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Cells and tissues

Overview of the main cellular components and tissues.

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If you had to guess, roughly how many cells do you think there are in the human body? Five million? No, that’s too little. A hundred million? No, probably more than that. How about three billion? Still too little. Hmmm. Five hundred billion? I mean, yeah, that seems plausible, right? Well, we’re actually still way off, because it’s estimated that we have over thirty-two trillion cells in our bodies. Yeah, you heard me – THIRTY-TWO TRILLION. And on top of that, our body produces an average over a billion new cells every hour which are needed to grow and replace dead or lost cells.

Despite there being so many in a healthy body, all of these thirty-two trillion cells magnificently come together and all work towards one common goal, and this is homeostasis, which is the process of maintenance and regulation of our bodies to provide stability and consistency within our internal environments while dealing with external changes.

Among these thirty-two trillion cells, we have roughly about two hundred types of cells, all of which are specialized for the function they play on our bodies. Groups of similar cells work together to perform their designated function and in doing so, they form tissues which essentially are the very fabric of our body which holds us together, gives us shape and – feeling overwhelmed? Don’t be. It’s not as complicated as it looks. It will all shortly make sense. Trust me.

So, welcome to our introduction on cells and tissues.

Luckily for us, despite our body containing over two hundred types of cells, all of this complexity can actually be summed up into just four primary types of tissue, and these are, epithelial tissue which lines the inside and outside of the body providing us with cover and protection, muscle tissue which of course provides us with means for movement and ability, connective tissue that supports our whole body and prevents us from looking like a giant and maybe like a pile of goo, and finally nervous tissue which our body uses for communication and control of bodily functions.

Before we begin exploring the four major types of tissue, let’s first 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. So, let’s explore each of the different major types of tissue in more detail, beginning first with epithelial tissue.

Epithelial tissue, also referred to as epithelia, is composed of sheets of cells which fall into one of two groups. The first of these is known as epithelial tissue proper, also known as covering epithelia, due to the fact that it covers the external surface or lines the internal surfaces of most organs. Examples of these would be the epidermal epithelia in your skin or respiratory epithelium lining your trachea and bronchi.

The second type is known as glandular or secretory epithelia which forms our glands. It functions to produce and secrete various macromolecules into our bloodstream or directly onto an epithelial surface via ducts. You’ll find examples of glandular epithelium in the gastric glands of your stomach as well as your sweat glands.

Epithelial cells live in direct contact with one another meaning that there’s little intercellular space between them. They’re also avascular which means that they don’t have a direct blood supply. Instead, they receive their nutrition from surrounding and underlying connective tissue.

In terms of the classification, epithelial cells are generally classified according to their morphology or shape – cuboidal, squamous or columnar – and according to how they are arranged – simple or stratified. As the name suggests, cuboidal epithelial cells are roughly cubish in shape, meaning that they’re roughly equal in height and width. Squamous refers to scale-like, thin, or flattened cells, while columnar describes these tall column-like epithelial cells. Simple epithelium refers to a single cell layer and stratified refers to multiple cell layers.

It is this shape and arrangement of the epithelial cells which typically determines the name for each epithelial tissue. For example, we can have simple cuboidal, simple squamous, simple columnar, stratified cuboidal, stratified squamous, and stratified columnar. There are a lot of epithelial tissues to discuss so pay close attention as we continue, and don’t forget to take notes.

So, simple cuboidal epithelium lines small tubules, ducts, and glands throughout the body. Its primary functions are secretion and absorption. In this micrograph showing a portion of a kidney cross-section, simple cuboidal epithelium can be seen 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. It can be found primarily in the ducts of sweat glands and lining larger tubules.

Simple squamous epithelium create 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, it can be seen just here composing the innermost of an artery wall.

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 up of a specialized form of this tissue pictured here and 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. It can be found lining the gastrointestinal tract. In this micrograph, simple columnar epithelium can be seen lining intestinal villi.

Stratified columnar epithelium, on the other hand, is not common. It has the same functions and can be found in certain large exocrine glands as seen here in a large duct of the sublingual salivary gland as well as the anorectal junction. Specialized tissue is often confused for stratified columnar epithelium because a single-cell layer can appear stratified. This common tissue is aptly named pseudostratified columnar epithelium. It can be found primarily lining the respiratory tract and has the same functions as simple columnar epithelium.

The last epithelial tissue that we’re going to be looking at today takes us back to the first micrograph we saw which is a cross-section of a ureter, and this unique tissue is called transitional epithelium and 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, the lumen is not dilated, however, and therefore most of the cells appear cuboidal. This tissue is also known as urothelium due to the fact that it is exclusive to the ureter and parts of the urinary bladder and urethra, and its function is to distend the tissues that collect and pass urine.

It is also common for cuboidal and columnar cells to present specialized projecting structures on their luminal surfaces. These are the function to increase the surface area of the cell for optimized absorption or to move substances along the epithelial surface itself.

And there are three main types of such specializations, and these are microvilli such as those which form the brush border found on your small intestine, stereocilia which can be found in the epididymis and ductus deferens, and finally, cilia which you can find in your trachea and bronchi. All epithelial tissue is separated from underlying tissue by a basement membrane, and this is shown here in green. This specialized extracellular matrix is responsible for providing support to tissues.

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 interchangeable terms. Basement membrane equals basal lamina plus the lamina propria.

Okay, so 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, what is connective tissue, and how is it different?

Well, for starters, connective tissue is the most abundant tissue in the body, and being the most abundant, it is also the most diverse type of body tissue. So much so that you might never even think it different types of connective tissue are even related to one another. You’ll see what I mean by that in just a moment.

The one thing that connects all types of connective tissue, though, is their origin. Yes, all connective tissue arises from the same multipotent tissue which is known as mesenchyme – a loosely organized and fluid type of embryonic tissue which some referred to as embryonic soup. Yes, I know, I’m never going to look at soup the same way again either.

In most cases, connective tissue consists of cells located in a sea of extracellular material known as the extracellular matrix. And this matrix consists of fluids and something known as ground substance, which is basically a jelly-like material that fills all the spaces between the cells. Within this ground substance, there are lots and lots of embedded fibers such as collagen which is actually stronger than steel and is that very same stuff that some people inject into their faces to plump up, elastic fibers which allow tissues to stretch and recoil like a rubber band, and reticular fibers which form a meshwork of spongy material which holds everything in place. And this all help determine the physical properties of the particular type of connective tissue.

So when comparing epithelial with connective tissue, one major difference between them is the relative abundance of this extracellular material as well as the fibers seen in connective tissue, but fewer cells by volume.

The primary function of connective tissue is to provide a structure and support as well as to connect or bind our tissues together just as its name suggests. It also works to provide protection to our body as well as insulation to our other tissues and organs and some types of connective tissue also have a role in storage of energy as well as working as a transport system within our body. Of course, since there are lots of different types of connective tissue, there are equally lots of different types of cell types, specific to each particular type of connective tissue. They do, however, share some common features and generally can be divided into two major types – immature connective tissue cells and mature connective tissue cells.

So immature cells are pretty easy to recognize by their names because they end in the suffix –blast – for example, fibroblasts in proper connective tissue, chondroblasts in cartilage, osteoblasts in bone, lipoblasts in your fat tissues, and even hemocytoblasts in your blood. And despite what their names might suggest, these cells are not involved in blowing anything up. In fact, it’s quite the contrary.

Among their many functions is to secrete the ground substance and fibers needed to build up their related connective tissue. And once they’ve done their job producing extracellular matrix amongst many other things, they transition into their mature form which tend to be less active and sometimes described as resting. These cell types give up their blasts surname and instead end up with the suffix –cyte. So our fibroblasts turn into fibrocytes, our chondroblasts into chondrocytes, our osteoblasts into osteocytes, and so on.

These cells can sometimes revert back into immature form should they need to repair and rebuild tissue, but in general, they function to maintain the tissue built by its progenitor blast cell.

So these are the cells that make and maintain your blood, ligaments, cartilage, and bone, and basically everything that holds you together and ensures that you don’t look like a giant blob of Jell-O.

Now, that’s not the end of the story when it comes to the cell types in connective tissue. There’s another class of cell types which aren’t involved in building connective tissue, but rather worked to protect you from, well, everything really. And these are your immune cells, such as macrophages and white blood cells like neutrophils, eosinophils, monocytes, and lymphocytes, and the list goes on. These guys are like the army, the marines, the navy seals, and the FBI of your body all combined who scout your connective tissues, fighting off invaders and external threats.

So, let’s take a closer look now at some of the different types of connective tissue.

So, there are three groupings of connective tissue based on the type of extracellular matrix present – tissues with liquid matrices, tissues with semi-sold matrices, and tissues with solid matrices. So, first, we have that one of the connective tissue types are tissues with liquid matrices and one of the most significant connective tissues with liquid matrices in the body is one of the key liquids of the cardiovascular system, and that is blood.

And blood is 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 one leucocyte example present in this image and that’s the one that’s circled and then shown in our breakout. And note that the dark green area represents the cell nucleus.

The red blood cells don’t have this dark purple staining that’s shown in the non-highlighted leucocyte in the circle to the left because they’re anuclear, meaning that they do not possess 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 mentioned, there are a few types of connective tissue with semi-solid matrices which are commonly referred to connective tissue proper.

And the first of these is the tissue we’ll call loose connective tissue, and this type of tissue can be found in three primary forms, the first of which is known as areolar tissue. Areolar tissue is widespread throughout the body and can often be found deep to the basement membrane of the epithelium. It contains a meshwork of collagen and elastin fibers and is therefore very flexible. Loose connective tissue is also the site of rapid fluid and gaseous exchange between tissues.

The second type of loose connective tissue is known as reticular connective tissue which acts like a scaffold in certain tissues providing other cell types with a home to mature or to develop. And you’ll find reticular connective tissue in structures like your lymph nodes, your spleen, and bone marrow.

One last type of loose connective tissue is adipose tissue which, to you and me, is a.k.a fat tissue. It’s 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 it’s made up of fat cells called adipocytes. Adipose tissue can be found all over the body including subcutaneously.

But before you go cursing your adipose tissue for fear of a dreaded potbelly and love handles, in the correct amounts, it is actually an extremely important tissue in our body because it provides insulation to protect us from excessive heat loss and it also provides protection for our organs such as our heart and kidneys, and finally and most critically, fat is our fuel tank – a calorie reserve that keeps our body ticking over during extended periods without food intake.

During fixation and staining, the fat containing adipocytes is removed leaving them empty in appearance with the nucleus at the periphery which as you can see gives them a very distinct ring-like appearance in histological section.

So, we’re moving on now to our second kind of connective tissue with semi-solid matrices which is loose connective tissue’s cousin – dense connective tissue – and as we can tell from the name, dense connective tissue is more dense than loose connective tissue, and just like loose connective tissue, dense connective dense connective has three main subtypes.

The first is dense regular connective tissue which you’re looking at now on your screen and this type of tissue contains more collagen than elastin and presents an organized or regular arrangement of fibers which provides it with greater strength. This tissue forms strong tissues such as ligaments and tendon which you can see now on your screen.

The second type of dense connective tissue is known as dense irregular connective tissue. Dermis which is mostly found in the dermis of the skin. And as you can see, it has a more haphazard arrangement compared to its tendinous and ligamentous cousins. The final type of dense connective tissue is known as elastic connective tissue, which true to its name, is full of elastic fibers which allow it to stretch and recoil. A good example of elastic connective tissue is that found in your larger elastic arteries just like you can see here in this micrograph.

So moving on to look at some examples of connective tissue with solid matrices, left’s first take a look at cartilage. And once again, we can identify several subtypes in this category of connective tissue. So, hyaline cartilage is the most common type of cartilage in the human body and can be found on the articulating surfaces of bones, the costal cartilage of the ribs, and surrounding the trachea. In this micrograph, we can see the trachea with hyaline cartilage visible in the purple-stained section shaped like a U, and this hyaline cartilage is responsible for the rigidity of the trachea which prevents it from collapsing.

Other types of cartilage include elastic cartilage like that found in your ears, fibrocartilage which you’ll find in your intervertebral discs, and pubic symphysis and physeal cartilage which makes up the growth plates in your bones. But regardless of the type of cartilage, remember that the resident cells found in this type of tissue are known as chondroblasts in growing cartilage and chondrocytes in developed.

Another type of connective tissue with solid matrices is bone, and it’s a form of calcified connective tissue which provides support and protection to soft tissues and, of course, it is the tissue that makes up the skeletal system. Bone tissue comes primarily in two forms. Compact or cortical bone which is a dense and highly organized tissue which contains Haversian systems like this one here and they each contain a Haversian canal which is surrounded by concentric layers of lamellae. Within this lamellae lie the osteocytes which are your resting bone cells.

The other type of bone is known as spongy or trabecular bone which is porous and not as dense as compact bone. It’s typically found in the heads of your long bones or in the center of your flat bones and is formed of struts or pillars of bone which are known as trabeculae. And the pores or spaces seen between these trabeculae is where your bone makes and stores bone marrow.

Okay, so we’ve now discussed epithelial tissues and connective tissues, let’s move on now to muscle tissue.

So, muscle tissue is exactly what you think it is. Yes, it’s that red, meaty tissue which affords mobility to many of our internal organs as well as to our body as a whole. So, muscle tissue is particularly unique because it is able to convert chemical energy which we obtained mainly from food and convert it into mechanical energy, thereby, allowing us to lift things, go running, and give your beloved a sweet lovely wet one. Oh, get a room you two!

The cells that comprise muscles are usually elongated and capable of contraction due to special proteins known as actin and myosin filaments which are the workhorse of all muscle tissues. And there are three types of muscle – smooth muscle, skeletal muscle, and cardiac muscle.

In this micrograph, you can see skeletal muscle – and let’s just increase the magnification. So, skeletal muscle is associated with the skeletal system through tendinous attachments and is primary responsible for the voluntary movements and posturing of the skeleton. Its cells commonly referred to as muscle fibers are long and non-branching and may be several centimeters in length. Each fiber is multinucleated containing thousands of peripheral nuclei 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 within a motor neuron.

Our next type of muscle tissue is cardiac muscle which, as its name suggests, is found only in the heart. The actin and myosin arrangement again gives this tissue a striated appearance which you can see here like skeletal muscle; however, unlike skeletal muscle, cardiac muscle cells contain only one or two nuclei, and cardiac muscle cells also form a complex three-dimensional network of branches. Between adjacent cells lie intercalated discs which have gap junctions and these allow the heart to beat in synchrony, and because of its specialized intercellular junctions, cardiac muscle is fortunately considered to be involuntary and contracts without any conscious input.

Our final type of muscle tissue to explore is smooth muscle, also called visceral muscle, and this is found encapsulating our arteries and the tubular organs of the digestive, urinary, and reproductive tracts. Smooth muscle cells contain a single central nucleus and are connective to one another via gap junctions which allow them to communicate with one another. These cells are spindle-shaped and contain randomly arranged actin and myosin filaments giving smooth muscle cell a smooth, non-striated appearance as opposed to the striated appearance of the other two types of muscles.

Like cardiac muscle, smooth muscle control is also involuntary which is handy as it means you don’t need to think about pushing that burger you ate for lunch through your nine meter digestive tract. It just goes in one end, and thanks to your smooth muscle, comes out the other end a few hours later.

The final type of tissue that we’ll look at is nervous tissue, and this, of course, makes up the nervous system. The nervous system can be divided into the central nervous system or the CNS which is comprised of the brain and spinal cord, and the peripheral nervous system or PNS which is comprised of nerves that transmits signals to and from the CNS and the rest of the body. Nervous tissue cells are called neurons. The neurons that transmit signals away from the CNS to the body are called efferent neurons such as your motor neurons while those that transmit signals from the body towards the CNS are called afferent neurons just like your sensory neurons.

So, of course, neurons come in different shapes and sizes, however, regardless of their morphology, they all share three common features. So, they all have a cell body which is the control center of the cell and that contains the nucleus. They all contain dendrites which are these branch-like structures here communicating with other nerve cells in a sort of input channel into the neuron, and finally, we have this axon which is the transmission cable for the electrical signal passing through the nerve, and it may synapse with another neuron or directly with its target structure.

In addition to neurons, the nervous system is also composed of specialized supporting cells called glial cells, and these are your Schwann cells in the peripheral nervous system or oligodendrocytes and astrocytes found in your central nervous system. And this help to provide support, protection, and insulation to various parts of the neuron.

And that’s it! You’ve now had a basic introduction to the four tissues of the human body, and you’re on your way to becoming a histology pro.

Remember the four types that we looked at today? These were epithelial tissue which we saw can be divided into two primary classifications – epithelial tissue proper or covering epithelium which lines the internal and external surfaces of our organs, and glandular epithelium which forms the ducts and secretory parts of several glands in our body.

Next stop was connective tissue, which we saw is grouped by the following classifications – connective tissue within a liquid matrix such as blood or lymph; connective tissue within a semi-solid matrix, examples of which include loose areolar connective tissue or dense fibrous connective tissue; and finally, connective tissue within a solid matrix such as bone or cartilage.

After looking at connective tissue, we moved on to muscle tissue where we discovered three primary types and these included skeletal muscle which allow our bodies to move around within our external environment, smooth muscle which affords our organs motility within our internal environment and, of course, cardiac muscle which is unique to the heart.

And last but not least, we finished off with nerve tissue, and this time we spoke about two main types of cells which comprised this tissue and these were neurons or nerve cells which carry electrical impulses, and glial cells, which support, maintain and protect the neurons.

And that’s it! This video tutorial is done and dusted. I hope you enjoyed learning about the four major types of tissue in our body. Catch you next time!

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