Tag Archives: photography

Book Excerpts – Introduction to the Cytoskeleton


Eukaryotic cytoskeleton shown using fluoresecence microscopy Actin filaments are shown in red, microtubules are green and the blue is the nucleus. Picture from

If one consults any of the standard developmental biology textbooks, the cytoskeleton will be briefly presented as the support structure of the cell and about as interesting to most biologists as the floor struts in a ballet school would be to an artistic director seeking the next prima ballerina. However, to think of cytoskeleton only as the structure on which cells do their remarkable things, leaves out a good part of the story of how cells work.

We prefer to think of the cytoskeleton as a troupe of acrobats. They run about in the cell, come together, stack to form some remarkable structure and then as rapidly as it appears, the structure falls apart and/or moves and the acrobats appear elsewhere on the stage. These acrobats are the individual proteins that make up the cytoskeleton, and the many proteins that bind to cytoskeleton, transiently hold it together, or move along it. The surface area available on the cytoskeleton filaments for molecules to electrostatically and chemically bind is huge, far exceeding that of all cell membranes in an organism. Ignoring the cytoskeleton while doing molecular biology would be like trying to choreograph a classical ballet on the same stage that has a troupe from Cirque de Soleil rehearsing.

This unfortunate attitude towards the cytoskeleton is beginning to change. For example, several of the pharmaceutical scientific supply companies have recently begun manufacturing kits designed specifically to detect the phenomenon of “cytoskeletal rearrangements” because all kinds of interesting other stuff seems to happen precisely when these rearrangements occur and therefore the rearrangements can be used to time such events. We find it ironic that only a few people seem to actually think this is more than a convenient coincidence.

The cytoskeleton is, as the name implies, the skeleton of the cell giving it shape and form. The concept goes back to Nikolai K. Koltsov in 1903. Koltsov formulated the idea that the deviation of the shape of a cell from the simple ball, that one would expect if it were a liquid drop, is caused by a stiff but elastic cytoskeleton within the cell. Cytoskeleton is nowadays more precisely defined as the various structures that are filamentous polymers of a single class of protein. These filamentous polymers have long-range order within the cell. They should have been called metapolymers, because the proteins they are made of are themselves polymers of amino acids. These cytoskeletal polymers are commonly classified as supramolecular structures. They are polymers of polymers, roughly topologically linear in their structure.

There are three types of cytoskeletal filaments, arranged in order of decreasing diameter as seen by transmission electron microscopy: microtubules (MTs, 25 nm), intermediate filaments (IFs, 11 nm), and microfilaments (MFs, 7 nm) (nm = nanometer = 10‑9 meter = 10‑3 µm). For things this small, we find it convenient to think in terms of the size of the hydrogen atom, whose diameter is 0.1 nm = 1 Å (Angstrom), so a microtubule, for instance, is 250 hydrogen atoms wide. For something so thin, microtubules can be incredibly long, with reported lengths up to 68 µm (micrometers). In Drosophila (fruit fly) species, the length of the sperm can be 5.8 cm (58000 µm), implying equally long microtubules. Such microtubules are therefore over 2 million times longer than their diameter. If we imagine a microtubule as a piece of cooked spaghetti with a width of 3 mm then the length of the spaghetti would be as long as 7 kilometers! The three cytoskeletal filaments are found throughout the cell. They are connected to each other at many points, as well as to the inner surface of the cell membrane , to the outer nuclear membrane and to the inner surface of the nucleus via specialized attachment proteins.

There is a growing awareness of other proteins that may well also be cytoskeletal in nature and deserving of addition to this triumvirate. The cytoskeleton, as a concept, can also go beyond just filaments to include structural sheets, patches and meshes. For example, septins, which are found in animals, are an example of these other cytoskeletal-like proteins. Septins use a protein called anillin to attach to microfilament rings and thereby form rings themselves. These rings line the inner surface of the cell membrane, increasing the membrane’s rigidity. Another example is CTP synthase, which forms filaments in bacteria, yeast and animals. We will nevertheless concentrate on the three best known, best characterized and universally accepted cytoskeletal components. However, the reader should keep in mind that there are likely to be more types of cytoskeletal-like structures that need to be considered in embryogenesis. We will also focus on the mechanical properties of cytoskeleton. There are poorly understood electrical and related potential memory/epigenetic properties of cytoskeleton. These properties may also become important in explaining aspects of embryogenesis.

The cytoskeleton is an extremely dynamic structure that can depolymerize, move and repolymerize within the cytoplasm in a matter of minutes. The three components are often interlinked in a meshwork that is required for cell movements, maintaining or altering cell shape, and for organizing and powering mitosis and meiosis. The cytoskeleton is required for transporting and sequestering proteins to specific regions of the cell. Cytoskeletal associated proteins act as levers and motors pulling cargo such as the vesicles involved in nerve transmission, chloroplasts and whole nuclei.

The cytoskeleton also serves as the host for a variety of enzymes. The cytoskeleton is directly involved in protein functioning of these enzymes by binding to (and thereby changing the functioning of) associated enzymes. The cytoskeleton can therefore direct a response to external physical or chemical signals in the form of movement of the cell towards or away from the signal by polymerizing or depolymerizing during cytoskeletal rearrangements. Due to the cytoskeleton, external signals can even travel down to and into the nucleus and trigger changes in gene expression.

The cytoskeleton found in the nucleus is also made of actin, microtubules and microfilaments but it requires special consideration. Bundles of microtubules ring the inner membrane of some nuclei. The nucleus contains additional unique protein attachments that connect it to the cytoskeleton. At the inner surface of the double nuclear membrane is a sheet of intermediate filaments called lamins, whose functions may include maintenance or change of nuclear shape, protection of the nucleus from mechanical shocks, and intranuclear rotation (rotation of the contents of the nucleus). Intranuclear rotation includes both nuclear membranes, but without causing the cytoplasm to rotate. Therefore there may be a layer just outside the outer nuclear membrane that has low viscosity, like the cell cortex. If this is the case, the motive force is occurring on the outside surface of the outer nuclear membrane. In some cells nuclear rotation and cell membrane rotation are coupled.

The cytoskeletal elements in the nucleus combine with a variety of nuclear proteins forming a “nuclear matrix” which acts like a scaffold on which the DNA is attached. For this reason the whole lot is referred to as the “nucleoskeleton”. The nucleoskeleton itself is then connected to the cytoskeleton that is outside the nucleus via proteins that bridge the two nuclear membranes.

These three filaments, tubulin, intermediate filaments and actin were once thought to be unique to eukaryotes. Many old textbooks included a table of differences between eukaryotes and prokaryotes and the presence of cytoskeletal elements figured prominently as a defining characteristic of the eukaryotic cell. This has proven to be incorrect, and as we showed in Chapter 1, many characteristics of eukaryotes are found in, if not inherited from, prokaryotes. Homologs of all three cytoskeletal proteins have been found in bacteria, albeit usually having different functions. There is a form of tubulin found in bacteria called FtsZ that is also required for cell division in most prokaryotes (but not all), which may represent the evolutionary precursor of all of the tubulins

.This book excerpt is introduction to Chapter 5, “The Cytoskeleton” in “Embryogenesis Explained”.

Video of the Honouring of Jack Rudloe during the Joel Sartore opening.

We previously blogged about our visit to the National Geographic museum and how we were guests where our friend Jack was honoured.

Here is a wonderful video of Jack Rudloe and his being honoured at the Joel Sartore PhotoArk exhibition opening. Dick and I were privileged to appear and we made what I like to think of as a couple of cameo appearances. What a great day!


Jack Rudloe Honoured at Joel Sartore’s Photo Ark Exhibit Opening

Dick & Natalie with Jack at Nat Geo

Today was one of those really nice days I will remember for a long time. Our friend and winter host Jack Rudloe was honored by National Geographic’s Joel Sartore in his Photo Ark Exhibit for Jack’s life long conservation and education efforts at Gulf Specimen Marine Laboratory. We were Jack’s guests for the event. We were even escorted in past crowds by a cheerful young woman who recognized us from Dick’s picture. Before the talks we wandered the halls and we found an image of the cover that Jack and his late wife Anne did on sea turtles, Race for Survival. We found out that their article on the grave state of sea turtles marked turning point for National Geographic. With that article National Geographic moved away from simply reporting on neat stuff around the world and started talking about the environment.


Joel Sartore’s exhibit is called Photo Ark. We were very privileged to be part of the reopening exhibit ceremony. Sartore’s  goal is to create pictures of every species in captivity which is about 12,000 different kinds of animals. He does them all, birds, crabs, octopus, bud and elephants. The pictures he creates are stunningly lovely and intimate. The purpose to to raise awareness about the environment and to try to get people to stop doing the things that cause extinction. Among the causes are habitat loss, poaching, eating animals for bushmeat, and the nonspecific catchall of climate change. The solutions Sartore offered in his lecture was to reduce consumption, recycle, eat less meat, donate money to good organizations that work for preservation of animals like zoos and rescue organizations, and his own National Geographic, and be aware of what you buy and how it impacts on the natural world. He gave two specific examples of awareness, palm oil from farms created by the destruction of old growth forests in Indonesia and buying furniture made from the wood harvested from old growth forests world wide. He was also a truly engaging speaker. We laughed, we were sad, we were uplifted and inspired.

In some ways the talk about solution was simply pap. I don’t see how recycling cans in Washington DC can stop poaching and slash and burn practices in Indonesia. I also noted a strange irony in that the National Geographic headquarters proudly proclaims it is a carbon neutral building powered entirely by wind generated electricity. That is a trick because it is in downtown DC with no visible windmills. So I assume this means they produce/purchase the equivalent electricity elsewhere. And, of course, one of Sartore’s other heroes is trying to preserve prairie grouse that are being gravely impacted by several dangers including windmills. Given all the trouble Germany has had in its failed efforts to live entirely on wind electricity, and how the result is using more coal, not less, and all the other negative impacts of wind power generation which cannot survive without massive government subsidies using money we could arguably be better using elsewhere, National Geographic may well have simply traded one kind of environmental damage for another that might be even worse. Still, I really liked how Sartore’s talks emphasized what dedicated individuals can do and his uplifting success stories. He is very right that being positive makes people want to be participants in change whereas constant doom and gloom makes people give up trying.

I have spent many hours in the wilderness myself. I know that people who live in the city simply have no idea about what diversity in habitat is. In 2001 my husband and I found a 152 acre parcel of land for sale for $14,500. We had to borrow to buy it, but we did and we have never regretted it. This little bit of land is in prairie parkland, the transition zone between boreal forest and tall grass prairie and it has species from both zones. The land has never been broken and the abundance and diversity to be found there is nothing short of astounding. Because the land is used by breeding red headed woodpeckers (along with four other nonendangered species of woodpeckers) we were able to put it into a conservation agreement. Once the agreement was in place, the property was given a detailed species survey by scientists and among the species were three tiny bladderworts and two types of sundews considered “species at risk”. This makes a very important point. We were able to put the property in a conservation agreement only because the government regulations allow us to preserve habitat for red headed woodpeckers. We inadvertently ended up getting government to protect five other species of plants that were in far more trouble. All the other solutions, like recycling your drink cans and eating less meat are insignificant next to the benefits of setting aside tracts of land and preserving habitat. And this can be done by lobbying government and simply going out and buying a chuck of land yourself. I was slightly irritated that Jack’s well deserved honour for environmental education did not include any mention of Jack and his late wife Anne’s tireless efforts to preserve salt marshes and how they have been attributed personally for saving 35,000 acres of salt marsh in Florida’s Big Bend and Forgotten Coast region.


All that being said, I still 100% support Joel Sartore’s work. There is a simple reason for it. Joel Sartore’s stunning images make all kinds of animals come alive. They make people care about the environment. While I may disagree with some of his emphasis on his proffered solutions and I might be willing to point out some of the inconsistencies of the National Geographic’s efforts, getting people to care is the most important things one can do. If people care, solutions will come even if we make a few mistakes (like windmills) along the way.

And I was delighted to discover that habitat preservation is one of National Geographic’s strongest positive aspects. In addition to the Sartore exhibit, they had an exhibit on the oceans and their effort to increase protected pristine ocean areas from 2% to 10% of the total. National Geographic “gets” it when it comes to habitat preservation. And so overall it was a wonderfully positive day being treated like a VIP,  seeing a dear friend get a well deserved honour, enjoying a chance to see how his work impacted a major organization and to be surrounded by people who care. And best of all was watching children, dozens of children, laughing and pointing and running around connecting with animals that came alive in their minds because of Sartore’s fabulous, intense, captivating images. You can see more, including many taken at or provided by Gulf Specimen Marine Laboratory, here.

Common octopus (Octopus vulgaris) at Gulf Specimen Marine Lab and Aquarium.

Common octopus (Octopus vulgaris) at Gulf Specimen Marine Lab and Aquarium. (with permission)