Histology Epithelium

Histology A cell is the smallest structural and functional unit of an organism. A tissue is a collection of cells and noncellular structures, which have the similar origin, structure and functions. Histology: (Gr. histo, web or tissue, + logos, study) is the study of cell and the extracellular matrix of tissues.

The human body has four types of tissue:   Epithelial Tissue   Connective Tissue   Muscular Tissue   Nervous tissue

Epithelial Tissue : sheet of cells that covers a body surface or lines a body cavity; also form most of the body’s glands. epithelium (pl., epithelia), : sheet of cells that covers a body surface or lines a body cavity; also form most of the body’s glands. They are derived from all three embryonic germ layers: from mesoderm [e.g., body cavity lining (mesothelium), blood vessel lining (endothelium)] from ectoderm (e.g., the epidermis); from endoderm (e.g., the lining of the gastrointestinal tract ); from mesoderm [e.g., body cavity lining (mesothelium), blood vessel lining (endothelium)]

Characteristics of Epithelium 1. Cells are close to each other. 2. Cells tend to form junctions. 3. Little intercellular material. 4. Lines surfaces and cavities or form glands. 5. Cells show polarity( apical, lateral, and basal surfaces). 6. Rest on a basement membrane. 7. Does not contain blood vessels. 8. Epithelia have the ability to undergo mitosis and replace damaged cells.

Special Characteristics of Epithelia Figure 4.1

Functions of epithelial tissues The primary functions of epithelial tissues are: (1) To protect the tissues that lie beneath it from radiation desiccation, toxins, and invasion by pathogens, and physical trauma. (2) The regulation and exchange of chemicals between the underlying tissues and a body cavity. (3) The secretion of hormones into the blood vascular system, and/or the secretion of sweat, mucus, enzymes, and other products. (4) To provide sensation.

The basement membrane The basement membrane is a thin sheet of fibers that anchors the epithelium to underlying connective tissue. The basement membrane is visible with the light microscope.

Electron microscopy revealed that the basement membrane is composed of two sub layers: 1. The basal lamina 2. The reticular lamina  The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. The main components of basal lamina are type IV collagen, the glycoproteins laminin and proteoglycan The basal lamina layer can further be divided into two layers:

A. A. lamina lucida The clear layer closer to the epithelium. B. B. lamina densa the dense layer closer to the connective tissue The lamina densa and the lamina Lucida together make up the basal lamina

  The reticular lamina located under the basal lamina of most basement membranes. The reticular lamina consists of reticular fibers embedded in ground substance. The components of the reticular lamina are synthesized by cells of the connective tissue underlying the epithelium.   The two layers (the basal lamina and the reticular lamina) are collectively known as the basement membrane.

Functions of The Basement membrane Structural support via cell-matrix adhesions Allow nutrients and waste to diffuse Filter for macromolecules (kidneys) Zone for differentiation and polarization of cells Plays a role in regeneration

Types of Epithelia Epithelia are divided into two main groups according to their structure and function: Covering Epithelia Glandular Epithelia.

Covering Epithelia Covering Epithelia are tissues in which the cells are organized in layers that cover the external surfaces or lines cavities of the body. Epithelia are generally classified by the morphology of their cells, and the number of layers they are composed of.

Epithelial tissue that is only one cell thick is known as simple epithelium. If it is two or more cells thick, it is known as stratified epithelium. There are three principal morphologies associated with epithelial cells. Squamous, cuboidal and columnar

Simple epithelium Simple epithelium is one cell thick, that is, every cell is in direct contact with the underlying basement membrane. Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: The four major classes of simple epithelium are: (1) simple squamous; (2) simple cuboidal; (3) simple columnar; (4) pseudostratified.

Simple Squamous Epithelium Consists of a single layer of thin, flattened cells that fit tightly together and have flattened or ovoid nuclei. Location examples: Air sacs of lung; kidney; lining of heart, blood vessels and lymph vessels; lining of ventral cavity. kidney; lining of heart, blood vessels and lymph vessels; lining of ventral cavity. Function: Allows passage of materials by diffusion and filtration, (its shape and arrangement permit exchanges of substances.

Simple Cuboidal Epithelium Consists of a single layer of cube- shaped cells that usually have centrally located, spherical nuclei. Location : Glands (such as the thyroid gland,the salivary glands, pancreas,), lines the kidney tubules, and covers the ovaries. Function: Simple cuboidal epithelium functions in secretion and absorption.When cuboidal cells are involved in absorption; they have microvilli, which increase the surface area.

Simple Columnar Epithelium Simple columnar epithelium Consists of a single layer of elongated cells that have oval nuclei usually located near the basement membrane. Some cells of simple columnar epithelium can be covered with cilia or microvilli. Simple columnar epithelium may contain mucus- secreting unicellular gland (goblet cells).

Locations Nonciliated simple columnar epithelium lines the digestive tract (where microvilli expand the surface area and aid in absorbing the products of digestion ),. Ciliated type lines small bronchi, uterus, and oviducts. Function: Absorption ; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by cilia action.

Pseudostratified Columnar Epithelium Pseudostratified Columnar Epithelium is so named because it appears to be layered (pseudo, false, stratified,layers).However,true layers do not exist because each cell touches the basement membrane.In particular,the irregular placement of the nuclei creates the appearance of several layers, where only one layer exists. This layer may contain goblet cell and bear cilia.

Location: Nonciliated in ducts of large gland. Ciliated type lines the trachea and most of respiratory duct. Function: secretion of mucus.Psuedostratified columnar epithelium lines the passages of the respiratory system, allowing the dust and microorganisms that enter with the air to be trapped in the mucus and swept out of the airways by the cilia.