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.
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.
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.
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.
The dermis on the other hand is deep to the epidermis
and is gonna be coming from mesoderm.
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.
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.
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.
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.
Now, these periderm cells coat the entire body.
They´re gonna die and also have a little bit of glandular secretions,
some sebaceous gland secretion associated with them.
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.
Amniotic fluid is actually just a little bit irritating
to the developing embryo and fetus.
And the vernix keeps it from getting annoyed
and irritable with that fluid.
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.
The bottom most one is the stratum germinativum or stratum basale.
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.
In addition to that, we have melanoblasts migrating into the developing skin.
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.
These melanocytes are gonna have multiple cytoplasmic processes
stretching into the cells of the epidermis and they release melanin.
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.
Melanin is gonna show up in our hair follicles, in our iris, and our skin.
And if we have failure of that melanin to be produced properly,
we can develop albinism.
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.
Albinism is not generally due to migration problems
but because those neural crest cell derived melanoblasts
cannot create functional melanin.
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.
Now, other problems can occur in the process of developing our epidermis.
Amongst these we have a range of conditions called ichthyosis.
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.
So ichthyosis has a variety of presentations.
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.
Much less severe are what are called, collodion babies.
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.
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.
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.
Speaking of sweat glands,
let´s talk about how these glands develop from the ectoderm.
A thickening of the ectoderm
is gonna grow down into the underlying mesoderm
and this is gonna be creating a bud.
This little sweat gland bud grows down and down, and it starts to coil.
And the coiling becomes more and more intricate
the further it moves down into the underlying dermis.
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.
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.
Now, just like sweat glands, we also develop mammary glands
but we develop them in a specific area.
There´s a mammary line running from the axillary fold
down the anterior or ventral surface of the body towards the inner thigh.
This mammary line is the place where mammary glands can develop.
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.
Although, it´s not uncommon to have accessory nipples
or even fully functional accessory breast tissue
develop anywhere along this line.
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.
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.
But secondary buds move off of this primary bud
and go every which direction into the underlying mesoderm.
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.
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.
Thank you very much.
And we´ll return and talk a little bit further on the next talk.