[048] Epidermal cell turnover (GB#117A02) | 基礎医学教育研究会(KIKKEN)Lab

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● Forefront to protect the body

It is said that 60% of the body is water. Though it is full of water in the body, it does not leak out of the skin thanks to a very thin epidermis that covers the outermost side of the skin. The epidermis also protects the cells inside from infiltration of water and various substances from outside, mechanical friction and ultraviolet attack. Furthermore, infiltration of bacteria is also watched here. The wonderful nature of the epidermis is that while cells are exposed to various attacks, the cells change little by little every day and are always fresh.
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● Epidermis swapped in 4 weeks

Skin refers to the epidermis of the part touched from the outside and the dermis underneath. Since the thickness of the epidermis is about 0.2 mm, it is only about one sheet of New Year’s postcard. About 20 to 30 layers of cell layers are packed in it. Histologically, it is a form of stratified squamous epithelium.The deepest layer is in close contact with the rough structure called the dermal papilla below it via a sheet called a single basement membrane.Except for a few special ones, all the cells that make up the epidermis are keratinocytes born of the cell division of basal cells stuck just above the basement membrane. While keratinocytes are pushed up sequentially from bottom to top, they change shape and properties between them. Later in the latter half of life activity is over, it turns into a sheet of thin thin piece called corneocyte made of epidermis-specific substance. And about 28 days after birth (there are some materials around 45 days ← this one seems to be new), peeling off from the outermost side. Every day in the whole body, roughly several grams of keratin is stripping. So even if a small prickle sticks, if the majority are above the basal cells, it will be lifted along with the cells and will naturally escape.

● Cells change greatly in their lifetime

The keratinocytes divided from the basal cells will no longer divide.As the cell is in an inflated form about 2 weeks in the first half while being pushed up little by little every day, because special connection between surrounding keratinocytes looks like a small thorn, it is called the spinous layer It is contained in the layer.After that, the keratinocytes become suddenly flattened, and granules made of a material called keratohyalin become conspicuous in the cells, so that it reaches a short stage of 2 to 3 layers called a granular layer.　

Later, the cell nucleus is lost soon, the whole cell almost becomes a sheet of a mass of protein called keratin, and it is peeled off after staying in the stratum corneum for the latter two weeks.Just as human beings were born, aged, and finished their lives by lifetime, there was a site already explained as if it is an image of the world changing appearance and role.

● Keratinize by scattering scattered keratin

Keratin is also famous as the main ingredient of hair and I realize that it is a very durable ingredient. It is also common sense that the horny skin is made of keratin. So, if I thought that the granular layer just before it changes to horny is about to produce the keratin, keratin itself is actually being made as a fiber of the cytoskeleton from among the spinous cell layer. The substance called filaggrin, which is formed by changing the keratohyalin granules made of the granular layer, functions to fix and solidify the keratin proteins scattered inside the cell, so that it becomes a firm and hard structure suddenly.　

● stratum corneum is surprisingly high function

Cells lining horizontally in the granular layer are adhered specifically by tight junction and strongly restrict the passage of various substances from above and below the granular layer. When going up from the granular layer, the other parts and structure inside the cell almost disappears and the cell dies. So the stratum corneum seems to be a mass of keratin sheets that wait for it to peel off later, but it was not that simple.Between the sheet and the sheet, it is filled with a substance called ceramide of cell membrane lipid and binds the sheets, which is also useful for waterproofing. Also, once keratin solidifies, the help of filaggrin seems to be unnecessary, and filaggrin is degraded in the stratum corneum. However, this degradation product now works well as it functions as a moisturizing factor to confine moderate moisture in the stratum corneum. Furthermore, the sebum secreted from the sebaceous glands of the skin covers the entire surface of the stratum corneum. With these elements cooperating, the stratum corneum becomes a powerful barrier to limit the ingress and egress of moisture while maintaining adequate moisture inside.

● No blood flows in the epidermis

Every day the epidermis is born a new cell, the born cells continue to live for at least two weeks to the granule layer, but there are no capillaries around the cells. The tip of the capillary blood vessel folds under the basement membrane at the dermal papilla. For this distance, there is no problem with the gas exchange and mass exchange with the granule cell layer.As far as gas exchange is concerned, direct exchange with the outside world through the stratum corneum may be possible if it is a granular layer, but for humans it may be, but it seems to be the extent that it is not necessary. On the other hand, because there is no blood vessel in the epidermis, it will not bleed even if it gets scratched up to the epidermis. Since there is no bleeding, bacteria and the like are also less likely to enter the blood vessel. Because nerve fibers of pain sense also come only to the dermis, we do not feel direct pain. Since the skin is often damaged, we can see the attitude that it is not treated as serious as it is.

● There are cells that patrol inside the epidermis

What we are surprised to learn about the epidermis is that if you thought like the outer walls that physically and biochemically protect the boundary without skin, blood vessels and nerves, innumerable special cells are always there It means that you are moving freely by watching the invasion of foreign enemies. This is a group of cells that acts as a dendritic cell called a dendritic cell. It has a special name of Langerhans cells, but there is no functional connection except for the fact that the discoverer is the same as the pancreatic islet of Langerhans.As Langerhans cells migrate laterally through the spinous layer, they sometimes stretch branches (dendrites) into granular layers, and it seems that they are checking for extraneous matter there. And when there is a change, it enters special warning posture (activate), it prolongs the protrusion and moves to the side of the granule layer, and swallows it if it is bacteria. Langerhans cells that swallowed foreign matter immediately will escape from the epidermis, merge into the lymphatic system of the dermis, and act to activate immunity activities such as antibody production.

● Langerhans cells come from the root of hair

Langerhans cells are originally a kind of blood leukocytes, which settled within the epidermis. Why is it in the epidermis although there are no blood vessels? After swallowing a foreign body, it can move to the dermis beyond the basement membrane, so it seems that this cell may come in anywhere, but there seems to be a fixed route when entering.The root of the fur protruding from the skin is stuck in the dermis. The circumference of the root is not cells of the dermis but cells surrounding the epidermis surround it. The root of the sweat gland is similarly penetrated deep into the dermis by the tissue of the epidermis.When leukocyte monocytes crawl out of the capillaries in the dermis, they turn into macrophages. Dendritic cells are macrophages that have settled in the dermis, and Langerhans cells are those in which macrophages have entered the epidermis from the root and sweat gland parts.

● Keratinocytes do not turn black when burned on the day

The longer the sunlight hits, the darker the skin. It is well known that black pigment called melanin is increasing to protect the body from ultraviolet rays. It is the keratinocyte of the epidermis that is dark.However, unexpectedly, keratinocytes do not react to ultraviolet rays to produce melanin.On the basement membrane, another cell, called melanocyte, exists in parallel with the basal cell, and this cell takes over in melanin production.Melanocytes do not change to keratinocytes. Upon being stimulated by ultraviolet rays, melanocytes extend the protrusions in all directions and pass the granules packed with melanin to adjacent keratinocytes. That is, melanin granules of keratinocytes are gifts from melanocytes.

● Epidermal cells are not born from the dermis

The epidermis is in close contact with the dermis and papilla via the basement membrane. If there is something damaged and the epidermis peels off and the dermis is exposed, the blood vessels are injured and naturally bleed. When the injury heals, will the epidermis regenerate from its dermis? Since it is a continuous structure, it seems to be okay with that, but in fact it does not happen. Epidermal basal cells and dermal cells are making skin together, but their birth and growth are completely different. Basal cells of the epidermis are epithelial tissue and the dermis is a different organization called connective tissue. More dating back, the epidermis is derived from ectoderm, the dermis is derived from mesoderm, at least in the natural flow, biologically, from the dermis (mesoderm tissue) to the epidermis (ectoderm tissue) (unless you do a special operation ), It is not born.However, even if the epidermis is peeled by appearance, if the cells of the same lineage as the epidermis survive the hair root portion buried in the dermis or the root of the sweat gland, the epidermis spreads out from the cell and is regenerated.Otherwise, we only have to wait until the basal cells gradually spread out and accumulate keratinocytes from the edge of the epidermis that remained without peeling. Small burns are almost restored with this mechanism.

● There is no epidermis in the leather wallet

If I thought that the surface of the leather that used supple and durable animal skin such as bags and shoes made use of the characteristics of this epidermis, unexpectedly, in the beginning of the leather making process, the epidermis is stripped cleanly. Leather is a work of connective tissue, the surface of leather that we touch directly becomes just part of the dermal papilla.Even if you leave the epidermis, it seems that it gets fuzzy, it gets peeled naturally and becomes dirty. In other words, the epidermis makes a strong barrier with dead cells, but if it really dies to the root, it will not be able to fulfill the role of barrier. In other words, it is a proof that the epidermis is alive, that the keratin of the dead cells will act as a barrier.

○ Referenced sites

· New dermatological science 1. Structure and function of skin (Hokkaido University School of Medicine Department of Dermatology)
· Skin tissue photos　⇒　WEB HISTOLOGY(UMIN)
Sensory organ system – hull (skin and its appendages)
・基底層が美肌を　February 19, 2009
・肌細胞の一生を知る　February 14, 2009
· New wound care (Mutsuki Natsui)
· Mystery covered with skin
· Dynamics of skin barrier and Langerhans cells (Ryoji Kubo)
· New significance of hair’s significance – hair follicle immune switch – Keisuke Nagao
· Common sense when applying to dermatology Chikakane Dermatology
· Structure and function of the epidermis (KAO skin care navi)

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○Referenced books

・カラー版 ボロン ブールペープ 「生理学」， 西村書店
・カラー図解 人体の正常構造と機能 全10巻縮刷版，坂井 建雄，日本医事新報社
・トートラ人体解剖生理学　原書8版，丸善
・柔道整復学校協会編「生理学」，南江堂
・東洋療法学校協会編「生理学」，医歯薬出版株式会社

rev.20150709. rev.20150710.　rev.20150813，rev.20161228，rev.20170506，rev.20170709.

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