MT 30 (LAB): EPITHELIAL TISSUE

GENERAL FEATURES

  1. Cover ALL body surfaces, line organs, are major tissues

    of glands.

  2. 1 side is exposed to open space, and the underside is

    attached to connective tissue by a thin, non-cellular

    basement membrane.

  3. Lack of blood vessels, fed by diffusion from well-vascularized connective tissues.

  4. Reproduce rapidly

  5. Tightly packed

  6. Good barriers: skin, the lining of the mouth

  7. Involved in secretion, absorption, excretion, sensory reception

Characteristics of Epithelial Tissue

  1. Cellularity

    • composed almost entirely of cells (with some extracellular matrix and sometimes other structures)

  2. Polarity

    • has specific top and bottom

    • apical surface - exposed to the external environment or internal body space

    • basal surface - attached to underlying connective tissue

    • lateral surfaces - connected by intercellular junctions

  3. Attachment

    • basal surface bound to the basement membrane

  4. Avascularity

    • no blood vessels; receive nutrients across the apical surface or by diffusion

  5. Innervation

    • lots of nerve endings

  6. High regeneration capacity

    • epithelial cells are frequently damaged or lost to abrasion, so they are replaced quickly

Functions of Epithelial Tissue

  1. Physical protection

    • protect exposed and internal surfaces from dehydration, abrasion, destruction

  2. Selective permeability

    • act as gatekeepers to let some substances in and keep others out

  3. Sensation

    • sense changes in the environment

  4. Secretion

    • applies in some epithelial tissues

    • contain glands that release sweat, scents, hormones, etc.

Specialized structures of Epithelial Tissue

  1. Basement Layer

    • the extracellular layer between the epithelium and connective tissue.

    • Often visible with a microscope

  2. Hemidesmosomes

    • basal cells anchored to the basement membrane with junctions

  3. Intercellular Junctions

    • Junctions between cells in epithelium

    • Tight Junctions

      • anchor cells to each other

      • prevents substances from passing between cells

      • materials must move through cells, or are blocked from moving past cells

      • found in the intestinal lining

    • Adhering junctions

      • often deep to tight junctions

      • Form all the way around a cell

      • Support apical surface

      • Allows passage between cells below the apical surface

  4. Desmosomes

    • cell to cell attachment

    • bind epithelium together

    • bind muscle cells

    • resist shear forces

Types of Epithelium

  1. Covering and Lining Epithelium

    • epidermis of skin

    • lining of blood vessels and ducts

    • lining respiratory, reproductive, urinary & and GI tract

  2. Glandular epithelium

    • originates from invaginated epithelial cells

    • secreting portion of glands

    • thyroid, adrenal, and sweat glands

Epithelium Germ Layers

  1. Ectoderm

    • oral and nasal mucosa, cornea epidermis, glands of the skin, mammary glands

  2. Endoderm

    • lining of respiratory and gastrointestinal tract, liver, pancreas

  3. Mesoderm

    • lining of the urogenital system, circulatory system, and body cavities lining-mesothelium

Classifications of Epithelial Tissue

  • According to number of cell layers

    • simple = one cell layer thick

    • stratified = less than 2 cell layers thick

    • pseudostratified = false layered (appears to be more than one

      layer, but only one); ciliated or with cilia

  • According to cell shape

    • squamous = flattened

    • cuboidal = cube-shaped or roundish

    • columnar = long and thin (like a column)

EPITHELIAL TISSUES AND ORGANS

  1. Lungs - simple squamous epithelial tissue

  2. Kidneys - simple cuboidal epithelial tissue

  3. Small intestine - simple columnar epithelial tissue

  4. Trachea Lining, Respiratory tract - Ciliated pseudostratified columnar epithelial tissue

  5. Mouth lining - Stratified squamous non-keratinized epithelial tissue

  6. Skin - Stratified squamous keratinized tissue epithelial tissue

  7. salivary glands, sweat glands - Stratified cuboidal epithelial tissue

  8. Male reproductive tract - Stratified columnar epithelial tissue

  9. Bladder - Transitional epithelial tissue

SIMPLE EPITHELIUM

  • Simple Squamous Epithelium

    • Single layer of flattened cells with disc-shaped central nuclei

      and sparse cytoplasm;

    • the simplest of the epithelia.

    • Function: filtration, passageway, secretion

    • Location: Kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, and lymphatic vessels; lining of ventral body cavity (serosae)

  • Simple cuboidal epithelium

    • Single layer of cubelike cells with large, spherical central nuclei.

    • Function: Secretion and absorption

    • Location: Kidney tubules; ducts and secretory portions of small glands; ovary surface.

  • Simple columnar epithelium

  • Single layer of tall cells with round to oval nuclei

  • may have cilia and goblet cells

    • Goblet cells - contain mucus-secreting unicellular glands

  • Function: absorption, secretion, and propelling (ciliated)

  • Location:

    1. Non-ciliated type: digestive tract, gallbladder, and excretory ducts of some glands

    2. Ciliated - small bronchi, uterine tubes, and some regions

      of the uterus.

  • Pseudostratified Columnar Epithelium

    • Single layer of cells of differing heights

    • some not reaching the free surface

    • nuclei seen at different levels

    • has mucus-secreting cells and bear cilia

    • Function: secretion, propulsion of mucus

    • Location:

      1. non-ciliated - male reproductive glands

      2. ciliated - trachea, upper respiratory tract

  • Stratified Squamous Epithelium

    • Thick membrane composed of several cell layers

    • basal cells are cuboidal or columnar and metabolically active

    • keratinized or non-keratinized

    • Function: protection

    • Location:

      1. Nonkeratinized - esophagus, mouth, and vagina

      2. Keratinized - epidermis of skin

  • Stratified Cuboidal Epithelium

    • 2-3 layers which line a lumen

    • Function: absorption and secretion

    • Location: Large ducts = mammary glands, sweat glands, salivary glands, pancreas, lining of ovarian follicle, seminiferous tubules

  • Stratified Columnar Epithelium

    • Superficial (shallow) cells are elongated, deeper cells (basal layers) are cuboidal

    • Function: Secretion, excretion, and absorption

    • Location: male urethra, vas deferens, parts of the pharynx

  • Transitional epithelium

    • made of cells that can stretch

    • cells look cuboidal when relaxed; look squamous when stretched

    • Function: Stretches readily and permits distension

    • Location: ureters, urinary bladder, and part of the urethra.

Epithelial Glands

  • Parenchyma

    • glandular epithelia that secretes substances

  • Stroma

    • connective tissue that supports the parenchyma

  • Exocrine

    • Can be Unicellular or multicellular

    • Location: salivary glands (secretory); pancreas (serous); mix (submandibular)

    • Classifications of Exocrine Glands

      1. Unicellular - goblet cells

      2. Multicellular

        • classified into: simple (ducts do not branch) and compound (ducts do branch - septa)

    • Glandular Epithelium

      • Epithelium that are inside particularly in the glands and organs.