Hello, we´re now gonna discuss the formation of hair and nails
which are both variation on both the ectoderm, the epidermis,
and other things like glands that develop from the ectoderm.
Now, initially, the thickening of the ectoderm that creates hair
is gonna grow downward into the underlying mesoderm as a hair peg,
a continuous group of ectodermal cells that´s moving into the mesoderm.
As it moves in, it´s gonna surround a little nub of the underlying mesoderm
called the hair papilla.
Now, that papilla is largely there to provide innervation and blood supply
to the developing hair follicle.
The follicle itself is derived almost exclusively from the ectoderm
and epidermis that it turns into.
So hair is an actual modification of skin.
Just as skin is released outward in a flat sheet and becomes keratinized.
Hair grows outward
but is made of concentric rings of keratin
and that´s what gives hair its typical appearance and toughness.
So the cells that actually lay down the hair are derived from the epidermis
are lining the hair papilla
which then supplies blood and innervation to all those structures.
Melanocytes migrate into the developing hair
and just as they release pigment into the skin,
they release pigment into the developing hair follicle
which winds up in the hair itself
and over time, as our melanocytes run out of activity
and are no longer able to divide and function appropriately,
our hair starts to get whiter and greyer
which unfortunately, I´m in the middle of experiencing right now.
So as the hair grows outward,
it´s continuously replenished by new keratinized hair
that´s pushing upward from the papilla.
So as we shed hair, new hair will continue to grow.
At the same time, we have sebaceous glands
and arrector pili muscles associate with the hair follicle.
Now, the sebaceous glands
are just an extension of the epidermis itself.
These glands are gonna release sebum into the area around the hair
and the arrector pili are muscles that move the hair follicle.
So sebaceous glands release sebum and that´s what gives hair,
unwashed hair in particular, its kind of greasy appearance
so that sebum is kind of greasy
but it also keeps the hair from getting fragile and cracking,
sort of moisturizes it.
The arrector pili on the other hand are derived from mesoderm
and they´re smooth muscles bundles
that connect to the hair follicle and the skin
and they´re involved in thermoregulation,
making sure that our skin and hair follicles are aligned properly
and allow us to have sweat evaporate
but most commonly, we experience the arrector pili
when the hair on the back of our neck stands up
and we have goosebumps.
That´s our arrector pili getting ready for flight or fight reaction.
Now, unlike a cat, the hair at the back of our necks
doesn´t stand up and make us look bigger
but we still have that same reaction
coming from our sympathetic nervous system
when we get into a threat situation.
Now, during early development we have a very fine coating of hair
called lanugo hair over our entire body.
These little fine hairs are just visible
in this picture if you look closely
but they keep the vernix, that little dead layer of skin
present over our body anchored in place
so that the amniotic fluid does not irritate our developing skin.
Now, at birth, the lanugo hair stops developing
and we have the mature hair on our head present at
or shortly after birth
and puberty is when we develop hair in other parts of our body
and it grows a little more wildly along the arms, legs, armpits, etc.
Now, fingernails and toenails are just modifications of hair.
They´re densely packed keratin.
So it´s as if we´ve got a bunch of hair follicles lined up
to produce a sheet of keratin that moves outward
and forms a hard protective covering of our nailbed.
The nailbed is initially just a thickened area of epidermis
at the distal ends of our fingers and the nail field is the place
where the nail will eventually grow into.
The proximal nail fold near our cuticle starts producing actual keratin
which pushes the developing nail plate forward
and covers the nail field.
Now, by the time we reach 32 weeks of development,
the nail plate has moved forward
and pretty much covers the mature spot.
And those who have children
may have noticed that they have fairly long fingernails
if they make it to full term
and sometimes have to have mittens or other coverings
on their hands to prevent them from scratching themselves
with inverting motions of their hands.
So the nail plate is resting on the nail bed
and it´s produced by a germinal matrix
which is located just behind and underneath the cuticle.
Other things that protect and anchor the nail
are going to be the eponychium on its superior surface
and the hyponychium on its underside
and these keep the nail anchored
so that they don´t bend back or get displaced.
And those of you who´ve had a fingernail get damaged
will know that you can accidentally take those nails off
and it takes quite a while for the nail to grow back out
from the cuticle to cover the actual whole nail field.
Alright, thank you very much and I´ll see you for our next talk.