Tissue are categorized into four principal types, or primary tissues:  (1) epithelial, (2) connective, (3) muscular, and (4) nervous.

A. Epithelial Tissue
Epithelial tissue, or epithelium (epi = upon; thelium = to cover), functions to protect, secrete, or absorb.  Epithelial membranes cover and line the body cavities, cover the outer surface of the body (the epidermis of the skin) and the surfaces of internal organs, and line the lumina (the hollow portions) of ducts, vessels, and tubes.  Most glands are derived from epithelial tissue.  All epithelial tissues share the following characteristics:

1. The cells are closely joined together and have little intercellular substance (matrix) between them.
2. There is an exposed surface, either externally or internally.
3. A basement membrane is present to anchor the epithelium to underlying connective tissue.

    Epithelial tissues may be classified as simple, if they are only one layer thick, or stratified, if they are two or more layers thick  Pseudostratified tissues are actually one one layer thick, but they give the impression of multiple layers because the nuclei of their cells are located at different levels.

    Epithelial tissues may be further classified by the shape of their surface cells as squamous (if the cells are flat and shingle like), cuboidal, or columnar.  Using these criteria, one can identify the following types of epithelia:

1.  Simple Squamous Epithelium.  This type is adapted for diffucion and filtration and occurs in such places as the lining of the air sacs (alveoli) withing the llungs (where gas exchange occurs); parts of the kidney (where blood is filtered); and the lining, or endothellium, of blood vessels.

2.  Stratified Squamous Epithelium.  This type of tissue is found in areas that receive a lot of wear and tear.  The outer cells are sloughed off and replaced by new cells, produced by mitosis in the deeper layers.   Stratified squamous epithelium is found in the mouth, esophagus, nasal cavity, and in the openings into the ears, anus, and vagina.  A specialized keratinized, or cornified, layer of dead surface cells is found in the stratified epithelium of the skin (epidermis).

3.  Cuboidal Epithelium.   This type of epithelium is usually simple and lines the surfaces of many ducts and tubules.

4.  Simple Columnar Epithelium.   This simple epithelium of columnar cells is found in the lining of the gastrointestinal tract, where it is specialized to absorb the products of digestion.  It also contains mucus-secreting goblet cells (indicated by arrow).

5.  Ciliated Columnar Epithelium.  These columnar cells support hair like cilia on the exposed surface.  This tissue is found in the uterine tubes of the female and , the case in this image, in the ductus deferens (vas deferens) of the male.

6.  Pseudostratified Ciliated Columnar Epithelium.  This epithelium is really simple but appears stratified because the nuclei are at different levels.   It is found in the respiratory passages of the trachea and bronchial tubes, and it, too, contains cilia.

7.  Transitional Epithelium.   This type is found in the urinary bladder and is uniquely stratified to permit periodic distension.  Transitional epithelium is a multilayered epithelium characterized by the presence of large dome-shaped cells in the apical layer. These cells flatten out into giant squamous cells as the bladder becomes distended with urine.
 

B. Connective Tissues
Connective tissue is characterized by abundant amounts of extracellular material, or matrix. Unlike epithelial tissue, which is composed of tightly joined cells, the cells of connective tissue (which may be of many types) are spread out. The large extracellular spaces in connective tissue provide room for blood vessels and nerves to enter and leave organs.
There are five major types of connective tissues: (1) mesenchyme, an undifferentiated tissue found primarily during embryonic development; (2) connective tissue proper, (3) cartilage; (4) bone; and (5) blood.

    Connective tissue proper  refers to a broad category of tissues with a somewhat loose, flexible matrix. This tissue may be loose (areolar), which serves as a general packaging material, or dense, as is found in tendons and ligaments.  The degree of denseness relates to the relative concentration of protein fibers versus fluid in the matrix.  Protein fibers may be made of collagen, which gives tensile strength to tendons and ligaments; they may be elastic fibers, which are prominent in large arteries and the lower respiratory system, or they may be reticular, providing structural support to the lymph nodes, liver, spleen, and bone marrow. Adipose tissue is a type of connective tissue in which the cells are specialized to store fat.

    Cartilage consists of cells called chondrocytes and a semisolid matrix, which imparts strength and elasticity to
the tissue. There are three types of cartilage.  Hyaline cartilage has a clear matrix that stains a uniform blue.  This is the most abundant form of cartilage, and is found in the articular surfaces of bones, the trachea and bronchi, the nose, and the costal cartilages between the ventral ends of the first ten ribs and the sternum.  Fibrocartilage is reinforced with coliagen fibers to resist compression. It is found in the symphysis pubis, where the two pelvic bones articulate, and between the vertebrae, where it forms intervertebral discs.  In this slide the fibrocartilage is located on the far right.  Elastic cartilage contains abundant elastic fibers for flexibility.  It is found in the external ear, portions of the larynx, and in the auditory canal (eustachian tube).

    Bone  contains cells called osteocytes.  These cells are surrounded by an extremely hard matrix impregnated with calcium phosphate. The osteocytes surround a central canal, containing blood vessels, and obtain nourishment via small channels in the matrix called canaliculi.

    Blood is considered a type of connective tissue because it contains abundant extracellular material (the fluid plasma) that suspends and transports the blood cells (erythrocytes, leukocytes, and thrombocytes).  This slide shows a number of erythrocytes and one leukocyte.  The slightly dimpled appearance of the RBC's is due to their biconcave configuration.  Contrast the appearance of human with blood with that of amphibian blood.  Note that the RBC's of amphibian blood are nucleated.
 

C. Muscle Tissue
Muscles are responsible for the movement of materials through the body, the movement of one part of the body with respect to another, and for locomotion.  Muscle tissues, which are contractile, are composed of cells, or fibers, that are elongated in the direction of contraction.  The three types of muscle tissues are smooth, cardiac, and skeletal.

    Smooth muscle cells are long and spindle shaped, with a single nucleus (denoted by the arrow)  near the center. They are found in the digestive tract, blood vessels, respiratory passages, and the walls of the urinary and reproductive ducts.

    Cardiac muscle is found in the heart;  its cells are branched and interconnected by intercalated discs (denoted by the arrow).

    Skeletal muscle, which is attached to the skeleton, is responsible for voluntary movements. Skeletal muscle fibers are long and thin and contain numerous nuclei. Both skeletal muscle fibers and cardiac muscle cells are categorized as striated muscle because they contain cross striations. Skeletal muscle is under voluntary control, whereas cardiac and smooth muscles are classified as involuntary.  This distinction relates to the type of innervation and not to the characteristics of the muscles themselves.
 

D. Nervous  Tissue
Nervous tissue, which forms the nervous system, consists of two major types of cells. The nerve cell, or neuron, is the functional unit of the nervous system. The typical neuron has a cell body with a nucleus, small projections called dendrites extending from the cell body, and a single long extension called an axon, or nervefiber. The neuron is capable of producing and conducting electrical impulses and releasing chemicals from the endings of the axon. A second type of cell found in the nervous system is a neuroglial cell. The neuroglia support the neurons both structurally and functionally.  This particular glial cell resides in the central nervous system and is called an astrocytye.  Its finger-like extensions surround the neruon.
 

E. An Organ: The Skin
Organs contain more than one type, and usually all four types, of primary tissues. The skin provides an excellent example. Epithelial tissue is seen as the epidermis, the hairfollicies, and the sebaceous glands that extend from the hair follicles.  One can also see sweat glands, which like all glands are a type of epithelial tissue.  The dermis, just below the epidermis, contains areolar connective tissue.  Muscle tissue is seen as the arrector pili muscle, a smooth muscle that attaches to the hair follicle and the matrix of the dermis.  Nerve tissue may be seen within an onionlike sensory organ, sensitive to pressure, called a pacinian corpuscle. Meissner's corpuscle  is a similar sensory structure that can be seen in the skin.