Development of Skin and Associated Glands

00:01 Hello, and welcome to our discussion of the development of the skin, its associated glands, and other skin derived structures such as teeth, hair, and nails.

00:10 We´re gonna start by discussing the skin and the truly interesting thing about it is that skin comes from both components of ectoderm and mesoderm that underlies it.

00:21 Now, structures like the glands, the hair, teeth, fingernails, toenails, all come from ectoderm that moves into the underlying mesoderm and then develops specialized features that allow it to actually do its work.

00:35 Now, skin is gonna come from the ectoderm and the epidermis, your outermost layer of skin comes from ectoderm and it´s covering the entire embryo.

00:47 The dermis on the other hand is deep to the epidermis and is gonna be coming from mesoderm.

00:53 Now, both lateral plate mesoderm and the dermatome from the somites contribute to the dermis but the dermis and epidermis form a united layer that keeps us safe from the external environment.

01:06 Now, take a look here and we can see the somite has split into its three components, the schleratome, the myotome, and most superficially, the dermatome.

01:15 The dermatome is going to extend, spread out, and cover a large portion of the embryo and take its nerve supply with it which is what creates the dermatome map of the body as nerves from the spinal cord innervate the dermatome.

01:30 Now initially, the ectoderm is gonna form a single layer on the surface of the body and during development in the amniotic cavity, it´s gonna release some cells onto the surface called periderm.

01:42 Now, these periderm cells coat the entire body.

01:45 They´re gonna die and also have a little bit of glandular secretions, some sebaceous gland secretion associated with them.

01:54 The periderm and gland secretion is gonna create the vernix or more properly, the vernix caseosa and this is a covering of the developing body that keeps the embryo safe from the amniotic fluid.

02:07 Amniotic fluid is actually just a little bit irritating to the developing embryo and fetus.

02:12 And the vernix keeps it from getting annoyed and irritable with that fluid.

02:17 Now, in addition to the periderm, the ectoderm is going to proliferate and as the epidermis develops, it´s gonna develop the five specialized layers.

02:27 The bottom most one is the stratum germinativum or stratum basale.

02:32 And then, the other four layers the stratum spinosum, granulosum, lucidum, and corneum will develop as the body gets thicker and thicker skin and the keratin on the outside keeps the body safe from dehydration, losing water from the inside, as well as getting infected by bacteria, viruses, and other pathogens from the outside.

02:56 In addition to that, we have melanoblasts migrating into the developing skin.

03:02 Now, these are derived from neural crest cells and they´re gonna take up residence at the bottom layer of the developing epidermis, the stratum basale or germinativum.

03:12 These melanocytes are gonna have multiple cytoplasmic processes stretching into the cells of the epidermis and they release melanin.

03:22 Melanin is what gives us our characteristic skin pigment and it is heritable and we have a variety of types of melanin that are released and a variety of places it appears.

03:34 Melanin is gonna show up in our hair follicles, in our iris, and our skin.

03:39 And if we have failure of that melanin to be produced properly, we can develop albinism.

03:45 Now, neural crest cells need to migrate all over the body and if we have failure of those neural crest cells to migrate properly, we have lots of problems.

03:53 Albinism is not generally due to migration problems but because those neural crest cell derived melanoblasts cannot create functional melanin.

04:03 Usually, this is due to a mutation in the tyrosinase enzyme and melanin is not gonna be functional and therefore, no pigment is released into the skin, eyes, and hair, and as you can see, this person has albinism and the iris appears pink because we can only see the blood vessels, there´s no intervening pigment, there´s no real pigmentation to the hair or skin as well.

04:26 Now, other problems can occur in the process of developing our epidermis.

04:31 Amongst these we have a range of conditions called ichthyosis.

04:36 Now ichthyose means fish and it´s so called because the scaly appearance of this over keratinized skin reminded people of fish scales at one point.

04:46 So ichthyosis has a variety of presentations.

04:49 The most severe is harlequin ichthyosis where there´s very, very thick hyper-keratinized skin and as the infant moves, there´s gonna be cracks that develop in the skin and they´re gonna be there for quite a long time.

05:02 Much less severe are what are called, collodion babies.

05:05 In this case, there´s a tight membrane of skin overlying the infant when they´re born and as the infant breathes and moves, they´re going to have that collodion little membrane rupture.

05:17 Now, this may just resolve right then and the skin may develop normally thereafter or there may still be some tight parts of the skin throughout the body.

05:27 Another condition is called lamellar ichthyosis and in this case, it starts like the collodion baby where there´s a tight membrane restricting movement but the scaling will persist thereafter and one hallmark of lamellar ichthyosis is an absence of sweat glands.

05:44 Speaking of sweat glands, let´s talk about how these glands develop from the ectoderm.

05:49 A thickening of the ectoderm is gonna grow down into the underlying mesoderm and this is gonna be creating a bud.

05:58 This little sweat gland bud grows down and down, and it starts to coil.

06:02 And the coiling becomes more and more intricate the further it moves down into the underlying dermis.

06:07 As development proceeds, the space within that duct hallows out to create a lumen and lining the duct and along the base of the gland are secretory cells.

06:18 These secretory cells are going to release sweat and then, nearby, myoepithelial cells, smooth muscle type epithelial cells are gonna contract, constrict the lumen, and propel sweat up the duct and out to the pore on the surface of the skin and that´s what allows sweat to leave the gland and cover our skin.

06:40 Now, just like sweat glands, we also develop mammary glands but we develop them in a specific area.

06:48 There´s a mammary line running from the axillary fold down the anterior or ventral surface of the body towards the inner thigh.

06:55 This mammary line is the place where mammary glands can develop.

07:00 Now, this mammary line is present in all mammals and you may note that some animals have a line of teats along this line but humans tend to develop it only at about the T4 area.

07:11 Although, it´s not uncommon to have accessory nipples or even fully functional accessory breast tissue develop anywhere along this line.

07:19 Most commonly, there´s a tiny little nubbin somewhere along this line, not a functional breast, or even really distinguishable as a nipple but they do tend to appear along that line.

07:30 So how do mammary glands form? Just like sweat glands, a thickening of the ectoderm then moves into the underlying mesoderm and that primary bud starts to form exactly like a sweat gland, an eccrine sweat gland would form.

07:45 But secondary buds move off of this primary bud and go every which direction into the underlying mesoderm.

07:53 And we wind up with a very sophisticated drainage pattern of all those secondary buds draining to the lactiferous ducts and those are gonna be carrying milk out of the gland into a mammary pit that underlies the nipple.

08:10 So when these glands become activated after puberty by estrogen and the carrying of an infant in the fetus, they´re gonna start shedding milk into the lumen, along with the cells themselves, they´ll move out to the mammary pit, to the nipple, and be available for nurturing and giving food for the newborn infant.

08:34 Thank you very much.

08:35 And we´ll return and talk a little bit further on the next talk.