Skin is the largest external organ of the human body. It acts as the mechanical barrier for many kinds of germs. It not only protects the internal organs from infections but also from harmful radiations by acting as an absorber. Despite the same functions, we see so many variations in its colour, tone, and texture. This is because of the histology of skin.

Can all the variations observed in the colour, tone, etc., be tagged to its structure and the tissues that make up the organ? Is skin made up of single tissue or many? In this article, we will discuss about skin in detail. Read on to explore about Histology of Skin.

Facts About Skin

Some of the important facts about skin are as follows:

1. It is the largest organ of the human body.
2. Skin covers about \(2.0\) sq meter area and weighs about \(5\,kg\) in adults.
3. Thickness varies from \(0.4\,mm\) at eyelids to \(4\,mm\) at the heels.
4. It is the major barrier between the internal side of the body and the outer environment.

What are the Three Layers of Skin?

The skin has a layered structure and is made up of many tissues. There are three main layers of skin, i.e. epidermis, dermis, and hypodermis.

1. Epidermis: It is the outermost layer made up of keratinized stratified squamous epithelial tissue. The outermost layer is of dead cells and is continuously scraped off. The basal layers of the epidermis are folded into dermal papillae. Dermal papillae provide adhesions between the epidermis and dermis and, in the areas of thick skin, nourish the epidermis.
2. Dermis: This layer is present below the epidermis. It is made up of connective tissues, nerves, the blood supply, fibroblasts, etc., as well as sweat glands. It also contains hair follicles on most parts except soles and palms. The apical layers of the dermis are folded into dermal papillae.
3. Hypodermis: This layer is present below the dermis. It mainly contains adipose tissue and sweat glands. The adipose tissue has functions like storage of lipids, protective cushioning and insulation.

Fig: Three Main Layers of Skin

Histology of the Skin

Each one of the main layers of skin is made up of different types of cells or tissues and contains some derivatives too. Let’s look at them one by one.

1. Layers of the Epidermis

The epidermis is the outermost layer made up of keratinized stratified squamous epithelial cells. These specialized cells are called keratinocytes. There are no distinct layers or division of the different layers present in the epidermis, but the keratinocytes show a gradual change in their structure. This process of gradual changes in the keratinocytes is known as keratinocyte maturation or keratinization. The keratinocytes are self-renewing cells and undergo continuous division. The cells in the basal layer divide and move up in the layers above. This process takes about \(2-4\) weeks. Based on the division properties and morphological differences, the epidermis is considered to have the following layers, starting from the innermost side towards the outermost layer.

1. Stratum basale– Also known as stratum germinativum, is the single layer of cells closest to the dermis. In this layer, cells continuously divide and move in the upper layers.
2. Stratum spinosum– This layer is present above the stratum basale and consists of \(8-10\) layers of cells. The cells are connected with each other very tightly with desmosomes and contain thick tufts of intermediate keratin filaments. As the cells move in the upper layers, they shrink, and the desmosomes of the neighbouring cells stand out like prickles. Hence this layer is also called the prickle layer of the epidermis.
3. Stratum granulosum– This layer is present above the stratum spinosum and is about \(3-5\) cell layers thick. As the cells start to move up in this layer, they start losing their nuclei and cytoplasmic organelles and become keratinized. The granules contain lipid-rich secretion, which acts as a water sealant.
4. Stratum lucidum– In the thick skin like at heels, soles and palms, this layer is usually found between the stratum granulosum and stratum corneum layer. It is thin and transparent, and in normal histological sections, it is hard to identify.
5. Stratum corneum: This is the outermost layer that has dead, keratinized cells, reduced to flattened scales or squames. They are highly packed with keratin.

Fig: Epidermal Layers of Skin

2. Layers of Dermis

The dermis middle layer is sandwiched between the outer epidermis and inner hypodermis. The dermis is made up of connective tissue. It also contains nerves, glands, and hair follicles. This layer of animal skin is used for making leather. Dermis shows two regions, i.e. papillary layer and the reticular layer.

1. Superficial/papillary layer– It is the region of the dermal papillae and makes up about 20% of the dermis. It has loose connective tissue, unmyelinated nerve endings, which are temperature sensitive and Meissner’s corpuscles which are the touch receptors.
2. Reticular layer– It is the deeper region of the dermis and has dense irregular connective tissue with collagen and elastin fibres. These protein fibres give the skin flexibility and strength. Fibroblasts, macrophages and adipocytes are also present in this layer.

3. Special Cells of Epidermis and Dermis

Many different types of cells are present in the epidermis and dermis.

1. Keratinocytes: They make up the bulk of the epidermis and are loaded with keratin protein.
2. Melanocytes: They are present in basal layers of the epidermis and contain a pigment called melanin. The levels of melanin decide the skin colour or pigmentation.
3. Langerhans cells– They are the skin macrophage cells and are present in all layers of the epidermis but most prominent in the stratum spinosum. They are antigen-presenting cells and participate in the surveillance function of the immune system.
4. Merkel’s disk: They are the small cells associated with the nerve endings in the epidermis. They are part of the ‘somatosensory apparatus’ of skin and are more prominent in the thick skin of palms and soles.
5. Meissner’s corpuscles: Also referred to as tactile corpuscles, are found in the upper dermis and project into the epidermis. They are mechanoreceptors for touch and are found on the palmar surface of fingers and plantar service of feet.
6. Pacinian corpuscles: They look like ‘onion bulbs’ and are found in the dermis and subcutaneous tissue. The afferent nerve ending in the region is surrounded by several concentric layers of flattened cells and enveloped in connective tissue. The corpuscles are pressure sensitive.
7. Ruffini corpuscles: They are found in the dermis and are slow adapting mechanoreceptors and detect the skin stretch and deformations with joints. They are also known as bulbous corpuscles.
8. Krause’s end bulbs: Found in the dermis, and these are thermoreceptors cell and detect cold.

Fig: Histology of Skin

Fig: Receptors of Skin

Hypodermis

It is deeper, below the dermis, and is also called the subcutaneous layer. It is the deepest layer of the skin, made up of well vascularised loose areolar connective tissue and adipose tissue.  It also has hair follicles, nerve endings, and blood vessels. This layer serves to connect the skin with the underlying fascia (fibrous tissue) of bones and muscles. Fat deposition and accumulation in hypodermis depend on hormones and genetic factors.

Skin Glands and Appendages

The skin has \(2\) glands, namely sweat or sudoriferous glands and sebaceous glands. It has hair follicles embedded in it and also has nails as appendages.

1. Sweat Glands or Sudoriferous Glands

a. These are simple tubular exocrine glands.
b. They are present in the superficial hypodermis that is bordering the dermis.
c. The glands discharge their content to the surface of the skin via a coiled secretory duct.
d. The duct opens onto epidermal ridges as sweat pores.
e. The secretory part is lined by cuboidal epithelial cells, and the secretary duct is lined by stratified cuboidal epithelium.
f. There are two types of sweat glands, i.e. merocrine and apocrine.
g. Merocrine or eccrine glands (glands that secrete their substances by exocytosis without loss of any cellular part) which open directly on the surface of the skin. They are found all over the body.
h. Apocrine glands (glands that secrete their substances by exocytosis with loss of a small part the cell membrane being bud off) are those which secrete their secretion into the pilary canal of hair follicles. They are found in axillary, pubic, and perianal regions.
i. Sweat contains 90% water and salts like sodium, potassium, chloride, excretory products like urea and lactic acid.
j. The glands play an important role in thermoregulation.

2. Sebaceous Glands

a. These are branched, spherical, and holocrine (the whole cell is secreted) glands.
b. They are present in the dermis region.
c. They secrete an oily substance called sebum.
d. Rounded cells of the gland are filled with lipid-filled granules.
e. The cells degenerate towards the end of the duct to release the content.
f. The oil coats the hair and skin surface to make it soft, supple, and waterproof.
g. The sebaceous glands are associated with hair follicles.
h. At puberty activity of sebaceous glands increases due to hormone androgen.
i. Blockage of the sebaceous gland can lead to acne.

3. Hair and Hair Follicles

a. These are tubular invaginations of the epidermis grown downwards into the dermis.
b. Follicles are lined by stratified squamous epithelium.
c. Hair is a column of dead keratinized cells.
d. Hair has three layers as a central medulla/core, keratinized cortex, and outermost highly keratinized cuticle.
e. The base of a hair follicle or bulb is the dermal papilla which contains the blood supply to the hair follicle.
f. The matrix present in the dermal papilla has dividing cells and generates hair and internal root sheath.
g. Internal root sheath is the outermost covering of the hair at the hair follicle.
h. Internal root sheath is surrounded by external root sheath, which does not take part in hair formation.
i. Ducts of sebaceous glands discharge sebum onto the hair.
j. A hair follicle is associated with a pair of arrector pili muscles (smooth muscles).
k. Contraction of arrector pili elevates the hair giving goosebumps.

The Glands of Skin

The skin is not the same at all parts of the body, and it differs in terms of colour, texture, and thickness. All the layers of the skin contribute to the variations. Let’s list the factors that contribute to the differentiation of skin:

1. Thickness of epidermis
2. Thickness of dermis
3. Size and concentration of hair follicles
4. Size and concentration of sebaceous glands
5. Size and concentration of sweat glands
6. Type and concentration of sensory nerve endings
7. Presence of muscles

Functions of Skin

The functions of the skin are as follows:

1. Containment: Skin prevents loss of fluids from the body.
2. Protection: Skin protects the internal organs from damage, UV radiations and thermal and stress shocks as well as abrasions.
3. Immunological surveillance and defence: Skin prevents microbial infections and acts as the first line of defence in our body.
4. Sensation: It is the major sensory organ and has receptors of touch, pressure, pain and temperature.
5. Wound healing: Skin has an extraordinary capacity of regeneration upon damage.
6. Thermoregulation: Various features of skin like sweat glands, hair and adipose tissue are involved in temperature regulation.
7. Metabolic functions: The adipose tissue present in the lower layers of skin is involved in the storage and release of lipids and secretion of various hormones.
8. Excretory function:  Excretes small amounts of wastes like urea and excess salts through sweat.

Summary

The most important and largest organ of the human body is the skin. It shows a complex structure in terms of layers and cells present in it. Epidermis, dermis, and hypodermis are the three main layers with many different types of cells and tissues present in them. Epithelial tissue and connective tissue form the major part of the skin. Special receptor cells- mechanoreceptors and thermoreceptors are also present in the layers of skin which enable it to function in diverse ways. The skin prevents microbial infections and acts as the first line of defence in our body.

FAQs on Histology of Skin

Q.1. What gives colour to the skin?
Ans: The pigment responsible for skin colouration is melanin and is present in cells called melanocytes. Melanocytes are present in the basal layer of the epidermis, and the concentration of melanin decides the dark or light colouration of the skin.

Q.2. What are the three main layers of the skin?
Ans: Epidermis, dermis, and hypodermis are the three main layers of skin. Further, the epidermis may have 4 or 5 layers of cells, the dermis has two layers, and the hypodermis is mainly adipose tissue (fat cells).

Q.3. What are the three main tissues found in the skin?
Ans: Epithelial tissue, loose connective tissue, and dense connective tissue are the three main tissues of the skin. Small bundles of smooth muscles (arrector pili) and sensory nerve endings are also present in the skin.

Q.4. Which layer of the skin does the hair grow from?
Ans: A hair grows from a hair follicle that is embedded in the skin. The hair follicle is the downward invagination of the epidermis into the dermis. A sebaceous gland is also associated with the hair follicle.

Q.5. Why is skin histology important?
Ans: Skin performs various functions like protection, thermoregulation, excretion, sensation, immunological surveillance, etc. To understand these functions better, it is important to know the histology of the skin. Also, the disease conditions of the skin can be better understood and treated if one knows the histology.

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