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Cells and tissues

Overview of the main cellular components and tissues.

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Hi everyone! This is Nicole from Kenhub, and welcome to this introductory histology tutorial on the topic of cells and tissues. So this tutorial will be an introduction to the four categories of tissues found in the human body and the cells that they're made up of. And these tissues are epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Before we begin, though, let's get familiar with the photos taken of cells and tissues through a microscope micrographs.

So as you may know already, histology is the study of microscopic anatomy, but since we can't provide you with your own microscope and samples to view the cells and tissues we'll be discussing, instead you'll see many micrographs in this tutorial. And in this micrograph, you can see the stained cross-section of a ureter. And we'll use staining when it comes to visualizing cells and tissues because staining creates a color contrast which can allow for cells and their internal structures to become more distinguishable. If we magnify a portion of this micrograph, you can identify numerous red spots scattered throughout the light purple area. And do you have any idea what these spots might be? Well, they each represent a cell nucleus. And together, these cells form transitional epithelium which is a type of epithelial tissue.

Epithelial tissue or epithelium is a particularly significant tissue as it covers all internal and external body surfaces. And its primary functions are protection, absorption, secretion, and filtration. The cells that compose epithelial tissues are called epithelial cells and these are tightly packed together with little intercellular space. In terms of the classification, epithelial cells are generally classified according to their shape – cuboidal, squamous or columnar – and according to how they're arranged – simple or stratified. Note that cuboidal means cube-shaped, squamous refers to thin and flattened cells, columnar means column-like, simple refers to a single cell layer, and stratified refers to multiple cell layers.

The shape and arrangement of epithelial cells typically determines the name for each epithelial tissue. There are also some types of epithelial tissue with more unique features named according to their specialization. And the main epithelial tissues are simple cuboidal, simple squamous, simple columnar, stratified cuboidal, stratified squamous and stratified columnar. And these all follow the easy-to-remember shape and arrangement pattern of naming. Some specialized epithelial tissues you should also know are keratinized stratified squamous, pseudostratified ciliated columnar, and transitional. And there are a lot of epithelial tissues to discuss so play close attention as we continue and don’t forget to take notes.

So the first type of epithelium we're going to talk about is simple cuboidal epithelium. And simple cuboidal epithelium lines small tubules, ducts and glands throughout the body. Its primary functions are secretion and absorption, and in this micrograph showing a portion of a kidney cross-section, we can see simple cuboidal epithelium just here lining one of the many collecting tubules that are responsible for concentrating urine. Stratified cuboidal epithelium has the same functions but is less common and it can be found primarily in the ducts of sweat glands and lining larger tubules.

Simple squamous epithelium creates a selective barrier for the diffusion of small molecules. It can therefore be found anywhere diffusion occurs such as the lining of alveoli and blood vessels. In this micrograph, we can see it just here composing the Bowman's capsule around this glomerulus. Stratified squamous epithelium meanwhile primarily provides protection. It can be found anywhere in the body where constant abrasion occurs such as the esophagus and the lining of the oral cavity. The epidermis where constant mechanical abrasion occurs is made of a specialized form of this tissue which is pictured called keratinized stratified squamous epithelium which is dead cells engorged in keratin on its outermost layers.

Simple columnar epithelium has the functions of absorption, secretion, protection and lubrication. And it can be found lining the gastrointestinal tract. And in this micrograph, we can see simple columnar epithelium lining the intestinal villi. Stratified columnar epithelium, on the other hand, is not common though it has the same functions and can be found in certain large exocrine glands. A specialized tissue is often confused for stratified columnar epithelium, however, because its single cell layer can appear stratified. This common tissue is aptly named pseudostratified columnar epithelium and it can be found primarily along the respiratory tract and has the same functions as simple columnar epithelium.

The last epithelial tissue we'll look at takes us back to the first micrograph we saw – a cross-section of a ureter. And this unique tissue called transitional epithelium is formed by stratified cells that can appear both squamous and cuboidal. For example, when urine passes through the ureter, the exertion of pressure forces the dilation of the lumen which in turn flattens the outer cuboidal cells into squamous cells. In this micrograph, however, the lumen is not dilated and, therefore, most of the cells appear cuboidal. This tissue is exclusive to the ureter and part of the urinary bladder and the urethra. And its function is to distend the tissues that collect and pass urine.

And while we're here, we're going to look at this one other structure called the basement membrane and as you can see, all epithelial tissue is separated from the underlying tissue by this basement membrane which is shown here in green. And this specialized extracellular matrix is responsible for providing support to tissues. And from time to time, you may find that the basement membrane is often called the basal lamina, however, it's important to note that the basal lamina is actually just one of the layers comprising the basement membrane and they're not actually interchangeable terms.

And now that we've reviewed epithelial tissue, let's move on to connective tissue. So the question I hear you ask now that we've moved on from epithelial tissue is how is connective tissue different? Well, for starters, connective tissue – an important supporting tissue – is the most abundant tissue in the body. And it's usually composed of fibroblasts, which are cells capable of repairing tissue over time. One difference between connective tissue and epithelial tissue is the abundant presence of extracellular material and fibers in connective tissue though it has fewer cells by volume. And the primary function of connective tissue is to provide structure by connecting different tissue types. And there are three groupings of connective tissue based on the type of extracellular matrix present – tissues with liquid matrices, tissues with semi-solid matrices, and tissues with solid matrices. And over the next few slides, we're going to have a look at some examples of connective tissue in each of these three groups.

So first off we mentioned that one of the connective tissue types are tissues with liquid matrices. And one of the most significant connective tissue types with liquid matrices in the body is one of the key liquids of the cardiovascular system – that's right, it's blood. And blood is, of course, composed of red blood cells called erythrocytes and white blood cells called leukocytes. And these cells are contained by a liquid matrix called the plasma.

In this micrograph, we can see the erythrocytes as reddish-brown discs and there's also one leukocyte example present in this image and that's the one that is circled and then shown in our breakout. And note that the dark green area which is our highlight represents the cell nucleus. The red blood cells don't have this dark purple staining that's shown in the non-highlighted leukocyte in the circle to the left because they don't contain a nucleus. And in this image, the small purple dots like this one here are platelets which aid in blood clot formation.

And as we've mentioned, there are a few types of connective tissue with semi-solid matrices. And the first of these is a tissue that we'll call loose connective tissue. And this type of tissue also called areolar tissue is widespread throughout the body and can be found deep to the basement membrane. It also contains collagen and elastin fibers and is therefore very flexible. Loose connective tissue is also the site of rapid fluid and gases exchange between tissues.

Another kind of connective tissue with semi-solid matrices is loose connective tissue's cousin that is dense connective tissue. And as we can tell from the name, dense connective tissue is more dense than loose connective tissue. It also contains more collagen than elastin and has a more structured arrangement of fibers for greater strength. And this tissue composes tendons and is found in the dermis of the skin as this picture on the right shows.

One last type of connective tissue with semi-solid matrices that I want to look at is adipose tissue. And adipose tissue is a unique type of connective tissue because unlike the others, it doesn’t have much extracellular material and is instead mostly cellular. Adipose tissue typically can be found encasing organs or other structures and is made up of fat cells called adipocytes. Adipose tissue can also be found all over the body including subcutaneously. During fixation and staining, the fat containing adipocytes is removed pushing the nucleus to the side and leaving behind empty-looking cells as depicted in this micrograph.

So moving on now to look at some examples of connective tissue with solid matrices, let's have a look at hyaline cartilage. And hyaline cartilage is a specialized type of connective tissue that is comprised of cartilage cells known as chondrocytes. Hyaline cartilage is the most common type of cartilage in the human body and can be found on the articulating surface of bones, the costal cartilage of the ribs and surrounding the trachea. And in this micrograph, the trachea is shown with the hyaline cartilage visible in the purple stain section shaped like a U. And this hyaline cartilage is responsible for the rigidity of the trachea which prevents it from collapsing.

And another type of connective tissue with solid matrices is compact bone. And compact bone provides support and protection to soft tissues and is the tissue that makes up the skeletal system. Compact bone consists of osteocytes or bone cells. Haversian systems like this one are the structural units of compact bone and they each contain a haversian canal which is surrounded by concentric layers of lamellae. And within these lamellae lie the osteocytes.

So now we've discussed epithelial tissue and connective tissue, let's move on to muscle tissue. Muscle tissue refers to the muscles of the body and of course we all know that muscles are responsible for movement. The cells that comprise muscles are usually elongated and capable of contraction due to actin and myosin filaments. And there are three types of muscle – smooth muscle, skeletal muscle and cardiac muscle. And in this micrograph, we can see smooth muscle which we'll begin with on our next slide.

Smooth muscle also called visceral muscle is found encapsulating arteries and the tubular organs of the intestinal and reproductive tracts. And smooth muscle cells contain a single central nucleus and are connected to one another via gap junctions. The cells of smooth muscle are spindle-shaped and contain randomly arranged actin and myosin filaments giving smooth muscle a smooth non-striated appearance as opposed to the striated appearance of the other two types of muscles. And smooth muscle control is primarily involuntary.

Skeletal muscle is associated with the skeletal system through tendinous attachments and is primarily responsible for the voluntary movements and posturing of the skeleton. Its cells called fibers are long and non-branching and may be several centimeters in length. Each fiber is multinucleated containing thousands of peripheral nuclei and is situated at regular intervals. Skeletal muscle has a striated appearance due to the perpendicular arrangement of actin and myosin filaments crossing over each fiber and each muscle fiber is controlled by a synapse with the motor neuron.

And coming in at the end of our muscle groups is cardiac muscle. And cardiac muscle is, of course, found only in the heart. The actin and myosin arrangement gives this tissue a striated appearance just like the skeletal muscle. However, unlike a skeletal muscle, cardiac muscle cells contain only one or two nuclei. Cardiac muscle cells also form a complex three-dimensional network of branches and between adjacent cells like intercalated discs which have gap junctions to allow the heart to beat in synchrony. And because of its specialized intercellular junctions, cardiac muscle is considered to be involuntary.

And the final type of tissue we will look at is nervous tissue which makes up the nervous system. And the nervous system can be divided into the central nervous system or CNS which is comprised of the brain and spinal cord and the peripheral nervous system or PNS which is comprised of nerves that transmit signals to and from the CNS and the rest of the body. And of course nervous tissue cells are called neurons. The neurons that transmit signals away from the CNS to the body are called the motor neurons while those that transmit signals from the body towards the CNS are called sensory neurons. In addition to neurons, the nervous system is also composed of specialized supporting cells called glial cells.

Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.

Now, good luck everyone, and I will see you next time.

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