Changes in the blood-brain barrier of the central nervous system in the blowfly during development, with special reference to the formation and disaggregation of gap and tight junctions. I. Larval development

In the central nervous system (CNS) of full-grown larvae of the blowfly Calliphora erythrocephala, the glial-ensheathed nerve cells are completely surrounded by a layer of perineurial cells which form a “blood-brain barrier” between the circulating haemolymph and the CNS. A variety of intercellular junctions, including gap and tight junctions, are found between adjacent perineurial cells and some also between apposing glial cells; these have been characterized by freeze-fracturing as well as by tracer studies and analysis of thin sections. They are found not to be present between such cells in the undifferentiated CNS in the newly hatched larvae, nor are the nerve cells encompassed by glial cells; ionic lanthanum can penetrate to the axonal surfaces at this stage. However, over the 5 days of larval growth and development the glial cells produce attentuated cytoplasmic processes that ensheath the nerve cells, and the perineurium is formed; junctional complexes are assembled and a larval blood-brain barrier is produced which excludes tracers. Freeze-fracture preparations suggest that the inverted gap junctions which develop have done so by migration of individual intramembranous EF particles to form, at first, linear arrays and small clusters and, ultimately, macular aggregations in the perineurium; these lie between the undulating rows of PF particles forming the septate junctions. These septate junctions are formed by the organization of arrays of PF particles into multiple rows. Extensive PF particles fusing into ridges with EF grooves to form perineurial “tight” junctions are also observed, seemingly in the process of development; entry of exogenous lanthanum followed by its exclusion parallels the completion of ridge formation. These ridges are simple linear arrays of particles which may be discontinuous, lying in parallel with one another and the surface. Clustered particle arrays as well as scattered short ridges on the axonal PF, however, appear to be present unchanged throughout larval life; their role may therefore be associated with neural membrane function although there are suggestions that some may form axo-glial junctions. This is the first report on the lateral migration of intramembranous particles as the mode of formation of gap junctions in the nervous system of an invertebrate.