Epithelium 2 PDF

Epithelium 2 • Amir Mhawi, DVM, PhD • Ross University School of Medicine • [email protected] Introduction This lecture describes the origin of the exocrine and endocrine glands. The students will appreciate the structures and functions of different types of exocrine glands and their mode of secret...

Epithelium 2 • Amir Mhawi, DVM, PhD • Ross University School of Medicine • [email protected] Introduction This lecture describes the origin of the exocrine and endocrine glands. The students will appreciate the structures and functions of different types of exocrine glands and their mode of secretion. This lecture will also highlight the differences between the mucous and serous membranes. Learning Objectives • Describe how the different types of exocrine glands are classified based on the shape of their secretory units and ducts • Identify the microscopic characteristics of mucous and serous glands. • Explain the different modes of secretion of exocrine glands. • Identify the microscopic characteristics of the mucous and serous membranes. Glands • Composed of epithelial cells. • Classified into 2 major groups. • Exocrine • Secrete product through ducts. • Endocrine • Lack duct system. • Secrete product into connective tissue and from there to blood. • Secretions called hormones. Formation of Glands from Covering Epithelia Useful definitions: Parenchyma: The parts of an organ that are directly concerned with its function. i.e., the epithelial components (secretory units and ducts) of a compound exocrine gland Stroma: The supporting connective tissue with the blood vessels and nerve fibers Exocrine Glands • Classified as: • Unicellular • Secretory component consists of single cells. • e.g., goblet cells • Located in: • Surface epithelium and glands of intestines • Respiratory tract Goblet cells Goblet cells within the epithelium of the small intestin. Mucus stored in the cytoplasm of the cell is washed out during the peparation of the tissue. H&E Goblet cells Higher mag. of goblet cells in the small intestine. The mucus stored in the cytoplasm of the cells is fixed and stained with PAS-PT (periodic acid SchiffPararosaniline Toludine blue Exocrine Glands cont. • Multicellular • Classified according to: • Shape of duct • Simple (duct is unbranched) • Compound (duct is branched) • Shape of secretory portion • Tubular (like tube) • Alveolar/acinar (like flask) • If secretory portion is shaped: • Like a tube, gland is tubular. • Like a flask, gland is alveolar/acinar. • If the tubular gland ends in sac-like dilation. • Called tubuloalveolar/tubuloacinar. Exocrine Glands cont. Exocrine gland classified as: • Simple tubular • Intestinal glands • Simple branched tubular Intestinal glands • Gastric glands • Simple coiled tubular • Eccrine sweat glands • Apocrine sweat gland duct Secretory unit Gastric glands Eccrine sweat gland Exocrine Glands cont. • Section of eccrine or apocrine sweat glands usually reveals multiple irregular sections of the secretory unit and ducts due to coiled nature of the gland. • The ducts appear more intensely stained than the secretory unit. Exocrine Glands cont. esophagogastric junction • Simple branched acinar • Cardiac glands of stomach stomach esophagus Exocrine Glands cont. • Compound acinar • As in pancreas • Compound branched tubular • Submucosal glands of Brunner (duodenum) • Compound tubuloacinar Compound acinar • Submandibular salivary gland Compound tubuloacinar Exocrine Glands cont. Mucous and serous glands Compound tubuloacinar glands of the soft palate, purely mucous • Names related to type of secretion product. • Mucous glands • e.g. goblet cells, sublingual glands and surface cells of stomach • Cells produce viscous, slimy secretion. • Result of extensive glycosylation of constituent proteins. • Mucin granules are thus PAS positive. • Water soluble, therefore, lost during tissue preparation. • Nucleus of the cell is usually flattened against base of cell. duct Higher mag of the mucussecreting acini. Note the clear cytoplasm and basal nuclei. Loose connective tissue surrounds a secretory duct. Exocrine Glands cont. • Serous glands • Produce poorly glycosylated or nonglycosylated protein secretions. • Nucleus of the cell usually appears rounded or oval. • Found in: • Pancreas (purely serous) • Parotid (purely serous) • Submaxillary (submandibular) (mixed) • Predominantly serous Exocrine Glands cont. • Acini of some glands contain both serous and mucous cells (mixed). • e.g., submandibular and sublingual salivary glands • Serous cells are more removed from lumen of acinus. • Shaped as crescents or demilunes. • At periphery of the mucous acinus • Serous secretions reach duct via small intercellular channels or canaliculi. • Between mucous cells Sublingual gland. Mixed but mostly mucous. Many serous demilunes seen in this micrograph. The arrows point to the canaliculus. Exocrine Glands-Mode of Secretion • Apocrine • Secretory material with adherent cytoplasm released from surface of cell • e.g., secretion of fat droplets by mammary gland • Holocrine (Gr. holos, all) • The entire cell is sacrificed in producing secretion. • Release of secretion accompanied by fragmentation and loss of integrity of entire cell. • Lost cells are replaced. e.g., sebaceous gland Exocrine Glands-Mode of Secretion cont. • Merocrine • Secretion is stored in membrane-limited vesicles. • Membrane of vesicle fuses with the membrane of the secretory cell. • i.e., the secretory material is exocytosed from cell surface • Different from apocrine secretion in that no cytoplasm is lost. • Most of the exocrine glands uses this mode of secretion. • Apocrine/Merocrine • Two different secretions released from the same cell by two different mechanisms. • e.g., mammary gland • Lipids – secreted by apocerine. • Proteins – secreted by merocrine. Mucous and Serous Membranes • Mucous membrane AKA mucosa • Lines cavities that connect with the outside of the body. • GIT, respiratory ducts, urogenital tract • Consists of diverse types of epithelium, basal lamina, and underlying lamina propria. • May contains glands: secrete mucus. • Serous membrane AKA serosa • Lines closed cavities of the body. • Peritoneal, pleural, and pericardial cavities • Consists of simple squamous epithelium (mesothelium), basal lamina, and underlying thin loose connective tissue. • Does not contain glands. • Extra reading if desired: The End of the Lecture! • Junquiera’s Basic Histology • Relevant portions of Chapter 4, • “Epithelial Tissue” • Digital Histology • https://digitalhistology.org/