Tissues are composed of cells and intercellular material produced mostly by the component cells. The cellular density and the amount and composition of extracellular matrix varies widely among different types of tissues. Tissues are specialized to carry out specific roles and are found not as individual units but rather together in various combinations as integral and functional components of organs and body parts. There are four basic tissue types found in the human body that can be further subdivided by differential features. Tissues may be classified as:
1. Epithelial Tissue
a. Surfacing epithelia
b. Secreting epithelia (glands)
c. Special epithelia
2. Connective Tissue
a. Embryonal connective tissue
b. Adult connective tissue
Connective tissue proper
Areolar connective tissue
Dense irregular connective tissue
Dense regular connective tissue
Other types: Adipose connective tissue
Fibrofatty connective tissue
Pigmented connective tissue
Elastic connective tissue
Special connective tissue
Cartilaginous tissue
Osseous(bony) tissue
Reticular connective tissue
3. Muscular Tissue
a. Smooth muscle tissue
b. Skeletal muscle tissue
c. Cardiac muscle tissue
4. Nervous Tissue
EPITHELIA are sheets of cells that cover the surface of animal's bodies and line their internal cavities such as the lungs and intestine. The cells may be columnar (taller than they are broad), or squamous (flat). To increase the area available for the absorption of nutrients, the intestine wall is folded to form fingers called villi (singular, villus). Focusing in on the columnar epithelial cells lining the intestine shows that their surface, too, is folded into much smaller fingers called microvilli (singular, microvillus). The bottom or basal surface of the epithelial cells sits on a supporting layer called the basement membrane. Many of the epithelial cells of the airways, such as those lining the trachea and bronchioles, have cilia on their surfaces. Cilia are hairlike appendages that actively beat back and forth, moving a layer of mucus away from the lungs. Particles and bacteria are trapped in the mucus layer, preventing them from reaching the delicate air-exchange membranes in the lung. The epithelium of the skin is referred to as stratified because it is composed of several layers.
CONNECTIVE TISSUES provide essential support for the other tissues of the body. They include blood, bone, cartilage, and adipose tissue in which fat is stored. In general, connective tissue tends to contain relatively few cells within a large volume of extracellular matrix consisting of different types of fibers embedded in amorphous ground substance. The most abundant of the fibers is collagen. Collagen forms extremely long, flexible fibers and accounts for about a third of the protein of the human body. Other fibers are elastic so that the supported tissues will return to their original position after they are bent out of shape. The amorphous ground substance absorbs large quantities of water, through which chemicals, oxygen and carbon dioxide can easily move to and from the cells in the other tissues and organs. Of the many cell types found in connective tissue, the two most important are fibroblasts, which secrete the ground substance and fibers, and macrophages, which remove foreign, dead, and defective material from it. A number of inherited diseases are associated with defects in connective tissue. Marfan's syndrome, for example, is characterized by long arms, legs, and torso and by a weakness of the cardiovascular system and eyes. These various symptoms have a single underlying cause, a defect in the organization of the collagen fibers.
MUSCLE TISSUE can be of two types, smooth and striated. Smooth muscle cells are long and slender; they are usually found in the walls of tubular organs like the uterus, stomach and many blood vessels. In general, smooth muscle cells contract more slowly and can maintain the contracted state for a long period of time. There are two classes of striated muscle: cardiac and skeletal, and these make up the big blocks of tissue that we recognize as muscle. The heart is made of cardiac muscle cells while the muscles we use to move our limbs and bodies are composed of skeletal muscle cells. Skeletal muscles are made of hundreds or even thousands of muscle fibers, each fiber being a giant single cell with many nuclei. This rather unusual situation is the result of an cells that give rise to the fibers fuse together, pooling their nuclei in a common cytoplasm. How muscles contract will be described in Chapter muscle.
NERVOUS TISSUE forms the brain and spinal cord as well as the nerves that run throughout our bodies. It is a highly modified epithelium that is composed of several cell types. Principal among these are the nerve cells, or neurons, and the glial cells. The job of neurons is to send and process electrical information. Neurons extend extremely long wire-like processes called axons which, in the leg, for instance, can be over a meter in length. Like wires, axons transmit electrical signals rapidly over long distances. One of the many jobs of glial cells is to make myelin, the electrical insulation around axons.
Plant cells are also organized into tissues. The basic organization of a shoot or root is into an outer protective layer or epidermis, a central vascular tissue that provides support and transport and a cortex that fills the space between the other two. The epidermis consists of one or more layers of closely packed cells. Above the ground, these cells secrete a waxy layer, the cuticle, that helps the plant retain water. Holes in the cuticle called stomata allow gas exchange between the air and the photosynthetic cells and are also the major route for water loss from the plant by the process of transpiration. Below ground, the epidermal cells produce root hairs that are important in the absorption of water and minerals. The vascular tissue is composed of xylem and phloem. The xylem transports water and its dissolved solutes from the roots; the phloem conveys the products of photosynthesis, predominantly sugars, to their site of use or storage. Photosynthesis occurs primarily in the parenchyma cells that form the bulk of the cortex. These cells are less obviously specialized and are the major site of metabolic activity in the plant. They generally lack thick cell walls.
Summary
1. The cell is the basic organizational unit of all living organisms. There are only two types of cells, prokaryotic cells and eukaryotic cells.
2. Prokaryotic cells have very little internal organization. They usually measure only I to 2 um across.
3. Eukaryotic cells contain a variety of specialized internal organelles. The largest of these, the nucleus, contains the genetic material. Eukaryotic cells usually measure 5 to 100 um across.
4. In eukaryotic cells the major cellular organelles such as the nucleus and mitochondria can be seen with the light microscope. It takes an electron microscope to reveal the detailed structure of these organelles and to resolve smaller cellular structures.
5. In multicellular organisms cells are organized into tissues. In animals there are four tissue types: epithelium, connective tissue, muscle tissue, and nervous tissue.
