TISSUE A. Definition: societies of cells and their associated extracellular substances(ECM), which are specialized to carry out specific functions. B. Four Primary Tissues of the Body 1. Epithelium 2. Connective Tissue 3. Muscle 4. Nervous Tissue

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body. Many glands are made up of epithelial cells. Functions of epithelial cells include secretion, selective absorption, protection, Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane .

Classification of epithelia 1. Surface epithelia In general, tissues are 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. However, when taller simple epithelial cells (see columnar, below) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium. Classification of epithelia 1. Surface epithelia 2. Glandular epithelia 3. Special epithelia  

Functions of Epithelia Major functions of epithelia include: 1. Protecting underlying structures. Examples include the skin and the epithelium of the oral cavity, which protects the underlying structures from abrasion. 2. Acting as barriers. Epithelium prevents the movement of many substances through the epithelial layer. For example, the skin acts as a barrier to water and prevents water loss from the body.

  3. Permitting the passage of substances. Epithelium allows the movement of many substances through the epithelial layer. For example, oxygen and carbon dioxide are exchanged between the air and blood by diffusion through the epithelium in the lungs. 4. Secreting substances. Examples include the sweat glands, mucous glands, and the enzyme-secreting portion of the pancreas. 5. Absorbing substances. The cell membranes of certain epithelial tissues contain carrier molecules that regulate the absorption of materials.

Cell Connections Lateral and basilar surfaces have structures that serve to hold cells to one another or to the basement membrane .These structures do three things: (1) they mechanically bind the cells together,(2) they help form a permeability barrier, and (3) they providea mechanism for intercellular communication. Epithelial cells secrete glycoproteins that attach the cells to the basement membrane and to one another. This relatively weak binding between cells is reinforced by desmosomes ,disk-shaped structures with especially adhesive glycoproteins that bind cells to one another and intermediate filaments that extend into the cytoplasm of the cells. Many desmosomes are found in epithelia that are subjected to stress, such as the stratified squamous epithelium of the skin. Hemidesmosomes, similar to one-half of a desmosome,attach epithelial cells to the basement membrane

Important characteristics 1. Covers or lines all body surfaces   Important characteristics 1. Covers or lines all body surfaces 2. Polarity (epithelial cells have specific apical, lateral & basal domains) 3. Characterized by specific intermediate filaments called cytokeratins 4. Attached to underlying CT via ECM basement membrane 5. Generally avascular 6. Gives rise to majority of glands 7. High regenerative capacity; plasticity 8. Great diversity of function

Simple epithelium Type Squamous Squamous cells have the appearance of thin, flat plates. They fit closely together in tissues; providing a smooth, low-friction surface over which fluids can move easily. The shape of the nucleus usually corresponds to the cell form and helps to identify the type of epithelium. Squamous cells tend to have horizontally flattened, elliptical (oval or shaped like an egg) nuclei because of the thin flattened form of the cell. Classically, squamous epithelia are found lining surfaces utilizing simple passive diffusion such as the alveolar epithelium in the lungs. Specialized squamous epithelia also form the lining of cavities such as the blood vessels (endothelium) and pericardium (mesothelium) and the major cavities found within the body.

Cuboidal   As their name implies, cuboidal cells are roughly cuboidal in shape, appearing square in cross section. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is commonly found in secretive or absorptive tissue: for example the (secretive) exocrine gland the pancreas and the (absorptive) lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium that covers the female ovary.

Columnar   Columnar epithelial cells are elongated and column-shaped. Their nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialized for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus, which acts as a lubricant.

Pseudostratified   These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading (hence "pseudo") impression that the epithelium is stratified when the cells are viewed in cross section. Pseudostratified epithelium can also possess fine hair-like extensions of their apical (luminal) membrane called cilia. In this case, the epithelium is described as "ciliated" pseudostratified epithelium. Cilia are capable of energy dependent pulsatile beating in a certain direction through interaction of cytoskeletal microtubules and connecting structural proteins and enzymes. The wafting effect produced causes mucus secreted locally by the goblet cells (to lubricate and to trap pathogens and particles) to flow in that direction (typically out of the body). Ciliated epithelium is found in the airways (nose, bronchi), but is also found in the uterus and Fallopian tubes of females, where the cilia propel the ovum to the uterus.

Stratified epithelium Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar. Stratified epithelial tissue also differs from simple epithelial tissue in that stratified epithelial tissues do not contain junctional complexes, and have their cells bound together only by desmosomes.Stratified epithelia (of columnar, cuboidal or squamous type) can have the following specializations

Specialization Description Keratinized In this particular case, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm, instead contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, so is found in the mammalian skin. The lining of the esophagus is an example of a non-keratinized or "moist" stratified epithelium. Transitional Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters and urethra.

GLANDULAR EPITHELIA Glands are formed from the invagination/infolding of epithelium and subsequent growth in the underlying connective tissue. There are two major classifications of glands: endocrine glands and exocrine glands. Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the blood vascular system. Exocrine glands secrete their products into a duct that then delivers the product to the lumen of an organ or onto the free surface of the epithelium. A. Classification 1. Types of glands a. Exocrine b. Endocrine (hemocrine) c. Paracrine d. Autocrine

B.Ducts: structure in exocrine glands 1. Simple glands (=unbranched ducts) 2. Compound glands (=branched ducts)   C. Secretory components 1. Number of secretory cells a. Unicellular (e.g., goblet cells) b. Multicellular glands (majority) 2. Structure of secretory units a. Tubular b. Acinar/alveolar

D. Types of secretion (secretory product) 1. Mucous 2. Serous 3. Seromucous   E. Mechanisms of secretion (methods of release) 1. Merocrine (e.g., Salivary glands,Pancreas) 2. Apocrine (e.g., Apocrine sweat glands, Mammary glands) 3. Holocrine (e.g., Sebaceous glands )

Myoepithelial cells 1. Contractile, branched epithelial cells 2. Located between glandular epithelial cells and basement membrane 3. Contain actin filaments; smooth muscle-like 4. Facilitate glandular secretion.