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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
The term pseudostratified columnar epithelium means falsely layered epithelium or epithelium that looks like more than one layer. Some texts consider this a separate classification of epithelium from simple epithelium. Viewed under a microscope, it appears that several rows of nuclei are present, which would suggest more than one row of cells. However, on closer examination, it is evident that all of the cells reach down to the underlying tissue, the basement membrane. Some of the cells are very short, but others begin on the basement membrane with a very narrow stem and then expand once they reach the upper part of the cell layer. This type of epithelium is seen in several areas of the body, but the most prominent is the respiratory passages. In the respiratory tract and in other places, the epithelium has many small, single-celled glands called goblet cells intermixed with the epithelium (Fig. 17-7). These glands secrete a mucous substance and lubricate the surface of the epithelium for a number of functions. Along with the goblet cells, this epithelium also has small, hairlike projections known as cilia, which perform a waving, beating motion. These cilia are coated with mucus from the goblet cells and then trap contaminants in the air passing through the respiratory passages. These trapped particles are passed along from cilium to cilium by means of the beating motion until they reach the nasal opening or the opening of the oral cavity where they are removed from the body. Some simple columnar epithelia also have cilia. These cells are also found in male reproductive passages.
A type of epithelium not commonly found is stratified cuboidal or stratified columnar epithelium. It consists of two or possibly more rows of cuboidal or columnar cells on top of one another and is generally only found forming large ducts of glands.
The term transitional indicates change, and that appropriately describes transitional epithelium. It changes in thickness and appearance as the need arises. Composed of multiple layers of cells and varying in thickness, it is found in the urinary system, with the primary concentration in the urinary bladder and ureter. When the bladder is empty, the epithelium is relaxed and about 8 to 10 layers of cells are present with the deepest layers composed of cuboidal cells and the surface layers slightly flattened but with rounded bulging nuclei (Fig. 17-8, A). When the bladder is full, the epithelium is stretched and may appear to be only 3 to 5 layers thick with the deepest cell layers somewhat flattened and the surface layers and the nuclei extremely flattened (Fig. 17-8, B). This change in appearance accounts for the name transitional and represents a very functional arrangement of cells.
The most common type of epithelium is stratified squamous epithelium. As skin, it covers the body and also makes up the mucosa of the oral cavity, pharynx, esophagus, and anal region. In a discussion of this type of epithelium it seems appropriate to consider the changes seen in the cells at different layers of the stratified squamous epithelium.
A single layer of cuboidal cells that rests on the underlying connective tissue of the basement membrane is known as stratum basale or stratum germinativum. It is in this layer that the cells divide and form more cells to maintain the supply and replace the cells that are lost.
As more cells form in the basal layer, they become displaced because of the crowding; thus they are pushed out of the basal layer into the layers above them toward the surface. The stratum spinosum varies in its number of cell rows from 2 or 3 to 10 or more. This layer, like the basal layer, is also sometimes referred to as a stratum germinativum, because some cell division can still be seen in this layer. In this layer the cells are no longer cuboidal but seem to be star shaped or having many small points; hence the adjective spinosum. The reason for the star-shaped appearance is that the cells are attached at points of intracellular modifications, called desmosomes, on the cell walls of adjacent cells. These points almost resemble a series of “spot welds” on the adjacent cell walls. During preparation of tissue for study under a microscope, the cells dehydrate and shrink away from each other but are held together at the points of desmosomal attachments and thus appear star shaped. The cells continue to be pushed toward the surface by the newly forming basal cells and eventually reach the next layer.
Not clearly seen in many areas of the mucosa of the oral cavity, stratum granulosum is particularly evident in thick skin. When seen, it appears as two or three layers of flattened cells that contain granules or spots within their cytoplasm. These granules are made up of a material called keratohyalin and eventually cause the cell to die when the amount of keratin becomes great enough.
The term corneum is the same as the term keratinized; it means hornlike, resembling the tissue of the fingernail, only less dense and therefore softer. In many instances stratum corneum is misnamed because the top layers of cells are not always cornified or keratinized. Three different situations can be seen in this layer:
2. The cells on the surface, although still showing signs of nuclei, may be in the process of dying and are referred to as partly or parakeratinized stratified squamous epithelium. Here, the stratum granulosum is very thin.
3. The cells on the surface may be dead and referred to as keratinized stratified squamous epithelium. Here, the upper layer has no nuclei, and they appear somewhat opaque when they are not stained with dyes for microscopic study (Fig. 17-9; see also Fig. 23-1).
The thickness of this upper dead layer varies, depending on the amount of trauma or rubbing to which the tissue is subjected. As you know, working with a shovel or rake will eventually cause a callus to form, a result of a thickening of the stratum corneum and stratum spinosum. In thick skin such as the palms of the hands or the soles of the feet, a clear layer known as the stratum lucidum can be seen between the stratum granulosum and the stratum corneum. The cells are continually produced in the basal layer of the epithelium and move up through the other layers until they reach the surface where they are shed. This process can be seen when one sits in a tub of hot water for a while. Without using soap, a ring begins to develop around the tub. This ring is not readily visible but can be felt. The primary component of this ring is dead epithelial cells, which slough off and float to the edge of the tub. Just think of the mechanism that regulates the rate of cells produced with the rate of those lost and that adjusts to meet any changes. Without this control, we would either have skin as thick as an elephant or have no skin whatsoever (Table 17-1).
|Cell Type||Cell Shape||Cell Modifications||Characteristics||Location|
|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|
|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|
From Avery JK: Essentials of oral histology and embryology, ed 2, St Louis, 2000, Mosby.
Another important mechanism relating to cell replacement in skin has to do with pigment in the skin and changes in that pigment level. Skin normally has a yellowish color because it contains the pigment carotene. Some pink color is also imparted to it by the underlying blood vessels. Immediately beneath the basal layer of cells in all people except albinos are cells called melan/>