Epidermal Cells

Keratinocytes

Keratinocytes are the most abundant cellsThe basic structural and functional units of life. in the epidermisThe outermost layer of the skin, made of stratified squamous epithelium.. There are other cells, but keratinocytesThe most abundant cells in the epidermis, responsible for producing keratin. being the most abundant lend their characteristics to the epidermis in general. As you can tell by their name keratinocytes are full of the protein keratinA strong, fibrous protein that forms the structure of skin, hair, and nails. which is a waterproofing protein. We learned from another mini lecture that the skinThe body’s largest organ, providing protection and regulation. provides an outer covering to prevent dehydrationA condition in which fluid loss exceeds intake, leading to a decrease in total body water.. The most damaging issue with burns is losing the ability to retain liquids in your body. Burns of second and third degree just essentially weep your fluids out leading you to low blood volume. Understand that keratinocytes is the name that we give these cells in the epidermis of skin. However these keratinocytes can be defined as an epithelial tissue containing stratified squamous cells. Remember that epithelial tissues kind of occur along the spectrum and that’s what we have here in skin. Epidermal cells begin as cuboidal in shape when they are created from a stem cell in the stratum basaleThe deepest layer of the epidermis, where new skin cells are formed.. As they rise up, they die. In the process, they become flattened into a squamous shape.

One thing that students usually get confused about is the borderline between alive and dead cells in skin. The borderline as you can see from this picture here is in the epidermis itself. The borderline is not the borderline between the epidermis and the dermisThe thick inner layer of the skin that contains blood vessels, nerves, and connective tissue.. The borderline of live and dead cells occurs about 1/3 stop the thickness of the epidermis. This distance is where keratinocytes are cut off from the blood vessels. These vessels of the subpapillary plexusA network of blood capillaries that vascularize the papillary layer of the dermis of skin. Loops ext loop up into each one of these dermal papillaeUpward extensions of the dermis that interlock with the epidermis, forming fingerprints.. As these cells die, their nucleusThe control center of the cell that contains DNA and directs cellular activities. becomes flattened. Their vesicles of melaninA brown-black pigment made by melanocytes in the stratum basal and given to keratinocytes as melanos explode. Additionally, the attachments holding the cells together also start to degrade.

I have a picture of one of those attachments here which is called a desmosome. Your skin is a protective layer. These desmosomes hold your cells together very tightly. They form an impenetrable layer. Desmosomes are not completely degraded as skin cells start to shed off from your epidermis. Active desmosomes cause skin cells to shed in sheets instead of individual cells. Desmosomes form what we officially call tight junctions between epidermal cells. These types of junctions and desmosomes are discussed in more detail in the tissues chapter of your book. We cover this chapter piece by piece in lecture.

Other Epidermal Cells

There are many other types of cells that exist in the epidermis. Although we say that the epidermis in general is not innervated, some sensory cells are present. This means there are no nerve fibers in the epidermis itself. However, the nerve fiber to which they are connected is not in the epidermis. It connects them to the brain. This leads us to say that the epidermis is not innervated.

This orangey cell right here at the top of an epidermal ridge is a tactile disk. This cell detects pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. In factA statement based on direct observation that is repeatedly confirmed., this cell is placed in the stratum basale of the epidermis. It senses light touch. Other pressure sensors deeper in the dermis sense more forceful sensations. This tactile disk is connected to a nerve fiber. The nerve fiber runs all the way to the brain. You can see this nerve fiber in yellow here. The nerve fiber is also connected to another sensory cell, something that might detect vibrations.

These cells up here are scattered throughout the keratinocytes in the stratum spinosumA layer of the epidermis with cells connected by desmosomes, giving a spiny appearance. and stratum granulosumThe layer of the epidermis where keratinization begins.. They are called dendritic cellsImmune cells in the epidermis that help fight infections.. The word dendritic means branching in these cells have a branched type appearance to them. These cells also do pinocytosis which as you may recall is still drinking. These cells roam your epidermis drinking the interstitial fluids. If these cells encounter something that seems like a pathogen or an invader, they will identify it as bacteria. They might also identify it as a virus, allergen, or fungus. Then, these cells will run to a lymph node. They will tell your B cells that you are being attacked by whatever it is. These cells are placed in the epidermis of the cutaneous membrane. However, they perform an immune function.

This glob of brown cells down here at the bottom of an epidermal ridge these are melanocytesCells in the stratum basale that produce melanin, the pigment responsible for skin color..  Melanocytes are also cells contained in the stratum basale. Their purpose is to make melanin give it to the keratinocytes.

You are Shedding

As the stem cells in the stratum basale continue to do mitosis, they create one cell to replace themselves. Another cell moves through the epidermis and sloughs off.  By doing so, the epidermis becomes a conveyor belt. The keratinocytes move from deepAway from the surface of the body. to superficialNear the surface of the body.. They rise through the strata spinosum, granulosum, lucidum (if thick skinSkin found on the palms and soles, containing an extra layer (stratum lucidum).), and corneum.  As it says here, keratinocytes rise through the strata quite quickly, in about 7 days.  However, it can then take the cells one to two weeks to shed completely off.  This is why wounds can take two weeks to fully heal as the damaged cells are sloughed off and replaced.