Epithelium

Tissue

A group of more or less similar cells and their extracellular products performing a variety of related functions

• Epithelial tissue

• Connective and

• supportive tissue

• Muscle tissue

• Nervous tissue

Tissue types

Intercellular substance

non-living materials produced by the cells

Fluids

liquid component of the tissue located between cells

Epithelium

Composed of closely aggregated cells in apposition over a large part of their surfaces and having a very little intercellular space.

1. Covering (surface) and lining epithelium

2. Special epithelium

3. Glandular epithelium

Types of epithelium

Simple

-only one layer of cell

Stratified

two or more layers of cell

Pseudostratified

(false) only a single layer that rest on basal lamina but not all reaching the apical surface.

Squamous

polygonal cells on surface view; thin, elongated on longitudinal view

Cuboidal

short, as tall as they are wide

Columnar

rectangular, taller than they are wide

Simple squamous epithelium

• Cytoplasm is spread out.

• Typically flattened nuclei may bulge above the free surface of the cell.

• Facilitates passive diffusion across internal bodily surfaces.

Endothelium

Internal lining of vessels.

Mesothelium

All the body cavities

Simple cuboidal epithelium

• The width of the epithelial cells is similar to their height.

• The usually spherical nucleus is located centrally.

Simple columnar epithelium

• Cells that are distinctly taller than they are wide.

• The nuclei are predominantly oval and may occupy a basal, central or apical position.

• Evident microvilli.

• Other cell types are present (goblet cells, hormone-producing cells).

• Typically associated with absorption or secretion.

SCE with typical columnar cells which are all similar

Found in large ducts of exocrine glands

SCE with mucous secreting columnar cells

• Differs from the typical columnar cells by their vacoulated cytoplasm due to dissolution of mucin granules during paraffin technique

• Found in the stomach, abomasum and cervix

SCE with both secretory and absorptive cells

• The secretory cell is the goblet and the absorptive is a typical columnar cell that have microvilli (tiny finger-like projections) on their apical/ luminal borders

• Found in the small and large intestine

Pseudostratified columnar ciliated

• All cells are in contact with the basal lamina, yet not all reach

the free surface.

• The cell nuclei lie at different levels, such that the cells appear to be arranged in two or more rows’

• Widespread in the respiratory tract (respiratory epithelium) and epididymal duct

Stratified squamous epithelium

Composed of several layers of cells

Superficial layer

squamous cells

Middle layer

polyhedral or polygonal cells

Deepest cells

columnar or cuboidal cells resting on the basement membrane

Keratinized

• Covers the outer surfaces of the body such as the epidermis of the skin

• Superficial cells undergo metamorphosis into tough non-living layer of keratin

Keratin

prevents fluid from evaporating and is impervious to bacteria, hence a first line of defense against infection

Nonkeratinized

Inner surfaces of the body such as the esophagus and vagina

Stratified cuboidal epithelium

• Composed of two or three layers of cuboidal cells with the basal layers of cells often appearing nonuniform in distribution

• Mainly found lining large ducts of exocrine glands

Stratified colunar epithelium

• Composed of two layers only

  • Basal layer of cuboidal cells

  • Superficial columnar cells

• Found only in the fornix of the conjunctiva, cavernous urethra; large excretory ducts of exocrine glands

Transitional epithelium

• Irregular ‘mushroom-shaped’ cells that become rounder towards the surface, forming caps over the deeper cell layers

• Deepest (basal) layer consists of cuboidal to columnar cells that are connected to the basal lamina.

• Found in the excretory passages of the urinary system (renal pelvis, ureter, bladder, urethra).

Transitional epithelium

Subjected to constant changes in pressure and distension; the epithelium can undergo rapid changes in height

Unstretched

Rounded surface cells

Stretched

Flattened, elongated cells

Microvilli

increase absorption surface (intestinal epithelium)

Stereocilia

non-motile (epididymis)

Kinocilia

motile (tracheal and bronchial epithelium; uterine epithelium)

Tight junctions

transmembrane proteins fused on outer plasma membrane

Gap junctions

connect the cytoplasm of two cells and allow for the passage of molecules freely between cells

Desmosomes

attach to the microfilaments of cytoskeleton made up of keratin protein.

Hemidesmosomes

similar to desmosomes in terms of function, however, they attach the epithelial cell to the basement membrane rather than to an adjacent cell

Glandular epithelium

Epithelial tissue involved in the synthesis and secretion of specific substances (secreta)

Endocrine and exocrine

Types of glandular epithelium based on the distribution of secretory product:

intra-epithelial, extra-epithelial

Types of glandular epithelium based on their relationship to the surface epithelium:

Exocrine

secrete out of the body, with ducts

Endocrine

without ducts; secretions poured into closely associated capillaries

Paracrine

target cell is nearby

Autocrine

target is the same cell

Intra-epithelial glands

Goblet cells are club-shaped cells with a narrow base.

Extra-epithelial glands

Located in the connective tissue underlying the epithelium, always multicellular.

• Merocrine (eccrine)

• Apocrine

• Holocrine

Extra-epithelial glands mode of secretion

Merocrine (eccrine)

released by exocytosis (sweat glands, salivary gland)

Apocrine

released by apocytosis (sweat glands)

Holocrine

whole cell (sebaceous gland)

Serous glands

• Produce a watery secretion rich in protein and enzymes.

• The cytoplasm is acidophilic.

• The nuclei are usually spherical and are located centrally or slightly towards the base of the cell.

Mucous glands

• Produce mucin, a glycoprotein with side chains containing oligosaccharides

• The cytoplasm has a foamy, vacuolated appearance.

• Densely staining flattened nuclei lie at the base of the cell.

Seromucous (mixed) glands

• Contain both serous cells and mucous cells.

• The serous secretory units form half-moon-shaped caps (serous demilunes, crescents of Gianuzzi) around the mucous units.

• simple glands

• compound glands

Structure of the duct system

Simple glands

Consist of a single multicellular secretory unit and an unbranched duct

Compound glands

• Secretory product enters a complex system of strongly branching ducts

• Groups of secretory units exhibit an organ-specific structure.

• tubular

• acinar

• tubulo-alveolar or tubuloacinar

Shape of the secretory unit

Tubular

(resembling a hose or pipe)

Acinar

(spherical, berry-shaped) or Alveolar (vesicular)

Tubulo-alveolar or tubuloacinar

(often associated with compound glands)

• intercalated duct

• striated duct

• intralobular duct

• interlobular duct

• lobar duct

Duct system of exocrine glands

Intercalated duct

Connects the secretory units with the secretory (striated) duct

Striated duct

Contributes to the secretory product and thus is also called secretory duct

Intralobular duct

Located in the center of the lobule to continue as interlobular ducts

Interlobularduct

Emerge from the lobule to enter the interlobular connective tissue and converge to form large lobar ducts

Lobar duct

Drains individual lobes of the gland and converges to form the main duct