RAT CEREBRAL MAST-CELLS UNDERGO PHENOTYPIC CHANGES DURING DEVELOPMENT

The evolution of rat cerebral mast cell phenotype during development w as studied using antibodies against the granule chymases, rat mast cel l protease I (RMCP-I) and rat mast cell protease II (RMCP-II) and thei r gene transcripts, as markers for serosal and mucosal mast cells, res pectively. In situ hybridization using specific oligoprobes for RMCP-I I permitted visualization of RMCP-II mRNA-containing cells as early as day 15 of embryonic development (E15). From E19 to day 4 postpartum ( D4) their number increased whilst they migrated from the pia mater to the choroid fissure; at D8 cells expressing RMCP-II gene transcripts w ere no longer observed. The 3'-end untranslated nucleotide sequence of the RMCP-I cDNA was established in order to design selective cDNA pro bes for Northern blot analysis of both enzymes, Northern blot analysis revealed a strong expression of RMCP-I and RMCP-II mRNAs at D2. At D4 , RMCP-I mRNA expression was still high, whereas that of RMCP-II was d ecreased. In adult brain, mRNA expression for both proteases was low, but detectable. Quantification of both proteases by ELISA showed that, from E19 to D4, levels of RMCP-II were maximal at E19 and remained co nstant until D4, whereas RMCP-I increased as a function of age. Therea fter, levels of both proteases decreased progressively, but were still present in the adult brain, with RMCP-II being uniformly distributed and RMCP-I concentrated in the thalamus. Immunohistochemical staining showed RMCP-II-immunoreactive cells within the pia mater at E19; on D2 and D4, cells with both RMCP-I and RMCP-II immunoreactivities were fo und within the choroid fissure and from D8, only RMCP-I-immunoreactive mast cells were observed. In the thalamus of adult rats, the latter h ad a perivascular localization. This study shows that in the adult, bo th types of mast cells are present, although in small numbers, except for RMCP-I-immunoreactive mast cells which are abundant in the thalamu s. The changes in the number and phenotype of cerebral mast cells may result from the influence of a number of growth factors during develop ment.