– where does skin in your body stop being skin? Is the roof of the mouth skin? The back of the throat? How does skin attach and transition to non-skin flesh?
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You essentially have two types of „skin“.
TLDR: in short you inner skin (mucosa) and outer skin (dermis) are fundamentally the same. Both have a connective tissue that holds it together and a layer above that protects it. Skin hardens and has nerve endings, whereas mucosa secretes slime and often doesnt have nerve endings. The transitions are in lips, eyelids, genitals, anus.
One is what we would normally refer to as skin, everything that covers your outside. The outer layer is called Epidermis, which roughly translates to outer skin. The top part is essentially dead cells that hardened to a degree and server as a shield. The part underneath replenishes these cells as they fall off iver time. The dermis is the next layer containing the hair, blood vessels, nerve endings, etc. it mostly consist out of connective tissue, giving the skin its elasticity.
The Mucosa is your internal skin. It‘s quite similar to regular skin, but a bit different. The top layer is called epithelial cells. Where skin builds a layer of dead cells for protection , mucosa secretes a slimy substance. Under that is the lamina propria, which is similar to the dermis. Its the underlying connective tissue that also has blood vessels, etc.
The junction between those two is called mucocutaneous junction. These are the areas where skin transitions to mucosa. In humans those areas are lips, nostrils, conjuntivae (inside part of the eyelids), urethra, vagina, foreskin and anus.
And now onto why we are this way:
Evolution started off as single cells. Over time those single cells became more complex and complex. Eventually they were too large to become even larger for the most part. So some cells attaches to one another, forming colonies.
The first large colonies where little balls of cells with a hollow inside. At some point however they folded in. Like you pushed your finger into a soft ball. Now this ball has two layers, the one facing outside and the one facing inside, as well as an opening.
This colony became more and more complex. Eventually the inside layer became less protective, using slime to keep itself safe. Its purpose was processing the food. The outside layer became harder and more resilient. This happened in countless different ways.
This pattern exists until today. The inner and outter skin are fairly similar but adapted to different features.
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