Blaschko: the invisible stripes we all have on our skin – 07/11/2023 – Science

Although we don’t see them, they are present from the moment we are born, running through our entire body, from head to toe.

They run up and down our limbs, swirl around our heads, wander across our faces, form stylized ‘v’s on our backs and squiggles on our hips.

Although these lines are invisible, the German dermatologist Alfred Blaschko began to suspect their existence when, at the end of the 19th century, he reviewed what he had discovered in his research on rashes, birthmarks and warts on the skin of his patients.

He had noticed that many followed similar patterns, as if there were already marked paths to follow.

Other lines were already known, such as Langer’s, which are those that surgeons usually follow when making incisions, because they mark the places of least tension on the skin and can help with the healing of wounds.

These were known to be parallel to the natural orientation of collagen fibers in the dermis and generally perpendicular to the underlying muscle fibers.

The curious thing about these lines was that they didn’t correspond with nerves or blood vessels, or with the muscular or lymphatic system. The mysterious pattern only became visible under very specific circumstances.

After collecting data from a group of patients with certain skin diseases and mapping the distribution of their lesions, in 1901, the German dermatologist presented his findings at a dermatology congress.

in publication Die Nervenverteilung in der Haut in ihre Beziehung zu den Erkrankungen der Haut (“The distribution of nevi on the skin in relation to skin diseases”), which includes his presentation at the congress, he highlights:

“Involuntarily, these nevi [lesões cutâneas] and line-shaped dermatoses became the main object of my work: in part, this is perhaps due to the fact that so much material was collected especially on this disorder.”

“I would like to show you, above all, (…) the diagram of the lines of the nevi, which, if you allow me, represent the quintessence of my work”, he said, pointing to the following image:

Although when he died in 1922, Blaschko was known above all for his fight against venereal diseases through concepts of social hygiene ahead of his time, his work on linear skin diseases was already recognized.

“For all subsequent studies in this field, his work will remain of fundamental importance, and that alone will be enough to preserve Blaschko’s name as a leading dermatologist, now and forever,” wrote his German colleague Abraham Buschke in the scientific journal Dermatologische Zeitschrift.

His words turned out to be prophetic. Patterns bear his name – Blaschko lines. Furthermore, his suspicions that they had an embryonic origin, since many skin diseases that followed these lines were present at birth, guided further research.

SWIRLS

A century after Blaschko revealed the imperceptible lines, the physician Rudolf Happle, together with Atessa Assim, from the University of Marburg, in Germany, added details to the map of our bodies.

In order to develop a comprehensive pattern, they analyzed 186 cases with head and neck injuries.

They found that, on the face, Blaschko’s lines “present an hourglass-shaped configuration that converges at the nasal root. However, in several areas these lines intersect at an angle of almost 90°. On the scalp, they form a configuration spiral”.

This is how the detected lines were drawn, according to the researchers:

Over the years, experts have arrived at a hypothesis, thanks to the research of British cytogeneticist Mary Lyon and German Happle, among others.

It is believed that they are cellular relics of our development.

We all started out as a single cell that kept reproducing itself.

As skin cells divided to multiply and cover our ever-growing body, each new cell line pushed the other, and they went round and round.

Our invisible Blaschko lines are the molecular evidence of what happened.

The routes of melanocytes, the deep cells of the epidermis responsible for pigment, for example, are marked in our body.

They form when we are just a handful of cells that have inherited an X chromosome from each parent.

Since they only need one, they select which one to disable.

Some get their father’s X chromosome and some get their mother’s, as do all the cells that divide from them; some will have one version of the DNA and some will have another version.

Thus, we can inherit two genetically distinct embryonic skins that swirl, but do not intermingle, in the coat of the embryos.

The results are these patterns, not usually visible in humans, but revealed by various skin diseases or simple rashes.

Understanding them helps doctors diagnose skin problems, as revealed in several studies.

CHIMERS

There are, however, dozens of skin conditions in humans that reveal Blaschko’s lines, at least on parts of the body, as lesions form where these two different cell lines meet.

One of the rarest that can show them is chimerism.

It happens when a single organism is formed by cells from two or more “individuals”, that is, two different sets of DNA.

As the specialized magazine Scientific American explains, a form of chimerism that can occur naturally is in the case of twins, if one of the embryos dies early in the pregnancy – and some of its cells are “absorbed” by the other embryo.

If different fertilized cells mix and form a human being among them, they can send out different waves of epidermal cells, which alternate with each other.

Often, the skin color of the two types is indistinguishable or subtle enough that it can only be seen under ultraviolet light.

Sometimes, however, the two different sets of genetic code for skin types are dramatically different, revealing the patterns of Blashko’s lines.

The fact is that, visible or not, they cover us from top to bottom, from side to side.

They look like the lines of a story written on our body.

A story most of us will never be able to read.

This text was originally published here.