Healthy Animals 1A - Histology of Epithelium

What are the four basic tissues?

1. Epithelial

2. Connective

3. Muscular

4. Nervous

Epithelium

An association of a specific cell type that acts as a lining or is organized as a glandular structure

Lining epithelium

A sheet of closely apposed cells, separating a space from underlying tissue; acts like a cover

Glandular epithelium

An invagination or aggregation of epithelium that forms a solid tissue structure

Common features of all epithelia (5)

1. Close apposition of cells

2. 2. Free surface of epithelial cells is adjacent to the space

3. Basal surface is adjacent to connective tissue

4. Sheets of epithelial cells may be modified into tubes forming glands

5. Absence of blood vessels within epithelial layer

What is the basal surface of epithelial cells adjacent to?

Connective tissue

True or false - the epithelial layer is lined with blood vessels

False

Basilar/Basolateral

Surface of the cell adjacent or attached to the basement membrane

Apical

Surface of the cell farthest away from the basement membrane (top surface)

Classification of epithelium (4)

1. The number of layers of cells

2. The shape of the cells at the free (top) surface

3. Function (what does the epithelium do?)

4. Surface modifications or specdializations

Simple epithelia

One layer of cells

Stratified epithelia

Two or more cell layers. Only cells at the basal surface of the epithelia contact connective tissue

Pseudostratified epithelia

Have their nuclei arranged at different levels in the epithelium, giving a stratified appearance. However, all the cells make contact with the basal lamina

Squamous cells

Flattened in the plane of hte epithelium (wider than they are tall)

Cuboidal cells

Heights approximately equal to width

Columnar cells

Taller than they are wide

Transitional (urothelium) epithelia

Capable of changing shape (when stretched)

What are the two classifications of functions of epithelium

Non-glandular and glandular

Non-glandular epithelium

Epithelial types that do not produce and secrete extracellular products.

What does non-glandular epithelium typically do?

Covers the outside of an organ or lines the inside of a tubular structure

Non-glandular epithelium cell type examples

• skin

• urinary bladder

• oral mucosa

• Esophagus

• Vasculature

Gland

Epithelial lined structures that produce a chemical that is secreted in an extracellular fashion

What are the two classifications of glands?

1. Endocrine

2. Exocrine

Endocrine

Secretes product directly into the bloodstream

Exocrine

Secretes product into a duct

Glandular epithelium

Ingrowth of epithelium at certain locations that produces an extracellular product

Exocrine glands of glandular epithelium

Secretion into a lumen or space. Production and excretio of product happens by multiple mechanisms

Three types of exocrine glands

1. Merocrine

2. Holocrine

3. Apocrine

Merocrine

Exocytosis of secretory vesicles to the lumen of the gland

Holocrine

Death of an entire cell within a gland releases content into the lumen of the gland

Apocrine

Release of budding vesicle off the epithelial cells into the lumen of the gland

Endocrine glands of glandular epithelium

Secretes product into extracellular space which enters the blood stream

What do modifications to epithelium do?

Allow epithelium to function properly in a particular enviornment

Types of modifications to epithelium

1. Microvilli

2. 2. Cilia

3. 3. Cellular connections

Types of cellular connections

1. Tight junctions

2. Desmosomes

3. Hemidesmosomes

4. Gap junctions

Microvilli

Cell surface modifications which increase surface area of a cell

Where are microvilli seen?

In organs where absorption of molecules is a primary function, such as the small intestine and kidney

Cilia

Projections and a type of organelle seen on the apical surface of epithelial cells

How do cilia work and why?

They beat in a coordinated fashion; this assists in the movement of material over the epithelial surface

Tight junctions

Hold cells together. When present in a sheet of cells, allows epithelium to forma barrier between two compartments

Gap junctions

Connection between cells which joins their cytoplasm. Allows the passage of molecules freely between cells

Desmosomes and hemidesmosomes

Specialized cell membrane modifications which join cells to each other or basement membranes

These structures bind cell surface proteins across cells and are anchored by filaments within a cell

What does a desmosome bind?

Cells to other cells

What does a hemidesmosome bind?

Cells to basement membrane

Simple squamous epithelium

A single layer of flat cells.

What kind of structure is simple squamous epithelium particularly important for?

Inside of blood vessels

Characteristics of simple squamous epithelium

Permeable and allows the passage of molecules

Internal organs are covered by an outer membrane, which is called:

Serosa

Mesothelium

Serosa and the cells forming it

What kind of epithelia forms mesothelium?

Simple squamous

Endothelium

The cellular inner lining of blood vessels

What kind of epithelia makes up endothelium?

Simple squamous

Simple cuboidal epithelium

A single layer of cuboidal epithelium lining a basement membrane

Simple cuboidal epithelium characteristics

Provide secretory and absorptive functions

Simple columnar epithelium

Epithelium in which the length is greater than the width

Where are nuclei in simple columnar epithelium located?

At the base of the cell

Common features of simple columnar epithelium

Cilia or microvilli

Pseudostratified columnar epithelium

A single layer of cells similar to columnar epithelium. Their nuclei are arranged at different levels giving the appearance of stratified epithelium

What is the relation between pseudostratified columnar epithelium and the basement membrane?

Every cell is attached to the underlying basement membrane

Pseudostratified columnar epithelium characteristic/functions

Absorptive or secretory functions

Stratified squamous epithelium

Epithelium arranged in layers. The most superficial layer is squamous.

Where can you find stratified squamous epithelium?

Suited to regions where protection from abrasion is necessary

Characteristic of stratified squamous epithelium

Layers of this epithelium can be shed and regrown`

Keratin

An intracellular filament which provides tructure to cells

Cornification

The cells in the uppermost layer of stratified squamous epithelium can lose their nuclei and organelles leaving behind a dead cell with keratin and other inert structural elements. Aka keratinization.

What is a characteristic of a keratinized layer on top of epithelium?

Provides a tough, waterproof barrier

Urothelium (transitional epithelium)

Unique type of stratified epithelium in which the cells can change shape in response to stretching of the organ

Where is urothelium found?

In the urinary tract