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What Causes a Pimple?

How Acne Breakouts Develop

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Updated April 16, 2014

Cross Section of the Skin

Cross Section of the Skin

Photo © A.D.A.M.

Acne is a chronic disorder of the sebaceous glands. While sometimes it seems pimples appear overnight, the development of an acne breakout is actually an extended process that begins at the cellular level. All pimples begin as a blockage of the hair follicle, or pore. Gaining an understanding of why a blockage begins and how a pimple develops will ultimately help you in treating your acne.

The Epidermis

The outermost section of the skin, the part you see every day, is called the epidermis. The epidermis consists of five layers. The deepest layer of the epidermis is the stratum germinativum. It is composed of a single layer of cells. Cell division, or mitosis, takes place in the stratum germinativum.

The new cells begin their journey up through epidermis to the skin's surface. First they are pushed up through the stratum mucosum, then the stratum spinosum. These three layers (stratum germinativum, mucosum, and spinosum) together are called the basal layer.

As the skin cells travel further, they reach the stratum granulosum. In this layer, the cells begin dying and hardening, giving them a granular appearance. By the time the cells reach the stratum corneum, they are dead.

The stratum corneum is made up of these tightly packed dead skin cells, which are continuously falling off and being replaced. This process is called desquamation. It takes approximately 28 days, from cell birth to sloughing off, to occur.

The Dermis

The dermis is the deepest, or inner section, of your skin and is made up of tough connective tissue. The dermis nourishes and supplies blood to the epidermis. It is also what gives the skin its elasticity. The dermis is much thicker than the epidermis but has just two distinct layers: the papillary layer and the reticular layer.

Within the dermis you will find blood and lymph vessels, nerves, arrector pili muscles (the muscles that make your hair stand on end), sudoriferous (sweat) glands, sebaceous (oil) glands, and hair follicles. It is within the hair follicle and sebaceous glands that acne begins.

The Hair Follicle

The hair follicle is a small, tube-like opening in the skin through which hair and sebum reach the skin's surface. The follicle consists of the pore opening, hair root and bulb, sebaceous duct, and sebaceous gland. Although it is contained within the dermis, the epidermis lines the inside of the hair follicle.

In a normal functioning follicle the sebaceous glands secrete oil, or sebum, into the pore. Typically, sebum and dead cells shed from the stratum corneum emerge at the skin's surface through the pore opening. In those with acne, however, this process goes awry.

In acne prone skin, sebum and dead skin cells easily become trapped within the follicle. This accumulation of cellular debris and sebaceous matter forms a hard plug that obstructs the pore opening. This obstruction is called a comedo. It manifests itself as non-inflamed bump or blackhead on the skin's surface.

Propionibacteria Acnes

Propionibacterium acnes (P. acnes) is the bacterium responsible for inflamed acne breakouts. P. acnes are regular residents within the hair follicle. Normally, they are harmless. However, in acne prone skin the P. acnes population grows out of control.

When a comedo blocks the pore opening, it creates an anaerobic environment, or a lack of oxygen within the follicle. This anaerobic environment, along with excess sebum within the pore, creates a favorable environment where P. acnes bacteria can thrive.

As the follicle becomes filled with sebum, dead cells, and bacteria, it begins to swell. The follicle wall ruptures and spills into the dermis. White blood cells rush in to fight the bacteria. Redness and swelling occurs, and pus is created. A pimple has now formed.

If the rupture in the follicle wall happens near the surface, the pimple is usually minor and heals quickly. It is when the break occurs deep within the dermis that more severe lesions, such as nodules and cysts, develop.

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