The average adult human body contains somewhere between 50 and 100 trillion cells. That means if Mark Zuckerberg took all of his $17.5 billion to the bank and had it converted into pennies, he probably wouldn't have enough coins to equal the number of cells in your left foot. So where do these trillions of tiny machines come from? If you trace them back far enough, they all originate from embryonic stem cells like the one above. Read on to see more of the amazing, infinitesimal workhorses that make your body what it is.
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Stem cells are pluripotent, which means they can copy themselves to produce various other types of body cells. And they're not just seen during fetal development -- mature human bodies have their own stem cells, fittingly known as adult stem cells. The body uses these cells when it needs patch-up work. For example, stem cells found in skeletal muscle tissue may produce new muscle cells to repair damage done by stress or injury.
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This is a human neuron. The central mass in the photo above is the heart of the neuron, known as the soma, and the branches extending outward are complementary components called dendrites and axons. Neurons are the standard building block of the nervous system. So what do the neurons in your body and a string of Christmas lights have in common? They're both electrically active, meaning they send, conduct and receive electrical current.
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This is a colored slide showing an olfactory receptor neuron and its smell receptors, also known as cilia. Olfactory neurons are the cells that let your brain know when you need to take the garbage out. Next, you'll see some charming cells from the inner ear.
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When sound penetrates the human ear, it causes waves to form in a tiny sea of fluid called endolymph. The ripples in the endolymph wiggle the inner ear hair cells shown above, and the cells send electrical signals to the brain, which interprets the motion as sound. Scanning electron micrographs (SEMs) like these give us incredibly detailed views of the smallest parts of the human body. Check out another neuronal SEM in the next slide.
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Have you ever wondered what body cells are responsible for the "you" part of you? Not your body, but your mind, your consciousness, your present self-awareness. If you have, look no further. This is a nerve cell embedded in brain tissue -- specifically, the grey matter that controls your conscious thoughts. Every time you reach a conclusion or make a decision, you can thank millions of these things for making it possible.
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The wires inside your electronic devices tend to have nonconductive sheaths, right? Well, the wires in the human nervous system need the same thing. Oligodendroglia like these are designed to form myelin sheaths to surround electroconductive axons in the brain and the spinal cord.
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It's the stuff that brains are made of. This is a human glial cell, magnified to X13,000. Neurons do the thinking, and glial cells keep them happy, providing the structural support and physical protection that neurons need. Glial cells also help supply myelin. Next, you'll see a fibroblast.
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Fibroblast cells are important in your body's connective tissue, cranking out collagen proteins that are the building blocks of many body tissues. Next, you'll see a very familiar cell up close.
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They're great swimmers, they change colors and they love oxygen. What are they? Red blood cells, of course -- but do you know what they actually do? Red blood cells are the high-speed couriers for all of the oxygen your body tissues need. Since your feet can't breathe for themselves, red blood cells carry freshly inhaled oxygen from your lungs all the way down to the tips of your toes.
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Leukocytes, better known as white blood cells, are essentially the body's security guards -- but not the friendly kind you might have met at a school or a shopping mall. These guys are more inclined to kill on sight. When dangerous foreign objects like viruses invade your body, the many various leukocytes work together to terminate with extreme prejudice. This particular cell is a B-lymphocyte, which produces natural antibodies.
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This is another immune system cell, the neutrophil granulocyte white blood cell. Neutrophils scout ahead of the main defense forces, swarming at inflamed areas of the circulatory system and literally eating bacteria alive. In this photo, a neutrophil extends a killer arm called a pseudopod to engulf the foreign body and eventually consume it.
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Sometimes, the immune system malfunctions and sabotages the human body. These are the B-lymphocytes of a patient with leukemia, which is a blood cell cancer in which the bone marrow produces a large number of improperly-formed white blood cells that hurt the body rather than help it.
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Another type of immune system cell is the dentritic cell, which is known as an antigen-presenting cell. One could consider these the spies or intelligence officers of the body's defense grid. They grab foreign objects, chew them up, and then present the pieces to other immune system cells so they know what they're up against. This process shapes the way the body responds to such threats.
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Just as the beams that hold up your house get weaker every year, the bones in your body suffer structural deterioration. Luckily, your body knows how to remodel its own skeleton: Cells known as osteoclasts and osteoblasts work as a team, breaking down and absorbing old bones while building new bones to replace them. This SEM shows an osteoclast reabsorbing bone material.
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This is an osteocyte at a magnification of X5520. When an osteoblast -- the cell responsible for building new bone tissue -- matures and becomes embedded in the mineral matrix of the bone, it becomes an osteocyte. Click over to the next page to see the human heart up close.
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Run far enough and eventually your calves will get too tired to keep going. No matter how many marathons you finish, the muscles in your legs will always look like wimps compared to the muscles in your heart, which never get too tired to keep pumping blood. This is a highly magnified image of the fibrils of cardiac muscle in a healthy human heart.
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Muscle cells are known as myocytes. In addition to the cardiac muscle cells from the previous page and the skeletal muscle cells that come to mind when you hear the word "muscles," there are also smooth muscle cells, which you can see on the next page.
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You can't flex them in the mirror, but they're just as important as your biceps or your abs. Smooth muscle cells make up various tissues inside the body, including the walls of blood vessels and the lining of the digestive tract. They do most of the internal exercise that you don't have to think about. Next, you'll see a dust-eater.
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If you can see the dust, soot and other particles in the air, you know you're inhaling all of it. But where does it go? An alveolar macrophage, also known as a dust cell, lives in the lungs and exists to gobble all the garbage you breathe in along with the air your body needs. Next, you'll see the cells that make teeth.
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There's a reason one of the most commonly reported nightmares is about inexplicable tooth loss -- in primitive circumstances, our teeth were crucial to our survival, and they're still an important symbol of general health and wellbeing. Humans develop teeth with the help of ameloblasts, which are cells that put down the layers of enamel that make teeth rigid and strong.
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Now here are some cells that many Americans wouldn't mind losing a few of. This is adipose tissue, a.k.a. fat, magnified to X70. As an adult human gains weight, fat cells like this tend to enlarge rather than multiply. While having oversized fat cells can be undesirable for health-related or aesthetic reasons, fat storage is also an important function of the body that has helped humans survive in hard times.
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This SEM image shows a kidney glomerulus, which is a structure that filters impurities and waste products out of the bloodstream and passes them on to be diluted and disposed of in urine. Next, you'll see a pancreas cell.
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Don't let its wart-speckled exterior fool you -- this is a very useful and friendly cell. Acinar cells like this one, located in the pancreas, secrete digestive enzymes that dissolve all of the foods you eat and turn them into globs of the basic nutrients your body tissues need in order to keep in good condition.
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Epithelial cells are numerous in the human body, typically covering the inside surfaces of cavities that are involved in secretion, absorption, or barrier protection. The cells above are shown lining an area of bronchus, which is a major airway of the lungs. Also pictured are several goblet cells, which are epithelial cells that secrete mucous.
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Maybe it's not worth fretting over that scar or pimple -- everybody's skin looks really weird up close. This is a highly magnified image of a human hair follicle extending from the skin, showing the scaly layers of squamous epithelium surface cells all around. Click ahead to see a sweat gland.
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It's no fun to get nervous and sweaty before a job interview or a first date, but sweat is a vital tool the body uses to excrete electrolytes and keep itself cool. The sweat gland secretory cells shown above are an excellent example of stratified cuboidal epithelium. Next, you'll see a rogue agent.
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Human cancer cells arise naturally in the body, sometimes caused by genetic predisposition and sometimes by environmental factors, such as exposure to toxins, radiation and certain viruses. In addition to what modern medicine has to offer, your body possesses internal defenses against cancer, such as a group of lymphocytes reassuringly called "natural killer cells."
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In terms of the species as a whole, one might say the sex cells shown above (sperm and an egg) are the most important cells in the human body. They allow us to propagate our genetic material and perform one of the most amazing magic tricks seen in nature: the creation of a wholly new and separate life.
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This is where it all begins. This fertilized egg cell will divide and form a mass of pluripotent stem cells, which will themselves go on to become the cellular ancestors of every other tissue in a new body. For now, the ovum rests in its mother's Fallopian tube before implantation in the uterus.
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