The question of hair in general has always intrigued me, well, truthfully, it has often grossed me out, quite frankly. I come from a rather large family with one older brother and younger twin sisters. We all shared one bathroom growing up and I was disgusted when I took a shower after my sisters, who have very long hair. The drain was constantly filled with masses of hair that they left behind for me to clean. As you may imagine, there were many screaming matches that ensued. Too Much Information possibly, but cleaning out hair that is more than 12 inches in length is just too much to handle early in the morning. So, why does hair on all other parts of your body stay short, while the hair on your head can grow very long?
I’ll start with a few fun facts about hair provided by the American Academy of Dermatology How Hair Grows are; that the only places hair doesn’t grow on our bodies are the soles of our feet and the palms of our hands; hair you can see is actually dead, which is why you don’t feel pain when you get your haircut; and, we lose anywhere from 50 to 100 strands of hairs a day!
To get to the answer to the question, it is important to first understand the science in hair growth. Every human is born with about 5 million hair follicles, the small cavities from which hair reaches the surface of our skin. Anagen, defined here, is the growth cycle and the first phase of hair growth. It occurs when blood flow enters into the hair follicle and feeds oxygen to stem cells. When these stem cells divide they form keratinocytes, defined here, which are the building blocks for the root of a hair. As keratinocytes continue to divide and push towards the surface of the skin, they die and form keratin, defined here, which is the protein that holds a strand of hair together. This process continues until the strand, which is then dead, pops through the follicle to the surface of your skin. The second phase of hair growth is called Catagen, defined here, or the regression phase, which lasts for up to two weeks. About 3% of our hair is in this phase at all times. In this phase, the blood supply gets cut off at the bottom of the follicle which stops the creation of keratinocytes. This phase stops the growth of hair in that follicle and the follicle shrinks and pushes the strand to the surface. The third phase is called Telogen, defined here, or the resting phase in which hair does not grow any further. About 8% of our hair is in this phase at all times. It is in this phase that the hair that can be seen above the skin falls out. This is the phase in which we lose those 50-100 strands of hair per day.
The answer to the question of why does the hair on all other parts of your body stay short, while the hair on your head can grow very long is in the Anagen, or the first phase of hair growth. Hair grows at a rate of about 1 cm every 28 days. The hair on our heads grows continually for 2 to 6 years in this phase as compared to other parts of our body like our eyebrows, eyelashes, arms and legs, which only grow for up to 45 days. Interestingly, hair on different parts of our bodies grow at different rates per month. For example, eyebrows grow at a rate of only 4.2mm per month.
In conclusion, scientists have figured out the way in which hair grows and the speed in which grows. They are, however, still trying to conclude how the Anagen phase is driven for each body part. Genetics play a role to some degree distinguished by different hair types. Chemical growth signals directed by stem cells in the skin is the most current determination as to how our hair grows. While we might not know exactly how each body part knows not to allow hair to grow too long, we can all be thankful that certain areas don’t grow hair like the hair on our scalps.
Sources:
https://www.aad.org/public/kids/hair/how-hair-grows
http://www.zmescience.com/other/science-abc/how-fast-hair-grows-042394/
Images:
http://www.belgraviacentre.com/wp-content/uploads/2012/09/Hair-Growth-Cycle-White.jpg
http://cdn7.connery.dk/image/box/236158/1290/10000.jpg?
oversize=1http://images.agoramedia.com/wte3.0/gcms/pg-baby-skin-conditions-bare-baby-feet-full.jpg
http://www.theseedofhope.com/assets/content/Fotolia_5208645_M%20hands.jpg
I liked how you explained how hair grows and the speed at which it grows at different parts of our body. I’m not sure if this is necessarily a good blog post topic though. You never mention a hypothesis so I’m not sure what you are trying to evaluate. However, I think that it is a cool topic and you were able to explain your findings well. I found a study that was performed at Columbia University Medical Center, which evaluated whether drugs that restrain an enzyme would promote hair growth in mice. The study was conducted in order to see if these drugs would be able to help people dealing with male pattern baldness and other types of hair loss.
Hey man I found your blog post to be very interesting being that I am a hairy person myself. I wasn’t aware of these certain areas in our body that we don’t grow hair. You brought up many different points on hair but at the end I was still confused what was your overall argument. There’s nothing wrong with being a bit hairy. Actually there is a correlation that the amount of body hair a man has is directly correlated to there intelligence. You can read about it at http://www.independent.co.uk/news/the-hirsute-of-higher-intelligence-1328285.html
I think that the topic of human hair is a good one for a blog topic- one that I had yet to see until now. Unfortunately, I feel that this post lacked any kind of insightful analysis seeking to answer a scientific question. Believe it or not, there are a lot of interesting hot topics relating to human hair. For example, hair loss prevention is a growing concerns among many aging males who lose theirs. Perhaps you could have written a blog about the validity of certain hair loss prevention techniques.
This recent experiment discusses the possibility of using scalp cooling methods to prevent hair loss as a result of chemotherapy. This question and many others are highly relevant and would make for great posts!
Hey Michael, I enjoyed reading your blog. I’ve never really thought about where hair grows before, and I was intrigued to learn that the only place that hair does not grown is on the palms of our hands and the soles of our feet. That is actually somewhat odd to think about. I was also interested to learn that we lose between 50-100strands of hair each day. That makes sense to me, because every time I brush my hair, I notice that strands are falling out. Your blog is great because it takes something that I experience everyday, and explains it more in depth.
Michael,
I read the blog and liked that there were many interesting facts about hair cycles and the areas on the body where hair does not exist! However I feel that the blog is missing a hypothesis and something you are trying to explain with the facts and evidence regarding hair. If the blog added a hypothesis supported by an argument of correlation versus causation, I believe the blog post would have benefited from the analysis.
-Sam Lee
Thank you so much for your feedback Sam! I think I have made the proper changes to my blog to satisfy a clear hypothesis. Your feedback is much appreciated.
Mike