secretory vesicles Dr.Jastrow's electron microscopic atlas

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Overview secretory vesicles (Vesiculae secretoriae):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)


serous secretion vesicle (monkey)	exocytosis of a secretion ve- sicle, pancreas (monkey)	sebaceous gland with holocrine secretion (monkey)	endocrine secretion in hypophysis (monkey)	endocrine secretion vesicles adenohypophysis (monkey)	herring bodies with stored ve- sicles neurohypophysis (monkey)	chief cell of the stomach with serous secretion vesicles (rat)	serous acinar cells parotid gland (rat)


submandibular gland serous acinar cell (monkey)	acinus with serous se- cretion vesicles 1 (monkey)	acinus with serous secre- tion vesicles 2 (monkey)	mucous und serous secretion vesicles sublingual gland (rat)	mucous epithelial cell Glandula sublingualis (rat)	detail: mucous vesicles (rat)	Secretion vesicles se- minal gland (rat)	serous acinus of a small human eccrine suet gland


neurotransmitter vesicles cerebral synapse 1 (rat)	type I synapse with transmitter vesicles Cortex cerebri 2 (rat)	type I synapse Cortex cerebri 3 (rat)	type I synapse Cortex cerebri 4 (rat)	type I synapse + mitochondrion cerebral cortex (rat)	dense core vesicles in vegetative nerve endings forming synapses en passant (rat)	secretion of a goblet cell of the colon (rat)

Secretory vesicles (Terminologia histologica: Vesiculae secretoriae) are vesicles bordered by a unit membrane that deliver secretory products to the extracellular space which in many cases is a glandular acinus or the lumen of an excretory duct. The wrong, but established term secretory granule should not be used since true granules like glycogen granules, in contrast to secretory vesicles never have a limiting membrane. Secretory vesicles may derive from the Golgi apparatus (Golgi vesicles) or from rough or smooth endoplasmic reticulum or in some cases directly from endocytotic or modified endocytotic vesicles as e.g., in membrane vesicles of the crusta of the ureter. They are transported to the cell membrane, their membrane fuses with the cell membrane whereby the latter diverges at the point of adhesion and the content of the vesicle is released. This process is termed exocytosis. Exocytotic vesicles in contrast to common endocytotic vesicles have no clathrin coat. The size of secretory vesicles may vary considerably neurotransmitter vesicles that serve in signal transduction processes on chemical synapses have a mean diameter of 50 nanometers (nm). Secretory vesicles of vesicular secreting endocrine gland cells, i.e. cells secreting directly into blood or lymph vessels usually are about 100 nm in diameter. Such secretions are called hormones (messenger substances delivered to their targets i.e. receptors via the blood). The secretory vesicles of exocrine gland cells that deliver their content into gland acini or excretory ducts are quite different in size in some cases over 1 µm and vary in electron-density depending on the content. This content virtually always contains water, ions and often several different soluted proteins e.g., zymogen vesicles of the exocrine epithelial cells of the pancreas are comprised of a mixture of different still inactive froms of enzymes. Examples of vesicluar secreting endocrine glands, i.e. those that deliver secretion products via vesiclular exocytosis are the pituitary, adrenal gland, thyroid gland (C-cells) and the pancreas. Examples of exocrine glands that release secretory vesicles are the pancreas, the salivary glands, the lacrimal gland, goblet cells and the mammary gland.
Regarding the consistency and viscosity of the secretory products secretory vesicles can be classified as:
1.) serous, i.e. vesicles have a water-like non-viscous content. Most of the proteins encountered in serous secretion vesicles are enzymes that often are secreted in inactive preforms. Due to the mostly relatively high protein content the vesicles are quite electron-dense and homogenous. They mostly to not join with each other before exocytosis and thus can be well differentiated from each other. The cytoplasm of serous secreting gland cells is homogenous. The cells are often seen bordering serous glandular acini. Here the cells show their round nuclei in the basal mostly basophilic (due to high content in ribosomes and ribosomal RNAs) cytoplasm. Typical glands with serous secreting cells are the parotid gland, the lacrimal gland, the pancreas and sweat glands.
2.) mucous, i.e. highly viscous secretion vesicles which contain acid mucins. The mucoid substances (glycoproteins or mucopolysaccharids in most cases) can be visualized using the PAS staining metod in light microscopy. They have not such a high affinity to electron-microscopic staining agents therefore vesicles appear much less electron-dense than serous ones. In many cases vesicles fuse before being released resulting in a less electron-dense honeycombed appearance of the apical cytoplasm. The nuclei of mucous epithelial cells usually are flat and located close to the basis of the soma. Mucous secretory vesicles are present in epithelial cells of the stomach, in goblet cells, in the sublingual- and submandibular gland. The last mentioned glands also have populations of serous epithelial cells that may be present either in acini alone or in form of vonEbner's half moons beyond mucous epithelial cells. In these cases the serous cells do not have direct contact to the lumen thus the intercellular clefts between the mucosus cell are dilated to allow the serous secretions to reach the lumen of the gland. Von Ebner's half-moons are only seen in the sublingual- and submandibular gland.
The delivery of secretions may happen in three different ways of which the eccrine is most common:
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.

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Some images were kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow.