II ANIMAL TISSUES
In animals, organs are composed of varying combinations of four basic tissue types -- epithelial tissue, connective tissue, muscle tissue and nervous tissue. Each of these tissues has distinctive structural features and specific functions which are again combined to produce functioning organs. The various tissues will be examined and their organization into one specific organ will be studied.
A. Basic Tissue Types
1) Epithelial Tissue
Epithelial tissue forms the covering or lining of free body surfaces, both internal and external. For example, the outer layer of the skin is formed from epithelial tissue as are the inner linings of the digestive tract and blood vessels. As a group, epithelial tissues perform a variety of functions, including protection, absorption, excretion, secretion and lubrication.
Classification of Epithelial Tissue
The cells of the epithelial tissue are tightly packed and rest on a thin basement membrane (Figure 3). The free surface of the epithelium is exposed to air or fluid. No blood vessels are present. Epithelial tissues are classified according to the shape of the component cells and the arrangement of these cells into one or more layers. The shapes of the cells may be (Figure 3):
i) squamous -- flat, scale-like cells (“fried-egg” appearance).
ii) cuboidal -- cells appear square in side view with nucleus in a central position.
iii) columnar -- cells appear rectangular in side view with the nucleus displaced toward the base of the cell.
If the epithelial cells are arranged in one single layer, the epithelium (plural, epithelia) is considered to be simple; if they are arranged in two or more layers, it is stratified. Combining these two forms of classification (number of layers and shape of the cells) leads to the following examples of epithelium:
simple and stratified squamous
simple and stratified cuboidal
simple and stratified columnar
Figure 3: Some types of simple epithelia.
EXERSCISE II: MICROSCOPIC OBSERVATIONS OF ANIMAL TISSUES
A. Basic Tissue Types
1) Epithelial Tissue
Animal tissues are organized into organs. Hence, to study animal tissues, especially epithelium, sections through organs often must be examined. In this case, only part of the histological sample is the epithelium of interest. Use the schematic diagrams to locate the position of the epithelium before trying to recognize the characteristics of the tissue.
a) Simple Columnar Epithelium (Slide #92)
Examine slide #92, which is a cross section through the small intestine. Locate the inner (next to the lumen or cavity) surface of the small intestine. This surface is lined with simple columnar epithelium. This type of epithelium appears as a single layer of tall, column-shaped cells with oblong nuclei. The primary function of this type of epithelium is absorption of nutrients, secretion of digestive juices as well as secretion of mucus by goblet cells.
The surface area of the small intestine is increased by outward finger-like extensions and inward indentations, known respectively as villi and crypts (Figure 5). Both of these structures are lined with simple columnar epithelium. In addition to villi (singular, villus) and crypts, find goblet cells, which appear as white, oval cells found among the columnar cells (Figures 4, 5).
Figure 4: Schematic representation of simple columnar epithelium.
Figure 5A: The inner surface of the small intestine showing villi and crypts.
Figure 5B: Microscopic view of the inner surface of small intestine.
Villus in cross section.
Structures labeled "G" are goblet cells.
What is the specific function of villi?
What is the specific function of goblet cells?
b) Simple Cuboidal and Simple Squamous Epithelium (Slide #39)
Examine slide #39, which is a thin section through the kidney. The kidney is composed of thousands of microscopic tubular structures called nephrons, which function in the formation of urine. Figure 6 illustrates the basic structure of the nephron. The most common types of epithelia present within the nephron are: simple cuboidal and simple squamous (Figure 7). The specific function of the simple cuboidal epithelium is secretion and absorption. This type of epithelium is also found in thyroid gland, salivary glands as well as ducts of glands, such as ducts of sweat glands, for example.
The flat shape of the simple squamous epithelium allows substances to either easily diffuse through the cells or to be filtered through them. In addition to the kidney, simple squamous epithelium is also found in other locations of the body. For example, it lines internal surfaces of the ventral body cavities, blood vessels, heart, and alveoli of the lungs.
On your slide, simple cuboidal epithelium is found in the walls of the nephron tubules. This type of epithelium, appears as a single row of cube-shaped cells. Simple squamous epithelium is found around glomeruli (singular, glomerulus). This type of epithelium appears as a single row of this, flat cells.
Locate nephron tubules, cut in cross section, and observe that the walls of these tubules are composed of cuboidal epithelium. On the same slide, find the glomerulus and observe simple squamous epithelium, located around the glomerulus. Keep in mind that your slide is a section through they kidney, hence you are looking at a side view of the simple squamous epithelium, as shown in Figure 7B - C.
Figure 6: Schematic representation of a single nephron.
Figure 7A: Schematic representation of simple cuboidal epithelium.
Figure 7B: Schematic representation of simple squamous epithelium.
Figure 7C: One simple squamous epithelial cell. A- top view; B- side view.
Figure 7D: Microscopic view of kidney.
Close up of a glomerulus.
c) Stratified Squamous Epithelium (Slide #15)
Stratified squamous epithelium provides protection against abrasion and pathogens to underlying tissues. It is commonly found on surfaces subject to abrasion, such as mouth, pharynx, esophagus, vagina, anus, and outer skin.
Examine slide #15, which is a section through the esophagus. The esophagus is the region of the digestive tract connecting the pharynx and the stomach. Use Figure 8 to locate the inner surface of the esophagus on your slide. This surface is lined with stratified squamous epithelium. As seen in Figure 8, the deeper layer (close to the basement membrane) of the stratified squamous epithelium is composed of cuboidal cells that are actively dividing by mitosis. Newly produced cells are pushed upward toward the surface of the tissue and are gradually transformed into flat, squamous cells. These cells are continually sloughed off. Thus, the organization of the stratified squamous epithelium ensures that abrasion affects the oldest (outermost) cells, while protecting the underlying tissues.
Figure 8A: Schematic representation of stratified squamous epithelium.
Figure 8B: Microscopic view of esophagus.
What is the specific shape and function of the cells in the deepest layer of this type of epithelium?
What is the specific shape of the cells on the surface of this type of epithelium?
d) Pseudostratified Ciliated Epithelium (Slide #16)
Pseudostratified ciliated epithelium lines the nasal cavities, trachea, and bronchi. Its function is to protect these structures, secrete mucus by goblet cells, and move mucus by cilia.
Examine slide #16, which is a section through the trachea. The trachea, or windpipe, is the tube leading from the larynx to the bronchi of the lungs. It possesses bands of cartilage to maintain its shape (as discussed shortly); and it also has an epithelial lining of pseudostratified ciliated epithelium. This type of epithelium consists of a single row of cells. Most of these cells have a columnar shape, while other, shorter cells, which are more cuboidal. The term “pseudostratified” literally means “falsely stratified”. In other words, the epithelium looks as if it has multiple layers but it only has one layer. This is due to the fact that the cells are of differing heights.
Use Figure 9 to locate the epithelium of the trachea on your slide. Observe cilia. Also, locate the oval, white cells scattered throughout this epithelium. These are goblet cells that secrete mucus into the respiratory tree. The function of mucus is to trap foreign material.
Figure 9A: Schematic representation of pseudostratified epithelium.
Figure 9B: Microscopic view of trachea.
What is the specific function of cilia in the trachea?
How many layers of cells are found in this type of epithelium?
What is the specific function of the goblet cells in the trachea?