A
tissue is a group of cells with similar structure and function. The tissue contributes to the function- ing of the organs in which it is found. You may recall that in Chapter 1 the four major groups of tissues were named and very briefly described. These four groups are epithelial, connective, muscle, and nerve tissue.This chapter presents more detailed descriptions of the tissues in these four categories. For each tissue, its functions are related to the organs of which it is a part.
Also in this chapter is a discussion of membranes, which are sheets of tissues. As you might expect, each type of membrane has its specific locations and func- tions.
STRATIFIED SQUAMOUS EPITHELIUM Stratified squamous epithelium consists of many layers of mostly flat cells, although lower cells are rounded. Mitosis takes place in the lowest layer to continually produce new cells to replace those worn off the surface (see Fig. 4–2). This type of epithelium makes up the epidermis of the skin, where it is called
“keratinizing” because the protein keratin is produced, and the surface cells are dead. Stratified squamous epithelium of the non-keratinizing type lines the oral cavity, the esophagus, and, in women, the vagina. In these locations the surface cells are living and make up
the mucous membranes of these organs. In all of its body locations, this tissue is a barrier to microorgan- isms because the cells of which it is made are very close together. The more specialized functions of the epidermis will be covered in the next chapter.
TRANSITIONAL EPITHELIUM
Transitional epitheliumis a type of stratified epithe- lium in which the surface cells change shape from round to squamous. The urinary bladder is lined with transitional epithelium. When the bladder is empty, the surface cells are rounded (see Fig. 4–2). As the
Free surface
Stratified squamous
Connective tissues
Transitional Simple squamous
Example: Lung (approximately 430X)
Alveolar sacs
Example: Esophagus (approximately 430X)
B
Free surface
Example: Urinary bladder (approximately 430X) C
A
Figure 4–2. Epithelial tissues. (A) Simple squamous. (B) Stratified squamous.
(C) Transitional.
QUESTION:Which two of these tissues seem to be most related in structure?
bladder fills, these cells become flattened. Transitional epithelium enables the bladder to fill and stretch with- out tearing the lining.
SIMPLE CUBOIDAL EPITHELIUM
Simple cuboidal epitheliumis a single layer of cube- shaped cells (Fig. 4–3). This type of tissue makes up the functional units of the thyroid gland and salivary glands. These are examples of glandular epithelium;
their function is secretion. In these glands the cuboidal cells are arranged in small spheres and secrete into the cavity formed by the sphere. In the thyroid gland, the cuboidal epithelium secretes the thyroid hormones; thyroxine is an example. In the salivary glands the cuboidal cells secrete saliva. Cuboidal
epithelium also makes up portions of the kidney tubules. Here the cells have microvilli (see Fig. 1–1), and their function is the reabsorption of useful materi- als back to the blood.
SIMPLE COLUMNAR EPITHELIUM
Columnar cells are taller than they are wide and are specialized for secretion and absorption. The stom- ach lining is made of columnar epithelium that secretes gastric juice for digestion. The lining of the small intestine (see Fig. 4–3) secretes digestive enzymes, but these cells also absorb the end products of digestion from the cavity of the intestine into the blood and lymph. To absorb efficiently, the columnar cells of the small intestine have microvilli, which you 72 Tissues and Membranes
Thyroid secretions (hormones)
Simple cuboidal
Simple columnar
Goblet cells
Cilia Connective tissue
Ciliated
Example: Small intestine (approximately 430X)
B
Example: Trachea (approximately 430X) C
A
Example: Thyroid gland (approximately 430X)
Figure 4–3. Epithelial tissues. (A) Simple cuboidal. (B) Simple columnar. (C) Ciliated.
QUESTION:What is the function of the cilia that line the trachea?
may recall are folds of the cell membrane on their free surfaces (see Fig. 3–2). These microscopic folds greatly increase the surface area for absorption.
Yet another type of columnar cell is the goblet cell, which is a unicellular gland. Goblet cells secrete mucus and are found in the lining of the intestines and the lining of parts of the respiratory tract such as the trachea. Mucous membranes will be described in a later section.
CILIATED EPITHELIUM
Ciliated epithelium consists of columnar cells that have ciliaon their free surfaces (see Fig. 4–3). Recall from Chapter 3 that the function of cilia is to sweep materials across the cell surface. Ciliated epithelium lines the nasal cavities, larynx, trachea, and large bronchial tubes. The cilia sweep mucus, with trapped dust and bacteria from the inhaled air, toward the pharynx to be swallowed. Bacteria are then destroyed by the hydrochloric acid in the stomach. The air that reaches the lungs is almost entirely free of pathogens and particulate pollution.
Another location of ciliated epithelium in women is the lining of the fallopian tubes. The cilia here sweep the ovum, which has no means of self-locomotion, toward the uterus.
The epithelial tissues are summarized in Table 4–1.
GLANDS
Glands are cells or organs that secrete something;
that is, they produce a substance that has a function either at that site or at a more distant site.
Unicellular Glands
Unicellular means “one cell.” Goblet cells are an example of unicellular glands. As mentioned earlier, goblet cells are found in the lining of the respiratory and digestive tracts. Their secretion is mucus (see also Box 4–1: Cystic Fibrosis).
Multicellular Glands
Most glands are made of many similar cells, or of a variety of cells with their secretions mingled into a collective secretion. Multicellular glands may be divided into two major groups: exocrine glands and endocrine glands.
Exocrine glands have ducts (tubes) to take the secretion away from the gland to the site of its func- tion. Salivary glands, for example, secrete saliva that is carried by ducts to the oral cavity. Sweat glands secrete sweat that is transported by ducts to the skin surface, where it can be evaporated by excess body heat. The gastric glands of the stomach lining contain different kinds of cells (see Fig. 16–5), which produce
Table 4–1 TYPES OF EPITHELIAL TISSUE
Type Structure Location and Function
Simple squamous
Stratified squamous
Transitional Cuboidal
Columnar
Ciliated
One layer of flat cells
Many layers of cells; surface cells flat; lower cells rounded;
lower layer undergoes mitosis Many layers of cells; surface cells change from rounded to flat One layer of cube-shaped cells
One layer of column-shaped cells
One layer of columnar cells with cilia on their free surfaces
• Alveoli of the lungs—thin to permit diffusion of gases
• Capillaries—thin to permit exchanges of materials;
smooth to prevent abnormal blood clotting
• Epidermis—surface cells are dead; a barrier to pathogens
• Lining of esophagus, vagina—surface cells are living; a barrier to pathogens
• Lining of urinary bladder—permits expansion without tearing the lining
• Thyroid gland—secretes thyroxine
• Salivary glands—secrete saliva
• Kidney tubules—permit reabsorption of useful materials back to the blood
• Lining of stomach—secretes gastric juice
• Lining of small intestine—secretes enzymes and absorbs end products of digestion (microvilli present)
• Lining of trachea—sweeps mucus and dust to the pharynx
• Lining of fallopian tube—sweeps ovum toward uterus
hydrochloric acid and the enzyme pepsin. Both of these secretions are part of gastric juice.
Endocrine glandsare ductless glands. The secre- tions of endocrine glands are a group of chemicals called hormones, which enter capillaries and are cir- culated throughout the body. Hormones then bring about specific effects in their target organs. These effects include aspects of growth, use of minerals and other nutrients, and regulation of blood pressure, and will be covered in more detail in Chapter 10.
Examples of endocrine glands are the thyroid gland, adrenal glands, and pituitary gland.
The pancreas is an organ that is both an exocrine and an endocrine gland. The exocrine portions secrete digestive enzymes that are carried by ducts to the duodenum of the small intestine, their site of action.
The endocrine portions of the pancreas, called pan- creatic islets or islets of Langerhans, secrete the hor- mones insulin and glucagon directly into the blood.