List of abbreviations
of micros-
specialist terms
explained in
English +

Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!

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Dr. med.
H. Jastrow

of use
Overview caveols (Caveolae):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
tubular caveols, inner
hair cell (guinea pig)
stereo image of a tubular hair cell caveole
(opening not visible in this section, guinea pig)
tubular caveols overview
outer hair cell (guinea pig)
smooth muscle cells static
alveolar caveols (monkey)
smooth muscle cell + static
alveolar caveoles, colon (rat)
static alveolar caveols, smooth
muscle cell, colon (rat)
alveolar caveola in capillary endothe-
lial cell over a heart muscle cell (rat)
alveolar caveols of an endo-
thelial cell, skeletal muscle (rat)
static alveolar caveols of a
smooth muscle cell (rat)
Caveols (Terminologia histologica: Caveolae) are short spherical to ovoid, i.e. alveolar or tubular invaginations of the cell membrane. They are surrounded by a fine meshwork of the integral membrane protein Caveolin located on the cytoplasmatic surface of the cell membrane which is hardly detectable in the electron microscope. Type 1 and 2 caveolin is present in all cells whereas caveolin type 3 is only seen in cartilage and straited muscle cells. Caveolin is responsible for the formation of caveols. Further there are mutiple different receptor proteins  and a special lipid composition (high concentration of glykolipis with their anchoring proteins, glycosphingolipids, cholesterol and sphingomyelin) of the cell membrane. Alveolar caveols invaginate only about 80 - 150 nanometers (nm) into the cell whereas tubular caveols may le longer than 1000 nm.
Tubular caveols are thin tubes running parallel to the cell membrane in a distance of 30-50 mn. Lots of these caveoles are present side by side parallel to the cell membrane of inner and outer hair cells of the organ of Corti in the inner ear. Their openings to extracellular space, however, are only rarely seen thus it may be assumed that they may be closed from time to time. Possibly tubular caveols concentrate calcium ions for quick release when needed for cellular contraction of hair cells. Perhaps tubular caveols which resemble smooth endoplasmic reticulum have connections to the latter.
Alveolar caveols resemble forming micropinocytotic vesicles that differ from these endocytotic vesicles by their lack of a clathrin coat. The highest concentrations of such caveols are seen in endothelial cells of capillaries in heart or skeletal muscles. Alveolar caveols close under influence of dynamin and GTP and become endocytotic vesicles. The latter transport extracellular fluid between vascular lumen and the extracellular loose connective tissue surrounding muscle cells in both directions. This kind of transport is called transcytosis or cytopempsis. Of importance in this context is the receptor protein Albondin, which binds to albumin of blood serum and which is concentrated in caveols of endothelial cells.
The receptor independent endocytosis via formation of caveols is called constitutive endocytosis in cells that recyle cell membrane material and resorb extracellular fluid by fusion of vesicles with smooth- or rough endoplasmatic reticulum or Golgi apparatuses. Choleratoxin and HIV viruses also use caveols as port of entry into cells. In epithelia the immunglobulin A receptor is concentrated in caveols. Potocytosis is the uptake of the vitamin folic acid into cells whereby folic acid binds to receptors concentrated in caveols. After enrichment of the vitamin the neck region of the caveoles close. The formed  endocytotic vesicle decays in pH due to ion transporters in its membrane. By this the folic acid solves from its receptors and finally is able to be transported by its specific transporter into the cytoplasm.
Alveolar caveols of smooth muscle cells are somewhat spherical with a maximal width of 100 nm. In sections they appear as omega profiles that are stable and NOT subject to endocytosis, i.e. a static type of caveoles is present here. It may be assumed that these cavelos, similar to the tubular caveols of hair cells, serve as calcium ion storage in analogy to L-tubules, i.e. smooth endoplasmatic reticulum of straited muscle cells.

--> endocytosis, smooth muscle tissue, endothelium, cytoplasm, cell membrane
--> Electron microscopic atlas Overview
--> Homepage of the workshop

One image was kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow.