Organs in animals are composed of a number of different tissue types. For example, the stomach:

Cells and tissues that comprise the stomach.


Organs are composed of tissues, which are in turn composed of cells. Animals have four: epithelial, connective, muscle, and bone.



Epithelial Tissue

Epithelial tissue

    1.  covers body surfaces and lines body cavities.

    2.  Functions include lining, protecting, and forming glands.


    Three types of epithelium occur:


        1.  can be simple or stratified.

            a.  Simple epithelium has only a single cell layer.

            b.  Stratified epithelium has more than one layer of cells.

            c.  Pseudostratified epithelium is a single layer of cells so shaped that they appear at first glance to form two layers.

Note the single layer of simple cuboidal epithelium lining either side of a tubule.

Functions of epithelial cells include:




        1.  can be single epithelial cells, such as the goblet cells that line the intestine.

    Exocrine glands

        1.  glands whose secretions pass into a system of ducts that lead ultimately to the exterior of the body. So the inner surface of the glands and the ducts that drain them are topologically continuous with the exterior of the body (the skin).

    Endocrine glands

        1.  place their secretions into the internal environment - the blood.

Many animals have their skin composed of epithelium. Vertebrates have keratin in their skin cells to reduce water loss. Many other animals secrete mucus or other materials from their skin, such as earthworms do.

Cell Junctions

Tight Junction

fluid tight seal

prevents fluid from leaving a cavity


Anchoring Junction (adhesion)

tissues that stretch, ex. Heart muscle


Gap Junction

passage of chemical & electrical signals

Connective Tissue

Connective tissue serves many purposes in the body:

Connective cells are separated from one another by an extensive extracellular matrix. The matrix may be solid (as in bone), soft (as in loose connective tissue), or liquid (as in blood).

 Loose Connective Tissue (LCT)

    1.   Fibroblasts (LCT) are separated by a collagen fiber-containing matrix.

        a.  Collagen fibers provide elasticity and flexibility.

        b.  LCT occurs beneath epithelium in skin and many internal organs, such as lungs, arteries and the urinary bladder. This tissue type also forms a protective layer over muscle, nerves, and blood vessels.


Adipose tissue has enlarged fibroblasts storing fats and reduced intracellular matrix. Adipose tissue facilitates energy storage and insulation.

Fibrous Connective Tissue

    1.  has many fibers of collagen closely packed together.

    2.  FCT occurs in tendons, which connect muscle to bone. Ligaments are also composed of FCT and connect bone to bone at a joint.

Cartilage and bone

    1.  are "rigid" connective tissues.

    2.  Cartilage has structural proteins deposited in the matrix between cells. Cartilage is the softer of the two. Cartilage forms the embryonic skeleton of vertebrates and the adult skeleton of sharks and rays. It also occurs in the human body in the ears, tip of the nose, and at joints.


    3.  Bone has calcium salts in the matrix, giving it greater strength. Bone also serves as a reservoir (or sink) for calcium. Protein fibers provide elasticity while minerals provide elasticity.   

    4.  Two types of bone occur.

        a.  Dense bone has osteocytes (bone cells) located in lacunae connected by canaliculi. Lacunae are commonly referred to as Haversian canals.

        b.  Spongy bone occurs at the ends of bones and has bony bars and plates separated by irregular spaces. The solid portions of spongy bone pick up stress.



    5.  Blood is a connective tissue of cells separated by a liquid (plasma) matrix. Two types of cells occur.

        a.  Red blood cells (erythrocytes) carry oxygen.

        b.  White blood cells (leukocytes) function in the immune system.

        c.  Plasma transports dissolved glucose, wastes, carbon dioxide and hormones, as well as regulating the water balance for the blood cells.

        d.  Platelets are cell fragments that function in blood clotting.



Human Red Blood Cells, Platelets and T-lymphocyte (erythocytes = red; platelets = yellow; T-lymphocyte = light green) (SEM x 9,900).


Muscle Tissue

Muscle tissue

    1.  facilitates movement of the animal by contraction of individual muscle cells (referred to as muscle fibers).

    2.  Three types of muscle fibers occur in animals (the only taxonomic kingdom to have muscle cells):

Organization of muscle.

Skeletal muscle fibers are multinucleated, with the nuclei located just under the plasma membrane. Most of the cell is occupied by striated, thread-like myofibrils. Within each myofibril there are dense Z lines. A sarcomere (or muscle functional unit) extends from Z line to Z line. Each sarcomere has thick and thin filaments. The thick filaments are made of myosin and occupy the center of each sarcomere. Thin filaments are made of actin and anchor to the Z line.

Bands and zones in a muscle fiber.

Skeletal (striated) muscle fibers have alternating bands perpendicular to the long axis of the cell. These cells function in conjunction with the skeletal system for voluntary muscle movements. The bands are areas of actin and myosin deposition in the cells.


Striated skeletal muscle fibers



Smooth muscle fibers lack the striated banding, although actin and myosin still occur. These cells function in involuntary movements and/or autonomic responses (such as breathing, secretion, ejaculation, birth, and certain reflexes). Smooth muscle fibers are spindle shaped cells that form masses. These fibers are components of structures in the digestive system, reproductive tract, and blood vessels.

Smooth muscle cells

Cardiac muscle fibers are a type of striated muscle found only in the heart. The cell has a bifurcated (or forked) shape, usually with the nucleus near the center of the cell. The cells are usually connected to each other by intercalated disks.

Cardiac muscle cells . Note the dark band of the intercalated disk that separates two muscle cells.

Heart Muscle Cell (nucleus, mitochondria, actin-myosin) (TEM x15,400).

Nervous Tissue

Nervous tissue

    1.  functions in the integration of stimulus and control of response to that stimulus.

    2.  Nerve cells are called neurons.

        a.  Each neuron has a cell body, an axon, and many dendrites.

    3.  Nervous tissue is composed of two main cell types: neurons and glial cells.

        a.  Neurons transmit nerve messages.

        b.  Neuro-Glial cells are in direct contact with neurons and often surround them.

The neuron is the functional unit of the nervous system. .

    1.  While variable in size and shape, all neurons have three parts.

        a.  Dendrites receive information from another cell and transmit the message to the cell body.

        b.  The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells.

        c.  The axon conducts messages away from the cell body.

Organization of a neuron.

 Large multipolar neuron (center of image)

Pyramidal Neurons from the Central Nervous System (SEM x3,960).