Sorry for the long absences, but my priorities lie elsewhere, especially since I’m not actively teaching these days. Disclaimer 1: This is an introduction, not a detailed analysis and review of all genetic, epigenetic and biochemical pathways we know of.
The standard ESs are derived from the blastocyst – the stage before an embryo is implanted into the uterus wall – since most of the cells there are undifferentiated. I will not go into embryology and morphogenesis to describe how the adult forms from the zygote. The other characteristic of stem cells is their ability to self-renew to maintain the stem cell population (or increase it in the embryo). One thing we know is that ES cells have a very fast cell cycle: the G1 (growth) phase is very short, so the S phase is reached quickly. Somatic stem cells are different, since they need to actively respond to environmental changes. Other stem cell types show lesser degrees of plasticity, but the fact that they can all be artificially triggered in a culture to go further than their counterparts living in an organism shows that the possibilities are great. But with all the excitement involving iPS cells, let’s not forget about other ways to induce pluripotency, outlined in the diagram below.
Nuclear transfer refers to removing the nucleus of an egg cell and injecting a somatic cell’s nucleus in its place. At the end of the day, the way forward is to further our understanding of how our treatments work at the molecular and genetic level (see picture below). One thing you may notice (and which I touched on already) is that many of these depend on growing cells on petri dishes or in test tubes. One aspect is the genetic regulation of stem cells: what is it about the gene expression in a pluripotent cell that gives it the flexibility to differentiate into all the other cell types? It’s known that several extrinsic factors, such as neighbouring cells, chemicals in the environment, stress and oxygen levels influence stem cells.
We know the fundamental principles behind how stem cell niches work, but we need to find out more exact details on the molecular interactions that go on there, the physical effect of the niche cells on the stem cells and basically how the niche cells manipulate the stem cells – so that we can copy their ways. The reason we can even hypothesise this is because stem cells and cancers share key characteristics, the most important being self-renewal. One source of mutation is reactive oxygen species (ROS), free radicals produced as a result of metabolism. The brain of a mammal grows during its lifetime as a result of adult neuronal stem cells (NSCs).
I will not go into the details of the exact biomolecular controls of the NSCs (overkill), but I do want to bring up integration of the new neurons. As can be seen in the diagram above, fresh neurons go through a specific developmental program, which varies depending on the neurogenic niche. The implications of this are clear for the study of cognition, learning and behaviour, in that we have a clear link between the origin of new neurons, i.e. This can all be studied using genetically-modified mice that produce too many, or too little, or defective neurons, and measuring their cognitive abilities using standardised tests.
You may be asking yourself about what can be achieved using this knowledge, and it’s important not to get carried away.
On the arthropod tree of life, spiders are just one group within the Chelicerata, a group of animals characterised by the possession of a chelicera (and, more informally, by having 4 pairs of legs). The main innovation of spiders is the splitting of the body into two parts: the prosome at the front and the opisthosome at the back, separated by a petiole.
The eight legs are fundamentally identical – there is no differentiation as with crustaceans.
There are 42000+ extant species of spider in 111 families (one was discovered recently and I haven’t updated the slide).
The basalmost extant spiders are the Mesothelae, of which there is only one extant family in SE Asia. The Opistothelae are the spiders we all know, and they fall into two groups: mygalomorphs and araneomorphs. The Mygalomorphae include tarantulas and trapdoor spiders – the big hairy spiders, if you will. Araneomorphae are characterised by several innovation in their silk apparatus, most importantly the acquisition of a cribellum, a plate over which silk is laid out, and the calamistrum, hairs on the back pair of legs that sculpt the silk. With the evolution of the araneomorphs, new types of traps could be built, from basic sheet webs to messy space webs to the highly-elaborate orb webs. As always with evolution though, some spiders eschewed such silly things and dumped silk use in prey acquisition altogether, preferring instead to hunt. But obviously, the biggest innovation in spiders is their use of silk to build traps – even hunting spiders will use silk to make draglines or help stabilise themselves or a prey. Viscid silk that forms the capture spiral, on the other hand, is more of an elastic product, able to handle a lot of strain without breaking.
This is why large insects can fly into an orb web at full force without the web breaking: the viscid silk absorbs the impact, and its rebounding from the impact is supported by the major ampullate silk of the frame. As already mentioned, spiders have different spinneret types for producing the various silks.
Taking the phylogenetically-determined variations in silk types, spinneret types, spinneret arrangements, and spigot modifications leads to the large diversity of spider webs that are observable, from the messy sheet and space webs (Eresus, Linyphia in the diagram above, respectively) to the highly-elaborate and infinitely customisable orb web (Araneus).
Regardless of how they gain their prey – by hunting, by ambushing, by trapping – all spiders have the same fundamental feeding mechanism (even that weird vegetarian one). The venom can contain many components but the active ingredient is a potent neurotoxin (or several, which is why it’s hard tog et an antidote sometimes). All that said, however, the majority of spiders are not strong enough to pierce human skin and are thus completely harmless. However, there are some spiders that do pose a threat by being able to pierce skin and thus are able to inject venom and be lethal.
That said, some spiders, notably tarantulas, have another way to hurt you: urticating hairs. After any mating ritual is done, the male puts some silk at the bottom of the pedipalp, then dips it in his gonads to fill it with sperm. The human equivalent is ejaculating into your hand and inseminating your partner by fisting her. However, mating is not as simple as that in many spiders, as the male runs the risk of being eaten in an act of sexual cannibalism. There is also pre-copulatory cannibalism where the male gets eaten during the mating ritual. Once the eggs are fertilised, they are ejected and develop in a coccoon that is either hidden somewhere (e.g. Such extensive childcare is a stepping stone towards sociality, which a few spider species have achieved – 25 as of last count.
However, new fossils like the one in bottom center made it clear that Megarachne was not a spider at all, but a eurypterid, a sea scorpion, reconstruction at the bottom right.
And just so you get an overview of the completeness of the spider fossil record, here is the most up-to-date diagram.
I will still post stuff, but it will be at a relaxed pace since I only have the time to write during random bouts of insomnia.
I will break my usual rules and actually pander to my audience by focusing more on implications for humans and medicine.

Interesting as they are, I do not feel they belong with a scientific introduction to stem cell research, since it might turn this into a festering cesspool of philosophical wankery.
This has several reasons, from the history of embryology to the overall diversity of stem cells. It can generate the three germ layers (endo-, meso-, ectoderm), germ cells and also the trophoblasts (the cells lining the blastocyst; they provide nutrients to the embryo and later develop into part of the placenta).
There is another, recently discovered source of pluripotent stem cells, namely from the epiblast (epiSC). In adults, most of the organs also have stem cells (called stromal stem cells or mesenchymal stem cells (MSC); I will be referring to them as somatic stem cells).
Their differentiation range stays the same after renewal: a unipotent stem cell will not generate a multipotent one. Going through such a reckless cell cycle would cause cancer, so their renewal is much more regulated.
Sometimes, it is as simple as putting one cell type in another tissue – as with fusing blood cells with liver cells to repair liver damage – but this is the exception rather than the rule, at least in humans (animals capable of regeneration are extreme cases). These are adult stem cells that have been artificially grown to become pluripotent (by changing their gene expression to match that of ES cells). IPS cells hold great promise for personalised medicine, since there is no risk of host rejection. Of course, this doesn’t make the task easy, and all these processes are rather inefficient. This egg cell can then be used to impregante an animal, giving rise to a clone , or it can be grown in culture as a source of embryonic stem cells (theis process is also referred to as SCNT: somatic cell nuclear transfer). The mechanism behind this is not fully understood, but it does have something to do with the epigenetic state of the ES cell. Therefore, what we find out using them does not reflect in vivo conditions – this is something that should always be kept in mind, and is one of the reasons why animal and human testing is so important. Before getting to any medical application, the biological underpinnings of any treatment have to be very clearly understood. Understanding the pathways behind these interactions is of paramount importance in order to develop effective stem cell therapies, and might even lead to understanding disease, as in the case of cancer, where many of these pathways are switched off. We know bone marrow is a niche for hematopoietic stem cells (more specfically, the surface of trabecular bone). This is another one of those hotly debated fields, with one side claiming that cancer stem cells are regular stem cells that have acquired mutations in its lifespan, while the other side claims that stem cells can be genetically predisposed to initiate cancer. Free radicals are the sluts of the chemical world, in that they will react with any chemical, including DNA. In stem cells, they can also show reduced self-renewal rates, reducing the repair rate of the involved tissue. Cancer notwithstanding, some have seen the potential to cure blindness, diabetes and neurodegenerative diseases.
The whole process consists of adult neuronal stem cells dividing, differentiating, maturing and finally getting integrated into the brain’s circuitry.
One tricky thing about them is that while they can be retrieved from all areas of the nervous system, it’s only in the SVZ and the SGZ that neurogenesis has been observed, making those two areas the neurogenic niches. We know that there are correlations between SVZ and SGZ neurogenesis and cognition, mood and various CNS disorders (notably epilepsy, strokes and Alzheimer’s). This egg cell can then be used to impregante an animal, giving rise to a clone , or it can be grown in culture as a source of embryonic stem cells (these are then referred to as SCNT: somatic cell nuclear transfer). The phylogeny above names all extant chelicerate orders; chelicerates evolved in the oceans, but only the basal Pycnogonida and Xiphosura remain marine animals. The Acari are the most species-rich with 55000 species, followed by the Araneae with 42000+ species. The prosome is the head, contains the brain, limbs, mouthparts, and eyes, and is covered by a carapace, the peltidium. They’ve mostly been lost and replaced by a tracheal system similar to the insectan one described in this post.
Going into their detailed systematics would require its own article, so I will just outline their most basic divergence. The diagram above showcases the tight correspondence between evolutionary history and predation style in spiders, the latter of which is directly linked to silk-weaving capabilities. This is the most basic dichotomy when discussing spider ecology: is it a hunter or a web-builder? All spiders can see (except those unique species that have gone blind), and the basic toolkit is to have six eyes, although some can have as many as eight, and others have four, and others only two. For example, in the salticids, the jumping spiders acknowledged as having among the best eyesight in all spiders, the immobile eyes are used to detect motion. Spider silk is made of fibroin proteins, and comes in 6 major types, each produced by a different spinneret type.
In fact, when humid, MA silk is around 5 times stronger than steel – this is why spider silk is under intense research for biotechnological and biomimetic pruposes, with the major stumbling block to industrial application being that we can’t get any spider to overproduce its silk on demand.
This is also what enables the orb weaver defence mechanism of shaking its web so violently that the spider becomes more or less invisible (try it next time you see an orb weaver: make your presence known and watch the spider vibrate the web!). Going into how each one works would go outside the scope and level of this article, but suffice it to say that differences between spider families arise from the arrangements of each spinneret type – like eyes, their positions can also vary.
Silk is first made as a liquid in the silk glands, where the tail is, and stored in the lumen. As you can see from that table of contents from a review paper, they can affect any possible section of the nervous system.
At most, these spiders can just bruise you – common examples of this type of spider are the only aquatic spider, the diving bell spider Argyroneta aquatica and the yellow sac spider Cheiracanthium punctorium.
The Sydney funnel web spider Atrax robustus and the Brazilian wandering spider Phoneutra fera are probably the most dangerous spiders, while the widow spiders of the Latrodectus genus and the brown recluse Loxosceles reclusa are dangerous for being rather common to find in urban habitats. These are sharp, needle-like hairs that they can eject at high speeds and that become embedded in your skin.
These are digestive enzymes, and they basically turn the insides of the animal into a pudding or a slushy. There is no chewing invovled – this is why you will find spider webs or spider burrows full of empty insect exoskeletons. Reproduction in spiders is interesting because while the female has a vagina, the male has no penis. As I explained in my post on natural selection, in some cases, males have evolved a behaviour where they actually jump into the mouth of the female and get eaten willingly, and this increases their chance at fathering offspring, therefore conferring a selective advantage to this seemingly destructive behaviour. This has the same advantage for the female, but means that the opportunity for reproduction is gone, which is risky since another one may not come again. These social spiders share a familial web, cooperate in foraging for food and in raising offspring.
This is when a web is shared but among unrelated aggregations that may not even be of the same species. As you can see, their earliest fossil record goes back to the terrestrialisation period in the Devonian, and no truly large gap exists, compared to the fossil record of insects. There is little developmental biology and I will not talk about organo- or morphogenesis (as I would normally do when talking about stem cells).
I will be using the definition of stem cells that the public understands to avoid confusion and overcomplication: a stem cell can self-renew and it can differentiate into one or more cell types.

They can be cultivated and maintained in their undifferentiated state in a cell culture, since there is no limit to how much they can renew themselves. They can also be taken from primordial germ cells (EG), which are the precursors to the gametes (sperm and egg). However, they are limited in their differentiation range, in that they can only generate mesodermal tissues (e.g. When discussing stem cell renewal, it is important to note the difference between potential and reality: neural stem cell cultures can be grown to enormous sizes, but those renewal rates are never observed in vivo. They need to be able to go into resting phases for a stretch of time and be activated when needed (activated means they need to get back to a regular cell cycle). Outside of the mesoderm, there is evidence that bone marrow stem cells can differentiate into neuronal cells (germ layer: neuroectoderm) and to epithelial cells (endoderm). The potential here is amazing and basically gets rid of the only argument moral opponents of stem cell research ever put forward, since  no ES cells will need to be harvested.
Talking about the possibilities iPS cells offer us will take up pages, so I’ll just let the following picture do it. However, it is generally not preferred due to its inefficiency and the resulting tetraploidy – making them diploid again is too risky (because of genome destabilisation). They are used to treat leukaemia and bone marrow failure,as well as to help rebuild the immune system in patients undergoing chemotherapy and radiation therapy. Pieces of the answers are pouring in every day thanks to efficient DNA sequencing technology and the associated growth of bioinformatics, and we now know about the genetic core controlling pluripotency (see figure below). It could be due to their gene expression patterns, which are controlled by transcription factors. They are found in specialised microenvironments, where stem cell growth and function is regulated and maintained. The importance of this field is crystal clear: it can lead to novel cancer-fighting strategies (see diagram below). Figuring the control mechanism in stem cells could give us hints about what causes cancer, or just tips for a cure that stops tumour growth. Stem cell mutations are more dangerous than any other ones, since they will get passed on to the most daughter cells – this may be a tumour-starting process!
Stem (and other!) cells have developed ways to minimise ROS damage, such as detoxification enzymes (all controlled by gene expression).
For example, depriving an organism of olfactory senses (smell) will prohibit the proper growth of olfactory neurons in the SVZ. This is the essential part of integration: these new neurons are not subjected to any different treatment, but go through the same biochemical control as the already-established neurons. But what must always be kept in mind is that there may be other sites of neurogenesis we haven’t discovered yet!
Finally, we can customise viruses to infect somatic cells and induce pluripotency (the iPS cells I mentioned earlier are the products of this treatment). The pedipalps are almost like an extra pair of legs, and the opisthosome still has a faint segmentation, unlike the Opistothelae. Mygalomorphs, with their rudimentary toolkit, are mainly burrowers, their use of silk usually limited to lining their burrows and to make simple traps or alarm lines (a line of silk that vibrates when touched by a prey, alerting the spider to its presence).
It should be noted, however, that hunting spiders have not lost the ability to use silk – many use draglines as stabilisation help when hunting, for example. On the last segment of the pedipalps, they have a tarsal organ, which is used for sensing temperature and humidity, and most probably for chemosensation as well. Fossil and extant species of the genus Leptopholcus in the Dominican Republic, with the first cases of egg?parasitism in pholcid spiders (Araneae: Pholcidae).
If something is detected, then the spider turns its head and uses the anterior median eyes to focus and judge the distance to the prey.
The spider uses two types of silk to build that: major ampullate silk (red in the diagram) and viscid silk (blue). When silk needs to be used, it gets squeezed out through the funnel and into the duct, where water and ions get absorbed out of it, similar to how ureine is processed in the kidneys. In order to do this, they bite the prey and inject a venomous cocktail through a hole in the fang.
There are other parts to the cocktail too, and I recommend reading the review linked below.
As the diagram on the right shows, they cause at least an inflammation (a rash), which can last up to a week (personal experience). This is a feeding method called extra-oral digestion, as opposed to typical internal digestion where the food is broken down into its nutrient parts inside the digestive system.
Some spiders carry their eggs on their backs or in their mouths, and some rarer ones even carry the first couple of instars on their backs too, as in the picture above. The picture at the bottom of the slide shows an example of a social spider web, from Anelosimus eximius. Besides sharing the web, no cooperation takes place, and the web is split into territories, but wars generally don’t break out.
A fossil spider (Araneae: Pisauridae) of Eocene age from Horsefly, British Columbia, Canada.
At the time of production, Megarachne was interpreted as a spider based on the original fossil, pictured above in the lower left – it is admittedly very spider-like. Of course, making stem cells achieve their full potential under physiological conditions is one of the ultimate goals, but in order to do that, we first need to understand how the renewal process is controlled.
A bone marrow transplant is nothing more than the injection of multipotent hematopoietic cells from the bone marrow or blood. One important element of this is determining how big the role of genetics is and how big the role of the environment is in controlling a stem cell’s behaviour. But the most integral part of the niche are the niche cells, which control the environment by secreting chemicals, regulating oxygen levels and physical factors – all in order to make the stem cells feel comfortable. By the end of it, you have solid silk that gets ordered throught he valve and sculpted by the spigot that shoots it out. They can be multipotent, like hematopoietic cells (stem cells that can only differentiate into the different types of blood cells) or unipotent, like skin cells (they can only differentiate into more skin cells, which is what happens when you cut yourself).
Stem cells, like cancer cells, express telomerase, defending them from telomere erosion (but placing other risks which we don’t yet understand). We still do not know how to control them: inducing cardiac stem cells will not automatically regenerate a heart. Nevertheless, we always must keep our feet on the ground, especially when messing around with the brain (granted, all this has been investigated only in rodent brains, not humans).
It’s at the last stage that final chemical modifications are made, from changes to the protein structure to adding glue droplets as needed.
If we find out what’s being repressed, we can get another way to manipulate stem cells.

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