Anatomy & Physiology Study Notes: Tissue Level of Organization

Tissue Level of Organization

Introduction to Tissues

Tissues are groups of similar cells working together to perform specific functions. The study of tissues is called histology. Tissues combine to form organs, such as the heart or liver.

• Definition: A tissue is a group or mass of similar cells working together to perform common functions.

• Types of Tissues:

  1. Epithelial

  2. Connective

  3. Muscle

  4. Nervous

• Example: Skin is a type of tissue, primarily epithelial and connective.

Epithelial Tissue

Epithelial tissue covers internal and external surfaces of the body and forms glands. It is avascular and attached to underlying connective tissue at the basement membrane.

• Functions:

  • Physical protection

  • Control permeability

  • Provide sensation

  • Produce specialized secretions

• Characteristics:

  • Polarity: Differences between apical and basal surfaces

  • Cellularity: Cells joined by cell junctions

  • Attachment: To basement membrane

  • Specialized surfaces: Microvilli, cilia

  • Regeneration: Continuously replaced by stem cell division

• Integrity Maintenance:

  • Intercellular connections (cell adhesion molecules, proteoglycans, cell junctions)

  • Attachment to basement membrane

  • Maintenance and repair

Cell Junctions

• Gap junctions: Permit free diffusion of ions and small molecules between cells

• Tight junctions: Interlocking membrane proteins bind plasma membranes, restrict movement

• Desmosomes: CAMs and proteoglycans link plasma membranes, provide strength

Basement Membrane

• Basal lamina: Superficial, selective filter

• Reticular lamina: Deeper, provides strength

Epithelial Classification

Epithelia are classified by the number of cell layers and the shape of cells.

• Number of Layers:

  • Simple epithelium: Single layer

  • Stratified epithelium: Multiple layers

• Cell Shape:

  • Squamous: Thin and flat

  • Cuboidal: Boxy, central nucleus

  • Columnar: Tall, slender rectangles

  • Transitional: Appearance changes as stretching occurs

Types of Epithelia

Type	Form	Function	Location
Simple Squamous	Single layer, flat	Diffusion, filtration	Air sacs in lungs, capillaries
Simple Cuboidal	Single layer, cube-shaped	Secretion, absorption	Kidney tubules, ducts, ovaries
Simple Columnar	Single layer, tall	Secretion, absorption	Digestive tract, uterus
Stratified Squamous	Multiple layers, flat	Protection	Skin, mouth, body cavities
Pseudostratified Columnar	Single layer, nuclei at different levels	Secretion, cilia-aided movement	Air passages, reproductive tubes
Transitional	Stretchable, variable appearance	Blocks diffusion	Urinary bladder
Glandular	Specialized for secretion	Secrete substances	Glands (salivary, sweat, endocrine)

Glands

• Glands: Collections of epithelial cells that produce secretions

• Types:

  • Endocrine glands: Release hormones into bloodstream

  • Exocrine glands: Discharge secretions onto epithelial surfaces

• Exocrine gland structures:

  • Unicellular (mucus, goblet cells)

  • Multicellular (organs)

• Exocrine gland secretion methods:

  • Merocrine: Exocytosis (e.g., sweat glands)

  • Apocrine: Released by shedding cytoplasm (e.g., mammary glands)

  • Holocrine: Released by cell bursting (e.g., sebaceous glands)

• Types of secretions:

  • Serous: Watery, contains enzymes

  • Mucous: Mucin-rich, forms mucus

  • Mixed: Both serous and mucous

Connective Tissue

Connective tissue supports, protects, and interconnects other tissues. It is the most abundant tissue type in the body.

• Functions:

  • Support and structure

  • Transport materials

  • Protect delicate organs

  • Store energy (fat)

  • Defend against microorganisms

• Components:

  • Specialized cells

  • Matrix: Extracellular protein fibers + ground substance (fluid)

• Categories:

  • Connective tissue proper: Viscous matrix, protein fibers

  • Fluid connective tissue: Watery matrix, dissolved proteins (blood, lymph)

  • Supporting connective tissue: Dense matrix, tight fibers (cartilage, bone)

Connective Tissue Proper

• Cells:

  • Fixed cells: Maintenance, repair, energy storage (fibroblasts, adipocytes, mesenchymal cells)

  • Migratory cells: Defense, injury response (macrophages, mast cells, lymphocytes)

• Fibers:

  • Collagen: Strong, resist force

  • Reticular: Stabilize organs, cells

  • Elastic: Stretch and return to shape

• Ground Substance: Viscous, fills space between cells and fibers

Types of Connective Tissue Proper

Type	Form	Function	Location
Loose	Fills internal spaces, more ground substance	Support, elasticity	Areolar, adipose, reticular tissues
Dense	More fibers, less ground substance	Framework, strength	Tendons, ligaments, fasciae

Loose Connective Tissue Types

• Embryonic: Mesenchyme (stem cells), mucous (umbilical cord)

• Areolar: Elastic, connects skin to muscle

• Adipose: Fat storage, insulation

• Reticular: Support for organs

Dense Connective Tissue Types

• Dense regular: Parallel fibers (tendons, ligaments)

• Dense irregular: Interwoven fibers (dermis, organ capsules)

• Elastic: Elastic ligaments

Fasciae

• Support organs, maintain positions

• Provide routes for vessels, nerves

• Types: Superficial, deep, subserous

Fluid Connective Tissue

• Blood:

  • Watery matrix: plasma

  • Formed elements:

    • Red blood cells (erythrocytes)

    • White blood cells (leukocytes)

    • Platelets

• Lymph:

  • Interstitial fluid entering lymphatic vessels

  • Lymphocytes, other cells and proteins

Supporting Connective Tissue

• Cartilage:

  • Avascular, matrix is firm gel

  • Cells: chondrocytes in lacunae

  • Perichondrium: membrane for support and growth

  • Types:

    • Hyaline: Closely packed collagen, flexible, respiratory tract

    • Elastic: Elastic fibers, ear, larynx

    • Fibrocartilage: Dense collagen, intervertebral discs, pubic symphysis

• Bone:

  • Mostly calcified matrix, collagen fibers

  • Cells: osteocytes

  • Covered by periosteum (outer fibrous, inner cellular layer)

Tissue Membranes

• Physical barriers formed by epithelium and connective tissue

• Types:

  • Mucous membranes

  • Serous membranes

  • Cutaneous membranes

  • Synovial membranes

Clinical Application: Epidermolysis Bullosa

• Genetic disease affecting COL7A1 gene, impacts collagen

• Collagen anchors epidermis to dermis; mutation causes skin to blister and tear easily

• Symptoms: Blisters, pain, scarring, nutritional issues

• Loss of collagen and elastin leads to wrinkles and aging

Summary Table: Epithelial Tissue Types

Type	Form	Function	Location
Simple Squamous	Single layer, flat	Diffusion, filtration	Lungs, capillaries
Simple Cuboidal	Single layer, cube	Secretion, absorption	Kidneys, ducts, ovaries
Simple Columnar	Single layer, tall	Secretion, absorption	Digestive tract, uterus
Stratified Squamous	Multiple layers, flat	Protection	Skin, mouth
Pseudostratified Columnar	Single layer, nuclei at different levels	Secretion, cilia movement	Airways, reproductive tubes
Transitional	Stretchable, variable	Blocks diffusion	Urinary bladder
Glandular	Specialized for secretion	Secrete substances	Glands

Key Equations

• Diffusion Rate (Fick's Law): $J = -D \frac{dC}{dx}$ Where $J$ is the diffusion flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info:

• Clinical examples (Epidermolysis Bullosa) and diagrams were inferred to provide context for tissue function and pathology.

• Tables were reconstructed to summarize tissue types and their properties.