Skin shedding in reptiles has long fascinated and perplexed humans. Several ancient civilizations had different beliefs about snakes. Some thought that snakes reincarnated when they were molting (“changing skin”), representing eternity or immortality just because of their ability to shed skin. As much as I love history and different mythologies: no, they are not being re-born. This phenomenon can be called shedding, sloughing, molting or ecdysis.
You probably have seen some reptile skin lying around already, and you probably know that they do this quite often. It’s commonly said that they do this to be able to grow. But is this true? There are actually many other reasons why reptiles can do this, and the histology behind this phenomenon is pretty awesome!
It’s Evolution Baby!
Imagine the slimy skin of most fish and amphibians. These guys have a mucogenic epidermis (mucous skin). The vertebrate colonization of terrestrial ecosystems wouldn’t have been that easy with this mucous exterior. Outside of the water, everything was drier, more rough structures could harm the skin, and there were new microbial and invertebrate threats that could easily harm this gooey exterior! So our first land-dwelling ancestors had to develop some adaptations in order to survive.
Horny layers
The epidermis of adult amphibians and all amniotes (reptiles, birds, and mammals) have more complex layering than fish. At the bottom, we have the dermis, which has dermal blood vessels (therefore, oxygen!). Right above the dermis, the epidermis starts with its basal stratum, where cells are in constant mitotic division. As cells divide (thus multiply!), the new cells push the previous cells upward into the outer layers of the epidermis.
As cells get displaced further away from the dermis (further away from the oxygen in the blood vessels), they start to die. They simply asphyxiate without oxygen. So the further away from the dermis, the older the cells, and at some point layers of dead cells accumulate. In fish, however, even the outermost epidermal cells are in constant mitotic division (so they just die normally and fall off).
Above all these dead cell layers, we get to the outermost layer of the epidermis. In histology, this is called the stratum corneum (which is Latin for “horny layer” – yes, you read that right, I did not make this up). This horny layer is made up of “corneocytes”, which are basically dead cells filled with proteins embedded in a lipid matrix (waxes and oils).
This proteic-waxy coat is what helps us against dehydration as it prevents the water within to evaporate easily. The types of proteins in this layer (like keratin) are also making this layer harder, protecting from mechanical stress. The formation of this corneocyte envelope is absent in fish and young amphibians.
Let’s scale up!
Now you know how the epidermis is layered. By now you might be wondering that if all of us amniotes have this similar epidermal layering, why don’t reptiles have our type of skin? Or, what would be way cooler, in my opinion: why don’t WE have awesome colorful reptile skin?! Well, the “cornification” (the dead cell accumulation and content) evolved differently in different vertebrates. The types and amount of proteins in the epidermis varies.
The keratinization in birds and mammals involves a lot of keratohyalins, which makes our skin softer. Actually, some human pathologies, like eczema, psoriasis, and ichthyosis, are partly caused due to a deficiency of keratohyalin. In mammals, hard keratinization occurs only in hairs, nails, claws, horns, or hooves. Nevertheless, we DO shed our skin! In humans (and most mammals, birds, and even some reptiles like chelonians and crocodylians) our horny layer is falling off every day and we completely replace our entire skin every four to six weeks.
But mammals are not that cool
In other reptiles, however, an accumulation of beta-proteins (e.g. beta-keratin) makes the “stiff and dry” scaled layers, and a still-not-so-well-understood signaling mechanism synchronizes their shedding.
In Lepidosaurs (snakes, lizards, amphisbaenids and tuataras), epidermal growth is cyclic and not continuous like ours. As you can see in the above diagram, these reptiles (squamates) have alternating alpha- and beta-protein layers in both the inner and outer “skin generations” (IG and OG).
After a certain number of beta-alpha layers (which is species-specific), the synthesis of beta proteins stops and only alpha layers continue to build up. This starts the “renewal” phase. After this, the OG separates from the IG, which might be initiated by enzymatic secretions and other liquid depositions in between the new and old skin layers. This is basically the shedding moment!
In between the layers some liquids containing lipids and enzymes are released, which initiate the shedding once given the correct environmental and genetic cues. The contents of this liquids basically help on the detachment of the outer layer into the external environment.
Lizards and tuataras shed their skin in patches, usually aiding themselves with their teeth. Snakes, on the other hand, will rub their heads or snouts against a rough surface to create a “breaking point” and then “crawl” out of their old skin as if they were getting naked. That’s why it is more common to find the tubular snake-skin in the forest than a lizard-shaped one!
Then what?
After shedding, the basal cells enter a resting phase. This has a different duration in each species. During the resting phase, mitosis ceases, and the renewal phase will start once again upon the synchronization of germinative cells based on environmental and genetic factors. These then begin to differentiate and proliferate and start pushing cells upward again, preparing the new layers that will “kick out” the old ones.
So skin shedding IS for growing?
This periodic shedding allows the somatic growth of these organisms, since the number of scales doesn’t change, but instead they just grow bigger scales that can no longer fit in the previous skin! This is also why scale number is commonly used in reptile taxonomy and systematics.
However, somatic growth is not the only reason! The protective proteins degrade over time, so skin shedding also plays an important role in maintaining these protective layers. This way they maintain their rough skin protected against UV-rays, evaporation, parasites and other potential threats by renewing keratin. Under certain conditions (this has been seen in captive -“pet”- reptiles) the shedding might happen more often because of stress, more food availability, or to get rid of parasites.
Fun fact!
Little is known about lizard behavior before molting. With snakes, however, we know that they become more secretive and lethargic. Some even immerse themselves in water bodies. This is, perhaps, because their eyes also get covered in this epidermal layers (like in the picture above, where the eyes look blue-ish) so their vision is compromised before their old skin is ready to shed off! From personal experience, I can tell you that handling pre-molting snakes is quite fun (don’t do it!) because they get way more defensive (aggressive)!
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References
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Alibardi, L. 2014. Transition from embryonic to adult epidermis in reptiles occurs by the production of corneous beta-proteins. International Journal of Developmental Biology. 58(2014):829-839
- Vitt, L.J. & J.P. Caldwell. 2013. Herpetology: An Introductory Biology of Amphibians and Reptiles. Elsevier Academic Press. 776pp.
- Lillywhite, H.B. 2006. Water Relations of Tetrapod Integument. Journal of Experimental Biology. 209:202-226.
- Alibardi, L. 2003. Adaptation to the Land: the Skin of Reptiles in Comparison to that of Amphibians and Endotherm Amniotes. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 298B(1):12-41.
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Maderson, P.f.A., T. Rabinowitz, B. Tandler, & L. Alibardi. 1998. Ultrastructural Contributions to an Understanding of the Cellular Mechanisms Involved in Lizard Skin Shedding with Comments on the Function and Evolution of a Unique Lepidosaurian Phenomenon. Journal of Morphology. 236(1998):1-24