Integumentary System definition
The integumentary system is made up of the organs that make up the skin and its many derivatives, the animal body’s outermost protective layer. Infection, desiccation, abrasion, chemical attack, and radiation damage are just a few of the dangers the integumentary system guards against. The skin serves as the main organ of the integumentary system in humans. This organ system includes skin in addition to numerous other glands and several sensory organs, including nociceptors and somatosensory receptors.
Organs of the Integumentary System (structure and functions)
The skin and its appendages, subcutaneous tissue, deep fascia, mucocutaneous junctions, and breasts make up the integumentary system.
- The biggest organ in our body is the skin.
- It has numerous glands and sensory organs and may cover up to 2m2 of the body’s surface.
- The mucous membrane that borders the buccal cavity is structurally distinct from the skin, which is continuous.
According to its structural makeup, the skin is a multicellular organ made up of two unique layers of tissues: an inner dermis that is formed from mesoderm and an outer epidermis that develops from ectoderm.
Depending on the environment, the two layers’ relative abundance varies.
- As compared to the dermis, the epidermis is a stratified epithelium that is often fairly thin.
- The cells of the squamous epithelium get blood by diffusion because the outermost layer of skin lacks a blood vessel supply.
- It is further divided into two regions: the outermost area, which is made up of numerous layers of dead, typically flattened (squamous) cells, creates a tough layer on the skin’s surface called the stratum corneum.
- Its cells progressively degenerate, amass the horny protein keratin, and finally fall off as scurf or dandruff.
- The Malpighian layer, also known as the stratum germinativum, is a single row of live columnar cells that make up the deepest or basal part of the epidermis. This layer is separated from the dermis beneath it by a foundation membrane.
- Its cells actively proliferate, replacing the cornified layer’s worn-out cells on a regular basis.
- Additionally, the epithelium includes a small number of melanocytes, which are in charge of giving the skin its colour.
- The keratinized stratum corneum defends against mechanical damage, fungal and bacterial infections, and loss of bodily fluids since keratin is strong and insoluble in water.
- The epidermis’ melanin shields the dermis and other internal organs from UV radiation and its harmful consequences.
- Under the influence of UV radiation from the sun, the epidermis is also in charge of producing Vitamin D.
- The skin’s immune system, which includes Langerhans cells in the epidermis, defends against foreign antigens.
- Skin feeling is produced by sensory cells and receptors in the epidermis.
- The inner layer of skin, known as the dermis or corium, is noticeably thicker than the epidermis.
- It is made of fibrous connective tissue and is filled with a lot of blood capillaries, lymph vessels, muscle fibres, nerve fibres, sensory organs, and elastic fibres that help the skin return to its regular shape.
- Although pigment granules can occasionally be detected in the epidermis, melanocytes, or pigment cells, are mostly found in the dermis.
- Deeper layers of the dermis and subcutaneous tissue may include specialised cells called adipocytes that store fat as a reserve fuel source.
- The dermis also contains several glands, including sweat and sebaceous glands.
- The dermis’s blood arteries provide nutrition for its cells and remove waste from the epidermis’s base.
- The dermis aids in thermoregulation because sweat glands encourage evaporation, which causes an excessive loss of body heat.
- The dermis also gives the epidermis stability and serves as the foundation for the cells.
- The deeper subcutaneous tissue composed of fat and connective tissue, known as the hypodermis, is the skin’s deepest and thickest layer.
- Cells including fibroblasts, adipose cells, connective tissue, bigger nerves and blood vessels, and macrophages make up this layer.
- The hypodermis’ functions include providing the other layers of skin with blood arteries and nerves, as well as connecting the skin to the bone and muscle beneath.
- Elastin protein and floppy connective tissue make up the hypodermis.
- This layer’s thickness varies greatly between males and females as well as in other body regions.
- Men’s shoulders and belly are where it is thickest, but women’s hips and thighs are where it is thickest.
- Large amounts of fat in the hypodermis serve as energy storage.
- The substantial layer serves as both a barrier against outside elements and an insulator against cold.
- This layer serves as a connection between the body’s internal organs and the epidermis and dermis.
- Fat cells in the hypodermis layer also produce the hormone leptin, which controls hunger.
- Appendages of the skin
- The skin is quite basic in and of itself, but its derivatives are many and intricate.
- These derivatives, which are known as skin appendages, are created from derived epidermal cells.
- Epidermal glands
- The Malpighian layer of the epidermis, which develops in the epidermis but frequently invades the dermis, produces integumental or epidermal glands.
- They might be simple, complex, or branching; tubular or alveolar in form; or multicellular and unicellular.
- They are often termed by their nature or function and are bordered by cuboidal cells or columnar epithelium.
- Common epidermal glands in people and other animals include:
- Sudoriferous Glands
- The epidermis of most animals has a large number of sweat glands, also known as sudoriferous glands (sudor = sweat).
- Sudoriferous glands can be classified as either eccrine or apocrine secretory skin glands.
- Apocrine glands open into connected hair follicles, whereas eccrine glands open directly onto the skin surface.
- Nearly every part of the human body has eccrine glands, with the palms and soles having the highest density.
- Apocrine glands are located in a smaller number of locations on the body, such as the areola, axilla, and anogenital region.
- They are lengthy ducts that open on the skin’s surface and are made up of thin, coiling tubes that are implanted deep inside the dermis.
- The sweat glands are made up of a glomerulus, a kind of secretory unit, and a lengthy duct that transports the sweat to the target surfaces.
- Adipose tissue surrounds the dermis or hypodermis, where these glands are buried.
- Myoepithelial cells that help in sweat excretion surround the secretory unit.
- The edges of the eyelids and the ciliary glands in the eyelashes are modified sweat glands.
- The sweat gland’s role in controlling body temperature is crucial. In heated conditions, the evaporation of watery sweat also aids in cooling and regulating body temperature.
- Additionally, a small amount of urea and a few ions are excreted in the sweat generated by these glands that aid in excretion.
- Sebaceous gland
- The secretory epithelial cells that make up the sebaceous gland are generated from the same tissue as hair follicles.
- Sebum, an oily antibacterial material, is secreted into the hair follicles by these glands.
- All regions of the body have sebaceous glands, with the exception of the palms and soles of the hands and feet.
- The scalp, face, axillae, and groynes have more of these glands than other body parts.
- The size of the gland changes inversely with the diameter of the connected hair since the majority of sebaceous glands are related to hair follicles.
- A tiny gland has a big diameter, and vice versa.
- These are real holocrine glands since they produce their secretion as a result of the whole glandular cell degenerating.
- Through the pilosebaceous canal, they may be seen on the skin’s surface.
- The acinar components of this holocrine type of gland are many. Each gland’s acinus gathers at a single excretory duct.
- Sebum gives the hair a lustrous sheen and keeps it smooth and malleable.
- On the skin’s surface, it helps to keep moisture in while also acting as a fungicidal and bactericidal agent to fight infection.
- Additionally, the sebum protects the skin from drying out and breaking, especially when it is exposed to heat and sunshine.
- Ceruminous glands, which are remodelled apocrine glands, work with sebaceous glands to generate cerumen, also known as ear wax.
- In the typical human ear, there are between 1,000 and 2,000 ceruminous glands, which are found in the cartilaginous portion of the external auditory canal.
- The ceruminous gland’s coiled tubules’ ducts go through the dermis before emptying into hair follicles or onto the epidermis.
- The cuboidal (inactive) or columnar cells of the coiled ceruminous gland have a wide lumen (active).
- A layer of secretory cells located on the myoepithelial cells lines the gland.
- The secretion initially exits the glandular cells’ individual ducts, then moves into bigger ducts, and eventually settles at the base of guard cells found in the auditory canal.
- Cerumen is crucial for the ear canal’s defence against harm from external factors and microbial invasion.
- The secretions in the inner/medial portions of the ear canal, which are about pH 5.7, keep the ear canal’s pH stable.
- Mammary gland
- These complex tubular glands, which are a feature of animals, generate milk for the young during the lactation period.
- All animals have these glands, but males only have primitive, inactive versions of them.
- Growth hormone and oestrogen work together to help these glands develop in humans throughout puberty. However, in other primates, breast development often occurs after pregnancy.
- A tear-shaped structure of glandular tissue, fibrous tissue, fatty tissue, and blood supply make up the mammary glands.
- Each of the approximately 20 lobes of glandular tissue that make up a mature female breast radiates outward from the nipple.
- Each of these lobules is made up of a collection of alveoli that open into tiny ducts that join together to produce lactiferous ducts, which are substantial excretory channels.
- These ducts converge in the centre of the breasts to produce milk reservoirs, also known as lactiferous sinuses, which are supported by the thick connective tissues. One duct that leads to the nipple emerges from each of these sinuses.
- Breastfeeding is the main way that mammary glands supply nutrition to the newborn.
- Breasts serve the basic purpose of feeding the newborn, but they also have significant social and sexual roles.
- Nipples, in particular, are an erogenous zone on the breasts.
- Hair and Hair follicles
- Mammals are known for their hair. They might be reduced to patches or stray hairs, or they could completely cover the body.
- The integument’s cornified epidermal products are hairs.
- Every so often, all of the hairs on the skin’s surface moult and are replaced by fresh sets of hair.
- Each hair grows at the base of a tubular invagination, also known as a hair follicle, that extends from the epidermis’ germinative layer into the dermis.
- At the base of the follicle, there is a collection of cells known as the hair papilla or bulb.
- The bulb’s cells divide to create the hairs, which are then keratinized when the old cells are pushed upward and away from their source of nourishment.
- The portion of the hair that is above the skin is referred to as the shaft, and the remaining portion as the root.
- The typical hair shaft is made up of three layers: the outside cuticle, which is composed of overlapping tiny scales; the middle cortex, which contains shrunken cells and colors; and the inner medulla, which has air holes in the case of thicker hairs.
- The primary roles of hairs appear to be as sensitive tactile organs and as a means of bodily insulation (e.g., vibrissae in rabbits).
- Additionally, hairs have a role in controlling body temperature and promoting sweat evaporation.
- Animal horns, hooves, and claws are analogous to human nails.
- The firm, horny keratin plates that cover the tips of the fingers and toes are derived from the same cells as the epidermis and hair.
- Connective tissue and epithelial tissue make up the nail organ.
- The nail is made up of the nail matrix, which is mostly under the proximal nail wall and generates the nail plate, and the nail bed epithelium, which provides a solid adherence to the nail bed’s dermis.
- The cuticle, which creates the hemispherical pale region known as the lunula, covers the nail’s root, which is implanted in the skin.
- The portion of the nail that has emerged from the nail bed is known as the nail plate.
- The fingertip and the soft tissues around it are shielded from harm by the nail as its main purpose.
- The counterpressure that nails apply helps with touch sensitivity and accurate movement.
Physiology of the Integumentary System
Development of skin colour (Skin Pigmentation)
- Human skin colour is influenced by a variety of pigments, including melanin, carotene, and haemoglobin.
- The most important pigment for skin pigmentation overall is melanin, which is exclusively produced in the cytoplasm of the melanocyte, the cell that produces melanin.
- There are two types of melanin: eumelanin, which produces the black and brown colour, and pheomelanin, which produces the red hue.
- On average, exposed skin has more melanocytes per square millimetre than protected skin. The nasal mucous membranes and the genitalia, however, have a high density of melanocytes.
- The quantity and rate of melanin produced by melanocytes, rather than the total number of melanocytes present, determines differences in skin tone across ethnic groupings and potentially even different people within the same race.
- The epidermal-dermal junction of the skin and mucous membranes contains these branching or dendritic cells.
- Once they enter the epidermis, these dermal melanocytes develop into epidermal melanocytes. Melanocytes that are still present in the human dermis progressively lose their activity.
- The keratinocytes are activated by exposure to the sun’s UV rays to release chemicals, which in turn excite the melanocytes to make melanin.
- The keratinocytes then accumulate melanin as a result of the melanin being created in the melanocytes being transported to them through a cell organelle known as the melanosome.
- Tyrosinase, an enzyme that includes copper and is present in melanosomes, catalyses the formation of melanin from tyrosine.
- The skin becomes darker as a result of melanin buildup. It takes around 10 days for melanin production to reach its peak following sun exposure.
- Similar to how epidermal cells travel outward to produce the stratum corneum, melanin granules within them are carried along and show up in the stratum corneum as small, irregular coloured particles rather than as granules when new skin is generated.
- The integumentary system includes the hair, which extends into the dermal layer and rests in the hair follicle.
- In a process similar to that of the skin, stem cells divide quickly and differentiate into keratinocytes, which migrate, flatten, and eventually die, producing keratinized cells.
- The final hair product that shows on the skin’s surface is made completely of keratin.
- The development of the hair follicle occurs in cycles. Three phases—anagen (growth), catagen (transition), and telogen—can be distinguished within this cycle (rest).
- The hair follicle’s active phase, known as anagengrowth, is when it assumes its onion-like structure and begins to create hair fibre. Most scalp hairs (between 85 and 90 percent) are anagen.
- Only during the anagen phase does the production of the hair shaft and pigmentation occur. The ultimate hair structure’s curvature is determined by the axial symmetry inside the hair bulb.
- By the conclusion of anagen, the matrix cells’ mitotic activity has decreased, and the follicle enters catagen, a tightly regulated involuntary phase.
- In humans, catagen lasts for around two weeks. During this time, the proximal portion of the hair shaft keratinizes and develops into club hair.
- In contrast, apoptosis causes the distal part of the follicle to lose 1/6th of its diameter.
- The telogen stage lies between the end of catagen and the beginning of the next anagen phase.
- About 10-15% of all scalp hairs are in the telogen stage, which lasts for two to three months.
- The hair shaft turns into club hair and is eventually lost during the telogen stage.
- This phase lasts until the dermal papilla sends signals that trigger the onset of anagen by inducing the hair germ to exhibit increased proliferative and transcriptional activity.
- The most important skin appendage is the nail. It develops continually, non-cyclically, throughout life and is not hormone-dependent in any way.
- The large toenail of the dominant foot develops just 0.03 to 0.05 millimetres per day, but the middle finger’s nail grows at a rate of about 0.1 millimetres per day.
- The size and form of the terminal phalanx bone, which determines how the nails develop, varies typically from finger to finger and from toe to toe.
- The nail is a keratin plate that grows continually throughout one’s life and is biochemically identical to the hair shaft.
- Nail growth happens by extrusion, in which the old cells are pushed to the tips while the new ones are introduced to the base.
- Old cells’ keratin gets tougher, and eventually the cells merely become structures made of hardened keratin.
- At the base of the nail, there are still some live cells visible as the white lunula that resembles the moon.
- Most of the cell proliferation takes place in the nail bed’s germinal matrix.
- On top of the nail fold are the cells that give the nail surface its gloss.
- The nail is pushed into the concave framework as it grows.
Functions of the Integumentary System
- Since it carries out several crucial tasks, including some of the following, the integumentary system of vertebrates is really a “jack of all crafts.”
- The animal’s integument, or skin, serves to keep its internal environment constant by insulating it from its surrounding environment.
- Skin inflammation cells act as a barrier against invasive antigens.
- Melanin, a pigment, shields skin from the sun’s damaging UV radiation.
- Different glands’ lipid and oil-like secretions serve as a further defence against toxins and stop heat loss.
- Sweat glands and the hair on mammal skin both work to keep the body temperature at a moderate level.
- In heated conditions, the evaporation of aqueous sweat from the skin aids in cooling and regulating body temperature.
- Similarly, hair assists in controlling body temperature and promotes sweat evaporation.
- Integumentary blood vessels dilate to make the skin become a radiator when heat is being lost, but the vessels constrict when heat is being retained.
- Urea and other ions that help with excretion are present in some quantity in the secretions of the sebaceous and sweat glands. Sweating is another way that extra vitamin B is expelled.
- When kidney function is compromised, and for some fragrant chemicals like garlic and other spices, the skin serves as a small excretory organ.
Formation of Vitamin D
- Sunlight transforms 7-Dehydrocholesterol, a lipid-based molecule in the skin, into vitamin D.
- Along with calcium and phosphate, this vitamin is essential for bone growth and preservation.
- The dermis contains sensory receptors that may detect pain, pressure, temperature, or touch.
- Sensory nerves are stimulated, which causes nerve impulses to be produced and sent to the cerebral cortex.
- Some substances, including some medications, hormone replacement treatment during menopause, nicotine used in transdermal patches to help people quit smoking, and some dangerous compounds like mercury, can be absorbed via the skin.
Integumentary System Diseases
- A highly contagious bacterial illness called impetigo is frequently brought on by Staphylococcus aureus.
- It starts off as small, superficial pustules, typically around the mouth and nose. It is common in kids and those who have compromised immune systems and is spread through direct contact.
- Some anaerobic bacteria, such as Streptococcus pyogenes and Clostridium perfringens, which enter through a break in the skin, produce cellulitis, a spreading illness.
- The development of enzymes that destroy the connective tissue that typically isolates an inflammatory region helps them spread.
- Bacteria may enter the blood and result in septicemia if left untreated.
Tinea pedis and ringworm
- These skin diseases are superficial.
- The most common areas of the scalp, foot, and groyne affected by ringworm infection are those mentioned above. It is also quite contagious.
- The skin in between the toes is impacted by tinea pedis (athlete’s foot).
Human papillomavirus (HPV)
Warts or verrucas brought on by HPV are contagious through physical touch. This results in the multiplication of the epidermis and the creation of a tiny, usually benign hard growth.
- The herpes zoster virus is what causes the rashes associated with shingles and chickenpox.
- Other herpes viruses, such as HSP1 and HSP2, are responsible for genital herpes and cold sores, respectively.
- Eczema is a common inflammatory skin condition that can be either acute or persistent.
- Redness, swelling, and serous fluid exudation are the hallmarks of acute dermatitis, which is also marked by pruritus (itching), eventually progressing to crusting and scaling.
- Chronic problems cause the skin to thicken and perhaps become leathery as a result of constant scratching, which can lead to infection.
- Male adolescents are more likely than females to get acne, which is assumed to result from higher testosterone levels following puberty.
- This is brought on by clogged and infected sebaceous glands (in hair follicles), which result in irritation and pustule production.
Base cell cancer
- The least aggressive and most prevalent kind of skin cancer is basal cell carcinoma.
- Since this tumour is linked to prolonged sun exposure, it is more likely to develop in sun-exposed areas, mainly the head or neck.
- Cancer first manifests as a glossy nodule, which eventually degenerates into a rodent ulcer—an ulcer with wavy edges.
- Malignant melanoma is a proliferation of melanocytes that has an abnormal shape and generally develops from an enlarged mole.
- Young and middle-aged people are more likely to have bleeding and ulcerating melanocytes.
- Recurrent periods of intense sun exposure, including multiple episodes of sunburn, especially in childhood, lead to the development of this tumour.
- Nagarajan P. (2018). Ceruminous Neoplasms of the Ear. Head and neck pathology, 12(3), 350–361. https://doi.org/10.1007/s12105-018-0909-3
- Stoeckelhuber, M., Matthias, C., Andratschke, M., Stoeckelhuber, B.M., Koehler, C., Herzmann, S., Sulz, A. and Welsch, U. (2006), Human ceruminous gland: Ultrastructure and histochemical analysis of antimicrobial and cytoskeletal components. Anat. Rec., 288A: 877-884. DOI:1002/ar.a.20356
- Hoover E, Alhajj M, Flores JL. Physiology, Hair. [Updated 2019 Aug 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499948/
- Bilgen Erdoğan (May 3rd 2017). Anatomy and Physiology of Hair, Hair and Scalp Disorders, Zekayi Kutlubay and Server Serdaroglu, IntechOpen, DOI: 10.5772/67269. Available from: https://www.intechopen.com/books/hair-and-scalp-disorders/anatomy-and-physiology-of-hair
- Volker, J.F. and Kenney, J.A., Jr. (1960), The Physiology and Biochemistry of Pigmentation. The Journal of Periodontology, 31: 346-355. DOI:1902/jop.1918.104.22.1686
- Yamaguchi, Y., & Hearing, V. J. (2009). Physiological factors that regulate skin pigmentation. BioFactors (Oxford, England), 35(2), 193–199. https://doi.org/10.1002/biof.29
- LOBITZ, W. C. (1957). The Structure and Function of the Sebaceous Glands. Archives of Dermatology, 76(2), 162.DOI:10.1001/archderm.1957.01550200006002
- Patel BC, Treister AD, McCausland C, et al. Anatomy, Skin, Sudoriferous Gland. [Updated 2019 Oct 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513244/
- Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
- Marieb EN and Hoehn K. (2013) Human Anatomy and Physiology. Ninth Edition. Pearson Education, Inc.
- Kotpal RL(2010). Modern Textbook of Zoology Vertebrates (Animal Diversity – ii). Rastogi Publications.
- 1% – https://www.ncbi.nlm.nih.gov/pubmed/29763123
- 1% – https://www.ncbi.nlm.nih.gov/pubmed/24718505
- 1% – https://www.createwebquest.com/integumentary
- 1% – https://reference.medscape.com/medline/abstract/24718505
- 1% – https://jamanetwork.com/HttpHandlers/ArticlePdfHandler.ashx?journal=derm&articleId=525114&pdfFileName=archderm_76_2_002.pdf
- 1% – https://doctorlib.info/anatomy/ross-wilson-anatomy-physiology-health-illness/14.html
- 1% – https://chnbaph.files.wordpress.com/2018/04/special-sensory-organs.pdf
- 1% – https://basicmedicalkey.com/the-skin-2/
- 1% – https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.a.20356
- 1% – http://encyclopedia.kids.net.au/page/sk/Skin
- <1% – https://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair
- <1% – https://www.unitypoint.org/livewell/article.aspx?id=9a64f6ba-8855-44dd-82d7-fe32b00f4e06
- <1% – https://www.typesof.com/types-of-dendritic-cells/
- <1% – https://www.sciencedirect.com/topics/neuroscience/melanin
- <1% – https://www.sciencedirect.com/topics/medicine-and-dentistry/apocrine-gland
- <1% – https://www.sciencedaily.com/terms/melanoma.htm
- <1% – https://www.researchgate.net/publication/6490908_Fuchs_E_Scratching_the_surface_of_skin_development_Nature_445_834-842
- <1% – https://www.researchgate.net/publication/260949719_Melanin_Transfer_The_Keratinocytes_Are_More_than_Gluttons
- <1% – https://www.nhs.uk/conditions/acne/causes/
- <1% – https://www.ncbi.nlm.nih.gov/pubmed/7449797
- <1% – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6773238/
- <1% – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3904096/
- <1% – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3834696/
- <1% – https://www.merckmanuals.com/home/skin-disorders/biology-of-the-skin/structure-and-function-of-the-skin
- <1% – https://www.mdpi.com/2079-9284/6/4/57/htm
- <1% – https://www.kenhub.com/en/library/anatomy/histology-of-the-sweat-glands
- <1% – https://www.healthyhairplus.com/sebaceous_glands_oily_itchy_scalp_s/4175.htm
- <1% – https://www.healthline.com/human-body-maps/mammary-duct
- <1% – https://www.health.ny.gov/diseases/communicable/athletic_skin_infections/bacterial.htm
- <1% – https://www.epainassist.com/skin/function-of-sweat-glands-and-benefits-of-sweating
- <1% – https://www.dummies.com/education/science/anatomy/the-anatomy-of-skin/
- <1% – https://www.dermnetnz.org/topics/skin-immune-system
- <1% – https://www.britannica.com/science/tuberculosis
- <1% – https://www.britannica.com/science/sweat-gland
- <1% – https://www.britannica.com/science/dermatitis
- <1% – https://www.biology-today.com/general-zoology/vertebrate-zoology/comparative-account-of-integumentary-glands-in-vertebrates/
- <1% – https://www.answers.com/Q/Where_is_the_dermis_layer_of_skin
- <1% – https://www.answers.com/Q/What_are_the_names_of_the_sensory_receptors_found_in_dermis
- <1% – https://trendydamsels.com/hair-follicle/
- <1% – https://steptohealth.com/hair-masks-shiny-silky-hair/
- <1% – https://quizlet.com/6095723/mammary-gland-flash-cards/
- <1% – https://quizlet.com/4588321/nutrition-flash-cards/
- <1% – https://quizlet.com/43673323/pathophysiology-11-221-flash-cards/
- <1% – https://quizlet.com/393251171/chapter-5-the-integumentary-system-comprehensive-medical-terminology-flash-cards/
- <1% – https://quizlet.com/296970344/exam-1-questions-1-30-flash-cards/
- <1% – https://quizlet.com/28618267/ap-exam-2-flash-cards/
- <1% – https://quizlet.com/252116291/4-flash-cards/
- <1% – https://quizlet.com/14462774/skin-aids-in-temperature-regulation-flash-cards/
- <1% – https://quizlet.com/109937090/hair-structure-flash-cards/
- <1% – https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1524-4725.2006.33039.x
- <1% – https://nailbees.com/nail-structure
- <1% – https://med.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Nutri_300_(Coppola)/Text/07%3A_Vitamins/7.2%3A_Fat_Soluble_Vitamins/7.2B%3A_Vitamin_D
- <1% – https://hubpages.com/education/5-Layers-And-Cells-of-the-Epidermis
- <1% – https://healthfully.com/what-makes-nails-grow-long-strong-fast-5905995.html
- <1% – https://eprints.soton.ac.uk/58862/
- <1% – https://en.wikivet.net/Ear_-_Anatomy_%26_Physiology
- <1% – https://en.wikipedia.org/wiki/Sebaceous_gland
- <1% – https://en.wikipedia.org/wiki/Nail_matrix
- <1% – https://en.wikipedia.org/wiki/Integumentary_system
- <1% – https://en.wikipedia.org/wiki/Hair_follicle
- <1% – https://en.m.wikipedia.org/wiki/Epidermis
- <1% – https://courses.lumenlearning.com/nemcc-ap/chapter/pigmentation/
- <1% – https://askthescientists.com/skin-layers/
- <1% – https://answersdrive.com/what-is-the-main-function-of-the-hair-7576656
- <1% – https://answersdrive.com/what-is-the-function-of-a-fingernail-3754455
- <1% – https://answersdrive.com/what-are-the-functions-of-adipocytes-8544022
- <1% – https://1knews.com/anatomy-and-physiology-of-the-female-breast/
- <1% – http://www.ratbehavior.org/CoatTypes.htm
- <1% – http://basskilleronline.com/tanning-sexual-peptide-melanotan-2-information-guide.shtml