History and development of cell theory

- [Voiceover] Alright, so in order to describe cell theory, I'm gonna go ahead and tell you the story of how cell theory developed. Now, over here on the right side of the screen, you notice a timeline that goes from the 1600's to the late 1800's and a couple boxes here. And these boxes will fill in as the three major tenets of cell theory, and they will correspond to the different periods of time in which these tenets were developed. Throughout this story, I'm gonna refer to examples of bacteria, plants, and animals to illustrate some of the points. So, the story starts off in the Netherlands in the 1600's with a scientist by the name Anton van Leeuwenhoek. Around this time, the microscope was actually invented in the Netherlands. And van Leeuwenhoek started using a microscope and he actually invented his own version of the microscope and started looking at everything he could get his hand's on under the microscope. And, so, one day this curiosity brought him to look at the gunk on his teeth. That's right, he looked at his dental scrapings under the microscope. And when he did so, he noticed many small animal-like creatures that were moving around and one of them may have looked something like this. And because he thought these kind of looked like little animals he named them "Animacules". And, now of course, we know that what he was looking at now were actually bacteria, but at the time bacteria hadn't been discovered. And for this discovery of these animacules, Anton van Leeuwenhoek is frequently referred to as the father of modern microbiology. Now, around the same time, there was an English scientist by the name of Robert Hooke and Hooke also looked at all sorts of things under the microscope. And this isn't probably how it actually happened, but it's a fun story that I like to think to help remind myself of this. I like to think that Robert Hooke was at a party one night and they decided to pop a couple bottles of champagne, and perhaps one of the corks hit Robert Hooke in the head. He then grabbed that cork, put it in his pocket, and the next morning he woke up and said, "I wonder what this cork would," "look like under a microscope?" So, he sliced it thin and looked at it under a microscope and what he saw was something that looked like this. And what he was seeing was the remnants of the cells in the plant that made the cork. And he thought that these spaces kind of looked like the little dormitory rooms that monks lived in in a monastery. And these rooms in Latin are known as "cellula" and so this is where the term cells came from. And then, similarly, other scientists were looking at animal tissues, and they also noticed that in different animal tissues, there were these similar cells, as in bacteria and plants, and from this, they developed the first tenant of cell theory. And that is that the cell is the basic unit of structure in life. Now, over the ensuing years, scientists continued to look at all sorts of things under the microscope. And in regards to bacteria, what they discovered was that there's all sorts of bacteria. One's like the one we saw above here, or can maybe kinda rod-shaped. Or there was bacteria that were spiral-shaped and there's bacteria that were more spherical. And what they discovered was that, regardless of the shape of these bacteria, they all had the same cell as their structure, this basic unit of structure. So, moving on to plants, in the 1830's there was a German botanist by the name of Matthias Schleiden. And like I said, Schleiden was a botanist, and he also was interested in microbiology, so he looked at all sorts of plants underneath a microscope. And what he realized was, it didn't matter whether he was looking at a specific type of flower or if he was looking at a maple tree. All the different plants he looked at had the same microscopic structure that Robert Hooke had described a couple hundred years earlier of these cells. Then, at the same time, another German scientist by the name of Theodor Schwann was looking at the nervous systems of different animals. And what he realized was it didn't matter if he was looking at a human or a cat. No matter what type of animal, they all had a similar structures that were these cells. And luckily for microbiology, Schleiden and Schwann in 1837 were actually at a dinner party together. And they began discussing each other's work. And I gotta imagine a light bulb just went off in both of their heads and they really discovered, you know, if all plants are made of cells and all animals are made of cells, it must be that all living organisms are made of cells. And the next year in 1838, Schwann published and in this book he stated this finding that all living things are composed of cells. Now, this was all well and good, because at the time no one really argued that all living things were made of cells. However, scientists didn't really know where the cells actually came from. Through observations, some of this was known. For instance, it was known for thousands of years that if a female and a male mated, you know over some period of time afterwards they would have an offspring. And so it was generally accepted that animals came from animals, and similarly in plants, it was well known that a tree or a plant, say this one right here, would produce a seed and that seed could then be planted in the ground and it would then produce a tree or a plant that looked similar to the one that produced the seed. So, it's pretty well accepted that plants came from plants. However, in bacteria it wasn't really clear where they came from. And the predominant theory at the time was something known as "Abiogenesis". So let me just erase some of our work here in order to describe this theory of "Abiogenesis". Now, "Abiogenesis" is actually a very old theory. It dates as far back as the 4th century BC with Aristotle. And up to the 1800's, this was really the predominant theory of how life came into existence. So let's just kind of describe "Abiogenesis" here briefly. Imagine you have, let's say, a rock. This is a rock. And what scientists thought at the time was that there was some unknown substance in the air and that it would combine with these non-living materials, such as a rock or whatever it might be, and from that, it would produce life. The mechanism of this theory was thought to be that of spontaneous generation. The idea that life was spontaneously made from non-life. Now, in the late 1800's, the scientists who were studying cells began to refute this theory. And, one such scientist was the German physician and pathologist, Rudolph Virchow. Now, Virchow was using his microscope to look at cells. And what he observed was that some bacteria, if you watch them at the right point of time, they actually divided and formed two bacteria that were identical, at least in appearance, to the first bacteria. Now today we call this mechanism binary fission and it's the means by which bacteria reproduce. But the time that wasn't known, and so Virchow observed this and he published this in a very famous work a phrase in which he stated, "Omnis cellula e cellula," which roughly, translated from Latin, means "Every cell originates from a cell like it." So, it's this idea that cells are not made by spontaneous generation, but cells produce cells. And it is important to note that although Virchow is generally given credit for this theory, the phrase "omnis cellula e cellula" was actually coined earlier by a French physician by the name of François-Vincent Raspail and this theory of binary fission was actually likely plagiarized from Robert Remak, a Polish physiologist. So, Virchow, although he didn't come up with the idea or the term, he still for some reason is generally given credit. But Virchow, he received a lot of criticism for this theory. And other scientists who believed in this "Abiogenesis" they stated, "Oh, this idea that cells produce cells," "That might be true, but it doesn't," "By any means, disprove Abiogenesis," "I mean, both of these mechanisms," "Could be going on in the world." And, so this is where the very famous French scientist Louis Pasteur enters our story in the 1860's. Now, during this time, Louis Pasteur did a very famous experiment known as the Swan-Neck Bottle experiment, which kind of finally laid to rest this theory of Abiogenesis. So, before I describe the Swan-Neck Bottle experiment, let me describe an experiment that some of the proponents of Abiogensis used as evidence for their theory. So, imagine you have a flask and you fill this flash with some broth and the broth may have some bacteria in it. Well, it was known that if they boiled the broth, they heated up and boiled it, that would kill the bacteria. You could sterilize the broth in this flask. And then, over time, if the flask was left alone, what would happen is that bacteria would start to grow in it and you'd get this growth and proponents of "Abiogenesis" said, "See, nothing was added to the flask," "And, yet, bacteria was formed," "It must have been formed through spontaneous generation," "As no living organisms were added to it." However, Pasteur disagreed with this and so he came up with his own flask, known as a swan-neck bottle. It looked something like this, and this swan-neck flask was also filled with broth and sterilized by boiling it, and what Louis Pasteur theorized was that there was actually maybe some bacteria, or living particles in the air, that would fall into the broth here, and that is what caused the growth. So, by creating this swan-neck, he was still allowing the broth to be exposed to air and this potential unknown substance in the air, but if there were any microorganisms in the air that fell in they would be collected in this curve of the neck and they wouldn't reach the broth. And, lo and behold, over time, there was no growth. Like I mentioned earlier, this kind of once and for all put to rest this theory of "Abiogenesis" and really established the third tenet of cell theory and that is that all cells come from preexisting cells. So, this is the story of cell theory and how scientists over a period of 200 years really discovered the three major tenets of cell theory that are really the foundation of much of modern biology. And these are that the cell is the basic unit of structure in life and that all living organisms are composed of cells and that all cells come from preexisting cells.