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9 pages, 6 figures.-- Printed version published Jun 2008., Microspore embryogenesis to form haploid and double-haploid embryos and regenerated plants is an efficient method of producing homozygous lines for crop breeding. In trees, the process is of special interest since classical methods are impractical in many cases, as in Olea europaea L. Recently, a convenient method has been developed for microspore embryogenesis induction by stress in olive isolated microspores in vitro cultures. In the present work, the switch of the microspore developmental pathway and the formation of microspore-derived multicellular proembryos have been achieved and a cytochemical and immunocytochemical analysis was performed in the early stages. The young microspore proembryos displayed defined features different to both, the in vivo gametophytic, and the in vitro non-responsive microspores. Reprogrammed microspores showed an absence of starch, the occurrence of a first symmetrical division and cytokinesis, the presence of an abundant ribosomal population, and changes in cellulosic and pectic cell wall components which constituted early markers of the embryogenic microspore process. They provided new insights on the molecular and cellular events associated with the microspore reprogramming of woody plants, and specifically in olive, providing interesting knowledge which could guide future selection and regeneration strategies in this fruit tree of high economic interest., Work supported by projects granted by the Spanish Ministry of Education and Science (MEC) BFU2005-01094 and AGL2005-05104, and it was performed in the frame of the CSIC-INIA Joint Collaboration Program (Convenio CC03-023). MTS is recipient of a predoctoral fellowship (FPI, BES-2006-14117) granted by the Spanish MEC. MJP was recipient of a contract granted by the project CAM 07G/0009/2003 1 funded by the Comunidad de Madrid. Partly supported by Spanish-Czech Joint project 2006CZ0006 granted by Spanish National Research Council (CSIC) and Czech Academy of Sciences and Czech grants MSM0021620806, LC535 and AV0Z50110509.