Ciliogenesis dependents on the bidirectional intraflagellar transport mechanism that supplies precursor molecules to the growing cilium.
During growth and elongation of the cilium, precursor molecules are delivered from the cell body to the most distal end of the elongating axoneme by intrafagellar transport (IFT). Since cilia have no molecular machinery for protein synthesis, the IFT is the only mechanism for delivering proteins required for cilia assembly and growth. In some ways, the IFT can be compared to the vertical lift assembly used at a construction site to move building materials and tools up and down a building. As the building increases in height, the track of the lift extends as well. Similarly, the IFT utilizes raftlike platforms assembled from about 17 different intraflagellar transport proteins that move up and down the growing axoneme between the outer doublets of microtubules and plasma membrane of the elongating cilium (Fig. 5.12). Cargo molecules (including inactive cytoplasmic dynein molecules) are loaded onto the IFT platform while it is docked near the base of the cilium. Utilizing kinesin II as a motor protein, the fully loaded platform is moved upward toward the tip of the cilium (anterograde transport). The “building materials” are then unloaded at the tip of the cilium (the site of axoneme assembly). Here particles turn around, and the platform heads back to the base of the cilium (retrograde transport) after picking up turnover products (including inactivated kinesin II). During this process, cytoplasmic dynein is activated and  utilized as a motor  protein to bring the platform back to the base of the cilium (see Fig. 5.12). Several proteins, including IFT raft proteins (kinesis, cytoplasmic dynein, polaris, IFT20, etc.), are important to ciliogenesis and subsequent maintenance of the functional cilium. Mutations in genes encoding these proteins result in loss of cilia or ciliary dysfunctions.
The lateral domain of epithelial cells is in close contact with the opposed lateral domains of neighboring cells. Like the other domains, the lateral domain is characterized by the presence of unique proteins, in this case the cell adhesion molecules (CAMs) that are part of junctional specializations. The molecular composition of the lipids and proteins that form the lateral cell membrane differ significantly from the composition of those that form the apical cell membrane. In addition, the lateral cell surface membrane in some epithelia may form folds and processes, invaginations and evaginations that create interdigitating and interleaving tongue-and-groove margins between neighboring cells.
Viewed with the light microscope, terminal bars represent epithelial cell-to-cell attachment sites.
Before the advent of EM, the close apposition of epithelial cells was attributed to the presence of a viscous adhesive substance referred to as intercellular cement. This cement stained deeply at the apicolateral margin of most cuboidal and columnar epithelial cells. When viewed in a plane perpendi cular to the epithelial surface, the stained material appears as a dotlike structure. When the plane of section passes parallel to and includes the epithelial surface, however, the dotlike component is seen as a dense bar or line between the apposing cells (Fig. 5.13). The bars, in fact, form a polygonal structure (or band) that encircles each cell to bind them together. Arrangement of this band can be compared to the plastic rings that hold together a six-pack of canned beverages.