Comprehensive Review Notes: Human Anatomy, Cell Biology, Histology, and the Integumentary System

Body Organization, Terminology, and Homeostasis

• Anatomy vs. Physiology: Anatomy studies body structures and their physical relationships; Physiology studies how those structures function. The Principle of Complementarity states that function always reflects structure.

• Levels of Biological Organization: Chemical level (atoms and molecules) $\rightarrow$ Cellular level $\rightarrow$ Tissue level $\rightarrow$ Organ level $\rightarrow$ Organ system level $\rightarrow$ Organismal level.

• Characteristics of Life: Energy production/consumption (ATP primary energy source, heterotrophic intake), Growth/repair, Adaptation/responsiveness, and Reproduction (essential for species survival).

• Survival Needs: Nutrients, oxygen, water, normal body temperature, and appropriate atmospheric pressure.

• Homeostasis: The active maintenance of stable internal conditions around established physiological set points.

  • Control Loop Components: Stimulus $\rightarrow$ Receptor/Sensor (detects change) $\rightarrow$ Afferent pathway $\rightarrow$ Control Center (integrates signal) $\rightarrow$ Efferent pathway $\rightarrow$ Effector (executes response).

*   **Negative Feedback**: Reverses or reduces the direction of the original stimulus to return the variable to its homeostatic set point (e.g., thermoregulation via sweating/vasodilation, insulin regulation of blood glucose).
*   **Positive Feedback**: Amplifies or accelerates the original stimulus, pushing the variable further from its initial state to control infrequent, non-continuous events (e.g., oxytocin during labor contractions, platelet plug formation during blood clotting).

• Anatomical Position: Standing upright, feet together, eyes facing forward, arms at sides, palms facing forward.

• Directional Terms: Anterior (ventral) / Posterior (dorsal), Superior (cranial) / Inferior (caudal), Medial / Lateral, Proximal / Distal (limb references relative to attachment point), Superficial / Deep.

• Body Planes & Cavities:

  • Planes: Transverse (horizontal), Frontal (coronal), Sagittal / Midsagittal.

  • Dorsal Cavity: Cranial cavity (contains brain) and Spinal cavity (contains spinal cord).

  • Ventral Cavity: Thoracic cavity (Pleural cavities for lungs, Pericardial cavity for heart, Superior mediastinum) and Abdominopelvic cavity (Abdominal cavity, Pelvic cavity). Anatomically divided into 4 quadrants or 9 regions.

Cellular Structure, Transport, and Communication

• Primary Biomolecules:

  • Nucleic Acids: Nucleotide monomers linked by phosphodiester bonds (DNA, mRNA, tRNA, rRNA).

  • Proteins: Amino acid monomers linked by peptide bonds; execute cellular tasks and structural functions.

  • Lipids: Phospholipids (membrane structure), Cholesterol (membrane stabilization and steroid precursor), Acylglycerols (energy storage), and Waxes (protective barriers).

  • Carbohydrates: Monosaccharide monomers linked by glycosidic linkages; function in energy storage and cellular recognition tags.

• Plasma Membrane Structure:

  • Fluid Mosaic Model: A thin ($7\text{--}10\,\text{nm}$) phospholipid bilayer embedded with cholesterol ($\sim 20\%$), integral proteins, peripheral proteins, and surface carbohydrates (glycocalyx).

  • Fluidity Factors: Higher temperatures increase fluidity; saturated fatty acids decrease fluidity; unsaturated fatty acids increase fluidity. Cholesterol acts as a temperature buffer.

• Membrane Transport:

  • Permeability: Permeable to small, nonpolar solutes (oxygen, carbon dioxide, steroid hormones); Impermeable to large, polar solutes and charged ions (carbohydrates, proteins, ions).

  • Passive Transport (down concentration gradient, no ATP): Simple diffusion, Osmosis (water movement causing isotonic, hypertonic, or hypotonic cell volume changes), and Facilitated diffusion (utilizing channel proteins or carrier proteins). Channel types include voltage-gated, ligand-gated, and mechanosensitive channels.

  • Active Transport (against concentration gradient, requires ATP): Primary active transport (e.g., Sodium-potassium pump pumping $3\,\text{Na}^+$ out and $2\,\text{K}^+$ in per ATP), Secondary active transport (cotransport), and Vesicular transport.

• Modes of Cell Signaling:

  • Autocrine: Soluble messenger binds to receptors on the same cell that secreted it.

  • Paracrine: Soluble messenger acts on nearby target cells.

  • Endocrine: Hormones travel through the bloodstream to act on distant target cells.

• Cell Junctions:

  • Adherens Junctions: Mechanical attachment using cadherins and actin filaments.

  • Desmosomes: Provide structural elasticity and cell-to-cell adhesion using cadherins and intermediate filaments.

  • Hemidesmosomes: Anchor the basal surface of epithelial cells to the extracellular matrix (ECM) using integrins and keratin.

  • Tight Junctions: Formed by occludin molecules to seal intercellular spaces and prevent passage of molecules between cells.

  • Gap Junctions: Composed of connexin monomers forming connexons that create hydrophilic channels for direct intercellular communication.

• Cell Life Cycle:

  • Interphase: Cell growth and protein synthesis ($\text{G}_1$), DNA replication ($\text{S}$ phase), and preparation for division ($\text{G}_2$).

  • Mitotic Phase ($\text{M}$): Mitosis (Prophase, Metaphase, Anaphase, Telophase) resulting in nuclear division, followed by Cytokinesis (cytoplasmic division) to generate two identical daughter cells.

Histology and Tissue Types

• Epithelial Tissue:

  • Characteristics: Tightly packed cells held by junctions, distinct apical and basal polarity, avascular but innervated, and highly regenerative.

  • Classification: Simple (1 layer), Stratified ($2+$ layers), or Pseudostratified; Squamous (flat), Cuboidal (cube-like), Columnar (tall), or Transitional (variable/distensible shape).

*   Glandular Classification: Endocrine (ductless, secretes hormones into capillaries) vs. Exocrine (secretes products into ducts or onto surfaces).
*   Exocrine Secretion Mechanisms: Merocrine (exocytosis; e.g., salivary glands), Apocrine (pinching off apical cytoplasm; e.g., mammary glands), Holocrine (whole cell rupture; e.g., sebaceous glands).

• Connective Tissue:

  • Characteristics: Cells embedded within an extracellular matrix (ground substance + fibers); derived from embryonic mesenchyme.

  • Cell Types: Fibroblasts/fibrocytes, Osteoblasts/osteocytes, Chondroblasts/chondrocytes, Adipocytes, and Blood cells.

  • Fiber Types: Collagen (thick, resists tension, most abundant), Elastin (long, branched, allows stretch and recoil), Reticulin (short, thin, forms flexible meshwork).

  • Classes: Loose CT (Areolar, Adipose, Reticular), Dense CT (Regular, Irregular, Elastic), Cartilage (Hyaline, Elastic, Fibrocartilage), Bone (calcified collagen matrix), and Blood (cells in plasma matrix).

• Muscle Tissue:

  • Skeletal Muscle: Voluntary, striated, multinucleated, elongated myotubes attached to skeleton.

  • Cardiac Muscle: Involuntary, striated, branched cells joined by intercalated discs with central single nuclei in heart walls.

  • Smooth Muscle: Involuntary, non-striated, spindle-shaped cells arranged in sheets within walls of hollow organs.

• Nervous Tissue:

  • Neurons: Excitable, conductive cells that receive and transmit electrical signals.

  • Glial Cells: Non-conductive support cells providing structural protection and nutrients.

• Body Membranes:

  • Cutaneous Membrane: Skin; dry epithelial/connective sheet exposed to air.

  • Mucous Membranes: Wet sheets lining body cavities open to the exterior (digestive, respiratory, urogenital tracts).

  • Serous Membranes: Wet double-layered sheets lining closed ventral body cavities (Pleurae, Pericardium, Peritoneum).

• Tissue Repair & Development:

  • Repair Sequence: Inflammation (clotting/scab formation) $\rightarrow$ Organization/Regeneration (granulation tissue/capillary ingrowth) $\rightarrow$ Fibrosis (scar formation).

  • Regenerative Capacity: Excellent (epithelia, bone, blood-forming, areolar, dense irregular CT); Moderate (smooth muscle, dense regular CT); Weak (skeletal muscle, cartilage); None (cardiac muscle, central nervous system tissue).

  • Germ Layers: Ectoderm, Mesoderm, Endoderm.

The Integumentary System

• Integumentary Functions: Protection (physical, chemical, biological barrier), Thermoregulation (sweating, vasodilation/vasoconstriction), Sensation, Vitamin D synthesis (UV conversion of 7-dehydrocholesterol to cholecalciferol/Vitamin $\text{D}_3$, converted to active calcitriol), Excretion, and Blood reservoir.

• Skin Structure:

*   **Epidermis**: Keratinized stratified squamous epithelium.
    *   Cell Types: Keratinocytes (main type, structural keratin), Melanocytes (melanin pigment synthesis), Dendritic / Langerhans cells (immune antigen presentation in stratum spinosum), Tactile / Merkel cells (mechanoreceptors for fine touch in stratum basale).
    *   Layers (deep to superficial): Stratum basale (single row of dividing cuboidal cells), Stratum spinosum ($8\text{--}10$ rows of keratinocytes), Stratum granulosum ($3\text{--}5$ rows accumulating keratohyalin), Stratum lucidum (dead clear layer present only in thick skin), Stratum corneum ($15\text{--}30$ layers of dead, flattened keratinized cells).
*   **Dermis**: Vascular connective tissue layer supporting the epidermis.
    *   Papillary Layer: Loose connective tissue with fine collagen/elastic fibers, dermal papillae, and capillary loops.
    *   Reticular Layer: Dense irregular connective tissue with thick collagen bundles forming cleavage lines (tension lines).
    *   Dermal Receptors: Meissner's corpuscles (light touch/vibration), Lamellated corpuscles (deep pressure/vibration), Ruffini corpuscles (pressure/distortion).
*   **Hypodermis**: Subcutaneous adipose tissue layer below the dermis (anchors skin, not part of integument).

• Appendages of the Skin:

  • Hair: Grows from hair matrix in the hair bulb. Shaft structure contains Medulla (soft keratin), Cortex (hard keratin), and Cuticle (hard keratin). Growth phases: Anagen (active growth), Catagen (end of active growth), Telogen (resting phase). Contraction of the arrector pili muscle pulls hair upright causing goosebumps.

  • Nails: Plates of hard keratin produced by the nail matrix beneath the eponychium (cuticle); average growth rate is $\sim 1\,\text{mm/week}$ for fingernails.

  • Sebaceous Glands: Holocrine glands that secrete lipid-rich sebum into hair follicles for lubrication and antimicrobial protection.

  • Sudoriferous Glands: Apocrine sweat glands (axillary and pubic regions, secrete into hair follicles, bacterial breakdown produces odor) and Merocrine / Eccrine sweat glands (widespread, secrete directly onto skin for thermoregulation).

• Skin Clinical Conditions & Aging:

  • Color Sign Diagnostics: Cyanosis (blue/low oxygen), Erythema (redness/inflammation), Pallor (pale/anemia), Jaundice (yellow/liver dysfunction), Ecchymosis (bruising).

  • Skin Cancers: Basal cell carcinoma (most common, least malignant, stratum basale invasion), Squamous cell carcinoma (keratinocytes of stratum spinosum, potential metastasis), Melanoma (melanocytes, highly metastatic; screened using ABCD rule: Asymmetry, Border irregularity, Color variation, Diameter > 6\,\text{mm}).

  • Burns: Evaluated using the Rule of Nines ($11$ regions at $9\%$ each, plus perineum at $1\%$).

    • Classification: 1st-degree (epidermal damage only, redness/edema), 2nd-degree (partial thickness, epidermal and dermal damage, blisters), 3rd-degree (full thickness, nerve endings destroyed, painless burn site, requires skin grafting).

    • Critical thresholds: > 25\% body with 2nd-degree, > 10\% body with 3rd-degree, or 3rd-degree on face, hands, or feet.

*   Aging Changes: Epidermal thinning, reduced dermal collagen/elasticity (wrinkling), decreased glandular activity, thinning hair, and slowed nail growth.