#actinophrys

Figure 1: Actinophrys taken by phase contrast microscopy. (Size about 70 microns in diameter; objective 10X.) Photo by: Chitchai Chantangsi.

This protist has a spherical body with many pseudopods, called axopodia, that branch out. Actinophrys are part of a group called heliozoa, or "sun animalcule," because their axopodia look like beams of light coming out of a sun. 

"In the 1960s, scientists wanted to understand how the microtubules arranged and rearranged themselves, and how this impacted the heliozoa's response to changes in the world around them. To get a much, much closer look that we can do here, they used electron microscopy, which revealed that the microtubules are arranged parallel to one another. When they looked at the cross-section of the axopodia, they found that the microtubules formed an interlocking double-coil formation."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

"While axopodia are, in a sense, the cytoplasm poking outwards, there needs to be something underlying those extensions to give them shape and structure. That "something" are hallow filaments called microtubules. Made from a protein tubulin, you can think of microtubules as building blocks for eukaryotic cells, though they're shaped like rods, not actually blocks. Microtubules show up in mitotic spindles, cilia, flagella, and many other pieces of cellular machinery. How they're arranged inside an organism--whether they are unicellular or multicellular--impacts everything from shape to movement. Heliozoa's axopodia are no different."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

Images Originally Captured by Jam's Germs

"Now it's not really hard to see how heliozoa got their name. With their sheroid body and spiky rays, it seems almost impossible to not pick a name related to the sun. Why not refer back to the Greek god of the sun Helios? Of course, heliozoa aren't exactly riding golden chariots across the sky, but they've been similarly difficult to pin down. That's partly because we've come to understand that labeling things as heliozoa might not be as meaningful as we once thought it was."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

Images Originally Captured by Jam's Germs

"After all, axopodia have quite a lot to do. They're not simply stagnant spikes on a heliozoan. They're an extension of the organism's cytoplasm, carry the extrusomes that make their surface adhesive. They contract when the organism needs them to, like when they're drawing in their prey. It's tempting to compare the axopodia to having just a bunch of arms, but the comparison doesn't do the axopodia justice. After all, it's not like we can retract and extend our arms out of our bodies at will."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

"In the 19th century, when heliozoa were first described, the label was thought to apply to just about any spiky, freshwater organisms that looked like spiky, freshwater organisms. The similarity in their appearance suggested a proximity in their categorization, so it made sense to group them together. That has of course changed in more recent years as scientists have turned to studies of organisms' DNA to revise those early classification systems so they better match genetic relationships. Today, "heliozoa" is less of a specific label and more of a description, referring to a set of organisms that are round with stiff pseudopodia or anopodia, the more technical term for those spikes. They are not necessarily closely related to one another, they just look like it."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

"The central, driving life of the heliozoan is in that central body, where food is consumed and the important biological molecules that keep the organism alive are made. What a fine, spherical body it is. But let's be honest: it's the axopodia that makes a heliozoa so striking to lood at. For as long as we've been looking at heliozoa, we have wanted to know more about how those axopodia work and how they're made."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

Images Originally Captured by Jam's Germs

" In the second half of the 60s, Lewis G. Tilney and an assortment of different collaborators published a multi-part series published a multi-part series of papers titled "Studies on microtubules in heliozoa".... "The effect of low temperature on these structures in the formation and maintenance of the axopodia". "Pressure analysis of the role of these structures in the formation and maintenance of axopodia". "The effected of colchine in the formation and maintenance fo the axopodia"... These experiments helped to not only see what happens to heliozoa in changing conditions, they helped our understanding of the microtubules inside them."

Journey to the Microcosmos- Heliozoa: Round, Sticky, and Covered in Spikes

Images Originally Captured by Jam's Germs

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