Unit 4: Animal Structure and Function
14.2 Animal Primary Tissues
Learning Objectives
By the end of this section, you will be able to:
- Describe epithelial tissues
- Discuss the different types of connective tissues in animals
- Describe three types of muscle tissues
- Describe nervous tissue
Multicellular, complex animals have four primary types of tissue: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells are called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. Table 14.2 summarizes the different types of epithelial tissues.
| Different Types of Epithelial Tissues | ||
|---|---|---|
| Cell shape | Description | Location |
| squamous | flat, irregular round shape | simple: lung alveoli, capillaries stratified: skin, mouth, vagina |
| cuboidal | cube shaped, central nucleus | glands, renal tubules |
| columnar | tall, narrow, nucleus toward base tall, narrow, nucleus along cell |
simple: digestive tract pseudostratified: respiratory tract |
| transitional | round, simple but appear stratified | urinary bladder |
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure 14.7a illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure 14.7b illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure 14.8, are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular