Anatomy and Physiology: Tissues

Tissue Definition: A tissue is a group of cells working together to perform a specific function.
Histology: The microscopic study of tissues.
Tissue Classification:
- Based on the structure of cells
- Composition of noncellular extracellular matrix
- Cell function
Major Types of Tissues:
- Epithelial
- Connective
- Muscle
- Nervous

Composed predominantly of cells, with minimal extracellular matrix.
Functions:
- Covers body surfaces and forms glands.
- Lines various systems including digestive, respiratory, and urogenital.
- Covers the heart and blood vessels, and lines many body cavities.
Surface Specialization:
- Distinguished Surfaces: Free (apical), basal, and lateral surfaces.
- Basement Membrane: Connects the basal layer to underlying tissue.
Avascular: Nutrient and waste exchange occurs via diffusion from the underlying connective tissue.
High Regeneration Capacity: Rapid cell turnover due to loss and damage.

Protection: Protects underlying structures (e.g., epithelium lining the mouth).
Barrier Role: Acts as a barrier (e.g., skin).
Selective Permeability: Allows the passage of substances (e.g., nephrons in the kidney).
Secretion: Secretes various substances (e.g., mucous glands).
Absorption: Absorbs substances (e.g., lining of the small intestine).

Based on Layers:
- Simple: Single layer of cells.
- Stratified: Multiple layers of cells.
- Pseudostratified: Appears stratified but is a single layer.
- Transitional: Specialized for stretching.
Based on Shape:
- Squamous: Flat cells.
- Cuboidal: Cube-shaped cells.
- Columnar: Tall, narrow cells.

Simple Squamous Epithelium:
- Structure: Single layer of flat, hexagonal cells.
- Location: Lining of blood vessels, heart, alveoli of lungs, serous membranes.
Simple Cuboidal Epithelium:
- Structure: Single layer of cube-shaped cells.
- Location: Kidney tubules, glands, choroid plexuses, ovaries.
Simple Columnar Epithelium:
- Structure: Single layer of tall, narrow cells.
- Location: Bronchioles, auditory tubes, stomach, intestines, gallbladder, bile ducts, ventricles of brain.
Stratified Squamous Epithelium:
- Structure: Several layers, cuboidal in basal layer, flattened at surface.
- Location: Keratinized (skin) and nonkeratinized (mouth, throat, larynx, anus, vagina, inferior urethra, corneas).
Transitional Epithelium:
- Structure: Stratified cells that change shape when stretched.
- Location: Lining of urinary bladder, ureters, superior urethra.

Types:
- Found on lateral and basal surfaces of cells.
- Function:
- Form a permeability layer.
- Bind cells together.
- Facilitate intercellular communication.
Types of Connections:
- Desmosomes & Hemidesmosomes:
- Desmosomes: Disk-shaped regions important for stress resistance; contain adhesive material and filaments.
- Hemidesmosomes: Half-desmosomes; anchor epithelial cells to the basement membrane.
- Tight Junctions: Hold cells together, forming a permeability barrier.
- Adhesion Belts: Below tight junctions; provide additional strength and prevent material passage.
- Gap Junctions: Protein channels that allow communication and transfer of small molecules and ions between cells.

Structural Support: Enclose organs and separate them into layers.
Tissue Connection: Joins and connects tissues (e.g., tendons and ligaments).
Support and Movement: Bones provide a rigid framework.
Storage: Energy storage (fat tissue).
Cushioning and Insulation: Provided by adipose tissue.
Transportation: Blood conducts nutrients and gases.
Protection: Immune response cells provide defense.

Specialized Cells Produce Extracellular Matrix:
- Blasts: Create the matrix (e.g., osteoblasts).
- Cytes: Maintain the matrix (e.g., osteocytes).
- Clasts: Breakdown matrix for remodeling (e.g., osteoclasts).
Types of Structural Cells:
- Osteoblasts/Cytes/Clasts: Bone cells involved in formation and maintenance.
- Chondroblasts/Cytes: Cartilage cells responsible for formation and maintenance.
- Fibroblasts/Cytes: Form and maintain fibrous connective tissue.

Adipocytes: Fat cells present in varying densities across tissues.
Mast Cells: Found under membranes and along blood vessels; release chemicals during inflammation.
Leukocytes: White blood cells that respond to injury and infection.
Macrophages: Specialized cells that engulf pathogens and dead cells.
Platelets: Fragments involved in clotting.
Mesenchymal Stem Cells: Undifferentiated cells capable of developing into various adult cell types.

Components:
- Protein Fibers:
- Collagen Fibers: Strong, flexible, and inelastic; most common protein.
- Reticular Fibers: Fine collagenous fibers forming networks between tissues.
- Elastic Fibers: Stretchable fibers composed of elastin that return to original form after compression.
- Ground Substance: Consists of:
- Hyaluronic Acid: Lubricant in joints and the eye.
- Proteoglycans: Trap water to provide gel-like consistency; consist of proteins and long polysaccharides (glycosaminoglycans).
- Adhesive Molecules: Such as chondronectin and osteonectin; help in holding the matrix components together.

Functionality: Contracts or shortens with force; responsible for movement or pump blood.
Types of Muscle Tissue:
- Skeletal Muscle:
- Attached primarily to the skeleton; striated and voluntary.
- Cardiac Muscle:
- Muscle of the heart; striated and involuntary.
- Smooth Muscle:
- Associated with tubular structures and skin; non-striated and involuntary.

Functionality: Capable of generating electrical signals known as action potentials.
Components:
- Cell Body: Contains the nucleus.
- Axon: Conducts impulses away from the cell body, typically a singular process per neuron.
- Dendrites: Receive impulses from other neurons, which can be numerous per neuron.

Role: Support cells of the nervous system.
Functions:
- Nourish, protect, and insulate neurons.
- Maintain homeostasis and form myelin sheaths around axons.