Tissues


     Tissues are groups of cells that are similar in structure and function. Tissues combine in various ways to form organs. There are four basic tissues:
Epithelium – forms surfaces and glands
Connective – binds, protects, resists gravity and distributes materials
Muscle – provides movement
Nervous – regulates and controls
Epithelium
     All the surfaces of the body are covered by epithelium. The amount of surface that needs to be covered in the body is enormous! It includes the outer surface of your body, the skin, passageways that open to the exterior of the body, digestive, reproductive, urinary and respiratory tracts, the internal lining of blood vessels and the outer surface of internal organs.
Characteristics of Epithelium
Cellularity
  Epithelial cells attach directly to one another to form a continuous surface.
Polarity 
  The surface that epithelial tissue forms has two sides. The side that faces the surface is called the apical surface. The side that is attached is the basal surface. 
Attachment
  The cells that form the epithelium attaches to the connective tissue underneath by a mix of molecules that form the basement membrane. 
Avascularity
  The epithelium has no blood vessels penetrating through it. Nutrients and oxygen is supplied by the connective tissue to which the epithelium is attached.
Renewal
  Epithelial cells divide to replace cells that get old or are lost by wear and tear.
     The nature of the epithelium on different surfaces reflects the primary functions performed by the epithelium. These functions that epithelium performs include protection, absorption, secretion and filtration.
     On surfaces that require protection and resistance against wear and tear the epithelium will have many layers of cells. The skin is an example of a location where the epithelium has many layers. The epithelium is stratified and can be very thick. The uppermost surface layers of the epithelium are dead and are densely packed with a protein called keratin. Keratin increases the toughness of the skin and gives it a water-proof quality that prevents water loss.
     In other surfaces of the body the epithelium is designed to permit the passage of a variety of molecules. In some locations, the lining has to be as thin as possible to allow for the rapid passage of molecules by diffusion. For example, the surfaces of the lung where gas exchange takes place is exceedingly thin. It is only one cell layer thick and the cells are exceedingly flat. This increases the speed by which gases such as oxygen and carbon dioxide can pass through the epithelium by diffusion.
     In other locations, molecules are transported across the surface by active transport and the cells that form the epithelium the cells need to expend energy to move molecules across the surface. For example, the inner lining of the small intestines is where nutrients and water is rapidly absorbed. The epithelium here is again only one cell layer thick but the cells have more bulk. These cells actively transport nutrients and water against their concentration gradients. This requires energy and cellular machinery. Hence, the bulk of the cells is due to the presence of organelles to perform this energy demanding function.
Classification of Epithelium
     Anatomists classify epithelium according to two criteria: 1) the number of cell layers and 2) the shape of the cells on the surface.
     Epithelium that is only one cell layer thick is called simple. Epithelium with more than one layer is called stratified. If the shape of the cells on the surface is flat the epithelium is called squamous. If the cells are shaped like cubes the epithelium is called cuboidal. If the cells are taller than they are wide the epithelium is called columnar.
     Let’s apply these criteria to describing the three examples cited above.
Epithelium Classification Comments
Epidermis (Epithelium of Skin) Stratified squamous   The complete classification of this epithelium is keratinized stratified squamous
Alveolar epithelium (exchange epithelium of lungs) Simple squamous   Delicate epithelium but in a protected place
Lining of the small intestines Simple columnar   Apical surface of these cells have microvilli that increase surface area for absorption
     Although most epithelia can be easily classified by this scheme there are two exceptions. We will mention only one.
Pseudostratified ciliated columnar epithelium (PCCE)
     PCCE is the epithelium typical of the upper respiratory tract. It is a simple epithelium in that all the epithelial cells are in contact with the basement membrane below. However, some cells are short and do not reach the surface, while neighboring cells are tall and do. This makes the epithelium appear stratified although it isn’t. Another distinct characteristic of this epithelium is that the cells bear cilia which move materials across the surface.
Glands
     Glands consist of cells that secrete a particular product. Secretions typically contain a protein in an aqueous (water) solution. All glands are part of, or originate from the epithelium.
     There are two basic types of glands:
Exocrine glands
     Exocrine glands release their secretions onto the surface of the epithelium. Although these glands may grow away from the surface, they remain connected to the surface by ducts.
Endocrine glands
     Endocrine glands develop from the epithelium but lose their connection to it and are ductless. Secretions of endocrine glands are released into the fluid that surrounds the glands and diffuses into the blood.

Connective Tissue
     Connective tissue is found throughout the body. As its name implies it holds together the other kinds of tissue. However, it also plays a protective and supportive role in a more general sense as we will see.
Characteristics of Connective Tissue
     The cells of connective tissue are not tightly joined to one another as in epithelial tissue but are more spread apart. Non-living material produced by the cells separates them and forms the extracellular matrix.
Extracellular matrix
  The extracellular matrix is usually produced by cells primarily responsible for the characteristics of the connective tissue. Extracellular matrix has two principal components:
Ground substance
  Ground substance is the structureless fluid that surrounds all other components of the tissue. It contains adhesion proteins that hold the cells in place and charged polysaccharides that hold water like a sponge.
Fibers
  Fibers are made from protein molecules secreted by the cells that self-assemble to become the long fibers that exceed the length of the cells themselves. The types of fibers include:
Collagen fibers – These fibers resist stretching and therefore have high tensile-strength.
Elastic fibers – These fibers allow themselves to be stretched under tension but recoil to the original length when the tension is released.
Reticular fibers – These are a type of collagen fiber that is more delicate and branches to form the support for the cells of soft organs such as the lymph nodes and the spleen.
Types of Connective Tissue
     The properties of connective tissue can vary greatly depending upon the nature of the matrix. Types of connective tissue include:
Bone Ground or compact bone
  In bone the matrix is very hard because crystals of calcium salts are deposited among the fibers of the matrix.
Cartilage Hyaline cartilage
  The matrix of cartilage forms a solid material that has more pliable and flexible.
Dense Connective Tissue Dense regular connective tissue
  The matrix of dense connective tissue is dense because it has a high concentration of fibers. The tissue then takes on the properties of the most abundant fiber. For example, ligaments and tendons resist stretching because of their collagen fibers.
Loose Connective Tissue
  Loose connective tissue has a more fluid matrix with a smaller quantity of fibers. There are three subcategories of loose connective tissue:
Areolar areolar connective tissue – serves as a universal packing and binding tissue.
Adipose adipose tissue – is essential areolar tissue in which fat cells (adipocytes) predominate. Adipose tissue serves as energy storage sites but also provides insulation and mechanical padding.
Reticular reticular connective tissue – consists of cells called reticular cells that produce and maintain reticular fibers that form the supporting framework of the cells of soft tissue.
Blood Blood
  Blood is an atypical connective tissue because the matrix, plasma, is entirely fluid and does not originate from the predominate cell, the red blood cell. Blood will be described in more detail later in the course.