Cytoplasmic organization during pattern formation in the alga Micrasterias

Area of Science:

• Cell Biology
• Developmental Biology
• Algology

Background:

• Pattern formation in unicellular algae like Micrasterias is crucial for morphogenesis.
• Previous studies indicated a lack of cytoplasmic organization in growing algal lobes.
• Understanding organelle distribution is key to deciphering growth mechanisms.

Purpose of the Study:

• To investigate the cytoplasmic organization during pattern formation in Micrasterias using improved electron microscopy fixation.
• To determine the role of specific organelles and ions in the tip growth of algal lobes.
• To elucidate the effects of ionophore A23187 on cytoplasmic organization and vesicle distribution.

Main Methods:

• Utilized an improved fixation technique for electron microscopy.
• Observed cytoplasmic organization at various developmental stages (bulge to 5-lobe stage).
• Applied ionophore A23187 with varying divalent cations (MgCl2, CaCl2) to assess effects on cytoplasmic structures.

Main Results:

• Identified ordered cytoplasmic organization, including peripheral electron-dense vesicles and internal mitochondria/Golgi, in early stages.
• Observed tip-associated clusters of electron-dense vesicles in growing lobes, with two clusters preceding branching.
• Demonstrated that ionophore A23187 disrupts organelle polarization in the presence of CaCl2 or MgCl2, and disperses vesicle clusters specifically with CaCl2.

Conclusions:

• The observed cytoplasmic organization in Micrasterias lobes is characteristic of tip growth, suggesting this mechanism drives lobe elongation.
• Tip-associated vesicle clusters appear to predict sites of new lobe outgrowth.
• High calcium concentrations, but not magnesium, disrupt the vesicle pattern, indicating calcium's role in regulating morphogenesis.