 This video will cover part one of an introduction to cytology using slides from OpenStacks Anatomy and Physiology. As we go, we'll cover the following study objectives, describe the cell theory, and describe the general structure of a cell and the diversity of cell types in the human body. Cells were first described in the mid-1600s. Usually Robert Hook is given credit for having first described the structure of a cell when he famously analyzed a section of quark tissue and recognized the small chambers he called cells in his text from 1665. We see an image here of an illustration he made after studying the structure of quark under his light microscope. You can see the picture of his light microscope that was used for these studies in the top left here. Robert Hook noticed this small repeating structure inside of living tissue and called it a cell, but it wasn't until later that the importance of the cell was fully appreciated and eventually led to the formulation of the three tenets of the cell theory in the late 1830s and these are that all living organisms are composed of one or more cells. Cells are the smallest structural functional units of life and cells arise from pre-existing cells. So how do we study cells and cytology, the structure of cells? A lot of work is done with the light microscope as we saw. Hook's light microscope was used for the original description of the cell and since then more advanced microscopes have been developed. The image that we can see here of neuron cells is taken with a light microscope. A light microscope allows us to see down to a certain level of resolution. That is, we can't see things any smaller than about two tenths of a micrometer, which is small enough to be able to get a good view of the shape of a cell. It's very good for being able to see the structure of a tissue for studying histology and we can study using a light microscope the shape of a cell and the shape of larger organelles inside of the cell. For example, we can see here the nucleus inside of these neurons and the nucleolus inside of the nucleus. A fluorescent microscope is a modification of a modern light microscope. Staining methods have been used for a long time in order to improve the contrast with a light microscope, but a fluorescence microscope uses a special type of dye, a fluorescent dye that absorbs UV light and emits a specific wavelength or a specific color of visible light and we can use very specific dyes in order to stain different structures inside a cell. Here we see fluorescent dyes that were used to stain the chromosomes blue and microtubules which are structures of the cytoskeleton or stained with green and then other structures of the cytoskeleton are shown in red in this image. That's a fluorescent microscope image that has these bright colors of red, green and blue standing specific structures within the cell. Electron microscopes allow us to view much higher resolution in order to see much smaller details. The resolution of an electron microscope is about a thousand times better than a light microscope allowing us to see down to about two tenths of a nanometer. The scanning electron microscope shown on the left here is excellent for taking images of a cell surface and getting a nice three-dimensional image of the surface of a cell. As we can see the picture of blood in the bottom left here, we can see the shape of the cell surface. A transmission electron microscope instead is useful for exploring the details of internal structures inside of a cell in order to study the structure of organelles. We can see in the image in the lower right here the Golgi apparatus is an organelle found inside of most cells. It has the structure of flat disks. We can see lots of flat disks that are made out of membrane. Flat membrane disks form the structure of the Golgi apparatus. Cells have two major regions. The two major structures that are found in any cell are the plasma membrane, the cell membrane that forms an outer border, a semi-permeable barrier that's the outer border around the cell, and then the region inside the cell called the cytoplasm. And the cytoplasm contains the organelles and also a liquid called cytosol. The cytosol or intracellular fluid contains mostly water with some things dissolved in that liquid. There are ions dissolved in there, proteins are dissolved in there, and other nutrients are also found in the cytosol. And then the organelles are the specialized cellular structures inside that perform specific functions. The nucleus is the largest organelle and so that's often considered a major compartment of its own separate from the cytoplasm because the nucleus is a very large compartment and inside of the nucleus is the genetic material called the DNA, which serve as the instructions to produce proteins in order to perform the functions that are necessary for the cell to sustain life. There are many different types of cells. Not all cells have all of the organelles, but all cells have the basic features of a plasma membrane surrounding a cytoplasm. There are different shapes of cells and different sizes. Here are some extreme examples. The longest cell in the human body is nearly equal to human height, so at least more than two meters in an individual that's more than six feet tall. This is a very small diameter of a very long cell. And so it's not very wide, but it's very long stretching the entire height from the tip of the toe all the way to the brainstem. This cell is called a pseudo-unipolar sensory neuron, important for carrying sense of touch. And the longest example would be from the tip of your toe all the way to the brainstem, nearly two meters long and a really tall person. The largest cell in the human body is the ovum or the female egg cell, which has a diameter of approximately 200 micrometers. So it's a very large cell, a very large amount of volume, large diameter. There are many cells that are nearly 100 micrometers, many neurons that are that large, but most cells are much smaller, closer to 10 or 20 micrometer diameter. Then the smallest cells in the body have a diameter of about four to five micrometers. And so the smallest cell in the body is a neuron called the granule neuron found in the cerebellum. And sperm cells are similarly about four to five micrometers, not including the tail. And so similarly with the granule neurons, there are long extensions that come out of the cell body, the large region surrounding the nucleus is only about four to five micrometers in diameter, making those the smallest cells in the human body. Common cells that we will study as we go through this course are neurons, which are the major cells found in nervous tissue. And there are other cells in nervous tissue called glial cells or neuroglia. You can see there's different types of neuroglial cells like astrocytes, oligodendrocytes, microglial cells. We'll study several different types of glial cells as we study the nervous system. And neurons are the larger cells found in nervous tissue. Neurons are specialized for rapid communication and they have long extensions that come out of their cell body. And these long extensions carry information rapidly through the body. Muscle tissue contains muscle cells that are important for contracting to produce motion. Muscle cells are also known as myocytes or fibers. A skeletal muscle fiber is found in skeletal muscles, the type of muscles that you can voluntarily control in order to move your bones to control the posture and position of your body. In order to move the skeleton, we use skeletal muscles that are long cells called muscle fibers that contain many different nuclei inside one cell. So it's an unusual cell in that it has more than one nucleus. Another type of muscle cell is called a smooth muscle fiber. And these smooth muscle cells have a small spindle shape, just one nucleus in each cell as is common. Most cells just have one nucleus. And smooth muscle cells can be found lining hollow organs. For example, the intestines and stomach are lined with smooth muscle. Another type of muscle is called cardiac muscle that contains cells called cardiomyocytes. And so cardiomyocytes are the muscle cells found in the heart wall. And so the special type of muscle found in the heart is called cardiac muscle made of cardiomyocytes. Muscle cells of cardiac muscle. Epithelial tissue is another major tissue type that we will study. Epithelial cells are cells that are tightly packed together to form sheets, to form barriers in the body. And so the skin is a simple example of an epithelial tissue made of epithelial cells. And we'll see lots of examples of epithelial tissues. There's epithelial tissues lining most of our organs. So the inner lining of the digestive tract contains epithelial tissue. There's epithelial tissue surrounding the heart lining the inner surface of the chambers and also surrounding the outside of the heart. The outer layer of the heart is epithelial tissue as well as the inner layer. The lungs are similarly lined with epithelial tissue both on the outside and the inside. And there is epithelial tissue forming membranes surrounding the digestive organs and the abdominal cavity as well. And so we'll see lots of different types of epithelial tissue. We can see the different shapes are shown here in the illustration that we could have squamous that are flat epithelial cells, cuboidal that are round epithelial cells, or columnar are tall shaped epithelial cells. And then we'll see that if there's just one layer of epithelial cells in a tissue, that's called a simple epithelium. Whereas multiple layers of cells is called a stratified epithelium. And then one weird exception is called a pseudo stratified epithelium where it's truly a simple epithelium. There's just one layer of cells, but there's lots of different heights. And so it has the appearance of multiple layers because the nuclei are not all lined up in one layer. However, all of the cells are contacting both surfaces of this epithelium and so it's a pseudo stratified epithelium. It's a false stratified epithelium is what it that means. It appears stratified but is actually a simple epithelium with only one layer of cells. Connective tissue is another major type of tissue and connective tissue is a very diverse group containing many different types of cells. One of the most common types of cells found in connective tissue is called a fibroblast or a fibrocyte is a mature cell that comes from a fibroblast. Mesenchymal cells are stem cells found in connective tissue that can mature or differentiate to form fibroblasts that can further mature into fibrocytes. Adipocytes are fat cells and so adipocytes contain lots of lipid droplets inside of their cytoplasm, lots of fat droplets that store energy inside of adipocytes. Whereas the function of the fibroblasts and fibrocytes in a connective tissue is to produce fibres, collagen fibres and elastic fibres that provide structure to the tissue in order to help with supporting functions of connective tissues. The adipocytes provide an energy storage function. Another cell we can see here in this illustration is called a macrophage. A macrophage is a type of leukocyte, a type of white blood cell and macrophages can wander in and out of other tissues performing the function of phagocytosis. That is that they perform cell eating that phagocytosis they will eat large particles or bacteria in order to help defend our body against infection. There are a lot of different types of connective tissue. Cardilage is a type of connective tissue that provides structural support in the body. The shape of the ear or nose results in part from the structural support of cartilage. We'll see there's also cartilage protecting our joints and the cells of cartilage are called chondrocytes. Another type of supporting connective tissue is the tissue of bone, oseous tissue and the mature cells of bone are called osteocytes. Blood is another type of connective tissue, a liquid connective tissue and the major type of cells that we see in blood, the most common cells in blood are red blood cells also known as erythrocytes. Red blood cells or erythrocytes are unusual in that they don't have most of the organelles found in other cells. There's no nucleus or mitochondria inside of erythrocytes and so they're very small and simple cells, essentially just filled with protein called hemoglobin that's important for transporting oxygen in the blood. And leukocytes are the other major type of cells called white blood cells and a lymphocyte is a type of leukocyte shown here. A lymphocyte is a specific kind of leukocyte, a specific type of white blood cell, which is important for the adaptive immune response in order to form immunity against infections.