Introduction to Human Anatomy

Definition: Human anatomy is the scientific study of the structures of the human body and the relationship between its various parts.
Rationale for Study: It provides the fundamental knowledge and essential foundation for the entire art and practice of medicine.

Subdivisions of Human Anatomy

Gross (Cadaveric/Macroscopic) Anatomy:
- Description: The study of large body structures that are observable using unaided or naked eyes.
- Methodology: Studied on dead, embalmed (preserved) bodies within a gross anatomy laboratory.
- Approaches:
  - Regional Anatomy: The body is studied in distinct parts or segments, such as the upper limb, lower limb, thorax, abdomen, and the head and neck.
  - Systemic Anatomy: The body is studied by individual systems, such as the skeletal system, respiratory system, and cardiovascular system.
Histology (Microscopic Anatomy):
- Description: Refers to the study of very small structures that can only be viewed through a microscope.
- Setting: Conducted in specialized histology laboratories.
Embryology (Developmental Anatomy):
- Description: The study of developmental anatomy from conception onwards.
- Key Processes:
  - Gametogenesis: Includes spermatogenesis (male) and oogenesis (female).
  - Fertilization: The process leading to the formation of a zygote.
- Stages of Development:
  - Pre-embryonic Stage: The first $2$ weeks of development.
  - Embryonic Stage: From week $3$ until day $56$ (approximately $8$ weeks).
  - Fetal Stage: From day $57$ ( $8$ weeks) until birth.
- Clinical Relevance: Focuses on development-related congenital malformations.
- Note: Development is a continuous process that does not stop at birth.
Radiological Anatomy:
- Description: Marks the transition from laboratory learning to the visualization of live anatomy in clinical or ward setups.
- Methods: Utilizes non-invasive visualization techniques,
  - X-rays.
  - MRI (Magnetic Resonance Imaging).
  - CT scan (Computer Tomography).
  - Ultrasound.
Applied Anatomy (Clinical Anatomy): Deals with the application of anatomical knowledge specifically to medical and surgical practice.
Comparative Anatomy: The study of the anatomy of other animals to explain changes in form, structure, and function (morphology).

Levels of Structural Organisation

Chemical Level:
- Subatomic/Nano Particles: Combine to form atoms. These are small aggregates of strongly bound/fused particles of different sizes with a specific surface-area-to-volume ratio (e.g., at the DNA level).
- Nanotechnology: An advanced approach for diagnosis (nano-diagnosis) and treatment (nano-therapy) targeting the body's nano-particles. It aims at site-specific and controlled release of drugs, favoring preferential distribution (e.g., in cancer lesions).
- Atoms: The smallest units of matter taking part in chemical reactions; atoms combine to form molecules.
Cellular Level: Molecules combine to form cells, which are the smallest basic functional units of a human being.
Tissue Level: Similar types of cells combine to form four basic tissues: epithelium, connective, muscle, and nerve.
Organ Level: Different types of tissues combine to form organs.
Organ System Level: Different organs with similar functions coordinate to form an organ system.
Organismal Level: Many organ systems working together form the complete organism.
Rule of Hierarchy: Higher levels of organization are always built from the components of lower levels.

The Four Basic Tissues

Epithelial Tissues: Form body boundaries and coverings (e.g., simple, stratified, or transitional forms of squamous, cuboidal, and columnar cells).
Connective Tissues: Interconnect different parts of the body (e.g., bones, ligaments, tendons, blood, and adipose tissue).
Nerve Tissues: Excitable tissues capable of generating and conducting (propagating) electrical impulses.
Muscle Tissue: Excitable tissues that contract to enhance movement and locomotion.

Detailed Classification of Muscle Tissue

Unicellular Muscles:
- Myo-epitheliocytes: Contain actin and myosin fibrils similar to smooth muscle; they line glands to expel secretions.
- Myo-fibroblasts: Cells with a contractile role that also secrete collagen proteins to maintain tissue framework; dominant in repair areas.
- Pericytes: Smooth muscle cells that line the capillaries.
Multicellular Muscles:
- Skeletal Muscle:
  - Function: Enhances movement and supports the skeleton.
  - Control: Voluntary.
  - Innervation: Somatic nervous system.
  - Structure: Many peripherally placed nuclei; long cylindrical fibers; contains many mitochondria.
  - Appearance: Striated (alternating dark and light bands) due to regular arrangement of actin (globular protein) and myosin.
- Smooth Muscle:
  - Location: Covers body viscera (urinary bladder, intestines, stomach, rectum).
  - Control: Involuntary.
  - Innervation: Autonomic Nervous System (ANS).
  - Structure: Spindle-shaped/fusiform (tapered at ends, widest in middle); single central ovoid nucleus.
  - Appearance: Non-striated.
- Cardiac Muscle:
  - Location: Present only in the heart.
  - Function: Causes mechanical motion to pump blood.
  - Control: Involuntary.
  - Innervation: Autonomic Nervous System.
  - Structure: Branching fibers; single central nuclei.
  - Appearance: Striated.

Organ Systems Overview

1. Integumentary System: Skin and appendages (hair, nails, glands). Protects deep tissues, synthesizes Vitamin D from cholesterol via sun exposure, houses cutaneous receptors, and assists in perspiration (heat loss).
2. Skeletal System: Comprises $206$ bones. Supports/protects organs, provides sites for muscle attachment and blood cell formation, and stores minerals (calcium and phosphorus).
3. Muscular System: Enhances locomotion (change in position/location) and movement (no change in location, e.g., peristalsis). Produces heat, propels blood, and maintains posture.
4. Respiratory System: From nose to lungs. Facilitates gaseous exchange, supplying blood with oxygen ( $O_2$ ) and removing carbon dioxide ( $CO_2$ ) for tissue survival.
- Upper Respiratory: Nasal cavity, Pharynx (naso-, oro-, laryngopharynx), Larynx.
- Lower Respiratory: Trachea, Bronchi (primary, secondary, tertiary), Bronchioles (terminal, respiratory), Alveoli.
5. Cardiovascular System: Involves the heart, blood, and vessels. Transports oxygen, nutrients, wastes, and carbon dioxide.
- Heart Valves: Pulmonary, Aortic, Tricuspid, and Mitral valves.
- Circulation: Right pump (Pulmonary circulation); Left pump (Systemic circulation).
6. Lymphatic System: Part of the immune system; complements the CVS. Comprises lymphatic vessels and nodes. Drains interstitial fluid ( $10\%$ of the total; the other $90\%$ goes to venules) back to CVS. Nodes release lymphocytes to destroy invaders (bacteria, viruses).
- Primary Organs: Thymus and Red Bone Marrow (site of B and T cell maturation).
- Secondary Organs: Lymph nodes, Spleen, and Tonsils (sites of immune response).
7. Endocrine System: Consists of glands that secrete hormones regulating growth, reproduction, and metabolism.
- Exocrine: Retain ducts; secrete to surface.
- Endocrine: Lose ducts; secrete directly into blood capillaries.
8. Nervous System: Fast-acting control system (CNS and PNS). Responds to change and activates muscles/glands.
- Components: $12$ pairs of cranial nerves and $31$ pairs of spinal nerves. Neurons consist of a soma, dendrites, and an axon.
9. Digestive System: From oral cavity to anus. Breaks down food, allows nutrient absorption, and eliminates indigestible material.
10. Urinary System: Kidneys, ureters, bladder, and urethra. Eliminates nitrogenous wastes, maintains acid-base balance, and regulates water and electrolytes.
11. Reproductive System: Production of offspring; involves male/female organs.

Anatomy of the Skin (Integumentary System)

General: The skin is the body's largest organ.
Epidermis (Epithelial Layer): Stratified squamous keratinized epithelium.
- Stratum Corneum: Outermost, dead cells filled with keratin; water-resistant; replaced every $2$ weeks.
- Stratum Lucidum: Translucent layer beneath corneum; found only in thick skin.
- Stratum Granulosum: Grainy layer where cells begin to die/harden; contain keratohyalin granules.
- Stratum Spinosum: Mature cells bound by desmosomes; contains Langerhans cells.
- Stratum Germinativum (Basale): Innermost layer; attached to basement membrane; contains germinal cells for regeneration and melanocytes for pigmentation.
Dermis (Connective Tissue Layer): Thickest layer; supports and nourishes.
- Papillary Layer: Loose areolar tissue with small capillaries and dermal papillae.
- Reticular Layer: Deep layer of dense irregular connective tissue; contains large vessels and nerves.
Hypodermis (Superficial Fascia): Loose areolar and adipose tissue. Connects skin to bone/deep fascia. Stores fat and provides insulation.
- Modifications:
  - Palms/Soles: Contains fibrous collagen strands forming fat-filled loculi.
  - Face/Neck: Contains muscles of expression.
  - Eyelids/Penis/Clitoris: Devoid of fat.
  - Scrotum: Contains Dartos muscle (smooth muscle).
  - Nipple Areola: Contains Meyerholz (radial) and Sappey (circular) muscles.
  - Joints: Forms bursae (fluid-filled compartments).
Skin Cells:
- Keratinocytes: Most abundant; make skin tough/waterproof.
- Melanocytes: Produce melanin (UV protection).
- Langerhans Cells: Immune defense.
- Merkel Cells: Light touch sensation.
- Fibroblasts: Primary dermal cell; produce collagen/elastin.

The Language of Anatomy

Body Positions:
- Anatomical Position: Standing straight, eyes forward, arms at sides, palms forward, feet together.
- Supine: Lying on back.
- Prone: Lying on face/abdomen.
- Lithotomy: Lying on back, legs up in straps (used in delivery).
Planes:
- Median Sagittal: Vertical plane through center, dividing body into equal right/left halves.
- Paramedian: Near and parallel to median sagittal.
- Coronal: Vertical at right angle to sagittal, dividing body into anterior/posterior.
- Transverse/Horizontal: At right angle to both sagittal and coronal, dividing body into upper/lower parts.
Directional Terms (Trunk):
- Ventral/Anterior (front); Dorsal/Posterior (back).
- Medial (near midline); Lateral (away from midline).
- Cranial/Superior (near head); Caudal/Inferior (near lower end).
- Superficial (near surface); Deep/Profunda (away from surface).
- Ipsilateral (same side); Contralateral (opposite side).
- Invagination (projection inside); Evagination (projection outside).
Regional Terms (Limbs):
- Upper Limb: Palmar aspect (front of hand); Dorsal aspect (back of hand); Flexor (front); Extensor (back); Medial border (little finger side); Lateral border (thumb side).
- Lower Limb: Plantar aspect (sole); Dorsal aspect (top of foot); Flexor (back); Extensor (front); Medial border (big toe side); Lateral border (little toe side).
Embryological Terms: Ventral (belly); Dorsal (back); Cranial/Rostral (head); Caudal (tail).

Terms of Movement

Gliding: Flat surfaces move back-and-forth/side-to-side; no change in angle.
Angular Movements:
- Flexion: Decrease in angle.
- Extension: Increase in angle.
- Abduction: Move away from midline.
- Adduction: Move toward midline.
Rotation: Bone revolves around its longitudinal axis (Medial vs. Lateral).
Circumduction: Sequence of flexion, extension, abduction, and adduction.
Specific Movements:
- Opposition: Thumb touches finger tips.
- Protraction/Retraction: Forward/backward movement (e.g., jaw).
- Inversion/Eversion: Sole faces medially/laterally.
- Supination/Pronation: Turning the forearm/palm.

General Histology and Cell Biology

Cell Theory: All living things are made of cells; cells are basic functional units; cells arise from existing cells.
Eukaryotic Animal Cell Organelles:
- Cell Membrane: Semipermeable lipid bilayer; selective transport.
- Nucleus: Membrane-bound; contains chromosomes (DNA + histones = chromatin).
- Nucleolus: Produces and assembles ribosomes.
- Mitochondria: "Power house"; generates ATP; contains its own DNA and cristae (folds for surface area).
- ER: Rough (protein synthesis/folding); Smooth (lipid/steroid synthesis).
- Golgi apparatus: Packages proteins/lipids into vesicles.
- Lysosome: Digestive enzymes; involved in apoptosis (programmed cell death).
- Cytoskeleton: Microtubules, intermediate filaments, microfilaments; provides structure and movement.

Cell Division

Chromosomes: Total of $46$ ( $23$ pairs) in somatic (diploid/ $2n$ ) cells. First $22$ pairs are autosomes; last pair is sex chromosomes ( $XX$ female, $XY$ male).
Somatic Division (Mitosis):
- Interphase ( $90\%$ of time):
  - $G1$ ( $11$ hours): Primary growth, normal metabolism.
  - $S$ ( $8$ hours): DNA replication.
  - $G2$ ( $4$ hours): Protein synthesis, preparing for division.
- $M$ -Phase Mitosis ( $1-2$ hours): Prophase, Metaphase (alinment at equator), Anaphase (chromatids separate), Telophase (nuclear envelopes form).
- Cytokinesis ( $C$ -Phase): Division of cytoplasm (cleavage furrow).
Reproductive Division (Meiosis):
- Produces four haploid ( $n$ ) gametes.
- Meiosis I: Homologs separate. Includes Synapsis (forming tetrads) and Crossing-over (genetic recombination) in Prophase I.
- Meiosis II: Sister chromatids separate.

Histological Tissue Processing Steps

Harvesting: Careful removal of specimen.
Fixation: Using $5\%-10\%$ formaldehyde ( $24$ hours) to preserve structure and harden tissue.
Dehydration: Removal of water using increasing concentrations of ethanol ( $50\%$ to $100\%$ ).
Clearing: Using Zyline (miscible with ethanol and wax) to remove fat.
Wax Infiltration: Molten paraffin wax infiltrates tissue.
Embedding: Creating a wax block.
Sectioning: Using a rotary microtome to cut slices ( $0.5\,\mu m$ thickness).
Staining:
- Haematoxylin (Basic): Stains nuclei/DNA/RNA purplish blue (basophilic).
- Eosin (Acidic): Stains cytoplasm/fibers red or pink (acidophilic).
Examining: Under a microscope (Light, Dissecting, or Electron).

Detailed Connective Tissue Elements

Connective Tissue Fibers:
- Collagen: Strong, inelastic; found in bone, tendons.
- Elastic: Elastin-based; allows recoil; found in large vessels.
- Reticular: Fine, branching meshwork; offers support.
Connective Tissue Cells: Fibroblasts (fiber synthesis), Adipocytes (fat), Osteoblasts (bone), Chondroblasts (cartilage), Macrophages (phagocytic), and Mast cells (histamine release).
Cartilage Types:
- Hyaline: Glass-like; respiratory tract, ends of long bones.
- Fibrocartilage: Tough; intervertebral disks, pubic symphysis.
- Elastic: High flexibility; ear pinna, epiglottis.
Bone Tissue: Calcified matrix; maintained by osteoblasts, osteocytes (in lacunae), and osteoclasts. Contains compact and cancellous (spongy) layers.