Ultrastructural studies of 2,4-D (2,4 dichlorophenoxyacetic acid) induced coconut calli and of untreated controls enabled us to characterize early events in cellular reorganization leading to embryogenic cell individualization and subsequent development into proembryos. Embryogenic cells were characterized by special features that chiefly affected the nucleus, cytoplasm and cell wall: deep invaginations of the nuclear envelope, proliferation of dictyosomes, with emission of Golgi vesicles, directly related to an increase in cell wall thickness. Modification of the cell wall structure was studied and particular attention was paid to the cytolocalization of bêta-1,4-glucans, and of callose and pectin epitopes, using gold-conjugated probes. The first changes (detected 7-14 d after 2,4-D increase) involved the closure of plasmodesmata, breaking of symplastic continuity, and callose deposition. The acquisition of embryogenic competence was linked to the appearance of an outer layer of fibrillar material containing pectin epitope (mainly un-methyl-esterified), fully coating the embryogenic cells (21 d after the induction treatment). Some of the ultrastructural changes observed during the reprogramming of somatic cells towards embryogenesis can be likened to those accompanying the maturation of female gamete cells in many plant species. The possible significance of these observations is discussed.