Glands: Types, Classification, and Functions
Glands are specialized groups of cells that synthesize and secrete substances or hormones. They are derived from epithelium and classified based on the manner of secretion, branching pattern of duct, shape of secretory unit, and nature of secretions in exocrine glands. Explore the different types of glands and their functions in this comprehensive guide.
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Presentation Transcript
GLANDS GLANDS GLAND IS A SPECIALIZED GROUP OF CELLS THAT SYNTHESIZES AND SECRETES SUBSTANCES/HORMONES. GLANDS ARE DERIVED FROM EPITHELIUM. CLASSIFICATION OF GLANDS- BASED ON MANNER OF SECRETION GLANDS ARE OF THREE TYPES. 1. MEROCRINE GLANDS- THEY SECRETE BY EXOCYTOSIS. 2. APOCRINE GLANDS-THEY SECRETE BY SHEDDING OF APICAL PORTION OF GLANDS. 3. HOLOCRINE GLANDS- THEY SECRETE BY DISINTEGRATION OF ENTIRE CELL.
EXOCRINE GLAND EXOCRINE GLAND THESE GLANDS CONVEY THEIR SECRETIONS WITH THE HELP OF DUCT. EXOCRINE GLANDS ARE FURTHER CLASSIFIED AS FOLLOWS. 1. NO.OF BRANCHES OF DUCT(SIMPLE OR COMPOUND). 2. SHAPE OF SECRETORY UNIT(TUBULAR,ACINAR,ALVEOLAR). 3. NATURE OF SECRETORY PRODUCT(SEROUS,MUCUS,AND MIXED). 4. MECHANISM OF SECRETION(MEROCRINE,APOCRINE,HOLOCRINE).
CLASSIFICATION BASED ON BRANCHING CLASSIFICATION BASED ON BRANCHING PATTERN OF DUCT. PATTERN OF DUCT. SIMPLE GLANDS- HAVE ONLY ONE UNBRANCHED DUCT. EXAMPLES- CRYPT OF LIEBERKUHN,SWEAT GLANDS,FUNDIC GLAND OF STOMACH,MUCOUS GLANDS OF URETHRA,MEIBOMIAN GLANDS. COMPOUND GLANDS-THESE GLANDS HAVE BRANCHING OF SECRETORY DUCT. EXAMPLES- BRUNNERS GLAND,MAMMARY GLAND,SUBMANDIBULAR GLAND.
CLASSIFICATION BASED ON SHAPE OF CLASSIFICATION BASED ON SHAPE OF SECRETORY UNIT. SECRETORY UNIT. 1. TUBULAR GLANDS- THE SECRETORY UNIT IS TUBE SHAPED. THE TUBE MAY BE STRAIGHT,COILED,OR BRANCHED. eg. INTESTINAL GLAND,SWEAT GLAND,GASTRIC GLANDS,BRUNNERS GLAND. 2. ACINAR GLANDS- THE SECRETORY UNIT IS SPHERICAL.eg. EXOCRINE PART OF PANCREAS,SALIVARY GLANDS. 3. ALVEOLARGLANDS- THE SECRETORY UNIT IS FLASK SHAPED.THE GLANDS WITH DISTENDED SECRETORY UNITS ARE ALSO CALLED SACCULAR GLANDS.
CLASSIFICATION BASED ON NATURE OF CLASSIFICATION BASED ON NATURE OF SECRETIONS SECRETIONS BASED ON THE NATURE OF SECRETIONS,EXOCRINE GLANDS ARE CLASSIFIED AS SEROUS,MUCOUS AND MIXED GLANDS. SEROUS GLANDS- 1. SECRETE PROTEIN RICH WATERY SECRETIONS. 2. SECRETORY UNIT IS SEROUS ACINI. 3. LINED BY PYRAMIDAL SHAPED SIMPLE COLUMNAR EPITHELIUM. 4. SEROUS CELLS HAVE ROUNDED BASAL NUCLEI,LARGE AMOUNT OF ROUGH E.R FOR PROTEIN SYNTHESIS.IT GIVES BASAL BASOPHILIA(STAINED WITH HEMATOXYLIN).
5.WELL DEVELOPED GOLGI SYSTEM.IT GIVES APICAL EOSINOPHILIA(STAINED WITH EOSIN). 6.SEROUS ACINUS HAS SMALLER LUMEN THAN MUCOUS ACINI. 7.EXAMPLES ARE PAROTID GLAND,EXOCRINE PART OF PANCREAS,LACRIMAL GLAND,VON EBNERS GLAND IN TONGUE.
MUCOUS GLANDS MUCOUS GLANDS SECRETE THICK MUCOID SECRETIONS. MUCOUS GLAND HAS MUCOUS ACINUS SECRETORY UNIT. MUCOUS ACINI ARE LINED BY PYRAMIDAL SHAPED SIMPLE COLUMNAR EPITHELIUM. THE MUCOUS CELL HAS FLAT,ELONGATED BASALLY-PLACED NUCLEUS.MUCOUS CELLS GIVE AN EMPTY APPEARENCE ON H&E STAINING. MUCOUS CELLS HAVE WIDER LUMEN THAN SEROUS ACINUS. EXAMPLES ARE SUBLINGUAL SALIVARY GLAND.
MIXED GLAND MIXED GLAND MIXED GLAND SECRETES BOTH MUCUS AND ENZYME RICH WATERY SECRETIONS. MIXED GLAND HAS MUCOUS ACINI WITH SEROUS DEMILUNES AS SECRETORY UNIT. MUCOUS ACINUS IS CAPPED WITH SEROUS CELLS THAT FORM SEROUS DEMILUNES. SEROUS DEMILUNE IS A HALF MOON SHAPED CAP OVER MUCOUS ACINUS THAT CONSISTS OF SEROUS CELLS. ALL THE SEROUS CELLS OF SEROUS DEMILUNES PORE THEIR SECRETIONS IN THE LUMEN OF MUCOUS ACINUS THROUGH FINE CANALICULI. EXAMPLES ARE SUBMANDIBULAR SALIVARY GLAND.
CLASSIFICATION BASED ON MODE OF SECRETION MEROCRINE GLAND- MEMBRANE BOUND VESICLES FUSE WITH APICAL SURFACE OF CELL AND GET SECRETED OUT BY EXOCYTOSIS. EXAMPLES ARE-PANCREATIC ACINAR CELLS,MEROCRINE SWEAT GLANDS. APOCRINE GLANDS- APOCRINE GLANDS SYNTHESIZE SECRETORY PRODUCT AND ACCUMULATE IT IN APICAL PORTION THIS APICAL PORTION OF CELL ALONG WITH SECRETORY PRODUCTS ARE SHED OFF. EXAMPLES ARE LACTATING MAMMARY GLAND,APOCRINE SWEAT GLAND CERUMINOUS GLANDS OF EXTERNAL EAR.
HOLOCRINE GLAND-HOLOCRINE GLAND SYNTHESIZE AND ACCUMULATE THE SECRETORY PRODUCT. WHILE DISCHARGING SECRETIONS ENTIRE CELL DISINTEGRATES AND UNDERGO PROGRAMMED CELL DEATH ON MATURATION. THIS MODE OF SECRETION IS CALLED HOLOCRINE. EXAMPLES -SEBACEOUS GLAND,MEIBOMIAN GLANDS OF EYE LID. PARACRINE GLANDS- CELLS SECRETE SUBSTANCES THAT AFFECT OTHER NEARBY CELLS(SECRETIONS DO NOT ENTER THE BLOOD STREAM).THIS MECHANISM OF GLANDULAR FUNCTION IS CALLED PARACRINE SIGNALING. CELLS SECRETE PARACRINE SUBSTANCES IN TO EXTRACELLULAR MATRIX AND THESE SUBSTANCES REACH THE TARGET CELLS BY DIFFUSION.eg. VASODILATORS RELEASED BY ENDOTHELIAL CELLS ACT ON VASCULAR SMOOTH MUSCLES TO PRODUCE RELAXATION OF VASCULAR WALL.
SOME CELLS IN EPITHELIA(UNICELLULAR GLANDS) FUNCTION AS PARACRINE OR AUTOCRINE SIGNALING MECHANISM. AUTOCRINE SIGNALING- CELLS SECRETE A MOLECULE THAT ACTS ON THE SAME CELL BY BINDING WITH RECEPTOR.THIS MECHANISM OF GLANDULAR FUNCTION IS CALLED AUTOCRINE SIGNALING. AUTOCRINE SIGNALING MAY ACTIVATE(POSITIVE FEEDBACK) OR INHIBIT (NEGATIVE FEEDBACK) ACTIVITY OF THE CELL. EXAMPLE- ACTIVATION OF CELLS OF IMMUNE SYSTEM BY INTERLEUKINS.
