Epithelia Lab

Introduction

Epithelia are tissues composed of closely aggregated cells that cover most body surfaces, cavities, and tubes. These include the outer surface of the body (skin), tracts traversing the body (gastrointestinal tract), dead-end tracts that have openings at the body surface (respiratory, urinary, and genital tracts), and ducts that open into these tracts (exocrine glands). The functions of epithelia are numerous, and a single epithelium may have several functions. The most important of these include physical protection and selective transport (diffusion, absorption, secretion).

Surface epithelia form continuous sheets that can have one or multiple cell layers. Several different types of cell junctions mediate physical strength and cell communication within the epithelium. A basement membrane lies beneath the epithelium and separates it from underlying tissue; because blood vessels do not penetrate the basement membrane, nutrients like oxygen and metabolites reach the epithelium by diffusion. Epithelia are polarized, with an apical surface that faces the external environment and a basal surface that faces the basement membrane.

Simple squamous epithelium

Simple squamous epithelia consist of a single layer of flattened cells. This type of epithelia lines the inner surface of all blood vessels (endothelium), forms the wall of alveolar sacs in the lung and lines the body cavities (mesothelium). The primary function of simple squamous epithelia is to facilitate diffusion of gases and small molecules.

Simple cuboidal epithelium

Simple cuboidal epithelium consist of a single layer of cells that are approximately as tall as they are wide. This type of epithelium lines collecting ducts and tubes and is involved in absorbing or secreting material into the ducts or tubes.

Simple columnar epithelium

Simple columnar epithelium consist of a single layer of cells that are taller than they are wide. This type of epithelia lines the small intestine where it absorbs nutrients from the lumen of the intestine. Simple columnar epithelia are also located in the stomach where it secretes acid, digestive enzymes and mucous.

Stratified Squamous Epithelium

Stratified squamous epithelia consist of multiple layers of cells with the outer most layer being squamous. The other layers may contain cells that are cuboidal and/or columnar, but the classification of the epithelium is based only on the shape of the outermost layer of cells. This type of epithelium provides protection to mechanical stress and dessication and is found in the skin.

Pseudostratified

Pseudostratified epithelia consist of a single layer of cells, but due to the different heights of the cells, it gives the appearance of having mutliple layers of cells, hence the name pseudostratified. Importantly, all cells are attached to the basement membrane. This type of epithelium is found in the respiratory tract and functions to secrete mucous and move material up the respiratory tract through the beating of cilia. Cilia are long extensions of the cell membrane that contain a microtubule-based structure called the axoneme. Dynein motors within the axoneme generate force that cause a wave-like motion in the cilia.

Basement membrane

All epithelia rest on a basement membrane. The basement membrane provides structural support and integrity to epithelia by providing a common framework of proteins to which epithelial cells adhere. The basement membrane separates the epithelia from the underlying or surrounding tissue. Importantly, because epithelia lack blood vessels and depend upon capillaries in surrounding tissues, all nutrients, hormones and other proteins must diffuse across the basement membrane before reaching an epithelia. Epithelial cells interact with the basement membrane via receptors in their cell membrane called integrins.

Virtual Microscope Slides

1. Trachea
Identify three different types of epithelia in this slide of the trachea. Try to relate the structure of the epithelia with its function.
2. Skin
Identify three different types of epithelia in this slide of the skin. Try to relate the structure of the epithelia with its function.

Lab Quiz

1. Classify the epithelium.

Answer: Simple squamous
2. Classify the epithelium.

Answer: Simple cuboidal
3. Classify the epithelium.

Answer: Pseudostratified with cilia
4. Classify the epithelium.

Answer: Simple columnar with microvilli