In this video we're going to go over the makeup, or structure of skin. Human skin, which is also
called the integument, or the cutaneous membrane, is made up of two layers, the epidermis, and the
dermis, which are labeled on this model of skin. The subcutaneous layer at the bottom has also
been labeled, however, it is not actually part of the structure of skin, but it's connected to the
dermis of the skin. I will also be drawing in some components that are not actually on this
model. Let's start by looking at the epidermis. The epidermis consists of four to five layers
depending on the type of skin. Thick skin has five layers and it is found in the palms of the hands
and on the soles of the feet. Thin skin has four layers and is what covers most of the body. The
skin model we're looking at has all five layers. The bottom, or deep layer is called the stratum
basale. It is made up of a single layer of cells attached to a basement membrane. There are three
types of cells in the stratum basal, keratinocytes, melanocytes, and tactile cells. Keratinocytes are
the most common cell in this layer and they go through cell division to replace cells that are
shed from the surface of the skin. These cells can produce a tough structural protein called keratin,
which strengthens the skin and makes it almost waterproof. Melanocytes are scattered among the
keratinocytes and they produce the pigment melanin, in reaction to exposure to ultraviolet light.
Melanin gets transferred to keratinocytes and surrounds the nucleus to protect DNA from mutating
from ultraviolet radiation. Tactile cells are also scattered among the keratinocytes and they serve
as light touch receptors. The next layer, moving upwards in the epidermis, is the stratum spinosum.
This layer is made up of daughter keratinocytes, made from dividing cells in the stratum basal
layer below, and epidermal dendritic cells. The daughter keratinocytes connect to neighboring
cells desmosomes, which are one of the ways cells connect to one another, giving them a prickly
appearance. The dendritic cells are immune cells that help fight infections in the skin. Moving
upwards, the next layer is the stratum granulosum. This layer is comprised of three to five layers of
keratinocytes. The process of keratinization begins in this layer of the epidermis.
Keratinization is where the keratinocytes fill with the keratin protein mentioned earlier.
This process continues as the cells move upwards in the epidermis, and as it continues, the cells
nucleus and organelles are eliminated, and the cell dies. The next layer up is the stratum lucidum.
This layer is only found in the thick skin in the palms and soles of the feet. The keratinocytes in
this layer are clear, flat, closely packed, and have no nucleus or organelles. They are also filled with
the protein called eleidin, which is eventually transformed into keratin. The last, or top layer
is called the stratum corneum. This layer is comprised of dead keratinized cells. It takes
about two weeks for the new keratinocyte to reach the stratum corneum, and it remains in this layer
for about another two weeks before it is shed. Now let's look at the dermis. The dermis is comprised
of connective tissue proper with collagen being the most plentiful type of fiber found throughout
the dermis. The dermis also houses other structures such as blood vessels, hair follicles, sweat glands,
sebaceous glands, which secrete a lubricating oil, sensory nerve endings, nail roots, and arrector
pili, muscles which affect hair follicles. We will cover these structures in depth in future videos.
The dermis has two layers, a papillary layer and a reticular layer. The papillary layer is the top
superficial layer of the dermis, it is composed of loose connective tissue, and forms bumps or
projections called dermal papillae that fit with the epidermal ridges of the epidermis. The form of
the dermal papillae and epidermal ridges increases the surface area of contact between the two layers.
The dermal papillae contain capillaries that supply nutrients to the cells of the epidermis
and they contain sensory nerve endings that help monitor touch on the surface of the skin. The
reticular layer is composed of dense connective tissue with some elastic fibers and many bundles
of collagen fibers, and it extends to the deeper subcutaneous layer. The subcutaneous layer, which
again, is not part of the skin, is located below or deep to the dermal layer of the skin.
This layer is often referred to as the hypodermis, and it consists of loose connective tissue and
adipose connective tissue, or fat tissue. Many times it is referred to as subcutaneous fat. This layer
helps bind the skin to underlying structures, acts as a cushion, protects the body, provides insulation,
and provides for energy storage. And that be the basics on the structure of the skin.