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Lesson 11- Introduction to Membranes and Glands

Student Performance Objectives
1. 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 Outline
A. Membranes
    1. Mucous membranes
        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)
        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
        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
            (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.

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