What is skin and how does it work?

Beneath the surface · Part one

 What is skin, and how does it work?

Welcome to Beneath the Surface, a new Upenya series exploring the biology of young skin - how it works, how it changes during puberty, and what those changes mean for everyday skincare.

Over the coming weeks, we will look at the structure of the skin, the role of the skin barrier and microbiome, what causes increased oil production and clogged follicles, how breakouts develop, and how young skin responds to products.

In Part One, we begin with the basics: What is skin, and how does it work?

Before we go beneath the surface, it helps to understand the organ we are talking about.

We see our skin every day, yet we rarely think about how much work it is doing.

Skin is often described simply in terms of appearance: oily or dry, clear or blemished, smooth or uneven. But skin is not merely the outer covering of the body. It is a living, active organ that protects us from the outside world, helps regulate body temperature and water loss, allows us to sense touch, heat, cold and pain, and continually renews and repairs itself.

Understanding the layers of the skin and how these different layers work together, makes it easier to understand what happens during puberty.

More than a covering

The skin is commonly described as the body’s largest organ. It covers the external surface of the body and forms the boundary between the body and the outside world.

That boundary must perform two seemingly opposite jobs.

It needs to keep potentially harmful things - such as microorganisms, irritants and some chemicals - out. At the same time, it must prevent too much water from escaping from the body.

The skin also responds continuously to heat, cold, pressure, sunlight, injury and changes taking place inside the body. It is not a passive sheet wrapped around us. It is a complex, responsive biological system (Proksch, Brandner and Jensen, 2008).

The three layers we usually talk about

Skin is commonly described as having three layers:

• the epidermis;

• the dermis; and

• the hypodermis.

Strictly speaking, the epidermis and dermis make up the skin itself. The hypodermis, also called the subcutaneous layer, is the supporting tissue beneath the skin. However, all three are closely connected and are usually discussed together when explaining skin anatomy (Yousef et al., 2024). Each layer has a different structure and a different job.

The epidermis: the outer protective layer

The epidermis is the thin outer layer that we can see and touch.

It does not contain blood vessels. Instead, it receives nutrients from the blood vessels in the dermis beneath it. Most of the cells in the epidermis are called keratinocytes. New keratinocytes are produced in the deepest part of the epidermis. As they mature, they gradually move upwards towards the surface.

During this journey, the cells change shape, produce keratin and form specialised proteins and lipids. By the time they reach the surface, they have become flat, protective cells called corneocytes.

These outer cells are eventually shed and replaced by newer cells moving up from below. This renewal happens continuously, although the rate differs according to age, body area and skin condition (Yousef et al., 2024).

The skin barrier

The outermost part of the epidermis is called the stratum corneum. This is the part commonly referred to as the skin barrier.

It is sometimes compared to a brick wall.

The corneocytes act like the bricks. A mixture of lipids - including ceramides, cholesterol and fatty acids - helps fill the spaces between them like mortar.

This structure helps slow the movement of water out of the body while limiting the entry of irritants, allergens and microorganisms. When the arrangement of cells and lipids is disrupted, the skin may lose water more easily and become dry, tight, irritated or more reactive (Elias, 2008; Proksch, Brandner and Jensen, 2008).

The barrier is therefore not a cosmetic idea or marketing phrase. It is a real anatomical and functional part of the epidermis.

Other important cells in the epidermis

Keratinocytes are not the only cells in the epidermis.

Melanocytes produce melanin, the pigment that contributes to skin colour and helps protect skin cells from ultraviolet radiation.

People with different skin tones generally have similar numbers of melanocytes. Differences in skin colour relate mainly to the amount, type, distribution and persistence of the melanin produced - not simply to the number of melanocytes.

The epidermis also contains specialised immune cells that help detect potential threats, as well as cells involved in the sensation of light touch (Yousef et al., 2024).

The epidermis may be thin, but it is biologically busy.

The dermis: the skin’s support system

Beneath the epidermis lies the dermis.

The dermis is thicker and is made largely from connective tissue. It contains collagen, which gives the skin strength, and elastic fibres, which allow it to stretch and return towards its original shape.

Unlike the epidermis, the dermis contains blood vessels. These vessels deliver oxygen and nutrients, assist with healing and help control body temperature.

The dermis also houses many of the structures we associate with the skin, including:

• hair follicles;

• sebaceous glands;

• sweat glands;

• nerve endings;

• small muscles attached to hair follicles; and

• blood and lymphatic vessels.

The sebaceous glands that produce sebum are therefore not located on the surface. They are found mainly within the dermis and usually release sebum into hair follicles (Brown and Krishnamurthy, 2022).

This becomes particularly important when we begin discussing teenage skin. Oil may appear on the surface, but its production begins deeper within the skin.

The hypodermis: cushioning and connection

Below the dermis is the hypodermis, or subcutaneous layer.

This layer is made primarily from fat and connective tissue. It helps attach the skin to the structures beneath it, cushions the body and provides insulation.

Its thickness differs greatly between body areas and between individuals. It also contains larger blood vessels and nerves that connect with structures higher in the skin.

Although the hypodermis is sometimes called the deepest layer of the skin, it is more accurately described as the supporting tissue immediately beneath the skin.

How the skin protects us

Protection is one of the skin’s most important functions, but the skin does much more than form a physical shield.

It helps limit water loss from the body. It provides mechanical protection against everyday friction and minor injury. It contributes to immune defense and helps prevent microorganisms from entering deeper tissues.

The surface of the skin also supports a diverse community of microorganisms, collectively called the skin microbiota. These organisms form part of the skin’s wider protective environment, although their composition differs across body areas and between individuals.

The skin barrier is therefore not a single wall. It includes physical, chemical, immune and microbial components that all work together.

How the skin controls temperature

The skin helps prevent the body from becoming too hot or too cold.

When the body becomes warm, sweat produced by the sweat glands can evaporate from the surface and release heat. Blood vessels in the dermis can also widen, allowing more heat to move towards the skin’s surface.

When the body is cold, these blood vessels narrow to reduce heat loss.

The tiny muscles attached to hair follicles can also contract, producing goosebumps. In humans, this response has a limited insulating effect, but it remains part of the body’s response to cold (Yousef et al., 2024).

How the skin allows us to feel

The skin contains specialised receptors and nerve endings that detect touch, pressure, vibration, pain, heat, cold and itch.

These sensations help us interact safely with the environment. They allow us to pull away from something hot, notice an injury or feel changes in pressure and temperature.

Different receptors are located at different depths. Some nerve endings extend close to or into the epidermis, while receptors for deeper pressure are located further down in the dermis or subcutaneous tissue (Brown and Krishnamurthy, 2022).

How the skin repairs itself

Skin is frequently exposed to friction, sunlight, microorganisms and minor injury. It must therefore be able to repair itself.

When the skin is injured, cells, blood vessels, immune signals and structural proteins work together through a carefully controlled healing process.

The epidermis produces new cells to restore the surface. Cells within the dermis help rebuild connective tissue, while blood vessels bring oxygen, nutrients and immune cells to the area.

The success of this process depends partly on the depth and severity of the injury. Damage limited to the epidermis may heal differently from an injury that extends deeply into the dermis.

Where skincare fits in

Most cosmetic skincare products work mainly on or within the epidermis, particularly the stratum corneum.

A cleanser can remove sweat, sunscreen, environmental debris and excess surface oil. A moisturiser can increase water content, reduce water loss and support the lipids between the outer skin cells. Certain cosmetic ingredients can also influence processes within the upper epidermis.

But skincare products do not need to reach every layer of the skin to be useful.

In fact, supporting the outer layers is important precisely because the epidermis forms the body’s main interface with the environment.

This is also why stronger is not automatically better. Excessive cleansing or exfoliation can interfere with the structure the skin relies on to retain water and protect itself.

Why this matters for teenage skin

During puberty, hormonal changes begin affecting structures within the skin - particularly the sebaceous glands and hair follicles in the dermis.

The results become visible at the surface as increased oiliness, blackheads and blemishes.

But those visible changes do not begin simply because the surface is dirty. They arise from biological processes taking place within and beneath the epidermis.

That is where our Beneath the surface series will begin.

In Part Two, we will look more closely at why teenage skin becomes oily during puberty, what sebum is meant to do and why trying to remove every trace of oil may not be the best approach.

Before we ask young skin to behave differently, we first need to understand how it works.

References

Brito, S., Baek, M. and Bin, B.-H. (2024) ‘Skin structure, physiology, and pathology in topical and transdermal drug delivery’, Pharmaceutics, 16(11), article 1403. doi: 10.3390/pharmaceutics16111403.

Brown, T.M. and Krishnamurthy, K. (2022) ‘Histology, dermis’, in StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing. Last updated 14 November 2022. Available via NCBI Bookshelf.

Elias, P.M. (2008) ‘Skin barrier function’, Current Allergy and Asthma Reports, 8, pp. 299–305. doi: 10.1007/s11882-008-0048-0.

Proksch, E., Brandner, J.M. and Jensen, J.-M. (2008) ‘The skin: an indispensable barrier’, Experimental Dermatology, 17(12), pp. 1063–1072. doi: 10.1111/j.1600-0625.2008.00786.x.

Yousef, H., Alhajj, M., Fakoya, A.O. and Sharma, S. (2024) ‘Anatomy, skin (integument), epidermis’, in StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing. Last updated 8 June 2024. Available via NCBI Bookshelf.

This article is intended for general educational purposes and is not a substitute for medical advice, diagnosis or treatment.