Levels of Organization
Tissue
Multicellular (large) organisms function more efficiently if cells become specialized for specific functions.
A tissue is composed of cells that function together in a specialized activity.
There are four types of tissues found in animals: epithelial, connective, nerve, and muscle tissue.
Sponges do not have tissues.
Organs
Organs are composed of two or more tissues which function together to perform a common task. For example, the heart contains all 4 types of tissues.
Sponges and cnidarians do not have organs.
Organ systems
An organ system consists of two or more organs which perform a specific task.
Some organ systems are: the integumentary, nervous, sensory, endocrine, skeletal, muscular, circulatory, immune, digestive, respiratory, excretory, and reproductive systems.
Embryonic Tissues
Ectoderm, mesoderm, and endoderm are embryonic tissues that give rise to all of the tissues, organs, and organ systems in the body.
Ectoderm forms the outer layer of skin and nervous system.
Mesoderm forms the muscles, connective tissues, skeleton, kidneys, and circulatory and reproductive organs.
Endoderm forms the lining of the gut, respiratory tract, and urinary bladder. It also forms the glands associated with the gut and respiratory tract.
Junctions
Cells are joined to each other by proteins. The point of connection between two cells is called a junction.
Junctions bind cells together. Some kinds of junctions prevent the passage of molecules between cells. Other kinds of junctions allow molecules to pass from one cell to another.
Epithelial Tissue
Epithelial tissue covers external surfaces and internal cavities and organs. Glands are also composed of epithelial tissue.
Epithelia forms boundaries. Most substances that move into or out of the body must pass through epithelial tissue.
One surface of the tissue is free and the other adheres to a basement membrane.
The photograph below shows kidney tubules. The cells lining the tubules are epithelial tissue. One surface is attached (the basal surface) and the other surface is free.
The apical surface of epithelial cells may have tiny projections called microvilli. These function to increase surface area. For example, microvilli on intestinal cells increase the surface area available for absorption.
Eipthelial cells may have cilia. Cilia can be seen on the cells lining the trachea in the photograph below. They function to move mucus and trapped particles upward toward the mouth where it will be swallowed, thus keeping the trachea clear of foreign particles.
Function of Epithelial Tissue
Protection
Epithelial tissue forms the skin of many animals.Absorption
Terrestrial vertebrates have keratin in their skin cells making them resistant to water loss.
Ciliated epithelium lines the respiratory tract. Numerous cilia on these cells sweep impurities toward the throat.
Absorption is an important function of epithelial tissue. For example, the gut is lined with epithelial tissue and it functions to absorb nutrients from food. The lungs are also lined with epithelial tissue and it functions to absorb oxygen.Secretion
Glandular epithelium secretes chemicals.
Endocrine glands secrete hormones directly into the extracellular space.
Exocrine glands often secrete through DUCTS; they secrete mucus, saliva, wax, milk, etc.Layers
Simple epithelium is one cell thick.
Example: Respiratory surfaces such as the lining of the lungs or the skin of a frog (below) are only one cell thick so that gasses can pass through quickly.
Stratified epithelium has more than one layer. It is found in areas of high abrasion such as the skin or the lining of the mouth.
Squamous Epithelium - Frog Skin Flat MountClick the photograph to view a larger photograph.
Cell division occurs in cells near the basement membrane, pushing older cells toward the surface. Cells lost by abrasion at the surface are replaced by cells underneath.
Example: the human skin shown below contains stratified epithelium.
Pseudostratified epithelium appears to be layered but each cell touches the same basement membrane. Some cells are elongate; they extend from the basement membrane to the free surface. Other cells are smaller, causing the tissue to appear stratified. The nuclei in the cells shown below appear to form multiple layers but the cells are all attached to the same basement membrane.
Stratified Squamous Epithelium, Human sec. X 100
Pseudostratified Ciliated Columnar Epithelium Shape
Epithelial cells are flat (squamous), cube-shaped (cuboidal), or elongated (columnar).
The words that describe layers (previous slides) can be used with words that describe shape. For example, simple squamous epithelium is one layer of flat cells.
Squamous
Simple squamous epithelium is a single layer of flat cells. It is found in the walls of small blood vessels (capillaries) and in the air sacs of the lungs (alveoli). Because it is thin, it permits diffusion of substances from one side to the other. For example, materials can diffuse out of the capillaries. In the lungs, oxygen can diffuse across the alveoli and into the blood.
Simple Squamous Epithelium
Below: The skin of a frog is used for gas exchange. The outer layer of skin is simple squamous epithelium. The thin, flattened cells promote rapid diffusion of gasses between the air and the blood vessels underneath the epithelium.
Cuboidal
Squamous Epithelium - Frog Skin
The cells that line the tubules of the kidneys are cuboidal. They function in secretion and absorption.
The ducts of some glands contain simple cuboidal epithelium.
Simple Columnar
Simple Cuboidal Epithelium
Simple columnar epithelium is a single layer of elongate cells. It is found in the lining of the gut and parts of the respiratory tract. It functions in secretion and absorption. The photograph below is a cross section of the small intestine.
Small Intestine (Jejunum) X 200
