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The predominant cell type in the epidermis, the outermost layer of the skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer (stratum germinativum) of the skin are sometimes referred to as basal cells or basal keratinocytes.
The epidermis is the outermost layer of our skin. It is the layer we see with our eyes. It contains no blood supply of its own—which is why you can shave your skin and not cause any bleeding
despite losing many cells in the process.
Assuming, that is, you don’t nick your skin to deep, where the blood
supply is actually found.
The epidermis is itself divided into at least four separate
parts. A fifth part is present in some
areas of our body. In order from the
deepest layer of the epidermis to the most superficial, these layers (strata)
The stratum basale, also called the
stratum germinativum, is the basal (base) layer of the epidermis. It is the layer that’s closest to the blood
supply lying underneath the epidermis.
layer is one of the most important layers of our skin. This is because it contains the only cells of
the epidermis that can divide via the process of mitosis, which means that skin cells germinate here, hence the
In this layer, the most numerous
cells of the epidermis, called keratinocytes, arise thanks to mitosis. Keratinocytes produce the most important
protein of the epidermis.
is appropriately called keratin. Keratin
makes our skin tough and provides us with much-needed protection from
microorganisms, physical harm, and chemical irritation.
Millions of these new cells arise in
the stratum basale on a daily basis. The
newly produced cells push older cells into the upper layers of the epidermis
with time. As these older cells move up
toward the surface, they change their shape, nuclear, and chemical
composition. These changes are, in part,
what give the strata their unique characteristics.
Stratum Spinosum and Granulosum
From the stratum basale, the keratinocytes move into the
stratum spinosum, a layer so called because its cells are spiny-shaped cells. The stratum spinosum is partly responsible
for the skin’s strength and flexibility.
From there the keratinocytes move into the next layer, called
the stratum granulosum. This layer gets
its name from the fact that the cells located here contain many granules.
The keratinocytes produce a lot of keratin in
this layer—they become
filled with keratin. This process is
known as keratinization. The
keratinocytes become flatter, more brittle, and lose their nuclei in the
stratum granulosum as well.
Stratum Lucidum and Corneum
Once the keratinocytes leave the stratum granulosum, they
die and help form the stratum lucidum. This
death occurs largely as a result of the distance the keratinocytes find
themselves from the rich blood supply the cells of the stratum basale lie on
top off. Devoid of nutrients and oxygen,
the keratinocytes die as they are pushed towards the surface of our
The stratum lucidum is a layer that derives its name from
the lucid (clear/transparent) appearance it gives off under a microscope. This layer is only easily found in certain
hairless parts of our body, namely the palms of our hands and the soles of our
feet. Meaning, the places where our skin
is usually the thickest.
From the stratum lucidum, the keratinocytes enter the next
layer, called the stratum corneum (the horny layer filled with cornified cells). This the only layer of skin we see with our
The keratinocytes in this layer
are called corneocytes. They are devoid
of almost all of their water and they are completely devoid of a nucleus at
this point. They are dead skin cells
filled with the tough protein keratin. In
essence, they are a protein mass more so than they are a cell.
The corneocytes serve as a hard protective layer against
environmental trauma, such as abrasions, light, heat, chemicals, and
microorganism. The cells of the stratum
corneum are also surrounded by lipids (fats) that help repel water as
well. These corneocytes are eventually
shed into the environment and become part of the dandruff in our hair or
the dust around us, which dust mites readily munch on.
This entire cycle, from new keratinocyte in the
straum basale to a dead cell flaked off into the air, takes between 25–45