Connective Tissue 9c
Welcome back. And to conclude our look at connective tissues, we're going to have a quick look at adipose tissue. Adipose tissue is considered a specialist form of loose connective tissue. We think about how we classify loose connective tissue, made up of lots of cells with few fibres. Well, adipose tissue is made up of lots of cells. In this case, they're called adipocytes, or fat cells, and there is very little extracellular matrix. We do have a little bit of extracellular matrix, but it's predominantly made up of cells. It consists of cells, as I mentioned, adipocytes that are tightly packed together. Each adipocyte is characterized by having a large lipid droplet that seems to fill the cytoplasm. You can see here, the little arrows are pointing to the nuclei of the adipocytes and this white space is where the lipid droplet used to exist. Now, when we process for microscopy, lipids get washed away so they end up with this sort of chicken wire type of appearance or netting type of appearance. The nucleus and cytoplasm are pushed to the extremities of the cell by the big lipid droplet.
This is an example of white adipose tissue or what we call unilocular, which means having one lipid droplet. Adipocytes are often found in areas of loose and dense irregular connective tissue wherever you find blood vessels. Because, of course, the lipids are about storage and then being able to mobilize those fats in times of need. So you need them near a blood vessel, stored near blood vessels, and then mobilized when they are needed. There is a second type of adipose tissue, called brown adipose tissue. It is predominantly found in infants. They start off as a stem cell, and then they either take a pathway. They either become a connective tissue cell like a fibroblast, or they form an early lipoblast. Remember, blast means immature cell, so lipoblast. Then they either become a large white adipocyte (unilocular cell) or a brown adipocyte which is called multilocular. It's called multilocular because they contain multiple lipid droplets.
The main form of fat storage in adults is adipose tissue. It's supporting tissue. We find it around our kidneys, for example, which helps to cushion the kidneys. We find it in the hypodermis and the sub-cutis, which is beneath our skin layers. So beneath the epidermis and beneath the dermis, we find the hypodermis, a little layer of fat beneath our skin that helps with shock absorption. Your buttocks have a amount of adipocytes so if you sit down hard, your adipocytes in your buttocks act as shock absorbers. As mentioned before, our kidneys have lots of adipose tissue around them that acts as a shock absorber, on our posterior abdominal wall and inside our eyes. Also, in the base of our feet, we have adipocytes that hold our eyes in our eye sockets and prevent them from jiggling around too much. Of course, we walk on our feet so the shock there is absorbed by the adipose tissue.
The soles of the feet, buttocks, kidneys, and orbit of the eye serve as energy sources and storage. The general theory of understanding at the moment is that you have a certain number of adipocytes that rather than multiply, actually just increase in size. If someone puts on weight, their adipocytes increase in size. There's another important aspect: these tissues can secrete hormones to help regulate metabolism. For example, there's a hormone called leptin that helps to regulate your metabolism. Being an energy store, adipose tissue needs to be mobilized quickly. They have a rich capillary network and are innovated by the autonomic nervous system. The autonomic nervous system is part of our unconscious control of our bodies through the nervous system, which can mobilize fat stores to provide energy if we need to run away or are severely malnourished.
Brown adipose tissue, on the other hand, is multilocular and contains lots of little lipid droplets. It contains many mitochondria, which give it its pink staining; remember, mitochondria have lots of membranes. The numerous mitochondria provide lots of energy, and the cells metabolize the fat to produce heat. That's why babies have lots of brown adipose tissues. They are often found around the shoulders, and babies can't normally regulate their body temperature when they're first born, so they can mobilize these lipid droplets in brown adipose tissue to help keep them warm. Neonates, meaning newborns, have a large amount of brown adipose tissue, which may be present in adults at selected sites. There is an area of research looking at ways to enhance or increase the amount of brown adipose tissue in people who are obese; this can help convert white adipose tissue into brown adipose tissue and burn it off, leading to weight loss.
Brown adipose tissue is brown because the enzymes give it a brown appearance when viewed in situ. If you were to section and saw brown adipose tissue, it would have a brown tinge due to the enzymes associated with the mitochondria. While we have primarily discussed humans, other animals, such as bears which hibernate, accumulate a lot of brown adipose tissue to keep warm during winter. Here are different views of brown adipose tissue in an electron micrograph. Here's its nucleus, and these are the lipid droplets with lots of mitochondria around the outside which help to metabolize the lipids and produce energy in the form of heat. You can see the capillaries here as well. This image shows the distribution of the brown adipose tissue in neonates, around the shoulders, kidneys, and chest. And this is a PET scan, a type of scan you can do to label the brown adipose tissue, allowing you to see the isolated areas in an adult. You can see the kidneys, urinary bladder, and dark areas where the brown adipose tissue has been highlighted in the scan. While adults have some brown adipose tissue, it is more abundant in neonates. This now concludes the pre-class resource on connective tissue. Next week we'll finish our look at the tissues module with muscle and nervous tissues.
Adipose tissue is a specialized form of loose connective tissue made mainly of adipocytes (fat cells).
Characterized by a minimal extracellular matrix and densely packed adipocytes.
Each adipocyte contains a large lipid droplet that pushes the nucleus and cytoplasm to the cell's edges.
Upon processing for microscopy, lipids wash away, leaving a characteristic chicken wire appearance.
Types of Adipose Tissue
White Adipose Tissue (unilocular):
Features a single large lipid droplet.
Found in various connective tissues around blood vessels, beneficial for energy storage and mobilization.
Brown Adipose Tissue (multilocular):
Contains multiple small lipid droplets and numerous mitochondria.
Predominantly present in infants, helping to regulate body temperature through thermogenesis.
Research is focused on converting white adipose tissue into brown adipose tissue to combat obesity.
Functions of Adipose Tissue
Serves as a major fat storage site, cushioning organs such as kidneys and providing shock absorption in body areas like the buttocks and soles of the feet.
Secretes hormones like leptin to regulate metabolism.
Largely composed of a stable number of adipocytes, which increase in size with weight gain.
Physiological Significance
Richly vascularized and innervated by the autonomic nervous system for quick mobilization of fat stores.
Brown fat is crucial for heat production and has a distinct brown color due to its enzyme content.
Important in various species for survival, seen in bears during hibernation.
Visual and Imaging
Brown adipose tissue has a brown tinge due to enzymes when viewed in situ.
Imaging techniques like PET scans can highlight areas of brown adipose tissue in adults, commonly found around the kidneys, chest, and shoulders.
Overall, while adults have some brown adipose tissue, neonates have a more significant amount.