Tissue Level of Organisation (I) PDF

TISSUE LEVEL OF ORGANISATION (I) Dr. P. De Gabriele MD DipWH(ICGP) MMCFD  Cells are highly organized units. BUT they do not function in isolation. They work together in a group of similar cells called a tissue.  TISSUES A tissue is a group of s...

TISSUE LEVEL OF ORGANISATION (I) Dr. P. De Gabriele MD DipWH(ICGP) MMCFD  Cells are highly organized units. BUT they do not function in isolation. They work together in a group of similar cells called a tissue.  TISSUES A tissue is a group of similar cells and their intercellular substance functioning together to perform a specialised activity. There are 4 principal types : EPITHELIAL TISSUE covers the body surfaces or tissues, lines body cavities and forms glands CONNECTIVE TISSUE protects and supports the body and its organs and binds organs together MUSCULAR TISSUE responsible for movement NERVOUS TISSUE initiates and transmits nerve impulses that coordinate body activities  EPITHELIAL TISSUE Epithelial tissue or epithelium may be divided into two subtypes: Covering and lining epithelium It lines the body cavities. It lines the interiors of the respiratory and digestive tracts, blood vessels and ducts. It forms the outer covering of the external body surfaces and of some internal organs. It makes up (along with nervous tissue) the parts of the sense organs that are sensitive to stimuli that produce smell and hearing sensations. It is the tissue from which gametes develop. Glandular epithelium This constitutes the secreting portion of the glands.  Characteristics of Epithelial Tissue The epithelium consists largely or entirely of closely packed cells with little or no intercellular material (also called matrix) between adjacent cells. The points of attachment between adjacent plasma membranes of epithelial cells are called cell junctions. These provide cell-to-cell attachments. They provide channels for communication between other cells. They inhibit the movement of materials into certain cells. Epithelial cells are arranged in continuous sheets that may be either single or multilayered. Nerves may extend through these sheets, but blood vessels do not and so epithelia are avascular. Both types of epithelium get their nutrients and remove their wastes from blood vessels located in the underlying connective tissue. The epithelia adhere firmly to the connective tissue and this holds them in position and prevents them from being torn. The surface of attachment between the epithelium and the connective tissue is a thin extracellular layer called the basement membrane. COVERING AND LINING EPITHELIUM Covering and lining epithelium is arranged in several different ways related to location and function. Simple epithelium is found in areas that have minimal wear and tear where the epithelium is specialised for absorption or filtration. Stratified epithelium is found in areas with a high degree of wear and tear. Pseudostratified epithelium has only one layer of cells but some cells do not reach the surface, giving the tissue a multilayered, or stratified, appearance. The cells that do reach the surface either secrete mucus or contain cilia that move mucus and foreign particles for eventual elimination from the body.  Covering and lining epithelium categorised by cell shape Squamous cells are flattened and scale like and are attached to each other like a mosaic. Cuboidal cells are usually cube-shaped in cross-section and sometimes appear as hexagons. Columnar cells are tall and cylindrical, appearing as rectangles set on end. Transitional cells often have a combination of shapes. They are found where there is a great degree of distension or expansion in the body. In the bottom layer of an epithelial tissue, transitional cells may range in shape from cuboidal to columnar. In the intermediate layer, they may be cuboidal or polyhedral. In the superficial layer, they may range from cuboidal to squamous, depending on how much they are pulled out of shape during certain body functions. Covering and Lining Epithelium SIMPLE Squamous Cuboidal Columnar STRATIFIED Squamous Cuboidal Columnar Transitional PSEUDOSTRATIFIED  SIMPLE SQUAMOUS EPITHELIUM This simple epithelium consists of a single layer of flat, scale like cells. Its surface resembles a tiled floor. The nucleus of each cell is centrally located and oval or spherical.  SIMPLE SQUAMOUS EPITHELIUM It is highly adapted to diffusion, osmosis and filtration. Thus, It lines the air sacs of lungs, where oxygen is exchanged with carbon dioxide. It is present in the part of the kidney that filters blood. It lines the inner surfaces of the membranous labyrinth and tympanic membrane of the ear. It lines the heart, blood vessels and lymph vessels and forms the walls of the capillaries. Here the epithelium is known as endothelium.  SIMPLE CUBOIDAL EPITHELIUM These cells possess a central nucleus. This tissue performs the functions of secretion and absorption. Secretion is the production and release by cells of a fluid that may contain a variety of substances such as mucus, perspiration or enzymes. Absorption is the intake of fluids or other substances by cells of the skin or mucous membranes.  SIMPLE CUBOIDAL EPITHELIUM Simple cuboidal epithelium covers the surface of the ovaries lines the anterior surface of the capsule of the lens of the eye forms the pigmented epithelium of the retina of the eye forms the kidney tubules (Here it contains microvilli and functions in water reabsorption.) lines the smaller ducts of some glands and the secreting units of the glands such as the thyroid. SIMPLE COLUMNAR EPITHELIUM This tissue contains cells whose nuclei are located near the bases of the cells. Simple columnar epithelium may be ciliated or nonciliated. The nonciliated type lines the digestive tract from the cardia of the stomach to the anus the excretory ducts of many glands the gallbladder The ciliated type lines a few portions of the upper respiratory tract the uterine (Fallopian) tubes the uterus some paranasal sinuses the central canal of the spinal cord  Modified Columnar Cells The luminal surfaces of simple columnar epithelial cells are modified in several ways, depending on the location and function. In the digestive tract, the cells of the columnar epithelium protect the underlying tissues. Many of them are also modified to aid in food-related activities. In the small intestine, the plasma membranes of the cells are folded into microvilli. This arrangement increases the surface area of the plasma membrane and thereby allows larger amounts of digested nutrients and fluids to be absorbed into the body. Interspersed among the typical columnar cells of the intestine are other modified columnar cells called goblet cells. These cells secrete mucus. The mucus accumulates in the upper half of the cell, causing the area to bulge out. The secreted mucus serves as a lubricant between the food and the walls of the digestive tract.  STRATIFIED EPITHELIUM Unlike simple epithelium, stratified epithelium consists of at least two layers of cells. Thus, it is durable and can protect underlying tissues from the external environment and from wear and tear.  STRATIFIED SQUAMOUS EPITHELIUM In the more superficial layers of this type of epithelium, the cells are flat, whereas in the deep layers, cells vary in shape from cuboidal to columnar. The basal cells continuously multiply by cell division. As new cells grow, they compress the cells on the surface and push them outward. They also become dehydrated, shrink and become harder. Nonkeratinized Stratified Squamous Epithelium This tissue is found on wet surfaces that area subjected to considerable wear and tear and does not perform the function of absorption: the lining of the mouth the tongue the oesophagus the vagina Keratinized Stratified Squamous Epithelium The surface cells are modified into a tough layer of material containing keratin which is a protein that is resistant to water and friction and helps to resist bacterial invasion. The outer layer of the skin consists of keratinized tissue.  STRATIFIED CUBOIDAL EPITHELIUM This is a relatively rare type of epithelium and is found in the ducts of the sweat glands of adults the fornix of the conjunctiva of the eye the cavernous urethra of the male urogenital system the pharynx the epiglottis Its function is mainly protective. STRATIFIED COLUMNAR EPITHELIUM Usually the basal layer or layers consist of shortened, irregularly polyhedral cells. Only the superficial cells are columnar in form. It functions in protection and secretion. STRATIFIED COLUMNAR EPITHELIUM This type of tissue is also uncommon. This kind of epithelium lines part of the male urethra some larger excretory ducts such as lactiferous ducts in the mammary glands small areas in the anal mucous membrane  TRANSITIONAL EPITHELIUM This epithelium is very much like nonkeratinized stratified squamous epithelium. The distinction is that cells of the outer layer in transitional epithelium tend to be large and rounded rather than flat. This feature allows the tissue to be stretched without the outer cells breaking apart from one another. When stretched they are drawn out into squamouslike cells. Thus, transitional epithelium lines the urinary bladder. Its function is to prevent a rupture of the organ. PSEUDOSTRATIFIED EPITHELIUM All the cells are attached to the basement membrane in a single layer. Some cells do not reach the surface. The nuclei of the cells are at varying depths. This gives the impression of a multilayered tissue. PSEUDOSTRATIFIED EPITHELIUM It lines the larger excretory ducts of many glands parts of the male urethra the auditory (Eustachian) tubes It may contain goblet cells. It may be ciliated. Most of the upper respiratory tract Certain ducts of the male reproductive system  GLANDULAR EPITHELIUM The function of glandular epithelium is secretion. This is carried out by glandular cells that lie in clusters deep to the covering and lining epithelium. A gland may consist of one cell a group of highly specialised epithelial cells that secrete substances into ducts onto a surface into the blood. The production of such substances always requires active work by the glandular cells and results in an expenditure of energy.  GLANDS EXOCRINE GLANDS Secrete substances into ducts Secrete substances directly onto a free surface ENDOCRINE GLANDS Secrete substances into the blood  EXOCRINE GLANDS These secrete their products into ducts that empty at the surface of covering and lining epithelium or directly onto a free surface. The secretions of exocrine glands include Mucus Perspiration Oil Wax Digestive enzymes. Examples are Sweat glands that eliminate perspiration to cool the skin Salivary glands which secrete a digestive enzyme Goblet cells which produce mucus  ENDOCRINE GLANDS They are ductless. They secrete their products into the blood. The secretions of endocrine glands are always Hormones Chemicals that regulate various physiological activities Examples include the pituitary gland the thyroid gland adrenal glands STRUCTURAL CLASSIFICATION OF EXOCRINE GLANDS UNICELLULAR Single-celled gland that secretes mucus such as the goblet cell of the digestive and respiratory systems MULTICELLULAR Many-celled glands A. Simple. Single, nonbranched duct B. Compound. Branched duct MULTICELLULAR EXOCRINE GLANDS A. Simple. 1. Tubular 2. Branched tubular 3. Coiled tubular 4. Acinar 5. Branched acinar B. Compound. 1. Tubular 2. Acinar 3. Tubuloacinar Simple Tubular Gland The secretory portion is straight and tubular. An example are the intestinal glands. Simple Branched Tubular Glands The secretory portion is branched and tubular. Examples include the gastric and uterine glands. Coiled Tubular Glands The secretory portion is coiled. An example is the sweat glands. Simple Acinar Glands The secretory portion is flask-like. An example is the seminal vesicle glands.  Simple Branched Acinar Glands The secretory portion is branched and flask- like. An example is the sebaceous glands. Compound Tubular Glands The secretory portion is tubular. Examples include: bulbourethral glands testes liver  Compound Acinar Glands The secretory portion is flask-like. Examples include: salivary glands sublingual submandibular Compound Tubuloacinar Glands The secretory portion is both tubular and flask-like. Examples include: the parotid salivary glands the pancreas FUNCTIONAL CLASSIFICATION OF EXOCRINE GLANDS Holocrine glands Merocrine glands Apocrine glands Holocrine Glands They accumulate a secretory product in their cytoplasm. The cell then dies and is discharged with its contents as the glandular secretion. The discharged cell is replaced by a new cell. An example is a sebaceous gland of the skin. Merocrine Glands They simply form the secretory product. They discharge it from the cell. Examples include the salivary glands the pancreas  Apocrine Glands They accumulate their secretory product at the apical margin of the secreting cell. That portion of the cell pinches off from the rest of the cell to form the secretion. The remaining part of the cell repairs itself and repeats the process. An example is the mammary gland.