Tissue Organization & Types Summary

Tissues are groups of similar cells and extracellular material with a common function.
Histology is the study of tissues.
The four types of tissues are epithelial, connective, muscle, and nervous tissue.

Epithelial tissue consists of closely packed cells with little to no extracellular matrix.
It covers body surfaces, lines body cavities, and forms the majority of glands.
Functions include physical protection, selective permeability, secretion, and sensation.

Classified by number of cell layers (simple, stratified, pseudostratified) and shape of cells at the apical surface (squamous, cuboidal, columnar, transitional).

Simple Squamous: Rapid movement of molecules. Found in air sacs of lungs, blood and lymph vessel walls.
Simple Cuboidal: Absorption and secretion. Found in thyroid gland, kidney tubules, and gland ducts.
Simple Columnar:
- Nonciliated: Absorption and secretion; contains microvilli and goblet cells. Lines digestive tract.
- Ciliated: Moves mucus; contains goblet cells. Lines bronchioles and uterine tubes.
Pseudostratified Columnar:
- Ciliated: Protective functions; contains cilia and goblet cells. Located in respiratory system.
- Nonciliated: Rare; lacks cilia and goblet cells. Located in male urethra and epididymis.
Stratified Squamous:
- Keratinized: Superficial layers of dead cells filled with keratin. Found in epidermis.
- Nonkeratinized: All cells alive and kept moist. Lines oral cavity, esophagus, vagina, and anus.
Stratified Cuboidal: Protection and secretion. Located in walls of ducts in exocrine glands.
Stratified Columnar: Protection and secretion. Located in large ducts of salivary glands.
Transitional: Stretching. Located in urinary tract; apical cells flatten when stretched; binucleated cells.

Physical protection, support, binding, storage, transport, and immune protection.

Resident cells: housed in the CT (e.g., fibroblasts, adipocytes, macrophages).
Wandering cells: continuously move through the CT (e.g., mast cells, plasma cells, leukocytes).
Protein fibers: collagen, reticular, and elastic fibers.
Ground substance: noncellular material produced by CT cells containing GAGs, proteoglycans and glycoproteins

Collagen fibers: Strong, flexible, resist stretching. Found in tendons and ligaments.
Reticular fibers: Flexible, form CT framework. Found in lymph nodes, spleen, liver.
Elastic fibers: Stretch and recoil easily. Found in skin, arteries, lungs.
Marfan Syndrome: Inherited disorder of fibrillin gene leading to abnormal elastic fibers

Loose CT: Fewer fibers, more ground substance.
- Areolar: Loose arrangement of collagen and elastic fibers; highly vascularized.
- Adipose: Mostly adipocytes; white and brown types.
- Reticular: Meshwork of reticular fibers, fibroblasts, leukocytes; structural framework of lymphatic organs.
Dense CT: More fibers, less ground substance.
- Dense regular: Parallel collagen fibers; stress applied in a single direction. Found in tendons and ligaments.
- Dense irregular: Clumps of collagen fibers extend in all directions; resists stress in multiple directions. Found in skin dermis.
- Elastic: Branching, densely packed elastic fibers; able to stretch and recoil. Found in walls of large arteries.

Cartilage: Collagen and elastic protein fibers; avascular in mature state; chondrocytes in lacunae.
- Hyaline cartilage: Most common, clear, glassy appearance.
- Fibrocartilage: Weight-bearing, resists compression; no perichondrium.
- Elastic cartilage: Flexible, springy; numerous elastic fibers.

Merocrine glands: Release secretions by exocytosis.
Apocrine glands: Apical membrane pinches off; secretion includes part of the cell.
Holocrine glands: Cells accumulate product, then disintegrate.