Development of Hair and Nails

00:01 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.

00:14 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.

00:27 As it moves in, it´s gonna surround a little nub of the underlying mesoderm called the hair papilla.

00:34 Now, that papilla is largely there to provide innervation and blood supply to the developing hair follicle.

00:42 The follicle itself is derived almost exclusively from the ectoderm and epidermis that it turns into.

00:49 So hair is an actual modification of skin.

00:54 Just as skin is released outward in a flat sheet and becomes keratinized.

00:59 Hair grows outward but is made of concentric rings of keratin and that´s what gives hair its typical appearance and toughness.

01:09 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.

01:20 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.

01:45 So as the hair grows outward, it´s continuously replenished by new keratinized hair that´s pushing upward from the papilla.

01:53 So as we shed hair, new hair will continue to grow.

01:56 At the same time, we have sebaceous glands and arrector pili muscles associate with the hair follicle.

02:04 Now, the sebaceous glands are just an extension of the epidermis itself.

02:09 These glands are gonna release sebum into the area around the hair and the arrector pili are muscles that move the hair follicle.

02:16 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.

02:32 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.

02:55 That´s our arrector pili getting ready for flight or fight reaction.

02:58 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.

03:09 Now, during early development we have a very fine coating of hair called lanugo hair over our entire body.

03:16 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.

03:31 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.

03:47 Now, fingernails and toenails are just modifications of hair.

03:53 They´re densely packed keratin.

03:55 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.

04:05 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.

04:16 The proximal nail fold near our cuticle starts producing actual keratin which pushes the developing nail plate forward and covers the nail field.

04:28 Now, by the time we reach 32 weeks of development, the nail plate has moved forward and pretty much covers the mature spot.

04:36 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.

04:48 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.

04:58 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.

05:13 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.

05:27 Alright, thank you very much and I´ll see you for our next talk.