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What is acne?
A normal follicle
A normal follicle looks like this:
1
For reasons no one completely understands, follicles, often called pores, overproduce cells and sometimes get blocked. Sebum (oil) which normally drains to the surface gets blocked and bacteria begins to grow.2-11 Both whiteheads and blackheads start out as a microcomedone.1,12-13 The picture below is a microcomedone:
There are two types of acne:
Non-inflammatory acne
Microcomedones become non-inflamed skin blemishes called comedones--either a whitehead or a blackhead:12-13
Whitehead
When the trapped sebum and bacteria stay below the skin surface, a whitehead is formed. Whiteheads may show up as tiny white spots, or they may be so small that they are invisible to the naked eye.1,13
Blackhead
A blackhead occurs when the pore opens to the surface, and the sebum, which contains the skin pigment melanin, oxidizes and turns a brown/black color. It is not dirt and can not be washed away. Blackheads can last for a long time because the contents very slowly drain to the surface.1,13
A blackhead or whitehead can release its contents to the surface and heal. Or, the follicle wall can rupture and inflammatory acne can ensue (see below). This rupture can be caused by random occurrence or by picking or touching the skin. This is why it is important to leave acne prone skin relatively untouched.1,12-13
Inflammatory acne
Papule
A papule occurs when there is a break in the follicular wall. White blood cells rush in and the pore becomes inflamed.13
Pustule
A pustule forms several days later when white blood cells make their way to the surface of the skin. This is what people usually refer to as a "zit" or a "pimple".13
An inflamed lesion can sometimes completely collapse or explode, severely inflaming the surrounding skin, and sometimes engulfing neighboring follicles. These lesions are called nodules or cysts:1,13
Nodule
When a follicle breaks along the bottom, total collapse can occur, causing a large, inflamed bump that can be sore to the touch.13
Cyst
Sometimes a severe inflammatory reaction can result in very large pus filled lesions.1,13
Milia
Milia are tiny white bumps that occur when normally sloughed skin cells get trapped in small pockets on the surface of the skin. Milia is common in newborns across the nose and upper cheeks and can also be seen on adult skin. The bumps disappear as the surface is worn away and the dead skin is sloughed. In newborns, the bumps usually disappear within the first few weeks of life. However, for adults milia may persist indefinitely.14
Treatment is usually not indicated in children. Adults can have them removed by a physician for cosmetic improvement.14
Diagrams courtesy of: National Institute of Health
References
1. "Questions and Answers About Acne." National Institute of Arthritis and Musculoskeletal and Skin Diseases Jan. 2006. National Institutes of Health. 2001. link.
2. Kligman AM and Plewig G. Acne and Rosacea. Berlin: Springer, 2000.
3. Bek-Thomsen M, Lomholt HB and Kilian M. "Acne is not associated with yet-uncultured bacteria." Journal of Clinical Microbiology. (2008); 46(10): 3355-3360.
4. Zouboulis CC. "Propionibacterium acnes and sebaceous lipogenesis: A love-hate relationship?" Journal of Investigative Dermatology. 2009; 129(9): 2093-2096.
5. Pappas A, et al. "Sebum analysis of individuals with and without acne." Dermatoendocrinology. 2009; 1(3): 157-161.
6. Akaza N, et al. "Effects of Propionibacterium acnes on various mRNA expression levels in normal human epidermal keratinocytes in vitro." Journal of Dermatology. 2008; 36(4): 213-223.
7. Picardo M, et al. "Sebaceous gland lipids." Dermatoendocrinology. 2009; 1(2): 68-71.
8. Pappas A. "Epidermal surface lipids." Dermatoendocrinology. 1(2): 72-76.
9. Youn SW. "The role of facial sebum secretion in acne pathogenesis: Facts and controversies." Clinics in Dermatology. 2010; 28(1): 8-11.
10. Dessinioti C and Katsambas AD. "The role of Propionibacterium acnes in acne pathogenesis: Facts and controversies." Clinics in Dermatology. 2010; 28(1): 2-7.
11. Makrantonaki E, Ganceviciene R and Zouboulis CC. "An update on the role of the sebaceous gland in the pathogenesis of acne." Dermatoendocrinology. 2011; 3(1): 41-49.
12. Bettoli V, Layton AM and Thiboutot D. Fast Facts-Acne. Oxford, UK: HealthPress Limited, 2004.
13. Fry L. The Encyclopedia of Visual Medicine Series: An Atlas of Dermatology. New York: The Parthenon Publishing Group, 1997.
14. Morelli JG. "Diseases of the neonate." In: Kliegman RM, Behrman RE, Jensen HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, PA: Saunders Elsevier;2007:Chapter 646.
Further Reading
Do TT, et al. "Computer-assisted alignment and tracking of acne lesions inidcate that most inflammatory lesions arise from comedones and de novo." Journal of the American Academy of Dermatology. 2008; 58(4): 603-608.
Jeremy AHT, et al. "Inflammatory events are involved in acne lesion initiation." Journal of Investigative Dermatology. 2003; 121(1): 20-27.
Morohashi M and Toyoda M. "Pathogenesis of Acne." Medical Electron Microscopy 2001; 34(1): 29-40.
