, Hamate).

    • Metacarpals ($10$ total, numbered $1$–$5$ lateral to medial).

    • Phalanges ($28$ total, $14$ per hand): Digits II–V contain proximal, middle, and distal phalanges; Digit I (thumb) lacks a middle phalanx.

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• Pelvic (Hip) Girdle ($2$ Coxal Bones):

  • Composition: Each hip bone forms from the fusion of three bones by age $23$: Ilium (superior), Ischium (inferior posterior), and Pubis (inferior anterior), uniting at the acetabulum socket.

  • Pelvic Regions: Greater (false) pelvis superior to pelvic brim; Lesser (true) pelvis inferior to pelvic brim.

  • Structural Sex Differences:

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    • Female Pelvis: Light and thin; shallow false pelvis; wide, oval inlet; small acetabulum facing anteriorly; pubic arch angle $>90^\bullet$; wide greater sciatic notch ($\approx 90^\bullet$).

    • Male Pelvis: Heavy and thick; deep false pelvis; narrow, heart-shaped inlet; large acetabulum facing laterally; pubic arch angle $<90^\bullet$; narrow greater sciatic notch ($\approx 70^\bullet$).

• Lower Limb Bones ($60$ Bones Total):

  • Thigh: Femur ($2$, longest and strongest bone in the body).

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  • Kneecap: Patella ($2$, triangular sesamoid bone).

  • Leg: Tibia ($2$, large medial weight-bearing shin bone) and Fibula ($2$, slender lateral non-weight-bearing bone).

  • Ankle & Foot ($52$ Bones Total):

    • Tarsals ($14$ total, $7$ per ankle): Talus, Calcaneus (heel), Navicular, Third Cuneiform, Second Cuneiform, First Cuneiform, Cuboid.

    • Metatarsals ($10$ total, numbered $1$–$5$ medial to lateral).

    • Phalanges ($28$ total): Big toe (hallux) contains proximal and distal phalanges; toes II–V contain proximal, middle, and distal phalanges.

    • Foot Arches: Supported by tarsals and metatarsals to distribute weight (Longitudinal Arch medial/lateral parts and Transverse Arch).

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Skeletal Pathology & Aging

• Age-Related Changes: As sex steroid levels decline with age, bone resorption exceeds bone formation, leading to calcium loss and brittle bones (Osteoporosis).

• Spinal Disorders & Pathologies:

  • Herniated (Slipped) Disc: Rupture of the surrounding fibrocartilage (annulus fibrosus) allowing posterior protrusion of the inner gelatinous nucleus pulposus, compressing spinal nerves.

  • Scoliosis: Abnormal lateral bending or curvature of the vertebral column, usually in the thoracic region.

  • Kyphosis: Exaggerated increase in the thoracic curvature producing a "hunchback" profile.

  • Lordosis: Exaggerated increase in the lumbar curvature producing a "hollow back" profile.

  • Spina Bifida: Congenital vertebral column defect where the laminae of L5 and/or S1 fail to develop normally and unite at the midline.