Microtubule organization in the differentiating transfer cells of the placenta inLilium spp.

Summary

Placental cells in the ovarian transmitting tissue ofLilium spp. are organized as transfer cells with inbuddings facing the ovarian locule. A detailed analysis of microtubule (MT) organization during development of these polarized cells is reported here. Formation of wall projections occurs at the apical part of the cell starting on the day of anthesis, and a fully mature secretion zone is found four days after anthesis. MTs are organized into distinct cortical and central arrays. The cortical array undergoes a unique transition at anthesis. MTs in the basal half of the cell remain in longitudinal bundles while in the apical half of the cell their longitudinal orientation is replaced by a transverse alignment. One day after anthesis, these transverse bundles become a meshwork of short, randomly organized MTs, while MTs in the basal half of the cell retain their longitudinal alignment. The realignment of MTs in the apical half of the cell coincides with the deposition of the secondary cell wall. The central array is composed of short, randomly arranged strands of MTs in the cytoplasm between the nucleus and the apical and basal periclinal walls of the cell. This array first appears as solitary strands in the apical part of the cell one day before anthesis. The central array extends during development and is eventually seen in the basal half of the cell. We propose that MTs in the cortical region near the apical wall act as templates for the deposition of cellulose microfibrils in the secondary cell wall. MTs in the central array in these transfer cells may be involved in the trafficking of vesicles and/or positioning of organelles near the secretion zone.