Glandular Epithelium Dr. Hersh Abdul Ham-Karim BVM&S, PG Dip, MSc and PhD
Glandular Epithelium Groups of surface cells differentiate, proliferate, and penetrate underlying connective tissue. Their main function is to synthesize and secrete extracellular products. These include hormone, enzyme, etc. Cells specialized to produce secretion. The molecules to be secreted are generally stored in the cells in small membrane-bound vesicles called secretory granules
Glands: a specialized cell, group of cells, or organ in the human or animal body which secretes particular chemical substances for use in the body. Glands of the body are classified as either exocrine or endocrine types 1-Exocrine Glands: - maintain connection with the surface epithelium via the tubular ducts through with the secretory product pass to reach the surface (skin, digestive tract).
Histologically, exocrine glands composed from two parts: secretory portion: contains the cells responsible for the secretory process Ducts Secretory portion system of ducts: transport the secretion to the exterior of the gland
2-Endocrine Glands: are ductless the connection with the surface was obliterated during development and they release their secretory product (hormones) into the bloodstream. * Paracrine Glands: These glands are similar to endocrine glands, but their secretions reach target cells by diffusion through the extracellular space or immediately subjacent connective tissue. Enteroendocrine cells of gastrointestinal tract (GIT).
What are Hormones? Hormones are special chemical messengers in the body that are created in the endocrine glands. These messengers control most major bodily functions, from simple basic needs like hunger to complex systems like reproduction, and even the emotions and mood. In the human body, hormones are used for two types of communication. The first is for communication between two endocrine glands. For example, the pituitary gland send signals to other endocrine glands to stimulate or inhibit their own hormone production. The second is between an endocrine gland and a target organ, for example when the pancreas releases insulin which causes muscle and fat cells to take up glucose from the bloodstream.
What are Enzymes? Enzymes are proteins (complex chains of amino acids) that play a role in all chemical functions in the body including digestion, energy production, and repair of tissues, organs, and cells. Enzymes can be divided into three groups. Metabolic enzymes (enzymes which your body produces that work in blood, tissues, and organs) Digestive enzymes (enzymes that break down food into usable material) Food enzymes (enzymes that are contained in raw food)
Classification of Exocrine gland This classification system is based on five different morphological criteria. 1. Number of secretory cells - Unicellular glands Goblet cells are the only example of these single-celled glands in humans. These goblet cells secrete mucus and are easily visualized in slides of the small intestine.
- Multicellular glands - These glands have many cells. In addition to the ways that multicellular glands are classified (listed below).
2. Nature of secretion: a) Serous A cell-type that produces a thin watery, protein-rich secretion. Serous cells are pyramidal, with round, basally located nuclei. Cytoplasm: granular (stain darkly from pink to dark purple with H&E stain) e.g. pancreas and parotid salivary glands
b) Mucous A cell type that produces is a strongly hydrophilic glycoproteins called mucins, viscous secretions that have a lubricating or protective function. Mucous cells are columnar, characterized by numerous large, lightly staining granules with basally located nuclei. Mucous cells are most often organized as tubules, consisting of cylindrical arrays of secretory cells surrounding a lumen. e.g GI tract
Differences between serous and mucinous cells Serous Serous cells are mostly pyramidal in shape. They are small in size. When stained with H&E stain, cells take dark stain Nuclei are rounded and placed near the centre but more toward the basal part of cells The apical portion is filled with secretory granules (zymogen granules) The base of serous cell is basophilic (blue) and apical portion acidophilic (pink) Mucous Mucous cells are short columnar to pyramidal in shape they are large in size compared to serous cells Mucous cells stain light; hence, appear empty Nuclei are flat and situated toward basement membrane The apical portion contains large number of secretory granules (mucinogen granules) Cytoplasm takes light pale stain and looks empty or vacuolated. This is because the mucous is lost in preparation of H&E stained slides Serous Mucous
c) Mixed These glands have both serous and mucous cells. The mucous cells are capped by serous cells called serous demilune. e.g. sublingual salivary gland. d) Sebaceous - Thick, lipid rich secretions of cuboidal cells in certain skin regions (e.g. face, nose and axillary regions)
3. Mechanism of secretion: on the basis how the secretory product is released a- Merocrine/Eccrine secretion This is the most common type of glandular epithelium secretion where secretory granules within the cytoplasm of the cell gather at the apical region of the cell. Then, the granule s limiting membrane fuses with the apical membrane and the contents of the granule are opened and released. This process of fusion and release are collectively referred to as exocytosis. The secretory granules leave the cell with no loss of other cellular material. (e.g. salivary gland)
b- Apocrine secretion A rare type of secretion dependent on sex hormones where secretory granules within the cytoplasm gather at the apical region of the cell. Then, a portion of the cytoplasm of the cell simply pinches off enclosing the granules. Within the lumen, this small secretory vesicle breaks down and releases the gland s products. (e.g. mammary gland)
c- Holocrine secretion This secretion consists of disintegrated cells of the gland itself. Granules fill the cell until the entire cell becomes bloated with secretory products. Instead of being released (merocrine) or pinched off (apocrine), the whole cell is discharged into the lumen. Once inside the lumen, the cell degenerates and the secretory products are released. (e.g. sebaceous gland)
4. Shape of secretory units: Remember exocrine glands have a secretory portion, and ducts. Tubular - An elongated group of secretary cells with a tube-shaped lumen. Mucous cells are most often organized as tubules. Acinar (or alveolar) - A small grape-like (acinus means grape ) or sac-like (alveolus means sac ) group of secretary cells arranged about a small lumen. Tubulo-alveolar Lumen of secretary units have both of the above listed shapes (seen in mixed glands).
5. Arrangement (branched or not) and Occurrence of Duct System Simple glands - Glands that have an unbranched duct into which the cells secrete. Branched glands These glands have several secretary units empty into an unbranched excretory duct. Compound glands - These glands have a highly branched duct system. Secretory portions empty into an elaborate branched duct system, which, in turn, drain into larger ducts.
Types of Exocrine Glands 1. Simple tubular glands: These glands are epithelial-lined tubules, which open on the apical surface. There are three types. Simple straight tubular glands: The long crypts of Lieberkühn, located runs a straight, unbranched course. within the colon that Simple coiled tubular glands: Within the dermis, sweat glands are located. Simple branched tubular glands: These simple branched tubular glands are found primarily in the stomach
2. Simple alveolar (acinar) glands: The paraurethral glands located in the penile urethra or the sebaceous glands located in the skin. 3. Simple branched alveolar glands: Some of the smaller glands of the respiratory tract; minor salivary glands located within the oral cavity are other examples.
4. Compound tubular glands: These glands have a highly branched duct system. The secretary cells at the ends of the ducts are in the form of tubules. Brunner s glands of the duodenum are compound tubular glands. 5. Compound alveolar glands: The duct system is similar to the compound tubular and compound tubulo-alveolar glands; however, compound alveolar glands differ from other compound glands in that the ducts end in alveoli with dilated sac-like lumina. The pancreas and parotid gland are the best examples of compound alveolar glands as they are entirely serous.
6- Compound tubulo-alveolar glands: These glands also have a highly branched duct system, but some of the ducts end as tubules and others end as alveoli. Two of the major salivary glands, the submandibular and the sublingual glands, are examples of compound tubulo-alveolar glands (as they are mixed glands).
Ducts of Exocrine Glands Several different types of ducts exist along the lumen of the glands. Intercalated ducts; receive secretion from acini. They have simple cuboidal epithelium. Striated (intralobular) ducts: cuboidal or low columnar cells with round nuclei. They receive secretion from intercalated ducts. Interlobular (excretory) ducts: are found between lobules, cuboidal to columnar cells. Transmit secretion from striated ducts to interlobar ducts. Interlobar ducts: pseudostratified columnar. Main duct: stratified columnar then stratified squamous at its end.
Myoepithelial Cells Specialized squamous epithelial cells with power of contraction. Usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the luminal cells in the secretory portion of glands. They are instrumental in moving the secretions toward the excretory duct.
Pancreas The pancreas is a glandular organ in the digestive system and endocrine system of vertebrates. In humans, it is about 6 inches long and sits across the back of the abdomen, behind the stomach. It is surrounded by the stomach, small intestine, liver, spleen and gallbladder.
Microscopic structure The pancreas has a thin cover of loose connective tissue from which septa pass into the gland, subdividing it into many small molecule. Each lobule is again composed of several rounded or tubular groups of pancreatic cells called acini. The connective tissue is very little between two acini. Among the acini are the scattered the islets of Langerhans
Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. The bulk of the pancreas is composed of exocrine cells that produce several digestive enzymes like trypsin, amylase, and lipase. These exocrine cells release their enzymes into a series of progressively larger tubes (called ducts) that eventually join together to form the main pancreatic duct. The main pancreatic duct runs the length of the pancreas and drains the fluid produced by the exocrine cells into the duodenum, the first part of the small bowel.
The second functional component of the pancreas is the "endocrine" pancreas. Inside the substance of pancreas there are groups of specialized cells surrounded by connective tissue, which form the endocrine part of the gland. These cells are called islets of Langerhans. In standard histological sections of the pancreas, islets are seen as relatively pale-staining groups of cells embedded in a sea of darkerstaining exocrine tissue. These endocrine cells don t release their secretions into the pancreatic ducts, instead they release hormones, into the blood stream.
The pancreatic islets each contain four varieties of cells: The alpha cell: secret Glucagon, which plays an important role in regulates blood glucose level. The beta cell: produces the hormone insulin, elevated blood glucose levels stimulate the release of insulin. The delta cell: secretes the peptide hormone somatostatin, inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin. The Pancreas Polypeptide cell: secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.