Human Tissue Types: Structure, Function, and Classification

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There are four basic tissue types in the human body:
1. Epithelial tissue
2. Connective tissue
3. Muscle tissue
4. Nervous tissue
Key functions of each tissue type:
- Epithelial: Covers body surfaces, lines cavities, forms glands.
- Connective: Supports, protects, and binds other tissues; stores energy; transports substances.
- Muscle: Specialized for contraction and movement.
- Nervous: Initiates and transmits electrical impulses; forms the brain, spinal cord, and nerves.

Epithelial tissues are distinguished by:
1. Cellularity: Composed almost entirely of tightly packed cells.
2. Polarity: Has an apical (free) surface and a basal (attached) surface.
3. Attachment: Supported by a basement membrane.
4. Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.
5. Regeneration: High capacity for cell division and repair.
Apical vs. Basal Surfaces: The apical surface faces the body surface or cavity; the basal surface is attached to underlying connective tissue.
Basement Membrane: A thin, fibrous layer that anchors epithelium to connective tissue.

By number of layers:
- Simple: One cell layer thick.
- Stratified: Multiple cell layers.
By cell shape:
- Squamous: Flat, scale-like cells.
- Cuboidal: Cube-shaped cells.
- Columnar: Tall, column-like cells.

Tissue Type	General Appearance	Function	Key Locations
Simple squamous	Single layer, flat cells	Diffusion, filtration	Air sacs of lungs, lining of blood vessels
Simple cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, glands
Simple columnar	Single layer, tall cells	Absorption, secretion	Digestive tract lining
Pseudostratified columnar	Single layer, appears stratified	Secretion, movement of mucus	Trachea, upper respiratory tract
Stratified squamous	Multiple layers, flat surface cells	Protection	Skin, mouth, esophagus
Transitional	Multiple layers, variable shape	Stretching	Urinary bladder

Simple squamous epithelium is adapted for rapid exchange due to its thinness.
Pseudostratified columnar epithelium appears layered but all cells touch the basement membrane.
Stratified squamous epithelium provides protection against abrasion.
Transitional epithelium is unique for its ability to stretch and change shape.

Endocrine glands: Ductless, secrete hormones into blood.
Exocrine glands: Secrete products into ducts (e.g., sweat, saliva).
Unicellular exocrine gland example: Goblet cell (secretes mucus).
Multicellular exocrine glands: Classified by duct structure (simple vs. compound) and secretory unit shape (tubular vs. alveolar).
Modes of secretion:
- Merocrine: Secrete by exocytosis (e.g., sweat glands).
- Holocrine: Entire cell ruptures to release product (e.g., sebaceous glands).

Four main classes:
1. Connective tissue proper
2. Cartilage
3. Bone
4. Blood
Distinguishing characteristics:
- Common origin (mesenchyme)
- Varying degrees of vascularity
- Extracellular matrix (ground substance + fibers)

Three main elements:
1. Ground substance
2. Fibers (collagen, elastic, reticular)
3. Cells ("-blast" for immature, "-cyte" for mature)
Ground substance: Unstructured material filling space between cells.
Collagen fibers: Strongest and most abundant; provide tensile strength.
Elastic fibers: Allow stretch and recoil.
Reticular fibers: Form supportive networks.

Tissue Type	Key Characteristics	General Function	Principal Locations in the Body
Areolar	Loose arrangement of fibers, many cell types	Wraps and cushions organs	Under epithelia, around capillaries
Adipose	Closely packed fat cells	Stores energy, insulates, cushions	Under skin, around kidneys and eyes
Reticular	Network of reticular fibers	Supports other cell types	Lymph nodes, spleen, bone marrow
Dense regular	Parallel collagen fibers	Attaches muscles to bones, resists tension in one direction	Tendons, ligaments
Dense irregular	Irregularly arranged collagen fibers	Withstands tension from many directions	Dermis of skin, joint capsules
Elastic	High proportion of elastic fibers	Allows recoil after stretching	Walls of large arteries, bronchial tubes

Cartilage: Types include hyaline (most abundant), elastic (flexible), and fibrocartilage (strongest, resists compression).
Bone: Rigid, supports and protects; stores calcium.
Blood: Fluid matrix (plasma); transports nutrients, gases, wastes.

Type of Membrane	Location	Is it a wet or dry membrane?
Cutaneous	Skin	Dry
Mucous	Lines body cavities open to exterior (e.g., digestive, respiratory tracts)	Moist
Serous	Lines closed body cavities (pleura, pericardium, peritoneum)	Moist

Regeneration: Replacement of destroyed tissue by the same kind of cells; restores normal function.
Fibrosis: Replacement by scar tissue (dense connective tissue); does not restore original function.
Steps of tissue repair (in order):
1. Inflammation
2. Organization (restores blood supply)
3. Regeneration and fibrosis
Granulation tissue: Delicate pink tissue composed of new capillaries and fibroblasts; forms during organization phase.

Muscle and nervous tissue will be covered in detail in a later exam.
Tables and matching exercises in the original file are designed to reinforce recognition of tissue types, their structure, and function.