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From basal layer to stratum corneum: How do skin cells transform into protective shields?
Skin is the largest organ of the human body and is mainly composed of three layers, in which the outermost epidermis plays an important role. The epidermis is not only the body's first line of defense against environmental pathogens, it is also responsible for regulating water loss to maintain a stable internal environment in the body. So, how do these skin cells change from the basal layer to the stratum corneum and evolve into a strong protective shield?
The epidermis is mainly composed of 90% keratinocytes, which gradually develop into mature keratinocytes through multiple levels of differentiation.
Structure of epidermis
The epidermis is made up of multiple layers of flat cells that develop from stem cells in the basal layer. The thickness of the epidermis varies according to different skin areas. The thickness of the sole of the foot can reach 596.6μm, while the thickness of the penis is about 31.2μm, and the overall thickness is about 90μm.
Cell composition
In addition to keratinocytes, the epidermis also contains melanocytes, Langerhans cells, Merkel cells, and some inflammatory cells. These cells work together to maintain the health and protective functions of the skin.
Hierarchical structure
The epidermis is generally composed of 4 to 5 layers, each layer has its own unique function:
- Stratum corneum: The outer layer, mainly composed of denucleated keratinocytes, provides the main protective function.
- Stratum lucidum: Found only on the palms and soles of the feet, it is part of the thick skin.
- Stratum granulosum: Keratinocytes begin to lose their nuclei and begin to produce water-retaining lipids.
- Stratum spinosum: Cells are connected by desmosomes. Langerhans cells in this layer contribute to immune responses.
- Stratum basale: Cell division continues to provide new cell sources for the epidermis.
Cell dynamics
The maintenance of the epidermis relies on cell division in the basal layer. These cells will gradually move upward from the basal layer, undergo multiple differentiations, and eventually become keratinocytes that have lost their nuclei.
Under normal circumstances, the production rate of keratinocytes is equal to the rate of their loss, and the entire epidermis is replaced by new cells in about 48 days.
Function of epidermis
The main functions of the epidermis include:
- Barrier function: Prevents microbial infections and other harmful substances from entering the body.
- Maintain moisture: Effectively control water loss in the skin to maintain skin hydration.
- Tactile perception: Specialized structures containing Merkel cells transmit tactile information to the nervous system.
Skin color
Skin color variation is mainly determined by the amount and distribution of melanin, and the synthesis and distribution of melanin are affected by ultraviolet light.
Clinical significance
In clinical studies, culturing keratinocytes to form 3D structures (artificial skin) provides an important tool for drug development and testing. In addition, certain skin conditions, such as skin hyperplasia, also provide insights into functional and structural abnormalities of the epidermis.
How do these skin cells develop from the basal layer into a powerful protective layer, and what changes do they undergo during this process? This undoubtedly triggers our deeper thinking about skin health.
