Ontogenesis of trichome-like cavities in Dictamnus dasycarpus

The ontogeny of a special type of glandular hairs, namely the trichome-like cavities in Dictamnus dasycarpus, characterized by many morphological similarities to non-glandular hairs, capitate glandular hairs, and secretory cavities, was studied using light and electron microscopy. These trichome-like cavities originate from a single, initial epidermal cell that undergoes a periclinal division, with one cell developing into the internal cells and the other into the outer, epidermal cells. A beak-shaped apex is formed on the head of the trichome-like cavity. The histochemical test shows that the trichome-like cavities are important sites for lipid production. By ultrastructural analysis it becomes evident that formation of these trichome-like cavities starts with a disorganization of the cytoplasm that is accompanied by formation of odd shaped nuclei with condensed chromatin. The process continues bringing about plasmolytic processes, and disintegration of the plasma membrane system follows leading finally to autolysis, when mitochondria and degenerated plastids with disorganized membrane systems are engulfed by vacuoles, multivesicular bodies, and double-membranous autophagosomes within the vacuoles. A strong structural twist and swelling of the internal cell walls ultimately leads to complete breakdown of the structures. Nuclei of the inner internal cells within the trichome-like cavity become TUNEL-positive and DAPI-negative first; later this is detected also in the outer internal cells, indicating a centrifugal nuclear degradation process. On the basis of this work, it can be assumed that the lysigenous formation of the trichome-like cavity is a typical programmed cell death process.