Lesson 11- Introduction to Membranes and Glands
Student Performance Objectives1. List 3 different types of biological membranes and explain the similarities and differences in their structure.2. Explain 2 functions of mucous membranes.3. Describe the location of serous membranes and explain their function.4. Describe the location of synovial membranes and explain their function.5. Explain how glands form.6. Explain the differences between exocrine and endocrine glands.Lesson OutlineA. Membranes 1. Mucous membraneshttp://www.meddean.luc.edu/lumen/MedEd/Histo/frames/h_frame1.html a. Structure (1) An epithelial layer that secretes mucus from goblet cells that are part of the epithelium. Mucus can also come from glands such as the salivary glands. (2) The epithelial sheet possesses a basement membrane to which the epithelial cells are attached and a deeper, loose connective tissue (areolar) layer that binds the epithelial sheet to the (usual) underlying smooth muscular layer. (3) Mucous membranes can be found in most passages that lead from the body's exterior to interior areas. E.g., the mouth and continuing gastrointestinal tract, the nasal passages and the following respiratory passages, the vagina and the genital tract, and the urethra and the urinary tract. (4) Some mucous membranes possess cilia, like the respiratory epithelium; some are non-ciliated, like the gastrointestinal (digestive) tract. b. Functions (1) In the respiratory passages the ciliated mucous lining traps bacteria and propels them upward, out of the lungs toward the pharynx. (2) In the digestive passages the mucous lining protects the stomach from its own acid, secretes intestinal enzymes for digestion of food and absorbs digested nutrients. (3) The mucous lining of the urogenital passages slows the movement of microorganisms helping to prevent invasion and infection. 2. Serous membranes a. Structure (1) These membranes are composed of a simple squamous epithelium attached to a basement membrane resting on a loose, irregular connective tissue layer (areolar connective tissue). Examples are the pleurae of the pleural cavities housing the lungs, the peritoneal membranes of the abdominal cavity, and the pericardial membranes surrounding the heart. (2) Although these membranes are relatively thin, they can be tough due to reinforcement by dense, irregular connective tissue (e.g., the pericardial sac). (3) The membranes possess parietal portions, on the walls of the cavities, and visceral portions, composing the outer surface of the organs in the cavities. b. Function - secretion of a thin, watery, lubricating fluid, serous fluid, reduces friction between the parietal and visceral portions of the membranes as the organs of the cavities move due to normal body functions. 3. Synovial membranes a. Structure (1) This membrane is the inner lining of joint capsules, bursae and tendon sheaths. (2) The membrane forms from mesenchymal cells (stem cells) that produce a connective tissue layer. This distinguishes synovial membrane from other types of membranes: all others are composed of epithelial tissue. Synovial membrane is a membrane composed of connective tissue. b. Function - secretion of synovial fluid - a thixotropic (multiple viscosity) liquid that renders joint movements virtually frictionless. 4. Cutaneous membrane - this is the skin and is considered in Unit 6 of this website.B. Glands 1. Glandular epithelium - this tissue is formed by the infolding (invagination) of epithelial sheets. This infolding produces some glands that maintain their contact with the surface, from which they invaginated, by a duct; these are the exocrine glands. Other glands lose their connection with the surface and are ductless; these are the endocrine glands. a. Exocrine glands - e.g., sweat glands, sebaceous glands, mammary glands. These are glands possessing ducts that carry the gland's secretion to a specific location such as the body's surface or the lumen of a digestive tract organ like the stomach. 1. Ducts can be simple (single and unbranched) or compound (branched). 2. If the gland and its duct are of uniform diameter the gland is called tubular. If the gland is dilated compared with the duct, the gland is called acinar. 3. Secretions can be thin and watery - serous glands produce these. Secretions can include mucus making them thicker - mucous glands produce these. 4. If a gland secretes its product through its duct by exocytosis from its cells, it is called a merocrine gland. If the gland secretes its product through its duct as some of its secretory cells self-destruct, it is called a holocrine gland. b. Endocrine glands - e.g., pituitary, thyroid, adrenals. These are ductless glands. Their secretions (hormones) exit from the gland, enter the fluid surrounding the gland, and then enter the blood and travel to all body regions where they interact with target cells. 2. Some organs act as both exocrine and endocrine glands: e.g., liver, pancreas. 3. Many organs have glandular functions even though they are generally thought of as performing other functions: e.g., heart, kidneys.