So here we have an example problem that says, imagine you're a pathogen trying to enter a human body via the skin on a person's arm. Now, if you look at the skin on your arm, you'll notice that it has some hairs on it, and that's an indication that this is going to be thin skin. And so we can go ahead and label the skin on a person's arm here as thin skin. Now, the problem here asks, which layers of the epidermis do you have to get through and in what order, and what might be some challenges you encounter along the way, and it ends with asking how would this journey differ if you were entering via the palm of their hand. Now, if you look at the skin on the palm of your hand, you'll notice that it does not have any hairs on it and that's an indication that it is thick skin.

And so recall that the skin on the palm of your hand and the skin on the soles of your feet is going to have thick skin. Now, we're going to approach this problem by breaking it up into 2 parts. The first part will focus on the epidermal layers and the second part will focus on the challenges that the pathogen can encounter. And so, of course we know that the epidermal layers in thin skin are going to be different than the epidermal layers in thick skin. And of course, because thick skin is thicker, we know it's going to have more epidermal layers than thin skin.

And so, recall the memory tool that helps us remember the epidermal layers in order from the deepest layer to the most superficial layer is "beautiful skin glows like crystals." And so, that reminds us that it is the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum. And of course, thick skin is going to have all of these layers. And recall that the stratum lucidum is unique to thick skin. It is only found in thick skin.

And so, in terms of the layers in thin skin, really it's going to have the exact same layers. The only difference is that once again, it's not going to have the stratum lucidum, so we can cross that one off. And so, in terms of the pathogen, of course the pathogen is going to start on the outside of the body. And that means it's going to start on the most superficial side of the epidermis. And so, that means that the pathogen is going to start at the top and make its way down.

And so, in thin skin, such as the skin on a person's arm, the pathogen will need to go from, starting from the stratum corneum to the stratum granulosum, then the stratum spinosum, and ending with the stratum basale. And then in thick skin, it'll make the same journey except again it has to go through the stratum lucidum as well. And so really that's the biggest difference between the journey the pathogen would need to take in thin skin versus thick skin. Now, in terms of some challenges that the pathogen will encounter along the way, of course, it's going to encounter these really tightly and firmly connected keratin-filled keratinocytes, which recall make up the vast majority of the epidermis. Something like 90% of the epidermal cells are keratinocytes.

And those really, really tightly packed keratinocytes are going to create a very difficult barrier or create a barrier that is very difficult for the pathogen to penetrate through. And so, the keratinocytes are found in all of these layers and so, that will create a challenge for the pathogen throughout its entire journey. Now, another challenge that the pathogen is going to encounter along the way are dendritic cells, which recall dendritic cells are cells that basically scout the epidermis for signs of pathogens. And then they can actually leave the epidermis and alert the immune system and activate components of the immune system.

And so, a pathogen would need to be able to evade these dendritic cells in order to be successful at invading the body. Now, recall that these dendritic cells are predominantly found in the stratum spinosum layer. And so, that is a challenge that the pathogen would encounter mainly when it gets to the stratum spinosum. And so, this here concludes this example problem and I'll see you all in our next video.