Skin: Appendages

00:01 All right. So that's epidermis and dermis.

00:04 Let's talk briefly about skin appendages.

00:06 And we'll talk briefly now.

00:08 But in subsequent talks this will be the main focus.

00:11 Skin appendages. You're thinking what. Arms and legs coming off the skin.

00:14 No. Skin appendages are the other structures that kind of poke through or are part of the skin and actually are part of skin function.

00:24 So let's think about this.

00:25 So we have nails, nail bed, that's almost, that is pure, almost pure keratin. We also have hair that pokes through.

00:34 We have the various glands that are associated with the hair follicles.

00:38 And we also have other glands not shown here yet that are associated with thermoregulation and apocrine production.

00:47 So let's talk about the hair.

00:49 Um, there is a hair root which sits below the level of the skin.

00:55 There is the hair shaft.

00:56 That's the part that sticks up above the skin.

00:59 There are also glands that are associated with the hair root.

01:03 And those are going to be very important for making secretions that help to lubricate the skin and help to maintain some degree of thermoregulation.

01:11 Sebaceous gland in particular is very important.

01:14 There is also a little column here of smooth muscle that is attached to the hair root, and also is attached to the basement membrane of the epidermis.

01:26 And you're saying, well, why do I need smooth muscle there? You, have your hairs ever stood on end because you were scared? Have you ever seen a dog or a cat try to demonstrate their, you know, their viciousness by raising the hair? That happens because of those arrector pili muscles.

01:45 So we have the hair proper.

01:47 But how does the hair get made? Hair is actually pure keratin.

01:50 Kind of like that nail bed.

01:53 And it is nourished by little capillary bed that sits right in the papillae at the at the very base of the root.

02:02 There is then an epidermal-like structure, basically, that is going to be responsible for secreting the keratins that make the actual hair shaft itself. So there's an internal and an external root sheath that is all continuous, entirely continuous with the overlying epidermis.

02:21 And it's the specialization of the epidermis that allows us to make hair.

02:26 Let's talk about some of the other glands.

02:28 So we talked about sebaceous glands. We'll come back to that in a minute. But we'll talk here about eccrine glands.

02:34 So eccrine glands are everywhere, they're on your soles, they're on your palms, they're on every part of your skin.

02:40 And this is going to be important for thermal regulation.

02:44 These are little tiny glands that are kind of coiled.

02:46 They don't dump their contents onto the hair shaft.

02:48 They dump them directly onto the skin surface.

02:51 And their job is to make water and secrete electrolyte.

02:55 So they're going to directly, when they are appropriately stimulated, say when you get too warm or you get too scared or a variety of other things, you're going to, under cholinergic control, secrete electrolyte followed by water in an obligate way and out to the surface to make sweat, part of the sweat. When that evaporates, you cool.

03:19 So that's why we have eccrine glands.

03:22 Then there are apocrine glands.

03:24 Different glands. This one does dump its contents onto the hair root so that it makes its way out along the hair shaft.

03:33 Okay. That's kind of like the sebaceous gland that we talked about previously, but it has a different role.

03:40 It probably was much, much more important when we were Neanderthals. It was part of pheromone production.

03:49 It was part of how we identified one another.

03:51 We would probably like dogs now sniff each other in a certain area.

03:55 We probably would sniff or sense each other by sniffing armpits or groins, believe it or not. And so that's where these glands exist and dump their contents.

04:06 They dump them in your axilla and your inguinal region.

04:11 They dump their contents, which contain partly electrolyte, partly water, but mostly kind of the apical contents of their cells.

04:20 And they dump them onto the hair shaft and into those areas.

04:23 They're under adrenergic control.

04:25 So different than the cholinergic control of the eccrine glands.

04:28 And because they're associated with kind of recognition pheromone control, we really only have them active after puberty.

04:37 But they are a mechanism by which the various cosmetic companies keep in business by making underarm deodorant, etc.

04:43 Okay. And then finally, let's come back to our apocrine glands.

04:47 Shown on the slide are where the eccrine glands are and the apocrine glands.

04:51 But now we're going to do sebaceous glands.

04:54 The sebaceous glands. These are special glands that dump their entire cell contents all at once onto that hair root.

05:03 So that's there a holocrine secretion as opposed to an apocrine secretion of those other glands. They're located everywhere that there are hair.

05:12 So everywhere except palms and soles.

05:15 And they secrete their contents along the hair shafts. They are part of lubrication.

05:19 So to think of a mechanism by which they're useful.

05:24 Ducks have very prominent sebaceous glands.

05:26 They secrete the oil and the fats that are in those cells onto the hair root or onto the the root of their feathers, comes out and allows them to float.

05:36 Well, we don't need it to float so much, but we do need it to lubricate the skin and maintain a certain level of moisture.

05:42 So it covers and provides a little bit more of barrier function over the surface of the skin. It's also an important source of acne.

05:51 Shown here is kind of the surface topography of the nail.

05:55 We have the lateral nail grooves coming down into the cuticle where the nail is being generated. Usually actually more through the proximal nail fold.

06:06 The clearing at the base of the nail is called the lunula.

06:11 Okay. With this we are concluded our survey of normal structure and function of the skin and skin appendages. Really important just to understand.

06:21 Very cool. Just to understand all of this.

06:23 But it's also going to be very important to refer back to this when we talk about pathology, because understanding normal structure and function, as always, will inform our way of thinking about the pathology, in this case of the skin and its appendages.

06:37 Thanks.