Structure of the Skin: Skin is the largest organ of our body that forms the protective covering of our body. It serves as a barrier between our body organs and the external environment. Beyond providing protection, it plays a crucial role in regulating body temperature. It possesses thermoreceptors and thereby serves as a sense organ. Sweat and sebaceous glands of the skin are involved in the excretory function.

A thin layer performing a variety of functions is astonishing and deliberately makes us know about the detailed structure of the skin. A study of the cross-section of the mammalian skin tells us that though skin appears like a thin sheet of paper, it is made up of different layers, each performing specific functions. Let’s read the article to know about the detailed structure of the skin.

Different Layers of the Skin

Skin is the largest organ of our body that covers a surface area of about 1.5-2.0 square metres. The skin comprises about 15 % of the total body weight. The thickness of the skin varies considerably over different parts of the body. The skin that covers the eyelids is the thinnest, measuring less than 0.1 mm in thickness, whereas the skin of the palm is the thickest, ranging from 1.3 mm to 1.5 mm. The skin comprises three different layers:

1. The outermost layer: Epidermis
2. The middle layer: Dermis
3. The innermost layer: Hypodermis

Learn About Skin and Its Functions

Fig: The Cross-Section Through the Skin

Receptors in the Skin

Skin serves as a sense organ and is also the largest sense organ. It possesses a number of sensory receptors. Different types of sensory receptors are specialised for different types of senses in our body. A few receptors found in the skin can be discussed as follows:

1. Thermoreceptors: These receptors detect the change in the temperature of the external environment. These receptors are located in the dermis layer of the skin.
2. Pain receptors: These receptors detect any extreme mechanical stimuli that induce pain. These receptors are found in the dermis around the base of the hair follicle.
3. Mechanoreceptors: There are four different types of tactile mechanoreceptors located in the human skin. These are named Merkel’s cells, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles.
   I. Merkel’s cells are located in the epidermis and possess senses for softness.
   II. Meissner’s corpuscles: These are found in the superficial layer of the dermis and detect light touch and vibrations.
   III. Ruffini endings: These receptors are located in the dermis sense for heavy and continuous pressure.
   IV. Pacinian corpuscles are located deeper in the dermis and determine heavy pressure.
   

Cross-Section Through the Skin

Skin is part of the integumentary system. Human skin remains covered with the hair follicle. The cross-section through the skin represents the following layers:

Epidermis

The epidermis is a Greek word that means ‘over’ or ‘upon.’ It is the outermost avascular (without blood vessels) layer of the skin. The epidermis consists of stratified keratinised squamous epithelium. The thickness of the epidermis varies at different body sites.

Cells of the epidermis: There are the following four types of cells found in the epidermis:

1. Keratinocytes: These cells form about 95 % of the total epidermal layer. The insoluble keratin protein present in the superficial cells makes the epithelium impervious to water and highly resistant to mechanical abrasion.
2. Melanocytes: These cells produce a pigment called melanin that is produced by an amino acid tyrosine found in the cellular vesicles called melanosomes. The pigment is further transported and delivered into the cytoplasm of the keratinocytes. Melanin occurs in the following two primary forms:
   I. Eumelanin: Provides black and brown.
   II. Pheomelanin: Provides a red colour.
   People having dark skin produce more melanin compared to pale skin members. Exposure to the sun’s UV rays stimulates the keratinocytes to secrete chemicals that stimulate the melanocytes to produce more melanin that leads to the darkening of the skin.

Importance of melanin
I. Beyond providing pigmentation, melanin also plays an important role in the protection of the DNA of epidermal cells from UV ray damage.
II. It causes the breakdown of folic acid.

There should be a balanced production of melanin in our skin as too much melanin interferes with the production of vitamin D.

Abnormal conditions of melanin production

Melanin is responsible for the pigmentation of the skin. There are the following two abnormal conditions of skin pigmentation:

1. Leucoderma: It is an abnormal condition of skin pigmentation in which the melanin is lost from any of the localised areas of the skin. This results in the formation of small or large depigmented patches on the skin. It is also known as vitiligo or achromoderma.

2. Albinism: It is a condition of complete loss of skin pigmentation of the entire body, including the colour of hairs, eyebrows, iris. The skin appears pinkish because of the underlying blood capillaries. It is a rare genetic disorder that is inherited from the albino couple to their offspring.

3. Langerhans cells: These are also called dendritic cells. These cells recognise the new allergens (antigens) and trap them to further develop an immune response through lymphocytes that can be carried out as follows:
I. These cells shatter the allergen into smaller pieces and migrate from the epidermis into the dermis.
II. From the dermis, they find their way to lymphatics and blood vessels.
III. The allergen is now recognised by the immune cells called T-lymphocytes that initiate an immune reaction to destroy the foreign invaders and stimulate the proliferation of more lymphocytes that recognise and remember the allergen in the future.

3. Merkel cells: These are also called Merkel-Ranvier cells or tactile epithelial cells. These cells are closely associated with the cutaneous nerves and play an important role in sensation, especially in palms, soles, and genitalia.

Layers of Epidermis: The epidermis is initially divided into two major layers:

1. An outer layer of keratinized, non-viable cells
2. An inner Malpighian or germinative layer of non-keratinized, viable cells.

However, there is a more familiar way of categorizing the layers of the epidermis by which the epidermis is described as a five-layered structure. Different cells of the epidermis remain distributed in the following layers of the epidermis arranged from the deeper to the superficial:

1. Stratum Basale (Basal or germinative layer): It is also known as stratum germinativum. It is a single layer of cells that is primarily made up of keratinocytes. The keratinocytes constantly undergo mitosis and produce new cells and push the existing cells away from the stratum basale. Based on its dividing ability, it is called the germinative layer. Besides keratinocytes, it consists of two other types of cells, the Markel cell that stimulates the sensory nerves and the melanocyte that produces melanin and is responsible for pigmentation.
2. Stratum Spinosum (Spinous layer):  This layer is spiny in appearance due to the protruding cell processes that join the adjacent cells via a structure called a desmosome. The stratum spinosum is composed of eight to ten layers of keratinocytes. The Langerhans cells remain scattered in between the keratinocytes. The Langerhans cells perform the function of phagocytosis.
3. Stratum Granulosum (Granular layer): This layer consists of modified keratinocytes that produce a large amount of keratohyalin (a protein structure found in cytoplasmic granules of keratinocytes). The granular cytoplasm of the keratinocytes gives a granular appearance to the layer.
4. Stratum Lucidum (Translucent layer): It is a smooth, translucent epidermal layer located between the stratum granulosum and stratum corneum. This layer is composed of dead and flattened keratinocytes. This layer of cells is found only in the thick skin of the palms, soles, and digits. An intracellular protein called eleidin is derived from keratohyalin, which gives a transparent appearance to the stratum lucidum and provides a barrier to water.
5. Stratum Corneum (Cornified layer): It is the outermost layer of the epidermis that is exposed to the outside environment. The keratinocytes become hard due to the deposition of excess keratin and therefore called corneocytes. This layer prevents the entry of foreign particles and the dehydration of underlying tissues. It provides mechanical protection against abrasion for the underlying layers. Cells in this layer are shed and are replaced by cells pushed up from the stratum granulosum or stratum lucidum in the case of palms, soles, and digits.

Fig: The cross-section of skin showing the layers of the epidermis

Dermis

The dermis lies below the epidermis. It is comparatively thicker than the epidermis. The epidermal appendages that connect with the external surface of the skin lie within the dermis, such as hair follicles and sweat glands.

The superficial region of the dermis that lies close to the epidermis is raised into numerous small projections called papillae that contain the blood capillaries and nerve endings and, therefore, the regions for receiving the touch and pain stimuli. The sharp sense of the touch in the dermal layer of the fingertips enables the blind person to read the braille characters. The dermis is supplied with blood through blood vessels. The layer consists of two main types of fibres called collagen fibres and elastin fibres.

Cells of the dermis: There are the following types of cells found in the dermis:

1. Fibroblasts: These are the major cells of the dermis that constitute about 75 % of the total dermis. These cells produce collagen and elastin fibres and a lubricating viscous gel. The collagen fibres provide strength to the skin, and the elastin fibres provide elasticity. Their functioning reduces with the increasing age due to the longer exposure to UV radiation. The viscous gel allows the nutrients, hormones, and waste products to pass through the dermis.
2. Mast cells: These are oval to spindle-shaped cells. These cells release histamine and other vasoactive molecules in the extracellular spaces. These cells play an important role during wound healing.
3. Smooth vascular muscle cells: These cells facilitate the contraction and dilation of the blood vessels and are mainly found in the dermis of the visceral organs. These cells regulate the body temperature by regulating the flow of blood through the blood vessels.
4. Specialised muscle cells: These are called myoepithelial cells and are found around the sweat glands. These cells contract to expel sweat.
5. Leucocytes: These are transient inflammatory white cells that leave the blood vessels to heal the wound.
6. Adipocytes: These are also called lipocytes that are primarily concerned with the storage of fat and therefore maintain the body temperature.

Layers of the dermis: The dermis consists of two distinct layers:

1. Papillary layer: It is the superficial layer of the dermis. The collagen and elastin fibres are loosely arranged in the layer. This layer of the dermis projects into the stratum basale of the epidermis and forms finger-like dermal papillae. It consists of fibroblasts, adipocytes, and leucocytes.
2. Reticular layer: It is underlying the papillary layer. The collagen and elastin fibres are tightly arranged and form a reticulated network of fibres. It consists of fibroblasts, mast cells, nerve endings, lymphatics, and epidermal appendages.

Blood vessels of the dermis:

The dermis consists of a complex network of blood vessels. These vessels form two distinct networks:

1. Superficial plexus: It consists of arterioles and venules that are interconnected. This network of blood vessels lies close to the deeper layer of the epidermis.
2. Deep plexus: It is located deeper into the dermis and remains connected to the superficial plexus through the vertical column of the blood vessels. This network of blood vessels lies close to the subcutaneous layer of the skin.

Fig: The cross-section of the skin showing the layers of the dermis

Hypodermis

It is also called the subcutaneous layer or panniculus. This layer is located below the dermis however, there is no clear demarcation between dermis and hypodermis. This layer is composed of adipocytes. It is a highly vascularised layer consisting of areolar connective tissue and adipose tissue. It serves as a fat storage layer and provides insulation and cushioning for the integumentary system.

The fat deposition and accumulation within the hypodermis vary in males and females. It depends on hormones such as testosterone, oestrogen, insulin, glucagon, leptin, etc. Fat deposition may also depend on genetic factors.

Skin Derivatives

Skin derivatives are the appendages that are derived from the skin. These are called epidermal appendages that originate from the epidermis but are deeply seated in the dermis. There are the following derivatives of the skin:
1. Hair
2. Hair follicles
3. Sweat glands
4. Sebaceous gland
5. Keratin structures: nails, horns.
6. Apocrine glands

Fig: Skin Derivatives

Summary

Skin is the protective covering of our body that appears to be a thin sheet. A cross-section through the skin explains that the skin of humans and other mammals is primarily composed of three distinct layers. The outermost layer is the epidermis followed by the dermis and hypodermis. The epidermis is non-vascular that consists of stratum Basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The keratinocytes constitute a major part of each layer of the epidermis.

The dermis is located under the epidermis that consists of the papillary layer and reticular layer. Both the layer comprises collagen and elastin fibres. The hypodermis is the deepest layer of the skin that consists of adipocytes and serves as a fat storage layer.

Frequently Asked Questions (FAQs) on Structure of the Skin

Q.1. How many layers does skin have?
Ans: Skin has three layers, epidermis, dermis, and hypodermis.

Q.2. What are the two layers of the dermis?
Ans: The two layers of the dermis are the papillary layer and the reticular layer.

Q.3. What are the five layers of the epidermis of the thick skin?
Ans: Stratum Basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum are the five layers of the epidermis.

Q.4. What is the most common type of cells in the subcutaneous layer?
Ans: Adipocytes are the most common types of cells in the subcutaneous layer of the skin.

Q.5. Which structure is responsible for skin pigmentation?
Ans: The pigment produced by the melanocytes called melanin is responsible for skin pigmentation.

We hope this detailed article on the structure of the skin helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!