As Many Exceptions As Rules: stem cell

Showing posts with label stem cell. Show all posts

Wednesday, July 31, 2013

Tough As Nails

Biology concepts – keratin, crystalline form, cornification, regeneration, stem cells

Horns are made from keratin, just as are nails, hooves, and hair.
Believe it or not, this is a real picture of a condition called
a cutaneous horn. It is a tumor of keratin producing cells and
can be benign or malignant. They can be removed surgically,
which makes me wonder why it is still on her head.

Thankfully, the cancerous ones are usually wider than they are long,
so this example is probably benign.

The first time I remember being amazed by biology was when I found out that our fingernails and hair were made of the same thing. My hair is (was) red, but my nails are pretty much transparent and colorless. Hair is thin and is broken easily - you should see my hairbrush. But fingernails and toenails are tough. How can they be made of the same material?

Question of the Day – How do fingernails grow to be so tough and why do they grow at all?

You use your fingernails everyday, but do you take the time to think about how amazing they are? It’s true that nails are made of a protein called keratin, just like the dead layers of your skin and your hair, but there are differences too.

You’d be surprised to find out what else they do. Your fingers and toes are basically tools; they grasp, push, pull, and gather information. Switching back and forth between gross and fine movements and manipulations requires that your fingers and toes be able to take a lot of punishment, but at the same time maintain themselves for delicate work.

Your nails protect the ends of your fingers and toes during work that could injure them, this keeps them in good shape for the precise things we need to do all day long. But nails are tools as well. They can slip under surfaces and pry them up, and they can act as a hard surface against which you can apply pressure.

This idea of pressure is important for fingers especially. You push down with your fingertip to deliver a precise amount of pressure. The nail provides a flat, rigid surface against which you can measure the pressure and fine tune your work. Nerves abound beneath the nail, you know that from trimming it too short or from having a splinter driven beneath it. Ouch.

But nails have other jobs as well. Did you know that your nails can be good indicators of your overall health? Healthy nails are smooth and have no ridges, although as you get older you may notice more vertical lines. Yellow nails may indicate a malignancy or fluid in your lung spaces (called pleural effusions). Pitted nails might indicate a connective tissue disease (like psoriasis or scleroderma).

Hippocrates was a Greek physician. His importance to the

profession of medicine is evident – new physicians take the

Hippocratic Oath, promising first to do no harm. In other

words, don’t make things worse. One of his great

contributions was to make the connection between an

outward sign (clubbing) and an internal disease. In this case,

it is usually something wrong in the chest (heart, lungs,

upper GI).

Clubbing of nails as a sign of lung or heart disease was recognized by Hippocrates 2300 years ago. However, clubbing is just an anatomic variation 60% of the time and reflects no underlying disease. What you need to look out for is a change from non-clubbed to clubbed fingers and toes. For a good review, read a 2012 review by Tully et al., you’ll be surprised at what you can learn from your nails.

Even more surprising is the job your nails could do for you if you happen to get the tip of your finger or toe chopped off. Unlike starfish and lizards, we don’t generally have the ability to regenerate lost anatomical units or tissues. One exception is the ends of our digits. If you get a portion of your finger cut off, it might grow back, nail and all. But it must be just the tip, some of the nail must be left. If not, all you can do is learn a new way to type.

A 2013 study has shown that nail beds contain stem cells that can grow back the skin, muscle, and even bone of the digit. A specific signaling pathway (Wnt) is key, and when blocked, there is no regeneration. But if you stimulate the Wnt pathway, you can get regeneration beyond the nail bed. This may be huge for future regeneration of lost limbs in humans.

So these are the things your nails do for you, but we haven’t tackled the question of how they can be tough enough to carry out these tasks.

The white semicircle at the base of your nails is the lunula (luna, as in moon). This is where your nails grow from and the mass of cells that produce the nail is called the matrix. Above the matrix, but below the nail is the cuticle. This connects the nail to the finger and hurts like heck if your cut into it while trimming.

The nail and the hair are made from keratin filaments joined

together. However, in nails they are wide masses of shorter

fibrils and in hairs they are fewer but longer. You can see that

both hair and nails have a matrix that produces the keratinizing

cells, a cuticle, and the new cells push the older cells along to

make the nail or hair longer.

The cuticle and matrix are white because the melanocytes there are inactive, so there is no pigment produced. The matrix cells divide and the new cells produce a lot of keratin protein. The older cells fill with keratin and die, and the new cells push them out of the way – this means toward the end of your finger. This is how your nails grow.

But our fingernail doesn’t look like individual cells. You are sloughing millions of skin cells each day, but for a nail the dead cells all mass together. Nails only wear away or must be trimmed. Individual cells are not lost. The solidity of the nail comes from the connecting of the dead cells together by junctions between the cells called desmosomes, and by the interlocking of the cells like jigsaw puzzle pieces. But there’s more.

Individual keratin protein filaments also become connected so that the entire mass of keratin becomes one solid structure. This is called cornification, like the stratum corneum (the outer, dead layers) of your skin.

One of the two main forms of keratin in your nails is crystalline keratin, which is rigid, stronger, and has an ordered structure - like a tinker toy cube. Transmitted light is less likely to strike an atom and bounce back when the atoms are all lined up, so this is why many crystalline lattices appear translucent. Precious gems have crystalline forms.

The other keratin is more gel-like and connects the different filaments of crystalline keratin together. There are crosslinks that join glutamine and lysine amino acids in one keratin filament to those in many other filaments; the crosslinking is performed by an enzyme called transglutaminase. There are billions more of these crosslinks in nails as compared to those in dead skin or in hair. This is why nails are much stronger than skin or hair.

Transglutaminase has become a favorite of chefs. They

can put different cuts of meat together to forma solid

mass. Also called “meat glue,” transglutaminase can be

used to fuse ground or cut meat into something sold as

a single piece. Health officials worry about this because

it takes outside surfaces that may have been

contaminated an puts them on the inside, which can

promote bacterial growth.

So if this is how nails grow, and it is the same for all your nails, why do they grow at different rates? The average rate of growth is about 3.5 mm (0.14 inch) each month (influenced by genetics, age, and health), while toenails grow about half has fast (1.5 mm or 0.06 inch/month). Toenails are thicker and more rigid, does it take longer to make their cornified structure?

Not really. The answer has more to do with the way the body responds to your behaviors and the environment. Live cells in the matrix produce the keratin before the cells die and are joined together as the nail. Being alive means that they need nutrients and oxygen – things carried in the blood.

Anything that increases the blood flow to an area will allow for faster cell grow and division. This includes heat; your superficial vessels dilate to release excess heat to the environment when your body is in a warmer environment. Dilated vessels hold more blood, so this would mean more growth in those areas. This is why your fingernails grow faster in summer than in winter. They do – you mean you don’t keep track of how often you trim your nails?

This is pianist Liu Wei from China. He lost his arms at

the age of ten when he was electrocuted. I wonder if

his toenails now grow faster than mine. It’s amazing

what people can do with their feet. Tisha Unarmed is

a fantastic video blog where she shows you how she

all her daily chores using her feet. You should check

it out.

There are other things that increase blood flow and nail growth rate. Activity is a big determinant. This is why your fingernails grow faster than your toenails. Muscular movements of the fingers are nearly constant during the day. There is little we do that doesn’t involve moving hands and fingers. All that muscular movement requires oxygen, so blood flow is increased – and the nails grow faster.

This idea is reinforced by the fact that fingernails grow faster on your dominant hand. More use, more blood, more growth. For people that have lost the use of the their arms and learn to write, eat, brush their teeth, etc. with their toes – do their toenails grow faster than their fingernails? If they don’t have fingers, you can’t compare the growth rates of their toenails versus their fingernails, but I bet their toenails grow faster than average.

Blood flow is also increased by trauma; part of the swelling when you whack your thumb with a hammer is due to increased blood flow to the area in an effort to start the healing process. This will also make your nails grow faster. Many scientists believe that everyday uses of fingers, tapping, typing, prying, etc., are all types of microtrauma, so the more you use your fingers, the more blood flow you are inducing.

On the left is Morton’s toe, also called Greek foot. Is my big toe

short, or is my second toe long? On the right, I just decided to

show another deformity I have, called Haglund’s deformity. It

is a bony bump on my heels, and makes it hard to buy decent

hiking boots. Neither picture is my foot by the way; nobody

wants to see that mess.

The one determinant I don’t understand - fingernails and toenails on longer digits grow faster. Your index finger’s nail grows faster than your pinky nail. Is it due to usage? Maybe, but then why do longer toenails grow faster- are you using them that much more? My second toe is longer than my big toe, a condition called Morton’s Toe. When I hike, my second toe pushes off the ground last, so maybe it is bearing more weight and doing more work. I can’t test it though, since my second toenails are always running into the front of my boots and the nails are always splitting and falling off.

By the way, since your nails come from the division of live cells, they only grow while you are alive. The old tale about hair and nails growing after you die is untrue. It may have started because other tissues lose water and retract after death, but proteinaceous (meaning made of protein) structures like hair and nails do not contract. Therefore, they may appear to have grown a little bit after death.

Next week - ever wonder why your grass grows back after you mow, but that tree you cut down probably won't? Believe it or not, it is related to your fingernails!

Takeo, M., Chou, W., Sun, Q., Lee, W., Rabbani, P., Loomis, C., Taketo, M., & Ito, M. (2013). Wnt activation in nail epithelium couples nail growth to digit regeneration Nature DOI: 10.1038/nature12214

Tully AS, Trayes KP, & Studdiford JS (2012). Evaluation of nail abnormalities. American family physician, 85 (8), 779-87 PMID: 22534387

Wednesday, February 20, 2013

Males – Can’t Live Without Them?

Biology concepts – parthenogenesis, gynogenesis, kleptogenesis, sperm-dependent parthenogenesis, pseudogamy, Muller’s ratchet

Last week we introduced the idea that species can be (facultative) or must be (obligate) parthenogenic. Both facultative and obligate species are diverse, interesting, and full of exceptions – what a surprise.

The pea aphid is a wonder of biology. Here, you see a winged male

with offspring nearby. It is hard to tell if these are clonal

offspring, but they are likely to be found in the Fall,

as winged males are produced from late summer eggs.

This is so they can fly to new food if necessary, before

mating and the females laying eggs that will overwinter.

Pea aphids are a wonderful example of facultative parthenogenesis. There are several different cues that trigger parthenogenesis in animals that can produce both sexually and asexually, including temperature, behavior and a lack of males. In the case of aphids, they are only sexually in the summer. The rest of the year they reproduce by parthenogenesis.

Overwintered eggs hatch in the spring and become wingless females. These individuals immediately begin to give birth to clones of themselves, apomictic, thelytokic, parthenogens. These are all females, due to the sex determination system that aphids use, the XX/XO system. When diploid develop, they double their haploid chromosomes, so all are XX females.

The parthenogenic females reproduce quickly, giving birth to dozens of females over a period of just days. These females immediately begin to give birth to more clonal females. The reason it can be so fast is that the females are born pregnant! The process is called telescoping generations, because there is less and less time between birth and birth. This is one form of paedogenesis (paedo = child), reproduction by sexually immature forms.

The life cycle of the pea aphid is complicated, having

both sexual and asexual components. In the spring to

summer, females will produce off spring by

parthenogenesis. In the late summer and Fall, the

parthenogenic females will mate with males and lay

eggs that will hatch in Fall and later eggs that will

hatch the next spring.

In the heat of the summer, the aphid females will undergo a change of their egg production. Adding an extra step to their meiosis reduces their XX to an XO and produce males. These males then mate with the females and they lay eggs that will overwinter to produce next year’s females. Many generations of parthenogenic offspring are interrupted by one generation of sexually produced offspring.

The result is that millions of offspring can be produced from a single female in the spring (although they live only about 10-40 days). A comparison is warranted. If all the offspring from a female lasted an entire summer and they were lined up in a single line, they could circle the Earth more than four times! Maybe there is something to this parthenogenesis.

Bees are also facultative parthenogens, but with a different twist. Bees are haplodiploid, meaning that all the males develop from unfertilized haploid eggs, while the females come from fertilized eggs. Even the sterile female workers are the result of fertilization. The twist comes when in some species, the queen dies without an heir. In this case, some of the sterile worker bees can start to lay eggs. It is a futile effort though, they produce only males because they are sterile and have not mated. The hive dies out anyway.

The exception to this unfortunate affair is one species of South African bee, Apis melifera capensis, who can repopulate by hiring a new queen. The female workers of this species will fight it out when a queen dies, and some will start to produce diploid eggs to produce a new queen by parthenogenesis. She will be a clone of a worker, but she will mate with a male and introduce more genetic diversity into the hive.

Some species of whiptail lizards are females only –

no males at all. But they need the stimulation of

feigned mating to start development of the

unfertilized eggs. So females who have just laid

eggs act as males and perform male behaviors.

Females that are acted on by these “male fakers” are

more likely to lay eggs and have the young survive.

In some cases, females need some help to stimulate egg development for parthenogenesis. In a few instances, this help insures that maximal reproductive success is met. In the whiptail lizard, this takes the form of feigned mating. “But wait,” you say, whiptails are obligate parthenogens – they’re all female! Yep, but after they give birth they have a short burst of male hormones, and start to mimic male behaviors, including mating. The funny thing is, females who are not “mated” by these other females do not produce as many offspring. Something in the behavior helps stimulate more egg development.

Other parthenogenic species need more help to jump-start the egg development. Many species require sperm in order to stimulate egg development. The sperm does not contribute any DNA to the embryo, but it contains a chemical, hormonal, or physical property that makes the egg develop into a whole animal.

If many obligate parthenogens are strictly female, where does the sperm come from? A male of a closely related species usually does the honor, but it doesn’t really matter, since the DNA is not incorporated into the egg. This process has many names, and they all mean pretty much the same thing - sperm-dependent parthenogenesis, kleptogenesis, pseudogamy, gynogenesis – more names than those two fellas on “Psych” (when are they going to bring that show back?).

The triploid Amazon molly fish (Poecilia formosa) is a good example of a gynogenetic species. It is the result of a hybridization of the Mexican and Atlantic molly species, and now lives in harmony with those species in an overlapping habitat It is good for P. formosa that they all get along so well, since they would die off with out the males of the other species. It is the mating process with those males that stimulates the amazon molly eggs to develop and hatch.

The amazon molly doesn’t live in the Amazon River.

It was named for the Amazon warriors of Greek

mythology, an all female warrior society. The amazon

molly is an all female species that reproduces by

gynogenesis. They mate with a closely related male,

but do not incorporate his DNA into the developing

embryo. The sperm is needed to stimulate egg development.

A 2011 study showed that male mollies of a close relative species fertilized P. formosa eggs about 50% as often as the eggs of females of its own species. The authors suggested that male-male competition for females was responsible for fertilization of the P. formosa eggs. These were the losers of the contest for females of their own species, but it really doesn’t matter, since the losers are not contributing DNA to the amazon molly offspring. Therefore, they are not weakening the species. Apparently this arrangement is enough to make P. formosa reproductively successful.

Many times, parthenogenesis is an animal’s only choice, but there are definite advantages to this mode of asexual reproduction. One, the offspring are clones, produced under a certain set of environmental conditions. Since the conditions were good enough to let the mother survive and reproduce. That means that offspring exactly like her should thrive in those conditions too. Little effort – maximum effect.

Two, we talked last week how rapid reproduction by parthenogenesis can help komodos colonize new territory quickly, much faster than they could by sexual reproduction alone. And three, parthenogenesis doesn’t waste community resources and energy on animals that don’t give birth – males. I don’t think I like this advantage.

But there are also definite disadvantages to parthenogenesis. One disadvantage is that the very clonality that helps them in steady state conditions is a hindrance if the environment changes. Genetic diversity is important for adaptation, but parthenogenesis offers no chance for genetic diversity.

Another potential disadvantage to parthenogenesis is the loss of traits that are needed for sex, like mating behaviors, mating calls, etc. An example is a facultatively parthenogenic fruit fly. In 1961 they were separated from males and raised separately. Ten years later they were reintroduced to males. Only some mated, but they still had the genes that controlled mating behaviors. I 1981 they were reintroduced again, and none of the females participated in the mating behaviors; they had been lost completely.

Muller’s ratchet has more to say than just that unused

genes will drift. In terms of becoming parthenogenic, it

does surmise that genes that have to do with sexual

reproduction will mutate at a higher rate. However, it

also states that there will be deleterious mutations in

asexual organisms, resulting in a drop off in births. As

such, the ratchet is a commonly held argument for

why sexual reproduction is so evolutionarily important.

This is evidence for something called Muller’s ratchet. Muller states that if positive evolutionary pressure is not kept on a trait, mutations will build in that trait until it is lost or non-functional. This seems to be what happened in the fruit flies.

One last disadvantage - parthenogenic species seem to last only about 100,000 years on average, probably due to the lack of genetic diversity. However, some salamanders have been gynogenic for 1 million years, suggests that they have had a few indiscriminate fertilizations along the way that have introduced new DNA, about 1 in a million births. Some orbatid mites (1 mm soil mites that help recycle dead material) have been parthenogenic for 100 million years!

Even though species have been parthenogenic for millions of years, it is only in the last few decades that we have really learned anything about these behaviors. Now that we have some knowledge, it seems time to put it to use.

For instance, human eggs can now be induced to develop in the absence of sperm. Before release, pre-eggs are frozen in time in metaphase II stage of meiosis. This means that they are still diploid, it isn’t until anaphase and telophase that the chromatids are pulled apart and the eggs become haploid.

In this stage, if you prick the eggs with a needle on their membrane, or treat them with some chemicals, or apply a mild electric shock, it seems to bring the same response that penetration of a sperm head does. This triggers the initial stages of development in the egg (blastocyst), regardless of the fact that it doesn’t have dad’s DNA.

Under these conditions in the lab, the eggs will develop to the 500-1000 cell stage, and then they will die out. Remember that they do not have the paternally imprinted genes available to them, so they can never become a full-fledged embryo.

Human stem cells are produced by teasing out the cells of a

blastocyst and growing them separately. Then you can treat

them with different growth hormones and make them into

different types of cells. One way to get the blastocyst cells is

from fertilized eggs. But to avoid those ethical headaches,

now scientists often stimulate the egg to develop

parthogenetically, and then harvest the stems cells.

But, they can be teased apart and used as stem cells. Using these human parthenogenic embryonic stem cells (hpESC’s) avoids the ethical issues of creating stem cells from fertilized eggs. In the past five years or so, many efforts have been made to get these pluripotent (can become any time of cell) stem cells to mature into different kinds of cell types so that they can be used for research and as medical treatments.

For instance, one 2012 study showed that hpESC’s could be used to generate mesenchymal stem cells, that had the ability to differentiate into several different type of cells, include bone making cells and fat making cells. They compared the hpESC’s to stem cells generated from embryos and found they expressed very similar marker proteins. Because they are homozygous for immune markers, it is hoped that hpESC’s will be good replacement cells in tissue therapies.

Next week – birds can undergo parthenogenesis, but it is usually not a happy ending, unless you like omelets.

Chen, Y., Ai, A., Tang, Z., Zhou, G., Liu, W., Cao, Y., & Zhang, W. (2012). Mesenchymal-Like Stem Cells Derived from Human Parthenogenetic Embryonic Stem Cells Stem Cells and Development, 21 (1), 143-151 DOI: 10.1089/scd.2010.0585

Alberici da Barbiano, L., Aspbury, A., Nice, C., & Gabor, C. (2011). The impact of social context on male mate preference in a unisexual-bisexual mating complex Journal of Fish Biology, 79 (1), 194-204 DOI: 10.1111/j.1095-8649.2011.03009.x

For more information or classroom activities, see:

Sperm-dependent parthenogenesis –

http://www.fao.org/docrep/005/B3310E/B3310E20.htm

http://www.examiner.com/article/unisex-salamanders-sometimes-say-yes-to-sex

www.crawfordparkdistrict.org/21_Who_s_Your_Daddy.pdf

http://www.pnas.org/content/108/24/9733.full

Mueller’s ratchet –

http://evolutionary-research.net/science/mullers-ratchet/principle

http://www.sciencedaily.com/releases/2012/08/120810083613.htm

http://pritch.bsd.uchicago.edu/publications/bottleneckabs.html

http://www.frozenevolution.com/stopping-mullers-ratchet-hypothesis-advantage-sexuality

http://www.uic.edu/classes/bios/bios101/mating/sld011.htm