Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
TISSUES AND ORGAN SYSTEMS
Unicellular organisms are cells that exist on their own. Each cell shows all the activities of life such as organization, irritability (response to stimuli), feeding and waste removal, metabolism, respiration, and excretion. Examples are chiefly members of the kingdoms Monera and Protista but include also yeast, a fungus, and the many unicellular algae.
In multicellular organisms cells cooperate for the well-being of the whole body. Groups of cells differentiate and emphasize certain functions while losing others. Basic categories of differentiated cells in animals include: secretory with highly developed powers of secretion for example of enzymes or hormones; fat for storage of fat; muscle which are highly contractile; nerve with high irritability; and reproductive that produce gametes. Each kind of cell has reduced or given up some of the functions of the other kinds.
The different cell types are not jumbled about the body, but those with the same function are arranged together to form tissues. In animals there are four basic types of tissues: epithelia or linings, connective or supporting, muscular, and nervous.
There are four basic cell shapes in epithelia: columnar, cuboidal, and squamous (scale-like). The deep columnar cells often have a secretory function, and the nucleus is pushed to the bottom by the made and stored secretions near the surface from which they will exit, e.g., the cells lining the stomach which secrete mucus. Cuboidal cells form the walls of small ducts as from salivary glands. Squamous cells are very flat, and the nucleus may form a bulge; they look something like a fried egg. The thinness permits diffusion of molecules across membranes, e.g., alveolar walls in lungs. Thick layers of cells, e.g., skin, prevent diffusion.
Connective tissues are cells in a matrix. Blood cells course through the body in a fluid matrix. In fibrous connective tissue cells are scattered among the collagen fibers they secreted. In bone and cartilage, cells are scattered throughout the hard or pliable matrix.
Muscle tissues are of three types. Striated muscle cells are large, multinucleate, and column shaped cells; they are chiefly attached to the skeleton. Smooth muscle cells are small and mononucleate; they are found in the walls of tubes such as blood vessels, glandular ducts, and the digestive system. Cardiac muscle cells of the heart are small, striated, and branched.
Nervous tissues consist of the neurons themselves that transmit impulses and the cells that act as supporting connective tissue in the brain and spinal cord.
Combinations of these tissues make up the organs in the human body. Organs are united into systems: digestive, circulatory, respiratory, excretory, endocrine, nervous, locomotory, and reproductive. In the sequence of animal phyla that follow, you will note different degrees of organ system development and sophistication.
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Last updated 30 August 1999 (JHW)