Epithelial Tissue-I

Question

• A male child is born with an absence of the normal structure labeled between the arrows; inclusions of that structure are found within the cells in the photomicrograph.

• He presents with refractory diarrhea and is chronically dependent on parenteral nutrition.

• What is the primary function of the structure labeled between the arrows in the photomicrograph below?

A. Extensive movement of substances over cell surfaces

B. Increase in surface area for absorption

C. Cell motility

D. Transport of intracellular organelles through the cytoplasm

E. Stretch

You should aim to:

• Recognize the presence of an epithelium and understand its defining characteristics.

• Distinguish the different morphological types of epithelium.

• Relate morphology to function where possible.

• Identify ultrastructural features of epithelial cells and their functional significance.

Epithelium

• Formed by closely apposed cells with little or no intercellular material

• Two arrangements:

– Sheets of cells - cover or line a surface

– Glands – specialized in secretion, are epithelial cell accumulations developed as downgrowths into connective tissue

Function

• Transcellular transport• Diffusion of oxygen and carbon dioxide• Carrier protein-mediated transport: Amino

Acids, glucose• Vesicle-mediated transport: IgA• Absorption: endo/pinocytosis• Secretion: exocytosis• Selective permeability: tight junction• Protection

Main Characteristics

1. Rests on a basement membrane.

2. Little intercellular space. 3. Avascular. Epithelium, with the exception of the stria

vascularis of the inner ear, contains no blood or lymph vessels.

4. Derived from all three germ layers.  5. Can undergo metaplasia.  6. Able to regenerate.  7. Nuclei tend to conform to shape of cells.

Classification of Epithelial Types• By the number of layers present

Simple – one layer Pseudostratified – one layer but looks as if

there are many layers Stratified – several layers

• By the shape of their cells Squamous – flat, width exceeds height Cuboidal – height and width almost equal Columnar – height exceeds width

Types of Epithelium

Some special Names

• Transitional: a specialized epithelium located in the urinary system

• Endothelium: simple squamous epithelium that lines blood vessels

• Mesothelium: A single layer of flattened cells; the layer lines serous cavities (peritoneum in abdominal cavity, pleura surrounding lungs, pericardial sac of heart

Simple epithelium is composed of one layer of cells.

• Simple Squamous

• Simple Cuboidal

• Simple Columnar

• Pseudostratified

Simple Squamous Epithelium

• Flattened thin cells that are adjacent to each other

• Nucleus protrudes into lumen• Forms:

– Endothelium (blood vessels)– Mesothelium– Alveoli (in lung)– Loop of Henle and parietal layer of

Bowman’s capsule in the kidney

Simple Squamous Epithelium

Transverse section (looking down on the epithelium)

Cross section, * marks the lumen

*

Note protrusion of nuclei into lumen

Simple squamous epithelium forms the endothelium that lines blood vessels.

Note how the nuclei protrude into the lumens of the vessels.

Simple Cuboidal Epithelium • Centrally located

nucleus

• Locations:

• Covers the ovary (germinal epithelium)

• Interior surface of tympanic membrane

• Ducts of many glands• Tubules in the kidney

Kidney tubules

Simple Cuboidal Epithelium

Ovary: outer covering & follicles

Ducts

Simple Cuboidal Epithelium

Ducts (asterisk marks the lumen)

*

Simple Columnar Epithelium

• Height is greater than width.

• Nuclei are near basement membrane.

Locations• Lining of the

stomach & intestines

• Larger glands & ducts

• Inner ear

Can have surface specializations

Simple Columnar Epithelium

Note the regularity of the epithelium and that goblet cells (arrows) may be present (left) image or may not be present (right image). Asterisks mark the lumens

*

*

*

Simple columnar ultrastructure

Pseudostratified Epithelium Several cell types

• All rest on the basal lamina, but not all reach the lumen

• The nuclei are located at different distances from the basal lamina

• Location– Respiratory system– Middle ear – Parts of male urethra

Usually ciliated and combined with goblet cells

Pseudostratified Ciliated Columnar Epithelium

H&E

trichrome

Form Follows Function(simple = one layer of cells)

• Simple Squamous Epithelium (flat, thin cells)Function: diffusion across the tissue, in & out of lumen

• Simple Cuboidal Epithelium (cube shaped cells)Functions: absorption, secretion, conduction involving various metabolic processes

• Simple Columnar Epithelium (height greater than width)Several cytoplasmic & membrane specializations (polarized)Functions: absorption, secretion, conduction involving complex metabolic processes

Stratified Epithelium

• Stratified Squamous

• Stratified Cuboidal

• Stratified Columnar

(very rare)

• Transitional

Stratified Squamous Epithelium• Several layers of cells• Highly resistant to irritation• 2 types:

– Keratinized (in touch with a dry environment)

– Non-keratinized (in touch with a wet environment)

• Locations – Mouth (non-keratinized)– Esophagus (non-keratinized)– Vagina (non-keratinized)– Skin (keratinized)

Stratified Squamous Non-Keratinized (“moist”) Epithelium

Found in the oral cavity (a “cheek scraping” can be used to collect cells for DNA analysis)

Stratified Squamous Keratinized Epithelium

This epithelium is found in the skin.It is resistant to abrasions and irritants of a dry (land) environment.NOTE THE OUTER LAYER OF KERATIN (ARROWS).

Stratified Cuboidal Epithelium

• Two layers of cells• Most prominent location is

in large ducts of glands (salivary glands, etc.)

• A singe, large duct

(L = lumen)

LL

Stratified Columnar Epithelium

Two layers of cells

• basal cuboidal • superficial columnarLocated in:

• Conjunctiva of the eye• Male urethra• Some larger excretory

ducts (ex: submandibular salivary gland)

Transitional Epithelium3 characteristics

• Apical cells bulge slightly into lumen

• Some cells bi-nuceated

• Apical cell surface stains slightly more deeply pink (actin filaments)

URINARY SYSTEM!

Transitional epithelium is found in the urinary system.

• Stratified• Bi – nucleated

cells• Slight apical

bulge• Dome-shaped

when relaxed, flattened when stretched

Transitional Epithelium

In bladder, exists in two states:

• Extended (flattened, few layers, more surface area, as in a full bladder)

• Contracted (taller, more layers, less surface area; membranes are stored within the cytoplasm as vesicles)

Transitional Epithelium

• Distended (right image)

• Relaxed (left image)

• Enormous elasticity

Bladder

Structure – Function Correlation(all types have several cell layers)

Stratified squamous keratinized epithelium• Protection against dry environments• Lines dry surfaces exposed to continuous

trauma

Stratified squamous moist epithelium• No protection against dry environments• Lines wet surfaces exposed to trauma

Transitional Epithelium• Highly elastic• Lines surfaces exposed to continuous

distension