Histogenesis

Transcription

Check out this amoeba. Pretty nice. Kind of a rugged, no-frills life form. The thing about amoebas is that they do everything in the same place. They take in and digest their food, and reject their waste, and get through everything else they need to do, all within a single cell. They don’t need trillions of different cells working together to keep them alive. They don’t need a bunch of structures to keep their stomachs away from their hearts away from their lungs. They’re content to just blob around and live the simple life. But we humans, along with the rest of the multicellular animal kingdom, are substantially more complex. We’re all about cell specialization, and compartmentalizing our bodies. Every cell in your body has its own specific job description related to maintaining your homeostasis, that balance of materials and energy that keeps you alive. And those cells are the most basic building blocks in the hierarchy of increasingly complex structures that make you what you are. We covered a lot of cell biology in Crash Course Bio, so if you haven’t taken that course with us yet, or if you just want a refresher, you can go over there now. I will still be here when you get back. But with that ground already covered, we’re going to skip ahead to when groups of similar cells come together to perform a common function, in our tissues. Tissues are like the fabric of your body. In fact, the term literally means “woven.” And when two or more tissues combine, they form our organs. Your kidneys, lungs, and your liver, and other organs are all made of different types of tissues. But what function a certain part of your organ performs, depends on what kind of tissue it’s made of. In other words, the type of tissue defines its function. And we have four primary tissues, each with a different job: our nervous tissue provides us with control and communication, muscle tissues give us movement, epithelial tissues line our body cavities and organs, and essentially cover and protect the body, while connective tissues provide support. If our cells are like words, then our tissues, or our groups of cells, are like sentences, the beginning of a language. And your journey to becoming fluent in this language of your body -- your ability to read, understand, and interpret it -- begins today. Although physicians and artists have been exploring human anatomy for centuries, histology -- the study of our tissues -- is a much younger discipline. That’s because, in order to get all up in a body’s tissues, we needed microscopes, and they weren’t invented until the 1590’s, when Hans and Zacharias Jansen, a father-son pair of Dutch spectacle makers, put some lenses in a tube and changed science forever. But as ground-breaking as those first microscopes were then, they were little better than something you’d get in a cereal box today -- that is to say, low in magnification and pretty blurry. So the heyday of microscopes didn’t really get crackin’ until the late 1600s, when another Dutchman -- Anton van Leeuwenhoek -- became the first to make and use truly high-power microscopes. While other scopes at the time were lucky to get 50-times magnification, Van Leeuwenhoek’s had up to 270-times magnifying power, identifying things as small as one thousandth of a millimeter. Using his new scope, Leeuwenhoek was the first to observe microorganisms, bacteria, spermatozoa, and muscle fibers, earning himself the illustrious title of The Father of Microbiology for his troubles. But even then, his amazing new optics weren’t quite enough to launch the study of histology as we know it, because most individual cells in a tissue weren’t visible in your average scope. It took another breakthrough -- the invention of stains and dyes -- to make that possible. To actually see a specimen under a microscope, you have to first preserve, or fix it, then slice it into super-thin, deli-meat-like sections that let the light through, and then stain that material to enhance its contrasts. Because different stains latch on to different cellular structures, this process lets us see what’s going on in any given tissue sample, down to the specific parts of each individual cell. Some stains let us clearly see cells’ nuclei -- and as you learn to identify different tissues, the location, shape, size, or even absence of nuclei will be very important. Now, Leeuwenhoek was technically the first person to use a dye -- one he made from saffron -- to study biological structures under the scope in 1673, because, the dude was a boss. But it really wasn’t until nearly 200 years later, in the 1850s, that the we really got the first true histological stain. And for that we can thank German anatomist Joseph von Gerlach. Back in his day, a few scientists had been tinkering with staining tissues, especially with a compound called carmine -- a red dye derived from the scales of a crushed-up insects. Gerlach and others had some luck using carmine to highlight different kinds of cell structures, but where Gerlach got stuck was in exploring the tissues of the brain. For some reason, he couldn’t get the dye to stain brain cells, and the more stain he used, the worse the results were. So one day, he tried making a diluted version of the stain -- thinning out the carmine with ammonia and gelatin -- and wetted a sample of brain tissue with it. Alas, still nothing. So he closed up his lab for the night, and, as the story goes, in his disappointment, he forgot to remove the slice of someone’s cerebellum that he had left sitting in the He returned the next morning to find the long, slow soak in diluted carmine had stained all kinds of structures inside the tissue -- including the nuclei of individual brain cells and what he described as “fibers” that seemed to link the cells together. It would be another 30 years before we knew what a neuron really looked like, but Gerlach’s famous neural stain was a breakthrough in our understanding of nervous tissue. AND it showed other anatomists how the combination of the right microscope and the right stain could open up our understanding of all of our body’s tissues and how they make life possible. Today, we recognize the cells Gerlach studied as a type of nervous tissue, which forms, you guessed it, the nervous system -- that is, the brain and spinal cord of the central nervous system, and the network of nerves in your peripheral nervous system. Combined, they regulate and control all of your body’s functions. That basic nervous tissue has two big functions -- sensing stimuli and sending electrical impulses throughout the body, often in response to those stimuli. And this tissue also is made up of two different cell types -- neurons and glial cells. Neurons are the specialized building blocks of the nervous system. Your brain alone contains billions of them -- they’re what generate and conduct the electrochemical nerve impulses that let you think, and dream, and eat nachos, or do anything. But they’re also all over your body. If you’re petting a fuzzy puppy, or you touch a cold piece of metal, or rough sandpaper, it’s the neurons in your skin’s nervous tissue that sense that stimuli, and send the message to your brain to say, like, “cuddly!” or “Cold!” or “why am I petting sandpaper?!” No matter where they are, though, each neuron has the same anatomy, consisting of the cell body, the dendrites, and the axon. The cell body, or soma, is the neuron’s life support. It’s got all the necessary goods like a nucleus, mitochondria, and DNA. The bushy dendrites look like the trees that they’re named after, and collect signals from other cells to send back to the soma. They are the listening end. The long, rope-like axon is the transmission cable -- it carries messages to other neurons, and muscles, and glands. Together all of these things combine to form nerves of all different sizes laced throughout your body. The other type of nervous cells, the glial cells, are like the neuron’s pit crew, providing support, insulation, and protection, and tethering them to blood vessels. But sensing the world around you isn't much use if you can't do anything about it, which is why we've also got muscle tissues. Unlike your nervous tissues, your muscle tissues can contract and move, which is super handy if you want to walk or chew or breathe. Muscle tissue is well-vascularized, meaning it’s got a lot of blood coming and going, and it comes in three flavors: skeletal, cardiac, and smooth. Your skeletal muscle tissue is what attaches to all the bones in your skeleton, supporting you and keeping your posture in line. Skeletal muscle tissues pull on bones or skin as they contract to make your body move. You can see how skeletal muscle tissue has long, cylindrical cells. It looks kind of clean and smooth, with obvious striations that resemble little pin stripes. Many of the actions made possible in this tissue -- like your wide range of facial expressions or pantheon of dance moves -- are voluntary. Your cardiac muscle tissue, on the other hand, works involuntarily. Which is great, because it forms the walls of your heart, and it would be really distracting to have to remind it to contract once every second. This tissue is only found in your heart, and its regular contractions are what propel blood through your circulatory system. Cardiac muscle tissue is also striped, or striated, but unlike skeletal muscle tissue, their cells are generally uninucleate, meaning that they have just one nucleus. You can also see that this tissue is made of a series of sort of messy cell shapes that look they divide and converge, rather than running parallel to each other. But where these cells join end-to-end you can see darker striations, These are the glue that hold the muscle cells together when they contract, and they contain pores so that electrical and chemical signals can pass from one cell to the next. And finally, we’ve got the smooth muscle tissue, which lines the walls of most of your blood vessels and hollow organs, like those in your digestive and urinary tracts, and your uterus, if you have one. It’s called smooth because, as you can see, unlike the other two, it lacks striation. Its cells are sort of short and tapered at the ends, and are arranged to form tight-knit sheets. This tissue is also involuntary, because like the heart, these organs squeeze substances through by alternately contracting and relaxing, without you having to think about it. Now, one thing that every A&P student has to be able to do is identify different types of muscle tissue from a stained specimen. So Pop Quiz, hot shot! See if you can match the following tissue stains with their corresponding muscle tissue types. Don’t forget to pay attention to striations and cell-shape! Let’s begin with this. Which type of tissue is it? The cells are striated. Each cell only has one nucleus. But the giveaway here is probably the cells’ branching structure; where their offshoots meet with other nearby cells where they form those intercalated discs. It's cardiac muscle. Or these -- they’re uninucleate cells, too, and they also are packed together pretty closely together. But…no striations. They’re smooth, so this is smooth muscle. Leaving us with an easy one -- long, and straight cells with obvious striations AND multiple nuclei. This could only be skeletal muscle tissue. If you got all of them right, congratulations and give yourself a pat on your superior posterior medial skeletal muscles -- you’re well on your to understanding histology. Today you learned that cells combine to form our nervous, muscle, epithelial, and connective tissues. We looked into how the history of histology started with microscopes and stains, and how our nervous tissue forms our nervous system. You also learned how your skeletal, smooth, and cardiac muscle tissue facilitates all your movements, both voluntary and involuntary, and how to identify each in a sample. Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course possible to themselves and also to everyone else in the world. To find out how you can become a supporter, just go to subbable dot com. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor and sound designer is Michael Aranda, and the graphics team is Thought Café.