between epithelial layers. Tight junctions are classified as a paracellular barrier which is defined as not having directional discrimination; however, movement of the solute is largely dependent upon size and charge. There is evidence to suggest that the structures in which solutes pass through are somewhat like pores. Physiological pH plays a part in the selectivity of solutes passing through tight junctions with most tight junctions being slightly selective for cations. Tight junctions present in different types of epithelia are selective for solutes of differing size, charge, and polarity. Proteins There have been approximately 40 proteins identified to be involved in tight junctions. These proteins can be classified into four major categories; scaffolding proteins, signalling proteins, regulation proteins, and transmembrane proteins. Roles Scaffolding proteins – organise the transmembrane proteins, couple transmembrane proteins to other cytoplasmic proteins as well as to actin filaments. Signaling proteins – involved in junctions assembly, barrier regulation, and gene transcription. Regulation proteins – regulate membrane vesicle targeting. Transmembrane proteins – including junctional adhesion molecule, occludin, and claudin. It is believed that claudin is the protein molecule responsible for the selective permeability between epithelial layers. A three-dimensional image is still yet to be achieved and as such specific information about the function of tight junctions is yet to be determined. Tricellular junctions Tricellular junctions seal epithelia at the corners of three cells. Due to the geometry of three-cell vertices, the sealing of the cells at these sites requires a specific junctional organization, different from those in bicellular junctions. In vertebrates, components tricellular junctions are tricellulin and lipolysis-stimulated lipoprotein receptors. In invertebrates, the components are gliotactin and anakonda. Tricellular junctions are also implicated in the regulation of cytoskeletal organization and cell divisions. In particular they ensure that cells divide according to the Hertwig rule. In some Drosophila epithelia, during cell divisions tricellular junctions establish physical contact with spindle apparatus through astral microtubules. Tricellular junctions exert a pulling force on the spindle apparatus and serve as a geometrical clues to determine orientation of cell divisions. Cell junction molecules The molecules responsible for creating cell junctions include various cell adhesion molecules. There are four main types: selectins, cadherins, integrins, and the immunoglobulin superfamily. Selectins are cell adhesion molecules that play an important role in the initiation of inflammatory processes. The functional capacity of selectin is limited to leukocyte collaborations with vascular endothelium. There are three types of selectins found in