Introduction to identifying epithelium

Characteristics of epithelium

Covers surfaces with an uninterrupted layer of cells.

Cells are attached to one another. Intercellular spaces are small. Epithelial cells are segregated from

underlying tissue by the basement membrane.

Epithelium cell shapes

1. Squamous-flattened shape

2. Cuboidal-box-shaped, not flattened

3. Columnar-cells that appear much taller than wide

4. Transitional- variation found primarily in urinary tract

Criteria used to identify epithelium:

1. The shape of the component cells.

2. The number of cell layers.

3. The presence of surface specializations, such as cilia, villi, or keratin.

4. The presence of secretory structures or cells.

Key feature of epithelium: polarity

Epithelium has a free surface, the apical surface, exposed to the outside;

and an attached surface, the basal surface, resting on the underlying connective tissue.

In a simple (single-layered) epithelium, each cell is polarized.  

The base of each cell is attached to an underlying basement membrane.

The apical end faces open space.  

Lateral surfaces are attached to neighboring epithelial cells.

Basement membrane

Principle layers of the skin

The skin has two principal layers.

1. The epidermis is the epithelial layer of skin.

2. The dermis is the connective tissue layer of the skin.

The epidermis

The epidermis displays several layers. These layers are not distinctly different tissues (unlike epidermis and dermis, for example).

But rather reflect visible changes or stages along the continuous process of keratinocyte maturation, or keratinization.

A continuous process that occurs in stratified keratinized epithelium.

The stages in keratinocyte maturation appear as layers within the epidermis; a single section the layers may show all the stages of keratinization.

Stratum corneum

Stratum granulosum

Stratum spinosum

Slide of stratified, keratinized epithelium showing epidermal layers

The dermis

The dermis lies beneath the epidermis, separated from the epithelium by the basement membrane.

The dermis consists of dense connective tissue; the primary component being collagen.  

The texture of the collagen serves as the basis for recognizing two layers of dermis.

Two layers of the dermis

1. The papillary layer of the dermis lies adjacent to the epidermis and consists of relative small, finely textured collagen fibers.  This layer is named after dermal papillae, the protrusions of dermal connective tissue which indent the base of the epidermis.

2. The reticular layer of the dermis lies beneath the papillary layer and consists of larger, more coarsely textured collagen fibers.

Papillary layer

Dermis Trichrome stain 40x

Simple Squamous Epithelium

Single, thin layer of flattened cells.

Irregular outlines that fit together to form a continuous membrane.

Often found in areas where diffusion or filtration take place.

The lung alveoli, the bowman’s capsule of the kidney are other sites this type may be found. Luminal surface (esophagus)

Simple cuboidal

Cube appearance refers to box shape seen when they are sectioned at right angles.

When it is stratified, lower layers also cuboidal.

Found in glandular ducts, the covering of the ovary.

Small ducts have simple cuboidal, larger ducts may have stratified.

Cuboidal in the upper eyelid

Simple columnar

Single layer of tall cells. Usually involved in active

secretion or absorption across the cell layer, often with striated borders and micro-villi.

Found lining the digestive tract, the female reproductive tract.

Modified simple columnar of the intestinal tract interspersed with mucous-secreting goblet cells for protective coating.

colon 40x

columnar

Stratified epithelium

Are able to withstand “wear and tear”

Not suited to absorption or secretion.

Secretion in this type is accomplished through glands.

Includes, stratified squamous non-keratinizing, stratified columnar, transitional.

Stratified squamous-the lip

Flattened squamous

Cuboidal basal

Psuedostratified columnar

Appears to be stratified, because the nuclei are in 2 or more distinct levels.

But because every cell rests on the basement membrane, it is classified as “simple”.

Found in the respiratory tract and the male reproductive system.

Respiratory Tract

Stratified squamous non-keratinizing

Usually protective. Multiple layers are too

thick for diffusion. The innermost (basal)

layers produce new cells to replace those lost at the surface.

no keratinized surface. Lines oral cavity, uterine

cervix, and esophagus (this slide).

Basal layer →

Transitional epithelium

Descriptive term for special stratified epithelium that is specialized to accommodate stretching without the membrane breaking apart.

Named because it has some features of both cuboidal and stratified squamous making it “transitional” or an intermediate type.

Found almost exclusively in the urinary tract.

Transitional epithelium in the Bladder

Specialized epithelium

Glandular epithelium is specialized for secretion and absorption.

Hair follicles, sebaceous glands, and sweat glands are all invaginations of the epidermis.

Some specialized epithelium is columnar epithelium with cilia for movement of secretions across membranes.

Sweat gland

Review

With practice you will learn to identify epithelial tissue under the microscope almost immediately.

However, we practiced eliminating obvious possibilities, and learning to identify the nuclei, cytoplasm and structural arrangements typical of epithelial tissue.

We identified the basic cell shapes of epithelium. By repeating this process with each section you

encounter, you will begin to develop a habitual method of identifying unique characteristics of different tissues under the microscope.