The skin is constantly renewing and regenerating itself, but do you know how it happens? Learning about the life cycle of skin cells can help us understand how our skin works to keep our skin looking its best.
Our skin protects our body from the outside world; it can weigh up to 4kg and stretch up to impressive 2 square meters, making it the largest organ in our body.
This organ protects the body's muscles, bones, and internal organs from infection and impact. And, to enjoy its 24/7 protection, our skin must undergo a continuous cycle of renewal and regeneration.
In this article, we will explore the life cycle of skin cells to better understand how this intricate process works.
The Structure And Composition Of Your Skin
To better understand the life cycle of skin cells, it is important first to be aware of its structure and composition.
Our skin consists of three main layers:
The outermost layer of our skin is called the epidermis, and it consists of five distinct layers; the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and finally, the stratum basale.
Each layer is composed of different types of cells: keratinocytes, melanocytes, Langerhans cells, and Merkel cells; the most abundant being keratinocytes, which make up around 90% of the epidermis.
Keratinocytes are responsible for producing proteins known as 'keratins,' the proteins that form the protective barrier against bacteria, viruses, fungi, infections, and environmental damage; melanocytes are responsible for producing melanin, the pigment that gives our skin its color to protect it from UV radiation; Langerhans cells are specialized immune cells that act as a first line of defense against any foreign antigens; and Merkel cells are specialized sensory cells involved in transmitting touch sensations to the brain.
The inner layer of our skin is called the dermis; this layer is composed of two layers: the upper papillary dermis and the lower reticular dermis.
The papillary dermis is composed of connective tissue made up of fibroblasts, which are responsible for producing a number of structural proteins that make up parts of the extracellular matrix — collagen, elastin, proteoglycans, and glycoproteins — blood vessels, nerve fibers, Meissner corpuscles, Mast cells, and Phagocytes.
Collagen works to give the skin its structure, strength, and flexibility; elastin allows the skin to stretch and retract; proteoglycans fill in the spaces between collagen and elastin; and glycoproteins help regulate water balance.
While the reticular dermis is composed of a network of dense connective tissue that contains blood vessels, sebaceous glands, sweat glands, nerve endings, and hair follicles, which acts as a foundation for the papillary layer and binds the epidermis to the hypodermis.
The innermost layer of our skin is called the hypodermis, also known as the subcutaneous layer, and is made up of adipocytes — fat cells — and collagen fibers that form a connection between our skin and underlying organs to provide insulation against heat loss from extreme temperatures, cushioning to the body against minor shocks and store energy reserves in the form of triglycerides.
Here's a complete guide on the skin, to know more about each layer of the skin in detail.
The Skin's Life Cycle
Now we understand the structure and composition of our skin, let's take a closer look at how skin cells go through their life cycle.
Skin cell renewal and regeneration is an ongoing process that occurs in three distinct phases: proliferation, differentiation, and desquamation.
The Proliferation Phase
Our skin is constantly renewing itself, with skin cells starting as stem cells in the deepest layer of the epidermis, known as the stratum basale — also called the basal cell layer — where they are nourished and filled with moisture.
The first phase, proliferation, is the process in which these stem cells' skin cells divide and multiply through mitosis — also known as cell division — where they divide to form two daughter cells, each containing a full set of genetic material from the original parent cell.
The Differentiation Phase
The second phase, differentiation, is the process in which the two daughter cells will then travel upwards toward the more superficial layers of the epidermis, where they start to differentiate into different types of skin cells and become specialized skin cells by taking on specific roles and functions within their layer.
The cells that are produced in the basal layer are called keratinocytes; they produce proteins known as 'keratin,' a protein found in hair, nails, and skin, which gives these newly formed cells their shape, strength, and structure and is responsible for providing structure and strength to our skin.
These newly-formed keratinocytes then move up the epidermis in a systematic fashion towards the surface of the skin; these immature cells will continue to divide until they reach the stratum granulosum, where they undergo changes from one layer to another; their shape and structure transform considerably, and they start to create keratin, cytokines, growth factors, interleukins, and complement substances.
Keratinocytes' nucleus begins to shrink, and their membrane starts to harden, gradually becoming thinner and flatter, losing their nucleus and ability to divide. As their cytoplasm fills with keratin proteins, which give them their distinctive look and functions, eventually forming a layer of dead skin cells called corneocytes through a process known as cornification or keratinization.
Corneocytes are dead skin cells filled with bundles of keratin protein, held together by lipids produced and secreted by the lamellar granules found within the stratum corneum; they are responsible for forming a protective barrier against dehydration and environmental stressors.
The Desquamation Phase
The third and final phase, desquamation — which is also known as skincell turnover, is when skin cells reach the surface of our skin, concluding their life cycle and getting pushed off by newer cells that are making their way up from below.
This process takes at least 6 weeks to complete, at which point it has completed its life cycle and enters a resting phase where skin cells have an average lifespan that lasts 14 to 28 days or two-to-four weeks until the cycle begins again with new stem cells produced in the epidermis.
While this is a very simplified version of skin cell renewal and regeneration, it illustrates just how complex and intricate the process can be.
That being said, the continuous life cycle of skin cells is essential for healthy skin; if this cycle is disrupted or interrupted, it could result in dry and cracked skin that may be prone to infection, leading to further health complications.
With regular care and protection, you are ensuring your skin's cells are able to go through their life cycle and keep your skin looking healthy to ensure proper function.