Organs such as hair, sweat glands and sebaceous glands that develop from the embryonic epidermis are labelled appendages of the skin or epidermal derivatives. Nail and the enamel of your teeth also derive from the epidermis.

In this lab, we will only study hair, sweat glands and sebaceous glands.

HAIR

The primary function of hairs is protection: in human, hairs protect the scalp from injury, from the sun ray and decrease heat loss. Eyebrows and eyelashes protect the eye from foreign particles. Hairs in the nostrils filter the air we inhale and hairs in the external ear canal also prevent foreign particles to penetrate there.

Hairs are made of dead keratinised cells.

The two regions of a hair are:

- 1 - the shaft -

It is the portion of the hair that projects from the surface of the skin.

The shape of the shaft determines the curliness of your hair:
• round shaft -> straight hair
• oval shaft -> wavy hair
• flat shaft -> kinky hair

- 2 - the root -

It is the portion of the hair embedded in the skin. It extends from the epidermal surface into the dermis but in the scalp, it can extend into the hypodermis.

The hair (shaft and root) is made of three tubes that fit into each other. The three tubes are made of keratinised cells.

The outer tube is the cuticle:

it is made of one layer of heavily keratinised cell.

The middle tube is the cortex:

It contains several layers of cells. The cells contain pigments in dark hair and air bubbles in white hair.

The inner tube is the medulla:

it is composed of 2 or 3 rows of cells containing pigments and air spaces.

The root of the hair is contained inside a bag called the follicle (follicle means bag). The base of this bag expends forming the hair bulb. A nipplelike extension of dermal tissue called the papilla protrudes inside the hair bulb and contains capillaries that supply nutrients to the growing hair. Sensory nerve endings called the root hair plexus surround the bulb.
The wall of this bag is made of two layers: the external layer is the connective tissue root sheath derived from the dermis and the internal layer is the epithelial root sheath derived from the epidermis. The epithelial root sheath thins as it approach the hair bulb so that only one layer of epidermal cells remains to line the papillae: the cells of the stratum germinativum. These cells are actively dividing and produce the hair cells. As the older hair cells are pushed upward by the newer hair cells produced, they become increasingly keratinized and die.

Associated with each hair follicle is a tiny band of smooth muscle: the arrector pili. This band of smooth muscle is attached at one end to the papillary layer of the dermis and at the other to the connective tissue root sheath of the follicle. contraction of the arrector pili in hairy mammals raises the hair, thus increasing the thickness of its coat. This provide increased protection against heat loss in cold weather or increase in body size to intimidate potential adversaries. In human, contraction of these muscles causes only "goose pimples".

SEBACEOUS GLANDS

Sebaceous glands or oil glands are simple branched areolar glands. Functionaly they are holocrine glands. They secrete the sebum (seb = oil) an oily product. Sebum is usually secreted into a hair follicle but in a few regions of the body (lips and mammary papilla for exemple) they are directly secreted onto the skin surface. Sebum is a natural skin cream: it helps hair from becoming brittle, prevents excessive evaporation of water from the skin, keeps the skin soft and contains a bactericidal agent that inhibits the growth of certain bacteria.

Sebaceous glands are scattered all over the surface of the skin except in the palms, soles and the side of the feet.

SWEAT GLANDS

Sweat glands or sudoriferous glands are simple coiled tubular glands. They are divided into two principal types: eccrine and apocrine.

Eccrine glands are the most common. Their secretory portion can be located in the dermis or in the hypodermis. They produce sweat, a watery mixture of salts, antibodies and metabolic wastes. Sweat prevents overheating of the body and thus helps regulate body temperature.

Apocrine glands are found mainly in the skin of the armpits, of the anogenital areas and of the areola of the breasts. Their secretory portion can be located in the dermis or in the hypodermis. Their excretory ducts open into hair follicles. Their secretion is more viscous than that of the eccrine glands. They start secreting at puberty and may be analogous to the sexual scent glands of other animals.

Ceruminous glands (or ear wax glands) and mammary glands are modified apocrine sweat glands.

NAILS

Nails are hard plates of tightly packed keratinized cells. They are clear and cover the dorsal surface of the last phalanges of fingers and toes. Nails protect the the ends of the fingers, allow us to scratch various parts of our body and help us grasping and manipulating small objects.

Each nail is made of three regions:
1- the nail body: portion of the nail that is visible. The nail body rests onto the nail bed: an epithelium made only of the stratum basale and the stratum spinosum. There are no stratum granulosum, stratum lucidum and stratum corneum in the nail bed.
2- the free edge: visible portion of the nail that extend past the end of the digit. It is the distal part of the nail that you clip every now and then.
3- the nail root: hidden portion of the nail that is embedded in a fold of skin (or nail fold). The cuticle is the stratum corneum of the nail fold that is pushed forward over the nail.

The site of nail growth is the nail matrix that is found beneath the nail root. The nail matrix is thick and is only composed of the deeper layer of the epidermis: the stratum basale (or germinativum). The keratinization of the cells of the nail matrix proceeds in the absence of a stratum granulosum and lucidum and this results into formation of a of a rigid and durable plate. As the nail matrix thus proliferates and differentiates, this hard plate is pushed forward onto the nail bed and the nail grows. Look at your nail, you will see a white crescent called the lunula that reflects the presence of the thick matrix underneath.