Epithelial tissues are composed of closely aggregated cells with very little extra-cellular matrix. These cells have numerous cellular junctions, tight adhesion and form sheets that cover the surface of the body and line cavities, tubular organs and blood vessels. The functions of epithelia are as a protective covering, absorption, secretion, sensation and contractility. Everything that enters or leaves the human body must pass through an epithelial layer. Embryonically, the epithelia derive from all three embryonic tissues depending on location. The linings of the mouth, nose, anus and skin are of ectodermal origin. The respiratory and digestive system are of endodermal origin. Other epithelial linings, such as those lining blood vessels, d derive from embryonic mesoderm.
Epithelial cells range in shape from columnar to cuboidal to flattened squamous cells. The cell nuclei vary from spherical to elongated or elliptical and often conform to the overall cell shape. Cells may be found as a single layer (simple) or as multiple layers called stratified epithelia. Single layers are found where a barrier is needed that minimally impedes diffusion such as in the lining of the alveoli of the lungs. Stratified epithelia are seen when protection is more important such as in the skin or lining of the mouth.
Epithelial tissue is often associated with a basement membrane made of thin fibers and ground substance. Basal lamina have several functions such as structural support, filtering, cell polarity, cell migration, cell growth and differentiation and have an influence on cellular metabolism. This basement membrane is attached to underlying connective tissue.
Epithelial cells often have adaptations at their luminal surface to increase surface area for absorption or to help move substances over the epithelial surface. A brush border is seen in the lining of the small intestine and is composed of hundreds of microvilli surrounded by glycoprotein coating. The microvilli are about 1 micron high and 0.08 micron wide and are covered by the plasma membrane. In the trachea, the epithelial lining is composed of ciliated cells. The rapid back-and-forth movement of the cilia helps to propel mucous and trapped particulate matter up the trachea.
Examples of Epithelial tissue – 1. Squamous
Examples of Epithelial tissue – 2. Cuboidal
Examples of Epithelial tissue – 3. Columnar
These tissues are responsible for cushioning, supporting and maintaining form within the body and include adipose, cartilage, bone, tendons and ligaments. The major constituent of connective tissue is the ground substance or extra cellular matrix which is composed of protein fibers, ground substance and tissue fluid. The cells make and are embedded in the matrix. Most connective tissue is derived from the mesoderm. Structural glycoproteins and glycosaminoglycans make up the transparent and colorless ground substance of connective tissue.
Glycosaminoglycans are long polysaccharide chains composed of uronic acid and hexosamines like glucosamine or galactosamine. The polysaccharide chains are usually bound to a protein core forming a proteoglycan. These molecules interact with collagen, are very hydrophilic and can bind a huge number of cat ions due to a great degree of sulfation. Examples include heparin sulfate, chondroitin sulfate, hyaluronic acid and keratin sulfate. The high viscosity of the ground substance makes it a fine barrier to the penetration of bacteria and other microorganisms.
Fibers present are made of collagen or elastin and are classified as collagen, reticular or elastic fibers. The different types are present in varying proportions dependent on the tissue type. Collagen is of several types is found in the human body. Collagen type I is the most abundant and is found in bone, tendons and dermis. Many cell types can produce collagen from the amino acids glycine, proline and hydroxyproline in a multi-step process involving many posttranslational modifications. The resultant thin fibrils spontaneously associate to form fibers and very stable bundles of fibers. Reticular fibers are thinner and made mostly of type III collagen. They are abundant in the framework of blood forming organs such as the spleen and bone marrow.
Examples of Connective tissue – 1. Loose
Examples of Connective tissue – 2. Fibrous
Examples of Connective tissue – 3A. Cartilage - Hyaline
Examples of Connective tissue – 3B. Cartilage - Elastic
Examples of Connective tissue – 3C. Cartilage – Fibro-cartilage
Examples of Connective tissue – 4. Bone
Examples of Connective tissue – 5. Blood
Body membranes are thin sheets of tissue that cover the body, line body cavities, and cover organs within the cavities in hollow organs. They can be categorized into epithelial and connective tissue membrane.
Epithelial membranes consist of epithelial tissue and the connective tissue to which it is attached. The two main types of epithelial membranes are the mucous membranes and serous membranes
Mucous membranes are epithelial membranes that consist of epithelial tissue that is attached to an underlying loose connective tissue. These membranes, sometimes called mucosae, line the body cavities that open to the outside. The entire digestive tract is lined with mucous membranes. Other examples include the respiratory, excretory, and reproductive tracts.
Serous membranes line body cavities that do not open directly to the outside, and they cover the organs located in those cavities. Serous membranes are covered by a thin layer of serous fluid that is secreted by the epithelium. Serous fluid lubricates the membrane and reduces friction and abrasion when organs in the thoracic or abdominopelvic cavity move against each other or the cavity wall. Serous membranes have special names given according to their location. For example, the serous membrane that lines the thoracic cavity and covers the lungs is called pleura.
Connective tissue membranes contain only connective tissue. Synovial membranes and meninges belong to this category.
Synovial membranes are connective tissue membranes that line the cavities of the freely movable joints such as the shoulder, elbow, and knee. Like serous membranes, they line cavities that do not open to the outside. Unlike serous membranes, they do not have a layer of epithelium. Synovial membranes secrete synovial fluid into the joint cavity, and this lubricates the cartilage on the ends of the bones so that they can move freely and without friction.
The connective tissue covering on the brain and spinal cord, within the dorsal cavity, are called meninges. They provide protection for these vital structures.
Cutaneous membranes of the skin cover the surface of the body. It consists of stratified squamous epithelium and the underlying connective tissues. Cutaneous membranes are thick, relatively waterproof, and dry.