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Microscopic appearance of the blood.

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Hey everyone! It's Nicole from Kenhub here, and in this histology tutorial, we'll be discussing probably one of the more motive parts of the human body – the blood.

Often when people think about blood, they think of something painful or unpleasant that may have happened like an injury and in fact, many people have an aversion to blood. But, blood, as you'll hopefully come to see during this tutorial, is a fascinating substance, if not only because it is so complex that we're still discovering new things about its composition but also because it's so vital for life.

Blood has fascinated us for millennia. In ancient Greece, blood was known as "hema" and was considered synonymous with life. To visualize the cellular components of blood using light microscopy, a simple preparation known as a blood smear or blood film is used. Here we see a micrograph showing a blood smear and as you can see, several cells can be seen and distinguished.

Blood is composed of cells and cell fragments suspended in an aqueous solution called plasma. This technique uses a single drop of blood which is placed onto a glass slide. The drop is spread thinly over the surface of the slide to produce a single layer of cells. The sample is then allowed to air dry. In order to visualize the cells, a stain or a dye is added. These stains usually contain methylene blue or other azure compounds which are basic dyes and eosin which is an acidic dye.

In this slide, you can see how the stain makes the blood cells different colors. For example, the light purple of the red blood cells and the dark purple of the lymphocytes. These little purple dots are platelets. The different compounds bind to different parts of the cells allowing them to be distinguished from one another. The stain is applied to the dried blood smear for a few minutes then rinsed off. Giemsa and Wright stains are two common stains used for blood smear preparations. Our image today is of a Wright's stain which is made up of a combination of methylene blue and eosin. Wright's stain is used to show blood cells and sometimes malarial parasites.

Blood plasma which is the aqueous solution in which the cellular components of blood is suspended is itself comprised of the proteins albumin, alpha, beta and gamma globulins, fibrinogens, and other complement proteins. In this slide, of course, the blood plasma is transparent and is therefore the white spaces you see in the image.

Let's talk briefly about what we're going to be covering in this tutorial. In this tutorial, we'll be focusing on the cellular components of blood starting off by looking at the erythrocytes or the red blood cells themselves followed by the leukocytes or the white blood cells and, finally, we'll look at the blood platelets, which are another important component of blood.

So, without further ado, let's get started by looking at erythrocytes.

Erythrocytes, more commonly known as red blood cells or RBCs, comprise about forty five percent of the total volume of blood. They're, of course, the cells you can see highlighted in green. The percentage of red blood cells in the blood is known as the hematocrit. As we can see from this little diagram here, the red blood cells themselves do not contain nuclei and are biconcave in shape. They measure about seven point five micrometers in diameter, about two point six micrometers at the rim or edge, and only zero point seven five micrometers in the center.

Their primary function is the transportation of oxygen to tissues and carbon dioxide away from the tissues. What allows this is the fact that red blood cells are filled with the protein hemoglobin, which is an oxygen-carrying protein. In fact, the entire cytoplasm of red blood cells is filled with hemoglobin which reversibly binds oxygen and carbon dioxide allowing the red blood cells to transport these gasses from the lungs to the tissues and vice versa for gaseous exchange.

The cytoplasm of red blood cells does not only lack a nucleus but also lacks organelles. And just to remind you in case you've forgotten, an organelle is an organized special structure that performs a particular function in a cell like a little organ. The lack of organelles in a red blood cell allows more hemoglobin to be packed into the cytoplasm and, therefore, more oxygen to be transported. The cells lose their nucleus and organelles during differentiation in the bone marrow shortly before their release into the general circulation. Whilst around that point, let's take a brief look at the precursor to red blood cells known as reticulocytes, some of which we can see in the micrograph here. Reticulocytes are immature red blood cells which develop in the bone marrow before being released into the circulating blood whereafter one or two days, they eventually develop into mature red blood cells.

In healthy individuals, reticulocytes account for only one or two percent of the cells in blood, however, this number increases in instances where the demand for oxygen outweighs the supply in which case reticulocytes are released from the bone marrow early before they mature into erythrocytes in order to increase the blood's oxygen-carrying capacity. As such, reticulocyte counts can be used as a marker for anaemia and to check for bone marrow disorders.

Moving back to our more mature erythrocytes, their structure means that they're very flexible and this allows them to easily deform or bend in order to pass through the narrow vessels of the capillary bed. Glycoproteins and glycolipids, both of which are kinds of antigens, are present on the surface of red blood cells. There are three types of antigens which differ slightly in their composition and are denoted by the letters A, B or O. The presence or absence of these antigens in an individual is what determines that person's blood type. For example, people with type A antigens on the surface of the red blood cells are described as having type A blood, and so on.

The presence of these antigens and knowing them is clinically relevant with respect to blood transfusion and blood type compatibility. The lifespan of red blood cells in circulating blood is about a hundred and twenty days. When red blood cells become old, they lose their ability to deform which is detected by specialized cells in the spleen. The spleen extracts them from the blood, breaks them down, and sends the components to the liver for recycling.

Now let's look at another group of cells that make up the blood known as leukocytes. Leukocytes are more commonly known as white blood cells or WBCs. The word leukocyte is derived from the Greek word "leukos" meaning white or clear and "cyte" meaning cell. These cells generally have a lifespan of around a hundred and twenty days. In this slide, do you know that we're looking at a leukocyte under an electron microscope as opposed to a light microscope as evidenced by these bright purple leukocytes you can see here.

There are five types of leukocytes which we'll be covering in this section of the tutorial. These are neutrophils, eosinophils, and basophils as well as lymphocytes and monocytes. These can be classified into two different categories. Granulocytes contain specific granules in their cytoplasm. Agranulocytes, on the other hand, lack these specific granules in their cytoplasm. However, it's worth noting that both granulocytes and agranulocytes possess nonspecific azurophilic granules known as lysozymes.

Let's take a look at each type of leukocyte in more detail starting with the neutrophils. In this image, note that we're looking at an artist's visualization of a neutrophil very, very close-up as we can't see them with the naked eye. As we just saw, neutrophils belong to a granulocyte group of white blood cells. They ranged between twelve and fifteen micrometers in diameter and make up between fifty four to sixty two percent of circulating white blood cells.

As you can see in this micrograph, the nucleus of a neutrophil is lobed and typically exhibits three to five lobes. That’s why in some literature, you may come across neutrophils being referred to as polymorphonuclear leukocytes or PMNs. These nucleoloids are linked together by thin nuclear extensions. In females, the inactive X-chromosome appears as a drumstick-looking appendage. On one of these lobes which we can see in our image here, the appendage is known as a Barr body. It may not always be visible but it's worth being aware of.

Neutrophils belong to the granulocyte group of white blood cells because they contain specific granules in their cytoplasm that stain a light pink after hematoxylin and eosin staining. These specific granules secrete ECM-degrading enzymes which are enzymes that degrade the extracellular matrix and the granules are also what provide neutrophils with their functional phagocytic capability. You can think of neutrophils like first responders. They're the first to be attracted to and arrive at the site of infection by chemical mediators moving to the site through a process known as chemotaxis.

As phagocytes, neutrophils engulf bacterial cells and other small particles. They are key players in the inflammatory response. Neutrophils release polypeptide chemokines and lipid mediators of inflammation. The release of polypeptide chemokines acts to attract other leukocytes inside the kines that direct the activity of neutrophils and other cells involved in the immune response. The lifespan of a neutrophil is short being only around one to four days.

The next type of leukocyte we'll look at are the eosinophils. Again, we're looking at an artist's impression. Eosinophils comprise about one to three percent of leukocytes and are roughly the same as neutrophils. These are also granulocytes and develop and mature within bone marrow after which they're released into the peripheral bloodstream.

Here in this micrograph, we see an eosinophil in a blood smear. As you can see, this cell type of granulocyte has a bilobed nucleus and like the other granulocytes, it contains oval granules in its cytoplasm. In addition to these, eosinophils also contain other substances such as major basic proteins or MBPs, eosinophilic peroxidases and enzymes and other toxins that act to kill helminths and parasitic worms.

Eosinophils can be found in abundance at sites where chronic inflammation occurs such as the lungs and in the connective tissue of the intestinal lining. This is because the eosinophils, among their other functions, modulate inflammatory response. Eosinophils are triggered into action by allergies and they release chemicals such as cytokines, lipid mediators, and chemokines.

The third type of leukocyte we have are basophils which are also granulocytes and which actually make up less than one percent of leukocytes. They are twelve to fifteen micrometers in diameter and contains specific granules that stain purple or basic on a blood smear. Here we return to our micrograph of a blood smear with a basophil highlighted here in green. The granules in basophils stain so highly because they contain heparin and other sulfated glycosaminoglycans. In addition to these, the granules present in basophils also contain histamine, proinflammatory factors called leukotrienes, and other mediators of inflammation which are released in response to certain allergies and antigens.

Now that we've looked at the granulocytes, let's look at the leukocytes that are classed as agranulocytes starting with the lymphocytes. Lymphocytes are the second most abundant leukocytes found in blood constituting around twenty five to thirty three percent of all leukocytes and are typically the smallest of all the leukocytes. They possess spherical nuclei which we can see here in this micrograph of the lymphocyte. As you can see here, at the center of the leukocyte is a round nucleus with only very minimal cytoplasm.

Lymphocytes can be subdivided into groups based on the surface molecules they possess known as cluster of differentiation or CD markers as you may know them. These CD markers which can be distinguished through immunocytochemistry using antibodies subdivide lymphocytes into four major classes – B-lymphocytes, helper T-lymphocytes, cytotoxic T-lymphocytes, and the natural killer or NK cells – all of which play a number of roles in immune defense against parasites, microorganisms and abnormal cells. The lifespan of lymphocytes varies according to their specific function and they can survive in circulating blood anywhere from a few days to several years in certain tissues.

Now, let's take a look at the other type of agranulocyte and the final type of leukocyte we'll be looking at in this tutorial known as monocytes. Monocytes are typically twelve to fifteen micrometers in diameter and possesses C-shaped nucleus. Their cytoplasm stains basophilic as you can see in this micrograph and contains small azurophilic granules called lysozymes. The high distribution of lysosomes is what causes the cytoplasm of monocytes to appear bluish in color as you can see in this micrograph. It should be noted that monocytes are precursor cells of the mononuclear phagocyte system including osteoclasts, microglia and macrophages like the one seen here in this micrograph. All monocyte-derived cells are antigen-presenting cells and as such play an important role in the body's immune defenses.

Now let's take a look at another important component of blood which are the platelets. These non-nucleated cells measure between two to four micrometers and function to promote blood clotting. Platelet cells also play a role in tissue repair such as the repair of minor tears and leaks in the walls of blood vessels. Here in this micrograph, we see platelets which are also known as thrombocytes. They are in fact fragments of the phospholipid membrane of a larger cell called the megakaryocyte found in the bone marrow which has released them into the bloodstream. In blood, the normal count of platelets or thrombocytes amount between one hundred and fifty thousand to four hundred thousand per microliter and they have a typical lifespan of up to ten days.

Now that we've had a look at the components of blood, let's briefly cover a blood-borne disease that affects millions of people and results in hundreds of thousands of deaths every year which is malaria. Malaria is an infectious disease transmitted by infected female Anopheles mosquitos to humans and to other animals. A bite from an infected female mosquito introduces the parasite plasmodium from the mosquito's saliva into the bloodstream which we can see in this magnification here. These parasites cause malaria symptoms occurring within ten to fifteen days after infection which include fever, vomiting, headaches, and fatigue. In several cases, this disease can lead to death.

Malaria can be diagnosed through the microscopic examination of blood films such as the one seen here or by antigen-based rapid diagnostic tests. Blood smears or blood films like the one seen here detect the parasite using light microscopy and specific staining techniques. To treat this disease, medications are prescribed such as chloroquine, mefloquine or doxycycline to kill the plasmodium in the blood.

This brings us to the end of the tutorial. Now, let's quickly recap.

We've learned that blood consists of blood cells suspended in aqueous solution known as blood plasma. We've also seen that blood consists of different cell types, the first one being erythrocytes also known as red blood cells. Erythrocytes are non-nucleated and their cytoplasm contains lots of hemoglobin which is what allows them to carry oxygen and carbon dioxide to and from the body's tissues. Erythrocytes mature from reticulocytes which are also non-nucleated and are formed in the bone marrow before being released into the general bloodstream. A high number of these in the blood can be an indicator of anaemia or blood marrow disorders.

We then looked at leukocytes which are also known as white blood cells. These can be subdivided into two groups, the first of which are the granulocytes which include neutrophils, eosinophils and basophils. The second group are the agranulocytes which include lymphocytes and monocytes. We also saw that the other cell types found in blood are known as platelets. These are responsible for blood clotting and play a role in the tissue repair of blood vessels.

And that's it! I hope you enjoyed this tutorial and have learned something more about the specialized connective tissue known as blood. Happy studying!

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