Betts, J. Gordon; etal. (2022). Anatomy and Physiology 2e. OpenStax. p.164. ISBN 978-1-711494-06-7. You lose collagen and elastin as you age. This causes the skin’s middle layer (dermis) to get thinner. As a result, the skin may sag and develop wrinkles. Tattoo ink is held in the dermis. Stretch marks, often from pregnancy and obesity, are also located in the dermis. Hives on darker skin can appear raised or inflamed and might be slightly darker or lighter than your natural skin color. On lighter skin, hives usually appear red. Del Rosso JQ, Levin J (September 2011). "The clinical relevance of maintaining the functional integrity of the stratum corneum in both healthy and disease-affected skin". The Journal of Clinical and Aesthetic Dermatology. 4 (9): 22–42. doi: 10.1111/j.1365-2133.1990.tb06268.x. PMC 3175800. PMID 21938268.

Oily skin is caused by over-active sebaceous glands, that produce a substance called sebum, a naturally healthy skin lubricant. [8] [44] A high glycemic-index diet and dairy products (except for cheese) consumption increase IGF-1 generation, which in turn increases sebum production. [44] Overwashing the skin does not cause sebum overproduction but may cause dryness. [44] Cutroneo KR, Sterling KM (May 2004). "How do glucocorticoids compare to oligo decoys as inhibitors of collagen synthesis and potential toxicity of these therapeutics?". Journal of Cellular Biochemistry. 92 (1): 6–15. doi: 10.1002/jcb.20030. PMID 15095399. S2CID 24160757. (subscription required) Many animals have skin which produces colors and patterns specially designed to blend in with their surroundings and avoid being spotted. This is important for both predator and prey species. Prey species can’t be eaten if their predator cannot see them; and predators can’t be avoided if they are invisible to their prey. a b Proksch E, Brandner JM, Jensen JM (2008). "The skin: an indispensable barrier". Exp Dermatol. 17 (12): 1063–1072. doi: 10.1111/j.1600-0625.2008.00786.x. PMID 19043850. S2CID 31353914. The below table identifies the skin cell count and aggregate cell mass estimates for a 70kg adult male (ICRP-23; ICRP-89, ICRP-110). [14] [15] [16]It might also appear as a mole that has changed color or gotten bigger over time, which is usually larger than a pencil eraser. Boelsma E, van de Vijver LP, Goldbohm RA, Klöpping-Ketelaars IA, Hendriks HF, Roza L (February 2003). "Human skin condition and its associations with nutrient concentrations in serum and diet". The American Journal of Clinical Nutrition. 77 (2): 348–355. doi: 10.1093/ajcn/77.2.348. PMID 12540393. Kligman A (2006). "A brief history of how the dead stratum corneum became alive". Skin Barrier. New York: Taylor & Francis. pp.35–44. ISBN 9780429163470.

Absorption through the skin: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts; some animals use their skin as their sole respiration organ (in humans, the cells comprising the outermost 0.25–0.40mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total respiration is negligible") [28] Some medications are absorbed through the skin. Many animals’ skins produce a static color pattern, such as the brown color of antelope and deer that mimics the color of dead grass and leaves, the bright green scales of the vine snake, or the dappled muddy scales of the flounder.Smith, Margaret Mary; Melrose, James (March 2015). "Proteoglycans in Normal and Healing Skin". Advances in Wound Care. 4 (3): 152–173. doi: 10.1089/wound.2013.0464. PMC 4352701. PMID 25785238. Scientists now know that humans do this ourselves. Although we don’t consciously process the scent information all around us, our behavior can change subconsciously based on whose sweat we are smelling. People are even more likely to be attracted to each other if they “smell” that they have complimentary immune systems, meaning that their children would have resistance to many diseases! Skin Structure

We’ll even talk about some functions our own skin performs that we may not know about! Functions of the Skin Vassar, R; Fuchs, E (1 May 1991). "Transgenic mice provide new insights into the role of TGF-alpha during epidermal development and differentiation". Genes Dev. 5 (5): 714–727. doi: 10.1101/gad.5.5.714. PMID 1709129. Reduction of tissue in this layer is what causes your skin to sag as well as wrinkle. This layer hosts sweat glands, and fat and loose connective tissue. The subcutis is responsible for conserving your body's heat and protecting your vital inner organs. Collagen and Your Skin A. Without the protective covering of the epidermis, the area might be more vulnerable to infection.National Institutes of Health, U.S. National Center For Biotechnology Information: StatPearls [Internet]. Histology, dermis.

Melasma is more common in pregnant people (chloasma) and individuals with a darker skin color or heavy sun exposure.Valentin J (September 2002). "Basic anatomical and physiological data for use in radiological protection: reference values: ICRP Publication 89". Annals of the ICRP. 32 (3–4): 1–277. doi: 10.1016/S0146-6453(03)00002-2. S2CID 222552 . Retrieved 30 October 2023.