endocytic vesicles

Clathrin-coated vesicles

Many endocytic vesicles originate at the cell surface as clathrin-coated pits. As clathrin assembles under a patch of plasma membrane, the clathrin-coated pit soon (about a minute) pinches off from the surface and forms a clathrin-coated endocytotic vesicle.

Soon after forming, the clathrin-coated vesicles release their associated clathrin and become competent to fuse with early endosomes.

Extracellular materials trapped in the endocytic vesicles can be either passed into the endosomal compartment or returned to the surface.

Microscopy indicates that, in some cells, the endosomal compartments comprise a network of membranous tubes and vesicles extending all the way to the cell nucleus.

The deep end of the endosomal compartment is called the late endosome compartment. It may take 5-15 minutes for materials to be transported from the cell surface through early endosome compartments and on to the late endosome.

The endosomal compartment is usually acidic due to the action of a proton-pumping ATPase of the endosomal membrane. Many receptors involved in endocytosis of extracellular substances change their conformation at low pH and release their bound substance. The empty receptor proteins can then be sorted back to the cell surface.

Some materials that reach the late endosomes are degraded in lysosomes. Parts of the late endosomal compartment may become lysosomes or temporarily fuse with lysosomes in order to transfer endoctosed materials into the lysosomes.

Transcytosis

In epithelial cells, a special process called transcytosis allows some materials to enter one side of a cell and exit from the opposite side. For example, the GI tract of babies can take protective antibody proteins from breast milk, and, via transcytosis, transport the antibodies into the blood stream (WP).

Animations

 Axonal transport: A movie from J Cell Biol

See also

 endocytic pathway
 endosomal compartment