In this video, we're going to talk about key features of the second epidermal layer in our lesson which is the stratum spinosum. And so the term spinosum actually means spiky or spiny which is why this layer is also called the spiny layer. Now, it gets its name from the spiky or spiny artifacts that appear during the preparation of cells in this layer for microscopy, but those spiky or spiny artifacts do not exist in the actual living tissue. Now, the stratum spinosum or the spiny layer is the second deepest layer of the epidermis right behind the stratum basale. And so you can think that the s in spinosum or the s in spiny layer is for the s in the word second to remind you of the second deepest layer.

Now, as you can see over here in this diagram where the stratum spinosum is colored like this, most of the stratum spinosum consists of many rows of these dividing keratinocytes. Now, it is worthy of noting that the keratinocytes that are deeper in the stratum spinosum are actually closer to the underlying vascular connective tissue and so they have more direct nutrients or more direct access to nutrients from those blood vessels. Whereas, the cells that are more superficial in the stratum spinosum are further away from the underlying connective tissue. And so they have less access to nutrients from those vascular tissues. And because that's the case, the cells that are closer to the underlying tissue have more direct access, and therefore have more capacity to divide.

Whereas the cells that are further away are going to have less access to nutrients and therefore less capacity to divide. And so, in fact, as the cells divide and get pushed up into more and more superficial layers, what we'll notice is that the next layer, the cells have lost their capacity to divide entirely because they are so far away and have very limited access to any nutrients from underlying connective tissues. Now, it is worthy of noting that the stratum spinosum or the spiny layer is usually the thickest epidermal layer, especially in thin skin. But in thick skin, usually the stratum corneum is the thickest layer, and the stratum corneum is the most superficial layer of the epidermis. Now keratinocytes in the stratum spinosum or spiny layer will actually begin to flatten at the top of this layer.

And so what you'll notice is that here in this diagram, the keratinocytes in the stratum spinosum are flattening toward the top of the layer, or the more superficial side of the layer. And the reason that they flatten is because of their keratin production. So recall that the deeper cells are newer cells since they originate in the stratum basale. But the more superficial cells are older cells, and so they've had more time to produce more keratin, and so that's why they start to flatten due to that keratin production. Now, it's also worthy of noting that in the stratum spinosum is where we can find dendritic cells.

Specifically, the Langerhans cells. And recall that these dendritic cells, or Langerhans cells, are immune cells that help to provide immunity by activating the immune system. And so later in our course, we'll get to talk more details about the exact functions of dendritic cells when we're focusing on the immune system. But really these dendritic cells are just scavenging through the stratum spinosum of the epidermis looking for signs of infection or invasion. And if they detect a sign of infection or invasion, they can actually migrate out of the epidermis into the lymphatic system, travel to a nearby lymph node to help activate other immune cells in that lymph node, and those other immune cells can generate a significant immune response to help prevent the infection.

And so what you'll notice here is that we've got this image of our analogy. And so notice that Cari the keratinocyte in the stratum spinosum is starting to sprout. And so now Cari the keratinocyte is this young adult. And so again, as cells divide, they will get pushed up into more superficial layers. And so the next superficial layer here is the stratum granulosum.

And so we'll talk more about those key features in our next video. So I'll see you all there.