In this video, we're going to talk about the 2nd dermal layer, which is the reticular layer. The reticular layer of the dermis actually lies deep to the papillary layer of the dermis, meaning that the reticular layer lies underneath the papillary layer. The reticular layer actually makes up about 4/5ths or about 80% of the total amount of dermis, making it the largest layer of the cutaneous membrane, or the largest layer of the skin.

Because the reticular layer is the largest layer of the skin, it largely defines the properties of the skin. The reticular layer of the dermis is made up of dense, irregular connective tissue. Recall from our previous lesson videos that dense irregular connective tissue is characterized by having densely packed and irregular or random arrangements of mostly collagen protein fibers, but also elastic protein fibers in the extracellular matrix. This irregular or random arrangement of fibers allows dense irregular connective tissue to resist forces in multiple directions and to display elasticity in multiple directions as well. Thus, the dense irregular connective tissue gives those properties to the skin.

The skin can resist forces in multiple directions, is quite stretchy, and can have elasticity in multiple directions as well. The reticular layer also contains a variety of accessory structures, including some sweat glands and oil glands. It also has hair roots and pressure receptors that are called lamellar corpuscles, also sometimes referred to as Pacinian corpuscles. The term "lamellar" refers to the onion-like layers or the cinnamon bun-like layers that these lamellar or Pacinian corpuscles make. These receptors lie in the reticular layer of the dermis and serve as pressure receptors to allow for pressure sensations.

Recall that the term "reticular" means net-like, and in the reticular layer of the dermis, it refers to the net-like arrangement of mainly collagen protein fibers, but also some elastic protein fibers as well. The collagen protein fibers allow for strength, and the elastic protein fibers allow for elasticity, or the ability to stretch significantly but then return back to its original shape after stretching. Because of this net-like or irregular random arrangement of these fibers, the skin can resist shrinking and forces in multiple directions, and also have elasticity in multiple directions.

Although many of the fibers have this net-like arrangement, it is important that there are also many fibers in the reticular layer of the dermis that may orient in one direction, in a parallel fashion to the surface of the skin. This creates what are known as cleavage lines, also sometimes referred to as tension lines. These cleavage or tension lines result from parallel orientations of many collagen fibers in the reticular layer of the dermis. These surgically relevant invisible lines can be indirectly detected based on the way that our skin resists tension, as well as the way that our skin behaves when it is cut. These lines are relevant to surgeons because when the skin is cut parallel to these cleavage or tension lines, it allows for faster healing.

Let's take a look at our image down below where we can piece some of these ideas together. Notice over here on the left-hand side, we have the diagram of the integumentary system. Notice that only the reticular layer of the dermis is being colored here, and notice that it makes up the vast majority of the dermis, and it is the largest layer of the cutaneous membrane, or the largest layer of the skin, thus largely defining the properties of the skin. It is mostly made up of dense irregular connective tissue, which has plenty of accessory structures within the reticular layer of the dermis, including hair roots, and sweat glands, and also sebaceous or oil glands as well. There are plenty of blood vessels here, making it vascular.

Notice these yellow structures here; those are the lamellar or Pacinian corpuscles, which are pressure receptors that allow for sensations of pressure. Although many of these collagen and elastic protein fibers form parallel orientations that create these cleavage lines, surgeons need to be very familiar with these cleavage lines because, when the skin is cut parallel to them, it allows for faster healing. And finally, notice here on the right, we have a leather jacket, which is important to note is made from the dermis of animal skins. The reticular layer makes up the vast majority of the dermis and is made up of dense irregular connective tissue, which can resist forces in multiple directions and have elasticity in multiple directions as well, properties we find in leather materials. This concludes our brief lesson on the 2nd dermal layer, the reticular layer of the dermis, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.