Basic Tissues

OBJECTIVES

• To describe a cell and the function of its components

• To define the function of epithelium, name the various types and their locations

• To describe the origin of glands and the ways in which they may be classified

• To describe the components, functions, and location of general connective tissues

• To describe briefly the structure of bone and the two ways in which it is formed

• To describe briefly the components and origin of blood cells, their functions and normal numbers

• To discuss the three types of muscles and their functions, shapes and locations

• To discuss the neuron, its parts, and function

This chapter on the basic tissues of the body is in no way meant to be a complete discussion, but rather an introduction to the basic concepts and structure of the body’s tissues. The body is composed of four basic tissues: epithelium, connective tissue, muscle, and nervous tissue. A tissue is an accumulation of cells, fibers, crystals, or fluids; any one or all might compose a tissue. The basic description of a cell should be the starting point for discussing basic tissues.

CELL STRUCTURE

A cell can be thought of as a bag of fluid, generally varying in size from 0.01 to 0.05 mm in diameter. The wall of this bag is called the cell membrane, and its function is to keep the cellular fluid in and unnecessary foreign materials out. The cell membrane does have the potential to allow molecules of different sizes to pass through, and it can incorporate other membranes into it or add to it by itself. Fig. 17-1 shows the area inside the cell membrane, a fluid medium known as cytoplasm. Within the cytoplasm are other components of the cell, but looking through a light microscope you can probably distinguish only one structure—the nucleus. The nucleus is the master control of the cell. It contains deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which control the operation of the cell. DNA is found in the chromosomes, which during nonmitotic times are found throughout the nucleus. Most of the RNA is found within the nucleolus. The nucleolus is a circumscribed dense area within the nucleus. There is generally one nucleolus per cell, but in some instances there may be more than one. The function of RNA is to carry genetic information or instructions from the DNA to the manufacturing parts of the cell.

FIG. 17.1 A cell showing all basic components, except for lipid or glycogen inclusions.

Organelles

Looking through an electron microscope, which further enlarges cellular structures, you can see other parts of the cell. Most cell parts are called organelles, which are small functioning parts. They allow the cell to remain alive and carry out its particular function.

Small, usually oblong organelles known as mitochondria are responsible for energy production and for the rate at which the cell uses energy—commonly referred to as the metabolism of the cell. If the mitochondria are injured, the cell will not be able to function and may die. The number and location of the mitochondria are an indication of cellular activity and where the majority of activity is located within the cell. The mitochondria in Fig. 17-1 show the infoldings of the inner membrane of the mitochondria that form leaflike projections called cristae. The cristae have enzymes on their surface that aid in cell metabolism. Just as an increased number of mitochondria indicates increased cellular activity, an increase in cristae also indicates a more active cell.

Another organelle, called the endoplasmic reticulum, is a sort of network within the fluid of the cell. The endoplasmic reticulum is a series of interconnecting tubules in the cell that are responsible for the manufacture of various products to be used inside or outside the cell. This function is controlled by several types of RNA from the nucleus and from the cytoplasm itself. Some endoplasmic reticulum have small granules of RNA known as ribosomes on its surface and are referred to as rough endoplasmic reticulum. In other instances the ribosomes are found floating in the cytoplasm. Rough endoplasmic reticulum is responsible for the production of protein. The RNA attaches to various amino acids and, by putting them in a certain order, forms different types of proteins. For example, the rough endoplasmic reticulum of salivary glands produces protein components of saliva.

Once the protein material is produced, it is often necessary to “package” it, as one would do in the shipping room of a factory. The organelle in the cell that takes care of this packaging is known as the Golgi apparatus. The Golgi apparatus, a series of flattened saccules, produces a thin membrane to surround the material produced by the endoplasmic reticulum so it can be moved around the cell and later out of the cell without mixing with the cell’s cytoplasm. To accomplish this type of cell secretion, known as merocrine secretion, the protein that has been surrounded by a membrane produced by the Golgi apparatus moves to the inner surface of the cell membrane and fuses to it. At the point of fusion, a rupture occurs in the cell membrane and in the membrane produced by the Golgi, and the contents are released without any loss of cytoplasm. The Golgi membrane is then incorporated into the cell membrane.

Another small organ found in many cells is called a lysosome, a circular structure that acts as a scavenger for the cell. If any other organelles of the cell die, or if the cell takes in some kind of foreign material, the lysosome will digest the substances. The one problem with this organelle is that it contains some very powerful digestive enzymes to perform its job, and if it is injured, the enzymes will leak out and consume the cell. The person who discovered these organelles called them suicide bags.

To some degree, cell shape is dictated by the pressure of cells around it if they are in tight contact. However, without that pressure and sometimes in spite of that pressure, most cells maintain their shape because of the presence of organelles known as microtubules or microfilaments. These structures are ultrastructural, meaning they are so small they can only be seen with an electron microscope. The microtubules are hollow rods formed of ball-like subunits of proteins called alpha-tubulins and beta-tubulins. They are often associated with the motile part of some cells called cilia or flagella. Microfilaments are solid rods and are found in all cells except mature erythrocytes. They are composed of the protein actin, which is also a component of muscle cells. They run in many directions throughout the cell and often bind to the cell membrane or to other microfilaments. Centrioles are found as a pair of multitubular rods that function in mitosis aiding in alignment of the poles of the dividing cell.

Cellular Inclusions

Organelles are intermixed in many cells with what are called cellular inclusions. This term indicates that the contents of the inclusions are not produced by the cell but rather are stored in the cell to be used at a later time and possibly another place. These inclusions may be little spheres of fat, known as lipid droplets, or multiple units of the sugar glucose, known as glycogen. Both lipid droplets and glycogen are storage forms of energy. When the body requires energy, they are released from the cell that stores them to travel to other parts of the body to be used as needed (Fig. 17-2). Many of these inclusions enter the cell by a process known as pinocytosis, which means a drinking in. The product pushes into the cell membrane from outside, and the membrane caves inward, finally pinching itself off and surrounding the inclusion without any loss of cytoplasm.

FIG. 17.2 A portion of an absorptive cell showing glycogen and lipid inclusions.

Although this is only a brief discussion of cells and their components, a number of different types of cells have different functions. This chapter further explores the similarities and differences of these cells and their functions within the four basic tissues of the body.

EPITHELIAL TISSUE

Epithelium is a group of cells that makes up the skin and lines the inside of the tubes and cavities of the body. Examples of these lining layers include the inside of blood vessels or the digestive tract and the lining of the walls of the thoracic (chest) and abdominal cavities. Glands such as salivary glands, sweat glands, pancreas, and liver also originate from epithelium. Shapes of these epithelial cells differ, and they are in a variety of relationships. Because of these differences, each type of epithelium has a different classification according to its number of cell layers—single-layered, or simple, epithelium and multiple-layered, or stratified, epithelium.

Simple Epithelium

Simple squamous epithelium

The word squamous means flat or platelike. If you looked at the surface of simple squamous epithelium, it would look like a collection of fried eggs side by side in a big pan. If you cut down through this epithelium and looked at a cross section, it would look like an overdone fried egg cut right through the yolk (Fig. 17-3). Looking at it from this side view you can imagine that such a thin layer would not be an extremely protective type of structure but might be thin enough for some materials to pass through between the cells. This type of epithelium is of the following two types:

1. Endothelial cells, found lining blood vessels; small, fluid-carrying tubes called lymphatic vessels; and lining the heart

2. Mesothelial cells, found lining cavities in the body such as pleural, peritoneal, and pericardial cavities

Simple squamous epithelium allows for the exchange of oxygen and carbon dioxide between the lining of the lungs and the capillaries of the lungs.

Simple cuboidal epithelium

As the names indicate, simple cuboidal are cuboidal in shape. Again, these kinds of epithelial cells are only one layer thick, but that one layer is much thicker than the squamous layer. They are located in a number of areas in the body such as the kidney, the glands, and the respiratory passages. There may be cilia on some of these cells, which are moveable and can trap contaminants as in the respiratory tract.

Simple columnar epithelium

The columnar cells line the digestive tract from the stomach to the anal region (Fig. 17-4). In this location the main function of the epithelium is absorption of the breakdown products within the digestive tract. To do a more thorough job in this process, the end of the cell facing toward the lumen has small ultrastructural projections known as microvilli. These microvilli increase the surface area of the cell end by many times its original surface, thereby increasing the amount of nutrients that can be absorbed in the digestive tract (Fig. 17-5). The cells are also found in the ducts of various glands. When these columnar cells are packed together to form small ducts, they tend to form a pyramid shape (Fig. 17-6) and are often referred to as pyramidal cells. These cells also often have microvilli for the same purpose as cuboidal cells.

FIG. 17.5 Simple columnar cell with microvilli. Microvilli are exaggerated in length and width so that you can see how they would increase the surface area of the cell membrane.

FIG. 17.6 Columnar cells forming a tube or duct pushed into a pyramidal pattern. Apical ends are narrower than basal ends. Secretions come from the apical ends.

Looking at the arrangement of these kinds of cells, one can see that the cells adjoin on two sides, and of the other two sides, one rests on underlying connective tissue and the other faces a free border or lumen of a duct. The side facing the underlying tissue is called the basal end of the cell. Facing the free surface is the side often referred to as the apical end of the cell. This terminology makes sense when you look at a pyramidal cell and see that the apical end is the apex of the pyramid and the basal end is the base of the pyramid. This chapter later discusses cells that secrete a product from their apical end. These cells are also found in the terminal portion of the respiratory tract.

Table I7-I

Classification of Epithelia

Cell Type	Cell Shape	Cell Modifications	Characteristics	Location
Simple
1. Squamous
a. Endothelial	Spindle			Lines heart, blood, and lymph vessels
b. Mesothelial	Oval to polygonal			Lines pleural, pericardial, and peritoneal cavities
2. Cuboidal	Cube		Cilia may appear	Kidney, glands, respiratory passages
3. Columnar	Rodlike		Microvilli, cilia may appear	Most glands, small intestines, respiratory passages
4. Pseudostratified	Rodlike with thin section		Cilia, stereocilia	Respiratory passages, male reproductive organs
Stratified
1. Squamous	Polyhedral		Intercellular bridges	Covering of the body, mouth, pharynx, vagina
2. Columnar	Columnar cells on cu boidal or columnar on columnar			Oropharynx, larynx, ducts of large glands
3. Transitional	Cube to pear		Distension causes cell flattening	Urinary passages, bladder