The components of the endocytotic pathways also function to transfer material within a cell. In vertebrates, most cells depend on nutrients from the blood even though they are not in direct contact with the blood. Diffusion of molecules from the blood to the tissues would in many cases be just too slow. Transcytosis is the transfer of molecules across cells from one side to the other, a process that entails endocytosis, vesicular transfer and exocytosis, and which speeds the bulk movement of molecules through tissues. In some cases, transcytosis is receptor-mediated, and is often carried out by vesicles called caveolae, which are distinct from those involved in endocytosis and exocytosis as described above.
Which cells in a vertebrate would you expect to contain abundant caveolae?
The endothelial cells that line blood vessels have large numbers of caveolae, which are particularly evident in clusters near the plasma membrane.
Figure 13 shows a cluster of caveolae adjoining the membrane of an endothelial cell from lung.
Receptor-mediated transcytosis can also carry specific molecules across cells. For example, in humans and some other mammals, antibodies are transported across the placental cells that form the interface between the maternal and fetal circulation. The transported maternal antibodies protect the fetus in utero and in neonatal life. A related process occurs in the gut epithelial cells of some neonatal animals, which have receptors that transfer antibodies from maternal milk intact across the gut epithelium and into the tissues, to protect the neonatal animal. The reverse process occurs in the gut of adult mammals that have a system for transferring a different type of antibody (IgA) from the tissues into the lumen of the gut, to protect against infection (Figure 14).
Transcytosis may also be important in moving proteins involved in differentiation and body pattern development. An interesting example is the Wg protein produced in a row of cells at the posterior of each segment of Drosophila, which is distributed throughout the segments to control the normal development of elements of the segment (mutations of the Wg gene which produces the Wg protein may result in a wingless phenotype). In this example, transcytosis distributes the protein laterally in the plane of the polarised epidermal epithelium. This differs from the basal–apical transcytosis of IgA, described above.