Cell Major Axis Research Articles

The opening mechanism of follicles of some species of Banksia

Banskia ornata F. Muell. is a common dominant in heaths in south-eastem Australia that are frequently swept by fires which kill the plants. Regeneration is only from seed, suggesting that the future of the species is almost completely fire-dependent. At maturity, a multiple fruit is formed consisting of a number of follicles attached to a central rachis. Each valve of the follicle consists of an exocarp comprising an epidermis with hair cells and some sclereid bundles; a mesocarp containing bundles of sclereids in which the microfibrils are almost transverse to the major cell axis; and an endocarp in which the innermost cells are fibrous in form and in which the microfibrils of cellulose are oriented mainly parallel to the longitudinal axis of the cell. Also present in the endocarp but abaxial to the fibrous sclereids are sclereid bundles that show similar microfibril orientation to the mesocarp sclereids. The junction between the valves of the follicle has a layer of interdigitating cells. A resinous substance together with phenols are present in and between these cells. Because of the difference in structure of the sclereids of the endocarp and of those of the mesocarp and exocarp, stresses develop between these tissues on desiccation. Events leading to the opening of the follicles are: (1) at maturity resin and phenols are secreted into the vascular elements at the base of the rachis, severing the vascular supply to the follicle; (2) with the advent of fire, the resin at the junction of the valves of the pericarp is destroyed, releasing the stress in the valves of the follicles, which reflex, allowing the release of seed. In B. marginata and B. Integrifolia, the anatomy of the valves of the follicle is basically similar to that of B. ornata. However, the follicles open without the intervention of fire. Thin-layer chromatography has shown that the resins extracted from the cells of the junction zone between the valves of the follicle of B. ornata are chemically different from those extracted from the non-fire-dependent species, suggesting that they may differ also in their physical properties.

Microfilaments and the mechanism of light-triggered sperm release in ascidians

In nature, the ascidian Ciona intestinalis sheds spermatozoa at dawn; however, gravid individuals can be induced to spawn at any time by a 1-hr dark-adaptation period followed by 25-min exposure to 415–430 nm irradiation. Spawning can be induced similarly from a preparation of the isolated sperm duct. A combined light and electron microscopic study was initiated to define morphological aspects of the sperm duct complex and, if possible, elucidate the receptor and effector systems. Results of this investigation indicate that the duct is composed of a tubelike epithelium with a distal closure apparatus consisting of approximately 12 tubules encased by, but not in contact with, subepithelial pigmented cells. There are no nerve or muscle elements associated with the complex. A microfilament system oriented circumferentially about the major cell axis and closely allied with numerous small vesicles is present within epithelial cells along the main portion of the duct. A similar system, oriented along the major cell axis, occurs in epithelial cells of the distal tubules. The potential roles of these epithelial cells as both receptors and effectors of the spawning response is discussed, and an hypothesis is presented to account for the mechanism of light-triggered sperm release.

Cell interactions and histogenesis in embryonic neural aggregates

Mixtures of cell suspensions from the optic lobe of 3-day-old and 6-day-old chick embryos form aggregates which show many ‘rosettes’ resulting from the invagination of peripheral ‘placodes’. However, in aggregates formed exclusively by optic lobe cells of either 3-day-old or 6-day-old embryos no rosettes are observed. By means of radioautographic studies of the combined aggregates, it was shown that rosettes are formed almost exclusively by cells from 3-day-old optic lobe, while those of 6-day-old embryos are predominantly found at the periphery of the aggregates. Cells from 3-day old optic lobe also form rosettes when cultured in combination with wing cells. Similar observations were done in aggregates formed by mixtures of 3-day-old optic lobe and 6-day-old liver cells. The cells from the optic lobe of 3-day-old embryos forming the concave surface of invaginating placodes and rosettes in the ‘combined’ aggregates appear polarized and wedge- or ‘bottle’-shaped. They are aligned in a highly ordered way, and abundant zonula adhaerentes, usually parallel to the cell major axis, are found at the cell apex where bundles of longitudinally oriented microtubules are present. The 3-day-old optic lobe cell which form the convex external surface of the ‘pure’ aggregates have a similar shape and organization, but in this situation apical cell bulges with randomly oriented microtubules interdigitate with those from neighbouring cells, and are joined by zonula adhaerentes usually perpendicular to the longitudinal axis of the cell.

Ultrastructural changes associated with an invagination phenomenon in embryonic neural aggregates

Aggregates formed by cell suspensions from the optic lobe of 4-day-old chick embryos have peripheral “placode-like” structures, which give rise by ulterior invagination to miniature neural tube-like structures, the rosettes. These events are accompanied by changes in cell shape, cell structure and cell contacts as shown by electron microscopy. Aggregates recovered after 1.5 h culture show a very poor histological organization. The neuroepithelial cells (NE cells) appear irregularly shaped, without any recognizable apico-basal polarity. Most of the cell surface faces wide intercellular spaces, although there is some apposition of adjacent plasma membranes, as well as a few junctional differentiations, between cell bodies, cell processes, and cell bodies and processes. The microtubules of the NE cells are scarce, and are apparently distributed at random. No particular orientation of NE cells with respect to each other is observed. Concomitantly to the formation and invagination of placodes the NE cells undergo several structural changes. They become wedge- or bottle-shaped, and are aligned in a highly ordered way within individual placodes and rosettes. Cell contacts involve most of the cell surface, while abundant zonula adhaerentes are present exclusively at the cell apex. The nucleus and some organelles of the NE cells are polarized; bundles of microtubules oriented along the cell major axis are found at the apical region of the cells.