Epithelium & Glands

Endothelium (definition)

Lining of blood- and lymph-containing structures

Mesothelium (definition)

Lining of true body cavities

5 characteristics shared by ALL epithelia

1) Line free surfaces 2) Express keratin 3) Avascular 4) Derived from all 3 germ layers 5) Polarized (apical free surface, basal surface on basal lamina)

Diagnostic marker for epithelia

Keratin intermediate filaments

What happens to keratin in carcinoma progression?

Usually lost or modified

Blood supply of epithelia

All are avascular, however large

Large avascular epithelium example (dental)

Enamel organ (makes enamel; entirely epithelial, no blood vessels)

Embryonic origin of epithelia

All three germ layers: ectoderm, endoderm, mesoderm

Epithelial polarity

Apical (free, lumen-facing) surface and basal surface attached to a basal lamina

Bottom line: epithelium and organ function

For most organs, if you know the function of the epithelium, you know the function of the organ

Carcinoma

Cancer of epithelial origin; ~80–90% of all cancer diagnoses (most common)

4 categories of epithelia by location

Mucosal, serosal, glands with ducts (exocrine), endothelial (~95% of epithelia)

Mucosal epithelium (mucous membrane)

Lines internal regions continuous with the outside world (e.g., GI tract, esophagus)

3 layers of a mucosa

1) Epithelial lining 2) Lamina propria (specialized CT) 3) Muscularis mucosa (smooth muscle; usually present)

Lamina propria

Specialized connective tissue deep to the epithelium of a mucosa

Muscularis mucosa

Smooth-muscle layer of a mucosa (usually but not always present)

Serosal epithelium (serous membrane)

Lines body cavities NOT continuous with the outside: pleural, pericardial, peritoneal

Why is serosal epithelium called mesothelium?

Derived from mesoderm

Function of serosal epithelium

Makes thin, slippery serous fluid that reduces friction between surfaces (e.g., visceral and parietal pleura)

Endothelium

Specialized simple squamous epithelium lining blood vessels and lymphatics

2 criteria that define epithelial type

1) Number of layers (simple vs stratified) 2) Cell shape at the FREE (apical) surface

Simple vs stratified epithelium

Simple = one layer; stratified = more than one layer

Shape of stratified epithelium is named by…

The shape at the free (apical) surface, NOT the bottom

First step when identifying an epithelium

Find the lumen — it marks the apical surface; the other side is the basal lamina

Simple squamous

Flat 'scale' cells, 'fried egg' (nucleus thickest part); nuclei easy to see, borders hard. Ex: endothelium

Simple cuboidal

As tall as wide; round nucleus fills the cell. Ex: kidney tubule, pancreatic duct

Simple columnar

Much taller than wide; nucleus often basal/middle. Ex: small and large intestine (microvilli brush border)

Stratified cuboidal

Usually only two layers; characteristic of DUCTS (sweat glands, large ducts of esophagus and small intestine)

'If you see stratified cuboidal, you are probably looking at a…'

Duct

Stratified squamous

Many layers (>2); flat surface cells, roundish basal cells. Ex: oral cavity, esophagus

An epithelium with many layers (>2) is almost always…

Stratified squamous

Stratified columnar

Two layers of nuclei; top layer columnar (tall thin cells, nuclei toward base)

Simple columnar vs stratified columnar (how to tell)

Simple: one row of lined-up nuclei. Stratified: two layers of nuclei

3 types of stratified squamous

Non-keratinized, keratinized, parakeratinized

Non-keratinized stratified squamous

Living cells with nuclei all the way to the surface. Ex: buccal mucosa (cheek)

Why is a cheek swab good for DNA collection?

Buccal mucosa is non-keratinized — living cells rub off

Keratinized stratified squamous

Flat dead cells (no nuclei) at the surface; tough barrier, waterproof, reduces cell loss from abrasion. Ex: skin, attached gingiva

Why does attached gingiva look pale?

Thick keratinized layer hides the blood vessels beneath it

Parakeratinized stratified squamous

Nuclei persist at the surface but cells and nuclei are condensed; some protection, less tough than keratinized

ID: many layers, flat surface cells, condensed density at the top

Stratified squamous, parakeratinized

ID: many layers, living nucleated cells at surface, no density on top

Stratified squamous, non-keratinized

ID: many layers, flat dead anucleate cells at surface

Stratified squamous, keratinized

4 specialized cells within epithelia

Goblet cells (unicellular glands), endocrine cells, lymphocytes, myoepithelial cells

Which specialized cells are keratin-positive (epithelial in origin)?

Goblet cells and myoepithelial cells

Goblet cell

Unicellular mucus-secreting gland; apical part full of mucus, nucleus pushed to base

Why do goblet cells look pale in H&E?

Mucus does not stain well with H&E

Where are goblet cells found?

Simple columnar epithelium (e.g., intestine)

Endocrine cells in epithelia: where do they secrete?

Basally into the ECM, taken up by capillaries (not into the lumen)

Hormones from epithelial endocrine cells (gut)

Gastrin, somatostatin, ghrelin, cholecystokinin, secretin

Why are endocrine cells hard to identify?

Scattered; need a specific marker

Wandering lymphocytes in epithelium: appearance

Dark round nucleus with a small white halo that doesn't match epithelial nuclei

Where are wandering lymphocytes common?

Stomach, small intestine, tonsil epithelium (palatine tonsil — so many it looks destroyed, but it is normal)

Myoepithelial cell (basket cell)

Keratin-positive epithelial cell on the basal lamina around secretory cells; contracts to expel secretions

Why are myoepithelial cells acidophilic (red)?

Abundant actin and myosin

Where do myoepithelial cells sit?

Between secretory (acinar) cells and the basal lamina

Stain showing myoepithelial cells are epithelial

Keratin immunohistochemistry (brown)

Origin of most glands

Epithelial ingrowth into underlying CT; cells differentiate into secretory part and duct

Exocrine gland

Keeps contact with surface epithelium via a duct; secretes into a lumen (e.g., salivary glands)

Endocrine gland

Loses contact with surface (no duct); secretes into interstitial space → blood

Unicellular exocrine gland example

Goblet cell

Simple vs compound exocrine gland

Simple: unbranched duct. Compound: branched duct system (e.g., salivary glands, pancreas)

3 modes of secretion

Merocrine, apocrine, holocrine

Merocrine secretion

Only the product released by exocytosis; little storage (small lumen). Ex: parotid acinus

Apocrine secretion

Product + portion of apical cytoplasm pinched off ('decapitation'); product stored (big lumen). Ex: mammary gland

Holocrine secretion

Entire cell and contents become the product (cell dies). Ex: sebaceous gland (sebum)

Holocrine progression (base → duct)

Stem cell mitosis → cell borders disappear → nucleus degenerates → lipid droplets fuse

Acinus

Cluster of secretory cells secreting into a central lumen

Example gland: merocrine / apocrine / holocrine

Parotid / mammary / sebaceous

Serous cells

Watery secretion; abundant RER, well stained; round basal nucleus. Ex: parotid, pancreas

Mucous cells

Thick secretion; clear, poorly stained cytoplasm; flat nucleus pushed to base. Ex: sublingual

Why is the mucous cell nucleus flat?

Artifact — mucus swells with water during slide prep and pushes nucleus to base

Seromucous (mixed) gland

Contains both serous acini (round nuclei) and mucous acini (flat nuclei), sometimes in the same unit

Parotid gland secretion

All (100%) serous

Sublingual gland secretion

All (100%) mucous

Submandibular gland secretion

Mixed (serous + mucous)

Which salivary glands are more likely to form stones and why?

Sublingual and submandibular — thicker mucus can block the duct

Apical epithelial specializations

Cilia (respiratory, reproductive tracts) and microvilli (GI tract)

Lateral epithelial specializations

Tight junctions, adhering junctions, gap junctions, desmosomes

Basal epithelial specializations

Basal lamina, hemidesmosomes

Why is basal lamina the only specialization covered in this lecture?

It is present in ALL epithelia

Basal lamina components

Type IV collagen, laminin, nidogen

Which basal lamina components form 'chicken wire' networks?

Type IV collagen and laminin

2 layers of basal lamina on EM

Lamina lucida (clear, next to cell membrane) and lamina densa (darker, denser)

Basal lamina functions

Physical support; development/morphogenesis/differentiation; barrier to metastasis; cell migration after injury; kidney filtration (glomerular basement membrane)

Basal lamina vs basement membrane

Basal lamina = EM term; basement membrane = light-microscope (H&E) term; used interchangeably

Stain that shows the basement membrane

PAS (hard to see in H&E)

External lamina

Basal-lamina-like structure on non-epithelial, non-polarized cells: adipocytes, muscle cells, Schwann cells, perineurial cells

Where does the word 'epithelium' come from?

Greek epi (on top of) and thele (nipple): papillae seen on a cadaver lip by Ruysch, 1703

List the six characteristics of all epithelia.

Line free surfaces (one or many layers), express keratin, avascular, derived from all three germ layers, polarized with an apical free surface and basal surface on a basal lamina

What is diagnostic for epithelium and what happens in carcinoma?

Keratin intermediate filaments; usually lost or modified

Give an avascular epithelium example from your future lectures.

The enamel organ

What is the bottom line about epithelia and organs?

Know the function of the epithelium and you know the function of the organ

What fraction of cancers are carcinomas?

Roughly 80 to 90%

Mucosal vs serosal epithelia?

Mucosal: cavities continuous with the outside (GI tract); layers: epithelium, lamina propria, muscularis mucosa. Serosal (mesothelium, mesoderm): closed cavities (pleural, pericardial, peritoneal); makes slippery serous fluid

What defines epithelial type?

Number of layers (simple or stratified) and the cell shape at the free surface

Describe simple squamous, cuboidal and columnar with an example each.

Flat (endothelium); cube, as tall as wide (kidney tubule, pancreatic duct); tall (intestine, with brush border)

Stratified cuboidal vs stratified squamous?

Two layers (ducts) vs many layers with a flat surface layer (oral cavity, esophagus)

Three types of stratified squamous and where?

Non-keratinized (cheek); keratinized (dead surface cells, skin, attached gingiva); parakeratinized (condensed nuclei at the surface)

How do you tell simple columnar from stratified columnar?

Simple: one row of lined-up nuclei. Stratified: two layers of nuclei

What does a goblet cell look like and why?

Pale mucus-filled apical cytoplasm; nucleus pushed to the base; mucus stains poorly in H&E