List of abbreviations
Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!
There are many ways to classify glands regarding different points
A.) regarding whereto the secretion flows:
1. endocrine glands: The target of the secretion which is called hormone in this case is the blood. After exocytosis the hormone passes the basal lamina of the gland cell, a minimal extracellular space filled with loose connective tissue, the basal lamina of an endothelial cell of a capillary and finally an open pore of the endothelial cell or a pore covered by a tiny diaphragm to reach the lumen of the capillary. The blood distributes the secreted hormone. In consequnce to this endocrine glands are always rich in capillaries. Examples of endocrine glands are the endocrine islands of Langerhans in pancreas, the pituitary, the pineal gland, the adrenal gland, the thyroid gland, the parathyroid gland, Leydig cells of the testis. The enteroendocrine cells of the gut deliver their secretion to nearby capillaries or lymph vessels.
2. exocrine glands: The secretion is delivered either directly into the lumen of an end piece (acinus, tubulus or alveolus) or a duct of an exocrine gland or into the lumen of the gut in case of intestinal gland cells like goblet cells. Examples of exocrine glands are the prostate, the exocrine pancreas, the lacrimal glands, sweat glands, salivary glands, sebaceous glands and the mammary gland.
B.) regarding the mechanism of secretion:
1.) eccrine = merocrine secretion, i.e. exocytosis, vesciles dock on and fuse with the cell membrane and deliver their content into the extracellular space without any notable loss of cell volume since no cytoplasm gets lost. The secretion product is water-like and non viscous in most cases. Eccrine secretion is typical for: salivary glands, pancreas and sweat glands
2.) apocrine secretion, i.e. apocytosis, "secretory vesicles" that are lipid droplets gather in the apical part of the cell close to the lumen, integral membrane proteins of the cell membrane like Butyrophilin bind to proteins associated to the border of the lipid droplets thereby the latter begins to protrude into the lumen. By further binding to the base of the spherical lipid droplets the adjacent cell membrane fuses and the lipid droplet, the surrounding small seam of cytoplasm and the covering cell membrane are released. Thus some cell membrane and a little cytoplasm gets lost with every secretion process resulting in a small reduction of the cellular volume. The fat droplet is set free, when later on the surrounding membrane collapses. This rare kind of secretion is only seen in the mammary gland, apocrine sweat gland cells and the glands of Moll in the eyelid.
3.) holocrine secretion, i.e. holocytosis, the whole gland cell fills up with more and more "secretory vesicles" that in fact are lipid droplets that may fuse. Thereby the epithelial cell looses its contact to the basement membrane and is pushed forward towards the lumen by proliferation of the basal cell layer. The farer the cell gets to the basement membrane the worse the situation for its nutrition by diffusion, thus the cell begins to degenreate. The nucleus and the cell organelles dissolve and the fatty, sebaceous secretion product fills the whole cell. When contact to neighbouring cells gets lost by desmosome disintegration, the whole cell becomes the secretory product in that the cell membrane finally disrupts and the rest of the cytoplasm as well as the lipid droplets are released. This type of secretion only occurs in sebaceous glands and the glands of Meibohm or Zeiss of the eyelid.
4.) avesicular secretion, The majority of glands shows secretory vesicles, however, in case the chemical properties of the secretion product (size, lipophily) do not require concentration and packing in vesicles, secretory vesicles are not produced. A lot of substances are secreted directly from the cytoplasm of exo- or/and endocrine cells via more or less specific transmembrane proteins. This is not visible in the electron microscope. The endocrine pineal gland is an example for an avesicular secreting gland. Its lipophilic melatonin can easily pass the cell membrane without need of any transporter. The liver is an exocrine gland that secrets gall without any vesicles into bile canaliculi and at the same time releases hundrets of other substances via Disse's space into the blood, i.e. it is exo- and endocine, in case one uses the expression endocrine not only for hormones. Further examples of avesicular secretion is delivery of testosterone by Ledig cells in the testis and the release of immunglobulins from plasma cells to surrounding connective tissue.
Considering A and B, the following classification of glands results:
|pineal gland||pituitary||liver (gall)||prostate||salivary glands||scent gland||sebaceous gland|
|liver||thyroid gland (C-cells)||plasma cells||exocrine pancreas||parotid gland (Gl. parotidea)||mammary gland||Meibohm's gland|
|fibroblasts||Brunner's glands (duodenum)||submandibular gland (Gl. submandibularis)||ciliar gland of the eyelid
|adrenal gland (cortex)||adrenal gland (medulla)||sweat glands||sublingual gland (Gl. sublingualis)|
|Leydig cells in testis||endocrine pancreas||lacrimal gland||small salivary glands of the palate (Gll. palatinae)|
|thyroid gland (main cells)||enteroendocrine cells||seminal gland||small lingual glands (Gll. linguales)|
|goblet cells||Cowper's gland (Gl. bulbourethralis)|
|alveolar epithelial cells type 2||accessory lacrimal gland (Krause's gland)|
|bronchial & treacheal glands||stomach (chief, parietal, mucous neck and surface cells)|
|further small glands|
C.) considering morphology of the entire gland:
1a.) simple tubular singular glands e.g., glands of the stomach or colon, sweat glands
1b.) branched tubular glands with several terminal pieces e.g., glands of the duodenum, cardia of the stomach, Cowper's gland (Gl. bulbourethralis)
2a.) simple alveolar glands with a simple wide end piece e.g., small sebaceous glands
2b.) branched alveolar glans e.g., larger sebaceous glands, bronchial glands
3.) tubuloalveolar (wide end piece) or tubuloacinous (narrow end piece) glands e.g., esophageal glands, mammary gland, prostate
4.) mixed tubular glands (many multiply branched end pieces on a larger duct), z.B. lingual glands, sublingual gland (Gl. sublingualis)
5.) mixed tubuloalveolar -acinous glands e.g., tubuloalveolar: lactating mamma; tubuloacinous: sublingual gland (Gl. sublingualis), submandibular gland (Gl. submandibularis), exocrine pancreas, parotid gland (Gl. parotidea)
6.) unicellular glands consisting only of one single cell e.g., goblet cells
D.) considering morphology of the end pieces:
1.) serous end piece: acinus (tiny lumen); round, mostly central nuclei; secretory vesicles concentrate in apical cytoplasm; water-like secretion rich in proteins; eg., parotid gland, exocrine pancreas
2.) mucous end piece: wider lumen in form of an alveolus (nearly spherical lumen) or tubulus (tubular ending), the flat nuclei seem to be pressed to the base of the cell, less electron-dense honeycomb-like cytoplasm, viscous secretion rich in acid mucins, e.g. goblet cells
3.) von Ebner's half moon = a single layer of serous cells is attached peripherically on a monolayer of mucous cells in an end piece; only present in sublingual gland (Gl. sublingualis) or submandibular gland (Gl. submandibularis).
E.) considering stimulation of secretion:
1.) glands with regulated secretion: secretion is evoked by chemical or electrical stimuli, i.e. by hormones, neurotransmitters or electrical depolarisation; e.g., pituitary, adrenal gland, pancreas, stomach
2.) glands with constitutive secretion: permanent secretion without defined stimuli; e.g. fibroblasts, hepatocytes, plasma cells
construction of a larger salivary gland:
The major part of the secretion is produced by the epithelial cells around the acini of the gland. Small intercalated ducts with a monolayered cubic epithelium transport the secretion to an intralobular duct whose monolayered columnar epithelial cells show a basal striping. This striping is caused by regularly stacked mitochondria in deep infoldings of the cell membrane (--> example image) and is visible in light- and electron microscopy. When the duct leaves the lobe of the gland, it becomes an interlobular duct and is surrounded by connective tissue. The columnar epithelium begins to show an additional basal cell layer, i.e. is bilayered. In case of larger glands such ducts join to form a main excretory duct which has a pseudostratified epithelium.
--> secretory vesicles, salivary
--> Electron microscopic atlas Overview
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