Layers of Epidermis

Skin is the body’s largest organ. It is the barrier between us and germs, dangerous temperatures and the organ that enables tactile sensations. In this blog post, you will everything about the skin starting with the anatomy of the skin and its layers. 

Skin structure and Histopathology is an important part of Dermatology preparation and essential part of the NEET PG syllabus. Read this blog post thoroughly to accelerate your NEET PG preparation and crack the exam. 

What is Skin and its layers?

Skin is the largest organ in the human body. The weight of the skin is 4-5 Kg with a body surface area 1.2-3 sq.m. Skin and its appendages are referred to as the Integumentary System. The human body also has glabrous skin, which is non-hairy. Glabrous skin is present in palms and soles. The appendages of the skin include Hair, Nails, and Glands

What are the Layers of Skin?

Although the skin is visible as single-layered, it is made up of different layers. Let’s learn each layer in detail. Epidermis is the first layer of the skin, below the epidermis, is the dermis, beneath that is the subcutaneous tissue, which is also called the hypodermis, and then the muscle layer. 

What are the Functions of Skin

The most important function of the skin is the formation of Vitamin D. It is formed in Stratum Basal or Stratum Spinosum. Other functions are: Temperature Control, Water Control, and Cushioning

Let’s study each layer separately! 

Epidermis 

Epidermis is the topmost layer of skin. The thickness varies from 0.5 - 1 mm. It varies because the skin is not uniform in all areas. For example, the skin on our eyelids is extremely thin (0.05mm), and the skin on our hands and soles is quite thick (1 mm). 

Important Function of Epidermis

Since Epidermis is the topmost layer of the skin, it acts as a major barrier-forming layer. Epidermis is formed of different layers that collectively build its complex structure. Each layer has its function and importance. Below are the different layers of epidermis : The topmost layer is called Stratum Corneum. Below that Stratum Lucidum, then Stratum Granulosum, then Stratum Spinosum, and Stratum Basale

Tip for learning: 

Mnemonics - COME LET'S GET SUNBURNT

The Stratum Lucidum layer is only present in Palms and Soles. Since palms and soles are the thickest, they need an extra layer. This layer is not present in any other body parts. 

Before we move into each layer, let’s understand a little bit about cell Differentiation.

What Changes happen after Cell Differentiation?

• In the human body, skin is formed from bottom to top i.e. Basale layer to the Corneum layer. As the cell moves from stratum basale to stratum corneum, it undergoes changes. The skin undergoes a process known as cell Differentiation. 
• As the cell Differentiate from the basale to corneum layer it loses nuclei and cell becomes flattened, This results in loss of mitosis, and cells are dead, which leads to an increase in surface area and the cell becomes dehydrated

What happens in a Preterm baby?

If a child is born preterm, his/her cell differentiation could not be completed and since cell differentiation happens from the basale to the corneum layer, hence the Stratum Corneum could not be developed. So, the stratum corneum would be absent in the preterm baby.

What is Epidermal Turnover time?

It is the time the epidermal layer takes to turn over. Cell cycle of Keratinocytes is 300 hours.  Cells take approximately 14 days to travel from Stratum Basale to Stratum Corneum. They stay in the Stratum Corneum for another 14 days. After that, cells exfoliate. This takes approximately 28 days. So, the skin remains in a continuous process of renewal, new skin is forming and old skin is shedding off. The process is so slow that one cannot see. This whole time period is called Epidermal Turnover time. The time the epidermis layer takes to turn over. The time cells travel from the basale layer to the top for differentiation. In total, it takes roughly 56 days. But the range can vary from 52- 75 days. This is important to know because in a condition like Psoriasis, the cell kinetics from 300 hours becomes 36 hours and the epidermal turnover time from 56 days becomes just 4 days. The cells multiply rapidly, so that’s why we see scaling, and exfoliation because the epidermis is turning over so rapidly that it cannot shed off. The skin starts to settle down on the stratum corneum. 

Let’s study each layer of the skin of the Epidermis 

Stratum Corneum 

• It is the topmost layer. Corneum means that it has corneocytes/keratinocytes. By the time cells reach into the stratum corneum, it is dead. So this layer has dead flattened corneocytes/keratinocytes, dehydrated dead cells, no nuclei/ no mitosis. But this is a very important layer because it acts as a barrier.

Histopathological findings related to each layer of skin

What Happens if Stratum Corneum start retaining Nuclei

• In the stratum corneum, the keratinocyte has no nuclei, so the retention of nuclei can lead to a condition called Parakeratosis. In normal conditions, parakeratosis is seen in the mouth and vagina. In Pathological conditions, parakeratosis is seen in Psoriasis, Eczema, Squamous Cell Carcinoma, Actinic Keratosis, Seb. Dermatitis. (Mnemonic PEAS2)
• Sometimes, there is an increased thickness in the stratum corneum, which is called Hyperkeratosis, seen in conditions like Lichen Planus and Psoriasis

Stratum Lucidum

This layer is also called the clear cell layer, found in palms and soles only because the skin is thick on these parts. The reason for being called a clear cell layer is that it has Refractile Granules of Elecidin.

Stratum Granulosum

According to the name, this layer has granule cells. What are the two important granules? The two most important granules of this layer are:

1. Keratohyalin Granules- They are responsible for forming Profilaggrin, which forms Filaggrin. Profilaggrin is filament Aggregating Protein. Filaggrin is very important because it binds the keratocytes together to form a structure. It has an important role in barrier functioning. If these granules are not formed, the filaggrin will not be formed, which will lead to certain disorders which affect the barrier. The disorders like Ichthyosis Vulgaris,  Profilaggrin is present in stratum granulosum, but these migrate up and form filaggrin and this filaggrin is present in stratum corneum. The disorder associated with this is ichthyosis vulgaris. Because these have a very important role in barrier function  
2. Lipid Coating Granules- Also called Odland bodies/ Lamellar Bodies. As the name suggests, these are granules that have lipids and are responsible for providing moisture, by coating the cell with lipids and if this is deficient, there will be dryness of the skin. That leads to a condition called Asteatotic dermatitis. Asteatotic means no fat, so no fat, no lipid cause very very dry skin. Usually, stratum granulosum is one or two cell layers thick. 

What are the Histopathological findings seen in the Granular layer?

Usually, the granular layer is 1-2 layers thick. But sometimes the granular layer increases in thickness called Hypergranulosis. It is seen in Lichen Planus. If the granular layer is absent, it is called Agranulosis which is seen in Psoriasis. In psoriasis, the epidermal turnover time is so quick that cells do not get time to differentiate. They straight away go into the stratum corneum from basale spinosum. No granular layer is formed. The keratinization process usually happens in the granular layer, but sometimes because of certain diseases, this will start happening in the lower layer in the stratum spinosum or basale. This disordered keratinization is called dyskeratosis. This condition is seen in certain benign conditions like Hailey Hailey disease and Darier's disease and it can be seen in certain malignant conditions like Bowen’s disease, Paget’s disease, and squamous cell carcinoma

Stratum spinosum

Stratum spinosum is the thickest layer of the epidermis. It is also called Prickle cell layers- boldly visible. Spinosum means spines. These spines are actually desmosomes. Keratinocytes which are present in the skin, are bound together with the help of some interconnecting filaments called desmosomes. In the layer of stratum spinosum, the cells are more loosely attached, able to see desmosomes more clearly. They appear as spines, that is why this layer is called stratum spinosum. In all the layers the keratinocytes are connected with desmosomes but histopathologically they are seen in this layer. 

What are the Histopathological findings seen in the Stratum spinosum? 

Sometimes cells present have fluid in them leading to intercellular edema. In between cells, there is edema, which is called intercellular edema and histopathologically it is called spongiosis. If the fluid is present in the keratinocyte, it will become like a balloon. So ballooning is nothing but intracellular edema. Spongiosis and Ballooning are seen in acute eczema. If the thickness is increased further in the spinous layer, that is called acanthosis.  This condition is seen in chronic eczema. 

What is Malpighian Layer?

Malpighian Layer is a mitotically active layer of the epidermis, which is found in stratum basale and stratum spinosum 

Stratum Basale

It is called stratum basale because it is the basale layer, the lowermost layer. It is the most mitotically active cell layer. This is usually one layer thick. It is a very important layer because from here skin (epidermis) is being formed. Sometimes the keratinocytes which are present in the basal layer get separated from each other and that is called acantholysis. This condition is seen in the pemphigus group of disorders.  Sometimes the basal layer gets degenerated, and there is damage and inflammation which causes degeneration of the basal cell layer, which is called Basal cell degeneration. This happens because there is some inflammatory infiltration, which comes from the dermis, targets the epidermis, targets the basal layer, and causes it to degenerate. This condition is seen in Lichen Planus

We have talked about the various layers in the epidermis. So what are these layers made of? These layers are made of cells. Let’s learn more about these cells.

Cells in Epidermis

There are 4 kinds of cells that are present in the epidermis. The first and foremost is the keratinocyte, which constitutes almost 95% of the epidermis. The next cell is present in the stratum spinosum called the Langerhans cell. Melanocyte is present in the next layer which is the basale layer and the 4th cell is called a Merkel cell, which is also present in the stratum basale.  The most predominant is keratinocyte. 

Before we move on to each cell, let’s learn from where the cell arrives

Development of Epidermal Cells

• The major development happens from the ectoderm from which keratinocytes are coming. Then comes mesenchyme and then neural crest. So there are 3 sources of development of epidermal cells. The major development happens from the ectoderm from where keratinocytes are derived. Next is mesenchyme from which Langerhans cells are derived and neural crest from which melanocytes are derived. 

What happens to Merkel cells?

• We are still not sure from where Merkel cells are derived. There are 2 theories of Merkel cells that are probably derived from neural crest or from ectoderm. 

Let's discuss each cell layer

Keratinocyte

These cells are present in the epidermis. They form 90-95% of the epidermis. They are connected to each other with desmosomes and these keratinocytes have keratin intermediate filaments, which are derived from profilaggrin, which forms filaggrin.

Langerhans Cells

Langerhans cells are present in the stratum spinosum. They are derived from mesenchyme. They are basically antigen-presenting cells. They have granules, called Birbeck granules, which are racket shaped granules. They are also dendritic cells.  They stained with an immunohistological marker which is CD1A, CD207and S100

Melanocytes

It is a very important cell present in the stratum basale. Melanin means pigment, so these are pigment-forming cells. They are basically dendritic cells.  The ratio of melanocytes is 1:10. So for every 10 keratinocytes there is 1 melanocyte. This melanocyte is pigmentary-forming cells. It forms pigment and packages them into melanosomes. These melanocytes will transfer their melanin to surrounding keratinocytes. One melanocyte transfers melanosomes to 36 keratinocytes. This whole unit of 1 melanocyte to 36 keratinocytes is called an EMU (epidermal melanin unit). The ratio is 1:36. EMU is responsible for uniform skin color. This melanocyte will transfer melanin through small packages called melanosomes. The melanocytes in light skin are distributed as membrane-bound clusters and in dark skin are larger and distributed individually. The changes in skin color are because of melanosomes and the type of melanin. It is not because of melanocytes. In each individual melanocyte is constant.

Merkel Cells 

It is the last cell in the epidermis. It is present in Stratum Basale. Development is from the ectoderm neural crest. These are basically slow-adapting touch receptors.

Touch receptors

They are fast adapting and slow adapting Merkel cells, Ruffini's ending and free nerve ending. Fast-adapting touch receptors like Pacinian corpuscle and hair endings 

Let’s talk about Receptors, Nerves, and Innervation in the skin

There is a rich network of nerves with 2 types of sensory endings – ‘The Corpuscles (Mechano-receptors)’ and ‘Free nerve endings (Nociceptors)’ . The Mechano-receptors 

Will take care of light touch and pressure. A light touch is by Merkel cells of the epidermis and Meissner’s corpuscle. But the location is different. The Merkel cell is present in the epidermis and the Meissner corpuscle is present in the dermal papillae.  Pressure is by Pacinian corpuscles in the deep dermis or subcutaneous tissue. There is a free nerve ending on Nociceptors , which takes care of pain and itch. The free nerve endings are located in the basal layer of the epidermis close to the dermo-epidermal junction. There are temperature receptors, Krause bulbs detect cold, Ruffini end organs detect heat. Heat, cold, and proprioception are also located in the superficial dermis.

Coming down from Epidermis is Dermo-Epidermal Junction

The part of the epidermis which is invaginating into the dermis is called Riti ridges. and the part of the dermis, which is vaginating into the epidermis is called the dermal papilla. The junction between the epidermis and dermis is called the dermo-epidermal junction also called the basement membrane zone.  This junction has a lot of components. This is predominantly found in type 4 collagen. The important role of the dermo-epidermal junction is to form adhesion between the epidermis and dermis and signaling between these two. 

Below this zone will be Dermis

Dermis is further divided into 2 main parts. The part of the dermis invaginating into the epidermis is called the Papillary dermis. The part which is not vaginating and has a lot of fibers is called Reticular dermis. 

Why is Dermis important?

Dermis is holding the major structure of the skin. It contains cells, collagen, and elastin fibers, a ground substance which is hyaluronic acid, and heparan sulfate. It has nerves, vessels, hair follicles, and lymphatics. The predominant fiber is collagen fiber. Cells present in the dermis are Fibroblast, Langerhans cells, mast cells, Lymphocytes, and phagocytes. The function of the dermis is to store all the components of the skin 

Below the dermis comes subcutaneous fat

Subcutaneous fat has blood and lymphatics which provides the cushioning effect. The subcutaneous fat is absent in the eyelids and genitalia. Histopathologically they are arranged in fat lobules and septa.

To read more informative and high-yield topics like 'Layers of Epidermis' keep following our blog page and our Telegram channel. Read quality content with high strike rate and boost your NEET-PG online preparation enroll in PrepLadder's Medical PG Dream Pack now.