CELL SURFACE JUNCTIONS
(1) Desmosomes (Maculae adherence) - Thickened circular areas on P.M. of adjacent cells with radiating fibrils in cytoplasm. These fibrils called tonofibrils. Desmosomes generally occurs in epithelial cells and provide cell adhesion. Desmosomes without tonofibrils are called terminal bars.
(2) Tight junction or Zona occludence - outer most layers of two P.M. at the site of attachment fuse completely.
Fused membrane reveals a 5- layered structure. Permeability of ions is more at tight junction. Tight junction occurs in nerve tissue at synapses. (electrical synapse)
(3) Gap junction or Nexus - Fine protein channel (connexon) join two P.M of adjacent cells. Exchange of ions between cells may takes place by protein channels. Desmosomes, Tight junction and Gap junction occurs in animal cell.
(4) Plasmodesmata - Name proposed by strassburger. Cytoplasmic connection bet. Two adjacent cells through pores in cell wall and cell membrane. Plasmodesmata are characteristic of multi-cellular plants and they maintain continuity of cytoplasm between adjacent cells. Plasmodesmata often contain an expanded E.R. termed as desmotubule.