Skincare science: Skin aging

Last week we talked about the anatomy of skin, and how its structures protect you from the slings and arrows of daily life. Of course, there’s no such thing as a free lunch. Skin aging is an inevitable result of that protection. This week let’s look more closely at each of the structural and molecular changes in aging skin.

An example of facial agingWhat is the scientific explanation for the difference between a young and an old Harrison Ford?

There are several changes that occur in skin as it ages, and all three layers of the skin (epidermis, dermis, hypodermis) are involved. Additionally, some of the changes we see in aging skin are actually changes in the deeper tissues of the body — like muscle and bone — which are being revealed through the skin. Starting from the deepest layers these change are:

Areas where bone is lost with age

Loss of bone/muscle

It is a key concept of the physiology of living beings that tissues are in a constant state of flux. For example, the hard matrix of your bones is constantly being absorbed and reformed. This is useful, because it allows tissues to respond to the stresses placed on them. For example, a tennis player’s racket arm has bones that are longer, thicker, and wider than the non-racket arm. Alternatively, if your leg is in a cast for a few weeks, you’ll notice that when the cast comes off, the leg looks smaller and you have lost muscle mass and strength.

This atrophy of tissues can occur very quickly with disuse, but aging has the same effect, albeit slowly. The changes in faces are subtle, but over time, we lose bone mass in our faces. So as you age, your face is literally shrinking, but your skin isn’t tightening up to match.

Loss of fat in the hypodermis

As I mentionled in the anatomy article, the hypodermis is a loose layer of fat between the dermis and the underlying muscle. With age, we lose fat cells, but this occurs unevenly – different compartments lose different amounts of fat. As we get older, our cheeks, eyelids, and temples become more hollowed, the upper lip loses volume. And again, we’re left with too much skin for the amount of underlying tissue.

Weak fat compartments are like weak plastic bagsWeak compartments allow fat to sag

Weakening of fat compartments

The fat in and under the skin is not moving around freely from area to area. The fat is organized into compartments that are separated by tough, thin sheets of tissue called septae. These sheets of tissue stretch and weaken with age, allowing the fat inside to bulge or sag. This is often the cause of baggy upper eyelids.

Loss of thickness and elasticity in the dermis

Dermis becomes about 6% thinner for every 10 years of age. It also changes at the microscopic level — we start losing the proteins that give skin its strength and elasticity, and the proteins that remain are more disorganized. This results in skin that is weaker and more easily injured (because of decreased or poorly functioning collagen), and also doesn’t snap back into place as quickly when it is stretched (because of decreased or poorly functioning elastin). As gravity pulls on your skin, this weaker, less elastic skin begins to droop. Treatments like chemical peels or lasers can stimulate the skin to produce more collagen and elastin and restore the strength and elasticity that has been lost.

Bubble wrap just isn’t the same once its popped

Decrease in the area of contact between the dermis and epidermis

As I mentioned last week, if you looked at a microscopic 3D picture of the border between the dermis and epidermis, it would look like bubble wrap. With age, the bubble wrap flattens, and the same 3D picture looks less and less bumpy, and more like a flat sheet. This decreases the area of contact between the dermis and the epidermis. In turn, the blood supply, and therefore delivery of nutrition and oxygen to the epidermis, is decreased. The rate at which new cells are made and old cells replaced in the epidermis slows down. Treatments that focus on removing old cells from the epidermis are designed to address this cause of aging.

Loss of pigment cells in the epidermis

A few specialized cells in the lowest layer of the epidermis, called melanocytes, produce melanin pigment. This pigment is absorbed by the surrounding cells of the epidermis, giving your skin its color. With age, the number of melanin cells decreases. Your body’s control over these cells is also decreased. This causes unevenness in skin color, and irregular areas of increased pigmentation (e.g., age spots, freckles). It is very counter-intuitive that an increase in pigmentation would be caused by a decrease in the number of pigment cells, but there you have it.

Accumulation of errors in cell DNA

Every time a cell divides, there is a risk of introducing an error into the DNA. So the more times a cell divides, the more likely it is that an error will occur and won’t be fixed. Ionizing radiation (such as naturally occurring X-rays from cosmic radiation, or radioactive elements in the earth’s crust) does the same thing. DNA errors can lead to cells that grow or divide out of control, giving rise to benign skin growths or even cancers.

Sun damaged desert floorSun damage makes skin less elastic

How is sun damage different?

Many of these signs of chronological aging are similar to the signs of photo-aging (sun damage). The ultraviolet radiation in sunlight (or from a tanning bed) damages collagen and elastin. Sun damaged skin actually has more of these proteins than aging skin, but the proteins are of lower quality. This has two effects

  1. it prevents you from making more “good” collagen and elastin, because your body thinks it has plenty, and
  2. it makes the skin weaker and less elastic, giving it a “leathery” look

In addition, ultraviolet radiation directly damages DNA in skin cells, just like ionizing radiation. But unless you live in Chernobyl, your skin will see a lot more UV radiation in your lifetime than ionizing radiation. So most of the DNA errors that accumulate in your skin are from sunlight. These errors can cause melanin cells to misbehave, causing age spots. They can also lead to regular skin cells misbehaving and making scaly or bumpy lesions like actinic keratoses.

Next week we’ll discuss common skin lesions you might see in aging or sun-damaged skin.