What causes melanoma to stay out of the bloodstream when it only needs to travel 1mm deep (src: [https://www.youtube.com/watch?v=6Giv9lopemY](https://www.youtube.com/watch?v=6Giv9lopemY)) to enter. If our cells are constantly dividing and cancer is a damaged cell that doesn’t know when to stop dividing, how exactly is it that humans don’t develop it instantly when greeted with high UV and sun exposure.
In: Biology
Are you saying melanoma can’t get to your bloodstream? Metastatic melanoma (metastatic means a cancer has spread to other areas) happens all the time, it’s horrible and has a very low survival rate. That’s why you always hear people saying you should get moles or other things on your skin checked out by a doctor.
If you mean why doesn’t everyone end up with melanoma traveling through their body, I guess sheer chance? The cells it develops in are called melanocytes and are scattered around in a layer of cells called the epidermis that acts as a barrier keeping the stuff inside you in and the stuff outside out. The bottom layer of the epidermis is called the basal layer; the cells in this layer are bound very tightly and it’s basically a wall. The melanocytes are on the opposite side of the wall from your blood; it’s possible for them to break through but only if they grow downward and which direction the mass of cells will grow is completely random (for eli5). And some will grow slower and some faster, also more or less random.
The other poster answered the last part correctly but I wanted to add the vast majority of mutations in the DNA of one cell won’t cause cancer. To give you an idea we estimate throughout all the cells in your body you probably acquire somewhere between 1 million and trillions of mutations every day. Most of the time your cell just repairs them. Even then most will occur in the 90% or so of your DNA that is not code to make proteins (genes) and likely won’t do anything at all. Most mutations that are in the genes will kill the cell, or do nothing to you overall. Most of the time when a cell gets a mutation that can lead to cancer and starts growing out of control your immune system detects it and kills the cell.
Melanoma starts in the melanocytes in your basale layer of your epithelium. These melanocytes rapidly divide. These melanocytes are very very tiny so a lot of them does not seem like much depth to us. However after enough time the cancer can break into the dermal tissue and enter the bloodstream.
However, severity of melanoma is determined by DEPTH not width. If the melanocytes grow sideways it’s really is not that big of a deal. It’s a huge deal if they start moving downward.
2nd question: your cells have machinery such as excision that can get ride of T dimers and other damages to DNA caused by UV radiation. As you keep hitting your DNA though you increase the RISK of a failure in your defense machinery and getting uncontrolled cell growth.
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