Skin Derivatives: Epidermis, Dermis, Hair, Nail and Glands

The integument or skin is an elastic and waterproof organ. It is continuous with the mucous membrane of the mouth, rectum, urogenital organs, nostrils and eyelids. The thickness of the skin is uneven at different parts of the body. Skin is thinner under the eyes, whereas it is thicker at the sole. The skin is made up of 3 distinct layers: epidermis, dermis and hypodermis.

Epidermis

The epidermis is the thin, uppermost superficial layer of the skin that covers almost the entire body surface. Anatomically the epidermis is differentiated into five layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum and stratum germinativum or Malpighian layer. The epidermis has a few specialized cells like Keratinocytes that make the epidermis waterproof. Melanocytes produce melanin and protect skin from UV rays. Merkel cells provide a light-touch sensation. And, Langerhans cells which are part of the immune system.

Dermis

The dermis is the underlying layer of the skin. It is much thicker than the epidermis and has a mesodermal origin. The dermis is the deep layer of the skin found under the epidermis. It is composed of loose connective tissue rich in collagen and elastic fibres. Dermis also contains nervous tissue, blood vessels and hair follicles. This layer provides strength and elasticity to the skin.

Hypodermis

It is present deep into the dermis. It is a layer of loose connective tissues. This layer acts as the flexible connection between the skin and the underlying muscles. Hypodermis also possess areolar connective tissue with collagen fibres.  This allows the skin to stretch and move independently of its underlying structures. Adipose tissue found in the hypodermis stores energy and helps the insulation of the body.

Function of Skin

1. Protection: Skin serves as the first line of defence and protects from direct exposure to the external environment and pathogens. Melanocytes present in the epidermis also protect the skin from UV rays by producing melanin.
2. Thermoregulation: Skin can help control body temperature based on the environment. Skin starts perspiration, vasodilation and sweating if temperature increases from normal. When the temperature goes down from normal, vasoconstriction and increased blood flow under the skin stop heat from radiating and cause goosebumps.
3. Storage of food: Adipose tissue stores fat, which serves as food storage which is utilized during fasting.
4. Excretion: Sweat helps excrete excess amounts of electrolytes along with a small amount of metabolic waste products such as lactic acid, urea, uric acid, and ammonia from the body.
5. Secretion: Glands present in the skin secrete various substances like sebum, sweat etc.
6. Vitamin D synthesis: Skin produces vitamin D on exposure to sunlight which is essential for calcium absorption.
7. Skin colour: Three pigments, melanin, carotene, and haemoglobin, control the skin colour of the human body. Melanin secretion increases during sun exposure and causes tanning.

Hair

Hair is a major skin derivative. Hair develops from the dermis, and it is a filamentous structure made of keratin. Hair is found all over the body except the palm and soles of most mammals. The colour of hair is formed by the pigment melanin. The colour, texture and length of hair vary in different parts of the body. Each hair has two parts: a hair shaft, a hair root and a hair bulb

1. Hair shaft: The shaft is the visible part of the hair that appears on the skin.
2. Hair root: The hair root is present in the dermis of the skin that is surrounded by the hair follicle. The root is surrounded by hair follicles. The hair follicle is a sheath of skin and connective tissue that is surrounded by the sebaceous gland. Hair follicles are connected to blood vessels, nerve ends, and tiny muscles called arrector pili. Hair bulb: It contains a projection of the dermis referred to as hair papillae. Papillae have a blood supply to them. Hair bulbs along with hair follicles regulate the growth of hair.

Mechanism of goosebumps:  Goosebumps is a phenomenon where hair is lifted up vertically. We often get goosebumps when we feel chills or experience certain emotions. The lifting of hair is called piloerection, which occurs due to erector muscles, also called arrector muscles. Contraction in the erector muscle at one end pulls the hair vertically, which causes goosebumps.

Function of Hair

1. It protects the scalp from direct exposure to sun rays.
2. Eyelashes and eyebrows keep dust, dirt and sweat away from our eyes.
3. The hair inside the nose and ears help to keep out germs and other things from the air.
4. Body hair helps in the regulation of our body temperature.

Nail

1. Nails are skin derivatives found at the end of toes and fingers. Nails are composed of dead Keratinocytes. A nail has three main regions: the bed, plate, and matrix.
2. The nail root or nail bed is the region below the skin where the nail begins. The nail begins from the deeper layer of the epidermis.  It is called the nail matrix. The nail matrix contains keratinocytes.
3. A nail plate is a body or visible part of a nail. It consists of dead keratinized cells. Lunula is the proximal part of the nail plate.  The nail plate ends at the margin, which is the edge of the nail that can be clipped. The dead epithelium surrounding the nail is called the cuticle.
4. Matrix is present at the base of the nail below the skin surface. It is visible as a whitish half-moon structure near the base of the nail.

Function of Nail

1. Nails protect the sensitive tips of fingers and toes.
2. Nails also help in the precision of movement.
3. Nails help in activities like holding small objects or scratching or itching.

Glands

Glands are skin derivatives that secrete various substances. Sweat glands, ceruminous glands, sebaceous glands are some of the examples of Skin derivative glands. These are all exocrine glands found in the dermis. Exocrine glands are those which release the secretion in the local area with ducts.

Sweat Gland

These glands are also called sudoriferous, and they release sweat.  The outer opening of sweat is called a sweat pore. The duct which connects the glandular region to the pore is called the sweat duct. The secretory part of the gland absorbs metabolic wastes from the surrounding capillaries. The secretion of sweat glands is called sweat, and it consists of 99% water,0.5% salt and urea in trace amounts. There are two types of Sweat glands.

Eccrine sweat glands: These are found in almost every skin region and produce a secretion of water and sodium chloride.

Apocrine sweat gland: These are found in the axillary and pubic regions. These glands are active only after puberty.

Function:  Sweat helps in regulating body temperature and helps in the excretion of some metabolic wastes.

Mammary Gland

The mammary gland is a modified sweat gland found in every human being. The gland remains in rudimentary form in males but develops to form breasts in females at puberty. These are milk-secreting glandular structures that are under hormonal regulations—hormones like prolactin produced during pregnancy riggers milk production. The produced milk is secreted out through a conical structure called the nipple.

Sebaceous Gland

These glands release oily secretion, also called sebum. These glands are present everywhere in the skin except the palm and soles. Sebum consists of squalene, esters of glycerol, wax and cholesterol. Secretion of sebum is increased in hot and humid environments and reduced in the cold. Excessive secretion or accumulation of sebum may cause acne.

Function: Sebum makes skin waterproof and keeps moisture of skin intact. It also increases the elasticity of the skin and lubricates hair follicles.

Meibomian Gland

These are modified sebaceous glands present at the margins of eyelids. The oily secretion at the eyelid serves as the lubricant and prevents the overflow of tears.

Ceruminous Glands

These are simple, coiled tubular glands found in ear canals. It produces waxy secretion called cerumen in the ear canal. This gland is always active.

Function: Cerumen protects the wall of the ear canal and eardrum by trapping foreign substances that enter.

Summary

The integument or skin is an elastic and waterproof organ. The skin is made up of 3 distinct layers: epidermis, dermis and hypodermis. The epidermis is the thin, uppermost superficial layer of the skin covering almost the entire body surface. This layer has a few specialized cells with distinct functions like Keratinocyte, Melanocyte, Merkel cells, Langerhans cells. The dermis is the underlying layer of the skin. It is much thicker than the epidermis and has a mesodermal origin. It is present deep into the dermis. It is a layer of loose connective tissues. This layer acts as the flexible connection between the skin and the underlying muscles.

Skin performs various functions like protection, thermoregulation, storage, excretion, secretion etc. Major skin derivatives are hair, Nail and exocrine glands. Hair develops from the dermis, and it is a filamentous structure made of keratin. Hair serves a protective function. Nails are composed of dead Keratinocytes. Nails have a protective function and help in the precision of movement. Glands are skin derivatives that secrete various substances. Sweat glands, ceruminous glands, sebaceous glands are some of the examples of Skin derivative glands. Sweat glands release sweat. The sebaceous gland secretes oil, and the ceruminous gland secretes cerumen or ear wax.

Frequently Asked Question (FAQs) from Skin Derivatives

Q.1. Which are the derivatives of skin?
Ans: Hair, nails and glands are major skin derivatives.

Q.2. What are the major layers of skin?
Ans: Skin has three main layers: epidermis, dermis and hypodermis.

Q.3. Which is the largest organ of the body?
Ans: Skin is the largest organ of the body.

Q.4. What system is formed by the skin and its derivatives?
Ans: Integumentary system is formed by the skin and its derivatives.

Q.5. What is the deepest layer of the epidermis?
Ans: Stratum basale is the deepest layer of the epidermis.

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