Hair, skin and nails

Hair, skin glands (including oil and sweat glands), and nails are all considered skin appendages, and they help with regulating body temperature and protection from the environment.

These appendages are found in a layer of skin called the dermis which is a layer sandwiched between two other layers called the epidermis and hypodermis.

The epidermis is the thin outermost layer of skin, the dermis is the thicker layer that lies below that, and the hypodermis is the lowest layer that’s made of fat and connective tissue and anchors the skin to the underlying muscle.

Hair is found on nearly every part of skin except the palms of the hands, soles of the feet, and the lips.

Every strand of hair is composed of the shaft, root, and bulb that sits in a pouch like structure called the hair follicle.

The hair follicle is epidermal tissue that dips down into the dermis, and is associated with other structures like apocrine glands, sebaceous glands, the arrector pili muscle, and nerve receptors.

Inside the bulb lies the hair matrix which serves as the active site of hair growth and gives hair its color.

It contains two different cell types - follicular keratinocytes and melanocytes, and these cells receive blood from a small cluster of capillaries called the papilla.

Similar to the epidermal keratinocytes in the skin, follicular keratinocytes in the hair replicate rapidly and die in a process called keratinization. During this process, keratinocytes produce keratin.

Keratin is a long filamentous protein that comes in many different forms depending on the type of keratinocytes producing it, but it is generally divided into soft and hard keratin; where soft keratin is produced by skin and hard keratin is produced by hair and nails.

Hard keratin is much denser than the soft keratin produced by epidermal keratinocytes in the epidermis, which gives hair and nails their durability.

During keratinization, the hard keratin fills up the entire cell, causing the organelles to flatten out.

Eventually, these follicular keratinocytes die - turning into compact little brick-like cells as they slowly get pushed further and further up into the hair follicle.

As this process continues different parts of the hair strand are made including the hair root and hair shaft that projects from the surface of skin.

Meanwhile, follicular keratinocytes at the bulb of the hair replicate only a set number of times, and once they have reached the maximum number, the hair follicle stops growing and falls out, which leads to baldness.

Now, melanocytes produce melanin, which is a protein pigment, or coloring substance, that gives each strand of hair its color.

Melanin is actually a broad term that constitutes several types of melanin found in people of differing skin color.

These subtypes of melanin range in color from black to reddish yellow and their relative quantity define a person’s skin color and hair color.

Melanocytes move melanin into small sacs called melanosomes, and these get taken up by newly formed follicular keratinocytes--giving hair its color. Over time these melanocytes stop producing melanin, which is why hair turns white as people age.

Looking at some of the structures in the follicle, we can see nerve receptors that wrap around the hair bulb, and these get stimulated when the hair shaft is moved like when a tiny bug walks over our forearm.

There’s also the arrector pili muscle that pulls on the hair making them stand up during cold weather or when you’re frightened - creating what are called goosebumps.

For many mammals - this was an evolutionary trick to make an animal look larger than it really is to scare away predators. Sort of like when a cat gets scared and all of it’s hair stand up tall!

Now, sebaceous glands, or oil glands, are located in the dermis layer of the skin and they’re found everywhere on the skin except on the palms, soles of the feet, and lips.

Sebaceous glands secrete an oily substance called sebum onto a nearby hair follicle or through pores that extend directly to the skin surface.

As it turns out, arrector pili muscles surround the sebaceous glands, so when these tiny muscles contract, sebum gets squeezed out.