Animal Tissues

Tissues Tissues are groups of specialized cells that work together for a particular function. In humans, combinations of different types of tissues make up organs, and groups of organs work together to form organ systems. Four Types of Tissue 1. Epithelial 2. Connective 3. Nervous 4. Muscle

Epithelial Tissue Epithelial tissues cover body surfaces. Skin and the lining of organs are epithelial tissues. Epithelial tissues play roles in absorption, secretion, and protection against foreign substances.

Simple Columnar Epithelium The cells of simple columnar epithelium (arrows) are taller, as their name suggests. This tissue is usually associated with secretion or absorption and is most often found lining the digestive tract. The section shown here is from an amphibian.

Simple Squamous Epithelium SS Simple squamous (SS) tissue is composed of flat, scale-like cells. It lines the walls of blood vessels, pulmonary alveoli (shown here), and the lining of the heart, lung, and peritoneal cavities.

Simple Cuboidal Epithelium This tissue is composed of a single layer of boxy cells (arrows). It lines the walls of kidney tubules. In describing epithelium, the term “simple” means one cell-layer thick.

Ciliated Epithelium BM Some epithelial membranes are made up of cells with cilia, tiny projections that beat in unison to move mucus along the surface. Ciliated epithelia in the trachea, for example, sweep debris out of the respiratory tract. This tissue lines the larger respiratory passageways. It is often ciliated (arrows).

Muscle Tissue Muscle is a contractile tissue. There are three types of muscle: skeletal, cardiac and smooth.

Skeletal Muscle Skeletal muscle is distinguishable by its striations and its long, unbranched, multinucleate cells or fibers. The striations are due to the arrangement of myosin and actin filaments, along the length of the fiber. Skeletal muscle is attached to bone and is responsible for voluntary movement. When signaled to contract, all the fibers in a particular bundle contract. The number and size of the bundles involved determines the strength of the contraction.

Skeletal Muscle Nucleus Striations are visible. Each fiber runs the entire length of the muscle. Nuclei can be found around the cell periphery.

Cardiac Muscle Cardiac muscle makes up the wall of the heart and is also striated. The cells are branched and usually contain only one nucleus. Cardiac muscle fibers are composed of a number of cardiac cells. Adjacent muscle cells are held together by intercalated disk. At each disk, the cell membranes of cardiac muscle cells are intertwined and held together by gap junctions. Because of these connections, impulses spread quickly through the muscular walls of each chamber.

Cardiac Muscle Striations are evident as well as intercalated disks, the light bands between cells in each fiber.

Nervous Tissue Nervous tissue, which occurs throughout the body, receives and transmits stimuli. It converts a stimulus, whether chemical or physical in nature, into an electrical impulse that is conducted by neurons. Neurons, also called nerve cells, are the functional unit of nervous tissues.

Neuron Axon Cell Body Dendrites A neuron consists of a cell body, dendrites, and axons. Dendrites carry impulses to the cell body, whereas axons transmit impulses toward another cell body or an effector, a muscle or organ that responds to the impulse.

Axons Axons are usually much longer than dendrites, sometimes reaching a meter in length. Many axons are enclosed in an insulating, lipid layer called a myelin sheath. This sheath is produced by Schwann cells, a type of supporting cell for the nervous system. The sheath not only protects the axon but also allows impulses to move more quickly along the axon. Axons arranged in ropelike bundles wrapped in connective tissue make up nerves. Nerves carry sensory impulses to the brain and transmit motor responses from the brain to effector organs. Nerves that make up the peripheral nervous system bring impulses from the entire body to the spinal cord, which then transmits them to the brain. The spinal cord and brain are the central nervous system.

Connective Tissue Connective tissue differs from other tissues in that it contains large amounts of intercellular matrix. Connective tissues function to bind other tissues together, provide support, provide nourishment, store wastes, or repair damaged tissues.

Loose Connective Tissue Loose connective tissue has few fibers (collagen protein fibers), a number of cell types, and a large amount of matrix. It functions to bind epithelia to underlying tissues.

Blood Blood is considered a connective tissue because it has an extracellular matrix called plasma. This liquid matrix is made up of water, salts, nutrients and an assortment of dissolved proteins. Suspended in the matrix are two types of cells, erythrocytes (red blood cells) and leukocytes (white blood cells), as well as specialized cell fragments called platelets that function in clotting blood.

Erythrocytes RBCs Red blood cells (RBCs) in humans are flattened disks because the cells lack a nucleus. The pigment hemoglobin, which gives blood its red color, binds to oxygen. Red blood cells are responsible for transporting oxygen to all the cells of the body.

Leukocytes White blood cells function mostly in fighting diseases. Some of them move through the walls of blood vessels and enter body tissues to engulf bacteria. There are five types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils.