Skin

• 1. Introduction

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  Skin is the soft, flexible outer tissue that covers the body of a vertebrate animal, and it does three jobs at once: protection, regulation, and sensation. It is the first thing an environment touches and the first line of defense against it. On the 11th of January 2024, biologists reported the discovery of the oldest known skin, fossilized about 289 million years ago, possibly belonging to an ancient reptile. That single fossil hints at how deep this organ runs through the story of life. Yet skin is not one thing. It guards muscles and bones in a human, lets a frog breathe and absorb chemicals, and hardens into the scales of a fish or the feathers of a bird. How does a barrier this thin keep pathogens out and water in? Why does a frog's skin let an anesthetic flood straight through? And how does this same tissue give rise to a rhino's horn, a deer's antler, and an armadillo's bony plates? The answers live in the layers.

• 2. Layers Of The Mammal

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  Mammalian skin is built from two primary layers stacked together. The outer layer, the epidermis, provides waterproofing and acts as a barrier to infection. Beneath it sits the dermis, which houses the appendages of skin. Keratinocytes dominate the epidermis, making up 95 percent of it, alongside Merkel cells, melanocytes, and Langerhans cells. These keratinocytes are born in the deepest stratum, the stratum basale, where they divide through mitosis. The daughter cells then climb upward through the strata, changing shape and composition as they differentiate, until they lose their nuclei entirely. Along the way they bind to each other through junctions called desmosomes and secrete keratin proteins and lipids that lend the skin mechanical strength. At the very top, in the stratum corneum, the spent cells are shed from the surface in a process called desquamation. The epidermis carries no blood vessels of its own. Its deepest cells survive on diffusion from capillaries reaching up into the dermis below. Separating the two layers is a thin sheet of fibers called the basement membrane, built by both tissues working together. It does more than divide them. By binding cytokines and growth factors, it acts as a reservoir that releases them in a controlled way during remodeling and repair.

• 3. The Dermis And Below

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  The dermis cushions the body from stress and strain through an extracellular matrix of collagen fibrils, microfibrils, and elastic fibers, embedded in hyaluronan and proteoglycans. Those proteoglycans sit in very specific places. Hyaluronan, versican, and decorin run throughout the dermis and epidermis, while biglycan and perlecan are found only in the epidermis. The dermis splits into two regions. The papillary region, made of loose areolar connective tissue, sends fingerlike projections called papillae up toward the epidermis, giving it a bumpy surface that interlocks the two layers and strengthens their bond. Below it lies the reticular region, usually much thicker, built from dense irregular connective tissue packed with collagenous, elastic, and reticular fibers that grant the dermis its strength, extensibility, and elasticity. This deeper region holds the roots of the hair, sweat glands, sebaceous glands, receptors, nails, and blood vessels. The dermis also harbors the mechanoreceptors behind the sense of touch and the nociceptors and thermoreceptors that register heat. Below the dermis, though not part of the skin, lies the subcutaneous tissue, also called the hypodermis. It anchors the skin to bone and muscle and supplies it with blood vessels and nerves. Its main cells are fibroblasts, macrophages, and adipocytes, and it carries fully half of the body's fat, which serves as padding and insulation. The skin's surface is not sterile either. Microorganisms such as Staphylococcus epidermidis colonize it, and a disinfected surface gets recolonized from bacteria living deep in the hair follicle, the gut, and the urogenital openings.

• 4. Skin Beyond Mammals

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  The epidermis of fish and most amphibians consists entirely of live cells, with only minimal keratin in the superficial layer. It is generally permeable, and in many amphibians it may serve as a major respiratory organ. The dermis of bony fish carries relatively little of the connective tissue found in tetrapods. In most species it is largely replaced by solid, protective bony scales. Cartilaginous fish go a different route, embedding numerous tooth-like denticles in their skin in place of true scales. Birds and reptiles sit closer to mammals, with a layer of dead keratin-filled cells at the surface to reduce water loss, a pattern echoed in terrestrial amphibians such as toads. Sweat glands and sebaceous glands are unique to mammals, but other vertebrates carry their own glands. Fish have numerous mucus-secreting cells that aid insulation and protection, and may also carry poison glands, photophores, or cells producing a more watery serous fluid. Birds and reptiles keep relatively few skin glands, though some reptiles have pheromone-secreting cells and most birds have the uropygial gland. Color follows different rules outside mammals too. In reptiles, amphibians, and fish, the epidermis is often relatively colorless, and skin color comes instead from chromatophores in the dermis. Beyond melanin, these may hold guanine or carotenoid pigments. Chameleons and flounders can even change their skin color by adjusting the relative size of their chromatophores.

• 5. The Amphibian Arsenal

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  Amphibians carry two types of cutaneous glands, mucous and granular, and both are built from three connected parts: the duct, the intercalary region, and the alveolar gland or sac. The duct, derived from keratinocytes, passes up through the epidermis to allow external secretions. The alveolus, a sac at the base of the gland, holds cells that specialize in secretion. Between them, the intercalary system forms a transitional bridge. Granular glands are the venomous ones, larger in size though fewer in number than the mucous glands. They sit in clusters whose concentration shifts across amphibian taxa, and their toxins can be fatal to most vertebrates or have no effect on others. As a granular duct matures and fills with fluid, pressure swells its base until the epidermis forms a pit-like opening through which the venom is secreted upward. Its intercalary region is well developed, a ring of cells around the duct's base argued to have an ectodermal muscular nature, dilating and constricting the lumen during secretion. The gland's sac is divided into three layers: an outer tunica fibrosa of densely packed connective tissue, a spongy intermediate layer holding elastic fibers and nerves, and an inner epithelium, the tunica propria. Mucous glands are non-venomous and cover the entire surface of the body, keeping it lubricated. They also control pH, aid thermoregulation, grip the environment, make the animal slimy against predators, carry chemical communication, and even fight bacteria and viruses. Their alveolar structure is far simpler, an epithelium and connective-tissue cover that lacks a tunica propria. The cells lining a mucous duct twist around it in a helical fashion, their longitudinal axes set at 90-degree angles. Such glands help explain why a frog sitting in an anesthetic solution is sedated quickly as the chemical diffuses straight through its skin.

• 6. Patterns From Embryo

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  Cutaneous structures arise from the epidermis: hair, feathers, claws, and nails all trace back to it. During embryogenesis the epidermis splits into two layers, the periderm, which is later lost, and the basal layer. That basal layer is a stem cell layer, and through asymmetrical divisions it becomes the source of skin cells for the rest of life. It stays a stem cell layer through an autocrine signal, TGF alpha, and through paracrine signaling from FGF7, the keratinocyte growth factor produced by the dermis below. In mice, over-expression of these factors leads to overproduction of granular cells and thick skin. Hair and feathers emerge in regular patterns, and this is believed to result from a reaction-diffusion system. The system pairs an activator, Sonic hedgehog, with an inhibitor, BMP4 or BMP2, to form clusters of cells in an orderly arrangement. Sonic hedgehog-expressing epidermal cells trigger the condensation of cells in the mesoderm, which then signal back to shape the right structure for that position, while BMP signals from the epidermis block placodes from forming nearby. The mesoderm appears to define the pattern, instructing the epidermis where it sits, and the epidermis answers with the structure to build. Transplantation experiments with frog and newt epidermis revealed something striking. The mesodermal signals are conserved between species, but the epidermal response is species-specific, so the same instruction yields a frog structure in a frog and a newt structure in a newt.

• 7. The Working Organ

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  Protection is the skin's headline function, an anatomical barrier against pathogens and damage, with Langerhans cells serving as part of the adaptive immune system. Sensation comes from nerve endings tuned to heat, cold, touch, pressure, vibration, and tissue injury. Thermoregulation runs through eccrine sweat glands and dilated blood vessels that shed heat, while constricted vessels cut cutaneous blood flow to conserve it, and erector pili muscles tilt hair shafts to change insulation. Evaporation is controlled by a semi-impermeable barrier that limits fluid loss, and the skin doubles as a storage center for lipids and water. Absorption is real but modest: oxygen, nitrogen, and carbon dioxide diffuse into the epidermis in small amounts, and some animals breathe entirely through their skin. In humans the outermost 0.25 to 0.40 mm of skin is almost exclusively supplied by external oxygen, though its contribution to total respiration is negligible. Sebaceous glands release sebum, an oily liquid that helps the skin resist water so essential nutrients are not washed out of the body. The mechanics are distinctive too. Skin shows a J-curve stress strain response, a region of large strain and minimal stress that corresponds to collagen fibrils straightening and reorienting. Intact skin can be prestretched, like a wetsuit around a diver, or held under compression, and small circular holes punched into it may widen into ellipses, close, or stay round depending on preexisting stresses. Thickness shifts across the body. In humans the skin under the eyes and around the eyelids is the thinnest at 0.5 mm and among the first to show crows feet and wrinkles, while the palms and soles reach 4 mm, the thickest on the body. With age, tissue homeostasis declines as stem and progenitor cells fail to renew, and TGF-beta blocks dermal fibroblasts from becoming the fat cells that provide support. Sun exposure deepens these changes in a process known as photoaging, and the speed and quality of wound healing in skin is promoted by estrogen.