Why Do Hair Follicles Produce Prolactin?

Most of us think of prolactin as the hormone responsible for milk production after childbirth.

It hardly seems relevant to hair.

So I was surprised to discover that human hair follicles don’t just respond to prolactin. They appear to make it themselves.

That immediately raises a question.

Why would a hair follicle produce a hormone best known for helping mothers breastfeed?

The answer isn’t obvious.

In fact, despite more than twenty years of research, scientists still don’t know for certain.

What they have discovered, however, has transformed how we think about the hair follicle itself.

A hair follicle is far more than a factory for making hair

For many years, hair follicles were viewed as relatively passive structures. Hormones arrived through the bloodstream, growth factors arrived from neighbouring tissues, and the follicle simply responded.

That picture has changed dramatically.

We now know that hair follicles produce many of their own signalling molecules. They are constantly communicating with surrounding cells and, in some cases, with themselves.

Melatonin.

Vitamin D.

Cortisol-related hormones.

Neuropeptides.

And, rather unexpectedly…

Prolactin.

The hair follicle is less like a factory and more like a busy town, with thousands of biological conversations taking place every second.

The discovery nobody expected

In 2006, researchers led by Ralf Paus cultured living human scalp hair follicles in the laboratory.

Their aim was to understand how prolactin affected hair growth.

They confirmed something that had already been suspected. Hair follicles contained prolactin receptors, meaning they were capable of responding to the hormone.

But then they discovered something much more interesting. The follicles weren’t simply listening to prolactin arriving from elsewhere in the body. They were producing prolactin themselves.

In other words, the hair follicle was both the sender and the receiver of the same message.

Biologists call this autocrine signalling when a cell talks to itself, and paracrine signalling when it talks to neighbouring cells.

Whatever the terminology, the implication was remarkable.

The follicle wasn’t simply following instructions.

It was participating in the conversation.

What happened when researchers added more prolactin?

The team then exposed living human hair follicles to prolactin. The results were surprisingly consistent - hair grew more slowly.

Cells within the hair matrix divided less rapidly.

More cells underwent programmed cell death.

The follicles entered catagen, the regression phase of the hair cycle, sooner than expected.

Rather than acting like an accelerator, prolactin behaved more like a brake.

Why would a hair follicle produce something that slows its own growth?

This is where the story becomes genuinely interesting.

The truthful answer is… We don’t know.

Nature rarely spends energy producing molecules without a reason. If hair follicles manufacture prolactin, evolution has probably preserved that ability because it serves an important purpose. But what purpose?

One possibility is that prolactin forms part of the follicle’s internal timing mechanism.

Growing hair is biologically expensive. Hair matrix cells divide extraordinarily quickly, demanding energy and nutrients. Every follicle must eventually decide when to stop growing and begin resetting for another cycle.

Perhaps prolactin contributes to that decision.

Or perhaps its main role has little to do with hair growth at all. Prolactin is also involved in immune regulation, inflammation and communication between different cell types. The follicle may use prolactin to coordinate many biological processes that we are only beginning to understand. At present, the evidence doesn’t allow us to be certain.

A new therapeutic idea

If prolactin can encourage follicles to leave the growth phase, another question naturally follows.

What happens if you stop the follicle hearing that message?

That question has inspired a completely new approach to treating androgenetic Alopecia (pattern hair loss).

Rather than targeting DHT, researchers are developing drugs that block the prolactin receptor, preventing the signal from reaching the follicle.

One of these experimental treatments is ABS-201, which has recently entered early-stage clinical testing.

It is far too early to know whether this approach will prove effective.

But the idea itself illustrates how our understanding of hair biology is changing.

The target is no longer just hormones circulating through the bloodstream.

It is the follicle’s own internal conversations.

Every answer uncovers another question

Science rarely works by replacing one mystery with certainty.

More often, solving one puzzle reveals another.

Twenty years ago, few people imagined that human hair follicles produced prolactin.

Today we know they do.

We know they can respond to it.

We know it influences how hair follicles behave.

But we still don’t know exactly why evolution gave hair follicles this remarkable ability.

Perhaps that is the most exciting discovery of all.

Because every time we uncover another conversation taking place inside a hair follicle, we move a little closer to understanding why hair grows, why it stops growing well, and how we might help it grow better.

References

1. Foitzik K, Krause K, Conrad F, Nakamura M, Funk W, Paus R. Human scalp hair follicles are both a target and a source of prolactin, which serves as an autocrine and/or paracrine promoter of apoptosis-driven hair follicle regression. American Journal of Pathology. 2006;168(3):748–756. doi:10.2353/AJPath.2006.050468.

2. Ramot Y, Bíró T, Tiede S, Tóth BI, Langan EA, Sugawara K, Foitzik K, Ingber A, Goffin V, Langbein L, Paus R. Prolactin—a novel neuroendocrine regulator of human keratin expression in situ. FASEB Journal. 2010;24(6):1768–1779. doi:10.1096/fj.09-146415.

3. Foitzik K, Langan EA, Paus R. Prolactin and the skin: a dermatological perspective on an ancient pleiotropic peptide hormone. Journal of Investigative Dermatology. 2009;129(5):1071–1087.

4. Langan EA, Foitzik-Lau K, Goffin V, Ramot Y, Paus R. Prolactin: an emerging force along the cutaneous–endocrine axis. Trends in Endocrinology & Metabolism. 2010;21(9):569–577. doi:10.1016/j.tem.2010.06.001.

5. Craven AJ, Ormandy CJ, Robertson FG, Wilkins RJ, Kelly PA, Nixon AJ, Pearson AJ. Prolactin signalling influences the timing mechanism of the hair follicle: analysis of hair growth cycles in prolactin receptor knockout mice. Endocrinology. 2001;142(6):2533–2539. doi:10.1210/endo.142.6.8179.

6. Foitzik K, Krause K, Nixon AJ, Ford CA, Ohnemus U, Pearson AJ, Paus R. Prolactin and its receptor are expressed in murine hair follicle epithelium, show hair cycle-dependent expression, and induce catagen. American Journal of Pathology. 2003;162(5):1611–1621.