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A morphological and cytochemical analysis of bud initiation and development in the filamentous brown alga Sphacelaria Furcigera Burns, Alan Robert


A study on the process of bud initiation and development in the filamentous brown alga Sphacelaria furcigera (Kutz) was carried out using techniques of light and electron microscopy, as well as cytochemistry. A localized thickening and subsequent protrusion of the outer cell wall of the axial mother cell characterizes the earliest detectable stage of bud initiation. This protrusion forms through the combined deposition of newly synthesized microfibrils together with the partial lysis/loosening of the existing cell wall. Evidence is presented that of the three enzyme activities localized, peroxidase, adenosine triphosphatase and acid phosphatase activity, only peroxidase activity is related to the lysis/loosening of the cell wall during the early development of the bud initial. Continued incorporation of new cell wall material into the outer cell wall maintains its structural integrity. However, there is a change in the layered appearance of the cell wall microfibrils. The cell wall of the bud initial is characterized by two cell wall layers instead of the four found in the pre-existing cell wall of the axial mother cell. This original cell wall is composed of an outer-most fucan layer, overlying an alginate layer, which in turn overlies another fucan layer and finally terminates in an inner-most alginate layer. In contrast the bud initial's cell wall has only a thin outer fucan layer and a thick inner alginate layer. Concomitant with the formation of the cell wall protrusion, there is a loss of cytoplasmic vacuoles, an increase in cytoplasmic mass and density and an increase in the number of organelles. The endomembrane system (endoplasmic reticulum, dictyosomes and the derivative vesicles) also proliferates. Organelle migration into the bud protrusion keeps pace with bud expansion. The movement of the nucleus, however, lags behind and it migrates towards the bud protrusion only after a "vacuole free" cytoplasm becomes established. As the nucleus approaches a medial position between the base of the axial mother cell and the tip of the bud protrusion, cytoplasmic vacuoles re-appear. They are confined, however, to the basal region of the axial mother cell. After karyokinesis, a cross wall is deposited between the two daughter nuclei resulting in the formation of a bud cell and a sister axial cell. The sister axial cell is highly vacuolated and structurally resembles the adjacent quiescent axial cells. The bud cell is dense and non-vacuolated, a feature characteristic of a mefistematic cell.

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