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Loose connective tissue

Different types of loose connective tissue and related structures.

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Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial about loose connective tissue. Before we start talking about loose connective tissue, let me first answer the question, what exactly is connective tissue? This is a term used to encompass a wide range of tissues with different functional properties but with key common characteristics that allow them to be grouped together. Connective tissue is a type of tissue found in the body that's located at virtually every site both internally and externally. It's important because it provides strength, elasticity and metabolic support to all other tissues of the human body. Connective tissue can be classified according to the composition, proportions and organization of its cell components as well as the organization of its extracellular matrix. As such, it can be classified as embryonic connective tissue, connective tissue proper and specialized connective tissue.

So now that you know what connective tissue is and the different classifications of connective tissue, let me give you a quick overview of what we'll discuss today.

The aim of this tutorial is to talk about loose connective tissue, its structure, components and where it's located throughout the body. In addition, we'll discuss some clinical notes relating to loose connective tissue. So let's describe what loose connective tissue is.

As we know, it's a type of connective tissue but what type exactly. Loose connective tissue is a type of connective tissue proper. It's also known as areolar connective tissue, an example of which can be seen in this micrograph. Its cellular content is relatively highly abundant and varied and it typically contains cells such as fibroblasts and adipocytes, fibers such as reticular fibers and ground substance which is made up of complex carbohydrates.

Now that we've defined loose connective tissue, let's move on to look at its cellular components. The most common type of cell found in loose connective tissue are the cells we can now see highlighted in green which are called fibroblasts. Fibroblasts are responsible for producing fibers and the complex carbohydrates of ground substance. Because the fibroblasts reside close to collagen fibers, it's hard to distinguish the cytoplasm clearly, however, their nuclei are quite visible after hematoxylin and eosin staining. If not for our green highlight, you would see that the nuclei of these cells stain a deep purple but as you can see in this image, the nuclei appear condensed, disc-like and elongated.

In an inactive or dormant form, these fibroblasts are known as fibrocytes, an example of which can be seen here in this micrograph. Structurally, fibrocytes are usually smaller and have less processes than active fibroblasts. The most common type of fibers found in loose connective tissue are the ones we can now see highlighted in green which are called reticular fibers. These fibers are produced by fibroblasts and consist of collagen fibrils that are composed of type III collagen. The delicate branched mesh formed by reticular fibers supports the resident cells in loose connective tissue and also provides mechanical strength. Reticular fibers can be found primarily in areas where new extracellular matrix is being produced by fibroblasts such as the site of wound healing and scar tissue formation.

The next component of loose connective tissue we'll look at are mesenchymal stem cells. Mesenchymal stem cells are a specific type of adult stem cell that give rise to fibroblasts. These cells are lineage-specific and along with giving rise to fibroblasts, they also give rise to new blood vessels.

As we just saw, fibroblasts produce fibers and loose connective tissue. An example of which is elastic fibers. Elastic fibers make up part of the extracellular matrix of loose connective tissue and provide it with flexibility or properties of stretching and elastic recoil. These thin strands of elastic fibers are arranged in a branching pattern forming a 3D network within the extracellular matrix. The protein elastin makes up the building blocks of the elastic fibers which are interwoven with collagen fibers to prevent the tearing of loose connective tissue from excessive stretching. Elastic fibers are highly present in the walls of blood vessels. They're also present in the lungs and more externally in the skin as well as in structures that frequently adjust their shapes such as the urinary bladder.

The other type of extracellular matrix that the fibroblast cells of loose connective tissue produce is ground substance which is a clear viscous gel with a high water content. Because ground substance is lost during routine hematoxylin and eosin staining, what we see here in this micrograph is a virtually empty background with cells and fibers. Interestingly enough, ground substance consists of mostly proteoglycans, glycosaminoglycans and structural glycoproteins. It occupies the largest proportion of loose connective tissue and along with elastic fibers, it's also responsible for giving loose connective tissue its flexibility.

Another type of cell that can be found in loose connective tissue is adipocytes. Adipocytes are specialized cells that gradually accumulate fat within their cytoplasm. Adipose tissue which is a specialized type of connective tissue is made up of adipocytes. Incidentally, there are two types of adipose tissue. White adipose tissue is predominant in adults and forms a subcutaneous layer within the loose connective tissue which is known as subcutaneous tissue. The subcutaneous tissue can be found underneath the skin as well as around the internal organs and peritoneal structures. In this micrograph, we can see a meshwork created by the cytoplasm of adjoining cells and some connective tissue. Each space represents the lipid droplet of an adipocyte that's been lost during tissue preparation.

Brown adipose tissue on the other hand is present during fetal life and diminishes significantly from the body within the first ten years of life. However, it's still present in adults in small amounts and aids in body temperature regulation by generating heat.

In the next histological slide, we can see one cell highlighted in green. This cell is called a histiocyte. Histiocytes are macrophages found in the loose connective tissue. They're also known as tissue-resident macrophages and they're derived from monocytes that migrate into the connective tissue to mature. In the next micrograph, we can see three cells highlighted in green here, here and here. These cells are called alveolar macrophages and are found in alveolar tissue of the lungs. Macrophages are phagocytes and they carry out phagocytosis clearing up debris and dead cells from the loose connective tissue. They also present antigens to lymphocytes which we can now see here highlighted in green and are a type of white blood cell.

Now let's look at the macrophages found in the liver. The macrophages that reside in hepatic tissue are known as Kupffer cells and they carry out the same phagocytic activity as other macrophages. In the central nervous system, microglial cells seen here on the right are specialized macrophages that mediate inflammatory response.

In the next histological slide, we can see two cells highlighted in green here and here. These cells are called mast cells. Mast cells are another resident cell type found in loose connective tissue. They arise from hematopoietic stem cells and bone marrow and migrate as immature cells through peripheral circulation into loose connective tissue where they mature. Mast cells contain granules and they act as vasoactive mediators releasing agents that are important in immune response and allergic reactions. These cells are present in connective tissue throughout the body with the exception of the brain and spinal cord.

Now that we've seen some of the cellular components that make up loose connective tissue, you might be wondering where specifically loose connective tissue is found. You may remember at the beginning of this tutorial, I mentioned that loose connective tissue is found in many areas in and around the body. It can be found beneath the dermis layer of the skin as well as beneath the epithelial tissue of all organs of the body that have external openings. The lamina propria lining the digestive and respiratory tracts is also composed of loose connective tissue.

There are many more sites around the body where loose connective tissue can be found. Loose connective tissue is found in the mucous membranes of the reproductive and urinary systems as well as the stroma of glands and the mesentery of the intestines.

Now that we're coming towards the end of our tutorial, let's go over some clinical notes relating to loose connective tissue. The lamina propria is a classical example of loose connective tissue and can be used as a reference point for determining the stage and prognosis of several carcinomas. Carcinoma is a type of cancer that originates from the epithelium and by looking at the extent of invasion of the carcinoma into the lamina propria, clinicians can determine whether the cancer is invasive or not. Urogenital cancer, for example, is considered noninvasive until the tumor completely penetrates the lamina propria after which is considered to be invasive.

Before we conclude this tutorial, let's briefly summarize what we've covered today. We've looked at examples of and describe what loose connective tissue is. We've also seen the cellular components found on loose connective tissue namely fibroblasts, reticular fibers, mesenchymal stem cells, elastic fibers, ground substance and adipocytes. Other cells we've talked about include histiocytes, alveolar macrophages, Kupffer cells and mast cells. In addition, we've also looked at examples of where loose connective tissue is found in the body.

So that brings us to the end of our tutorial on loose connective tissue. I hope you enjoyed it and thank you for listening.

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