histo-epithelial tissue

EPITHELIAL TISSUE

- Rest on a continuous extracellular matrix – basal lamina –

meshwork of fine filaments

FUNCTIONS

• Protection

- Skin protects from sunlight & bacteria & physical damage

• Absorption

- Lining of small intestine, absorbing nutrients into blood

• Filtration

- Lining of Kidney tubules filtering wastes from blood plasma

• Secretion

- Different glands produce perspiration, oil, digestive

enzymes and mucus

ORIGIN: PRIMARY GERM LAYERS

1. Ectoderm

- Gives rise to the corneal epithelium, epidermis of the skin.

- Invagination – glandular appendages of the skin,

sudiparous, sebaceous, and mammary glands

2. Endoderm

- Intestinal glands liver and pancreas

o Exocrine glands

o Endocrine glands

3. Mesoderm

- Kidneys and reproductive organs, lining of your blood and

lymph vessels, peritoneal cavity and other serous cavities

• Apical Surface

- Topmost surface of epithelial tissue

- All epithelial cells have a top surface that borders an open space – known as a lumen

• Basement Membrane

- Basal surface of epithelial tissue

- Underside of all epithelial cells which anchors them to

connective tissue

• Avascularity (a = without)

- Lacks blood vessels and it is nourished by a connective

tissue.

CLASSIFICATIONS

Cell Shape:

o Squamous

- Flattened like fish scales

- Their heights being very little as compared to their

width.

o Cuboidal

- Cubes

- The height and width of the cells of the epithelium

are more or less equal in size

o Columnar

- Columns

- The height of the cells of the epithelium is

distinctly greater than their width

• Cell Layers:

o Simple

- One layer

o Stratified

- Many layers: Named for the type of cell at

the apical surface

GLANDS

1. ENDOCRINE GLANDS

- No duct, release secretion into blood vessels

- Often hormones

- Thyroid, adrenal and pituitary glands

2. EXOCRINE GLANDS

- Contain ducts, empty onto epithelial surface

- Sweat, Oil glands, Salivary glands, Mammary glands

- Shapes/Braches of Exocrine Glands:

• Branching

- Simple: single, unbranched duct

- Compound: branched

• Shape:

- Tubular – shaped like a tube

- Alveolar – shaped like flasks or sacs

- Tubuloalveolar – has both tubes and sacs in gland

MODES OF SECRETION

- How the gland’s product is released

1. Merocrine

- Just released by exocytosis without altering the gland at all

- Ex: Sweat glands and salivary glands

2. Holocrine

- The gland ruptures and releases secretion and dead cells as well.

- Sebaceous (oil glands on the face) only example

- Includes whiteheads, blackheads and cellular debris on the skin.

BASAL LAMINA

- Sheetlike extracellular structure

- Visible only with electron microscope (20-100 nm thick)

- Consists of a delicate network of fine fibrils – lamina densa

- Electrolucent layers on both sides – lamina rarae or lamina lucida

- Composed of type IV collagen, laminin and proteoglycan

(heparan sulfate)

• Type IV collagen: Monomers of type IV collagen self-

assembled into a two-dimensional network of evenly

spaced subunits

• Laminin: large glycoproteins that attach to

transmembrane proteins called integrins at the cells’

basal surface and project through the network of type IV

collagen

• Nidogen and Perlecan: Respectively a short, rod-like

protein and a proteoglycan, both of these cross-link

laminins to the collagen network and help determine the

porosity of the basal lamina and the size of molecules

able to filter through it.

BASAL LAMINA

Attached to the underlying connective tissues by anchoring structures (fibrils) – type VII collagen

- May also be closely related to reticular fibers – forms a

layer(reticular layer) – produced by connective tissues

- Basement membrane vs Basal lamina

• Basal lamina: denotes the fine extracellular layer seen

ultrastructurally

• Basement membrane: entire structure beneath the

epithelial cells visible with the light microscope

INTERCELLULAR COHESION

- Due to the binding action of glycoproteins in the plasma

membrane and calcium ions (weak covalent bond)

- Intercellular junctions:

• Tight junctions (Zonnulae Occludentes)

• Zonula adherens

• Gap junction

• Desmosome (macula adherens)

• Hemidesmosomes

ZONNULAE OCCLUDENTES

- Tight junctions (singular zonnulae occludens)

- Forms a band completely encircling the cell

- Closes off the intercellular space

• On electron microscope – pentalaminar appearance

• On cryofracture – replicas show anastomosing lines of

ridges (P face) and grooves (E face)

- The number of fusion sites or grooves correlate to the

“leakiness” of the epithelium

- More fusion sites, less permeable epithelium

- Functions as a tight seal to prevent flow of materials

between cells

- The seal between the two cell membranes is due to tight

interactions between the transmembrane proteins

claudin and occludin

- In some epithelia there is an electric potential – for

transfer of molecules

- Ring of plasma union between cells

ZONULA ADHERENS

- Encircles the cell – distance is greater than the usual 20

nm in these areas

- Provides some adhesion of one cell to another

- Insertion of numerous actin-containing microfilaments

into dense plaques on the cytoplasmic surfaces

- These microfilaments arise from web of filaments

(terminal web) – provide rigidity to the terminal apex.

- Terminal apex - found in cytoplasm of cells and gives

overall rigidity It also connects it to one lateral side to

other lateral sides.

GAP JUNCTION

- Nexus: can occur almost anywhere on the lateral surface

- Close apposition of cell membranes (2 nm)

- On cryofracture – aggregates of intermembranous

particles found in circular patches

- Polypetide- major protein, MW 26,000 – 30,000

- Proteins (Connexins) form hexamers with a hydrophobic

pore (1.5 nm) – Connexon

• Connexons - aligned to form a hydrophilic channel

between 2 cells → intercellular communications

DESMOSOME

- Macula adherens – complex disk-shaped structure on

the surface of one cell – matched to another cell

- Membranes are very straight in their regions and farther

apart (>30 nm)

- There is dense material intercellular plaques

(attachment plaques) – groups of intermediate filaments

of cytokeratin

HEMIDESMOSOMES

- On the contact points of epithelial cell and the basal

lamina

- Morphologically, half a desmosome

- Adhering junctions→ zonulae adherentes,

hemidesmosomes, and desmosomes

- Impermeable junctions→ zonulae occludentes

- Communicating junctions →gap junctions

- Hemi - means half - desmosomes

microvilli

cilia (stereocilia, kiniocilia)

flagella

SPECIALIZATION OF THE APICAL SURFACES OF EPITHELIA

MICROVILLI

- Few to numerous projections arising from the surface

(short or long)

- Microplicae

• Longer folds lining of small intestines and proximal

renal tubules

- 1 μm high and 0.8μm wide

- Glycocalyx - filamentous coat of variable thickness,

contains glycoproteins

- Microvillus – extension – covered by plasma membrane

- Interiorly – cluster of 20-30 actin-containing

microfilaments that are cross-linked to each other and to

the surrounding plasma membrane – basal ends

intermingle with the filaments of the terminal web

SPECIALIZATION OF THE APICAL SURFACES OF EPITHELIA

CILIA

1. STEREOCILIA

- Long non-motile processes of cell (epididymis and

hair cells of the inner ear)

- Longer than microvilli

- Parallel to their base but become sinous at their tips

SPECIALIZATION OF THE APICAL SURFACES OF EPITHELIA

CILIA:

KINOCILIA

- Found on the cells for special transport of a mucous

film or fluid over a surface

- Rapid oscillations

- 7-10 μm in length and 0.2 μm in width

- Rapidly stiffen forward → effective stroke then more

slow relaxation → recovery stroke

- May beat together (isochronal stroke) or

metachronal stroke (successive rows in sequence) it is more effective in transporting and usually found in

respiratory tract.

- “Kino” means kinetics

- Axoneme

• A core complex on EM

• Two single microtubules in the center with nine

doublet microtubules uniformly spaced around –

Central microtubules – 13 protofilaments, similar

to those in the cytoplasm

SPECIALIZATION OF THE APICAL SURFACES OF EPITHELIA

CILIA:

KINOCILIA

- Peripheral microtubules

• Subunit A

o Complete microtubule (13 protofilaments)

• Subunit B

o Incomplete microtubule (10 protofilaments)

• A radial spoke extends from each subunit A to the

central pair

• Subunit A of each doublet is connected by fine

nexin to the subunit B of the next doublet

• Short dynein arms project from subunit A to the

subunit B of the next doublet

o 24 nm intervals

- Walls of capillaries, air sacs in lungs

- Form serous membranes in body cavity

- Lines the inside of the heart, where it is called

endocardium; and of blood vessels and lymphatics,

where it is called endothelium.

- Lines some parts of the renal tubules, and in some

parts of the internal ear

-Glands and ducts (pancreas & salivary), kidney

tubules, covers ovaries (germinal epithelium)

- Follicles of Thyroid gland

- Choroid plexuses

- Inner surface of the lens, and the pigment cell layer

of the retina

- Linings of entire digestive tract

- Ciliated columnar epithelium lines most of the

respiratory tract, the uterus, and the uterine tube

- It is also seen in the efferent ductules of the testis,

parts of the middle ear and auditory tube

- Ependyma lining the central canal of the spinal cord

and the ventricles of the brain

- Special Features:

•Microvilli: bumpy extension of apical surface,

increase surface area and absorption rate.

•Goblet cells: single cell glands, produce protective

- Respiratory Linings & Reproductive tract

- Some parts of the auditory tube

- Skin (keratinized), mouth & throat

Stratified Squamous Epithelium