Dictionary

pigment A substance that gives color to tissue. Pigments are responsible for the colors of skin, eyes, and hair.

pigmentation The coloring of the skin, hair, mucous membranes, and retina of the eye. Pigmentation is due to the deposition of the pigment melanin, which is produced by specialized cells called melanocytes. Other pigment-related terms include hyperpigmentation (too much pigment), hypopigmentation and underpigmentation (too little pigment), and depigmentation (loss of pigment).

piles See hemorrhoids.

pill, the See oral contraceptive.

piloerection Erection of the hair of the skin due to contraction of the tiny arrectores pilorum muscles that elevate the hair follicles above the rest of the skin and move the hair vertically, so the hair seems to “stand on end.”

pilonidal cyst An abscess that occurs in the cleft between the buttocks at the base of the tailbone (coccyx). Pilonidal cysts are common in adolescence, often after long trips that involve sitting.

pimple An inflamed area of the skin with pus formation that results from an oil gland being infected with bacteria. Pimples are due to overactivity of the oil glands located at the base of the hair follicles, especially on the face, back, chest, and shoulders.

pineal gland A small gland that is located near the center of the brain. This gland secretes melatonin, and it may therefore be part of the body’s sleep-regulation apparatus. Also known as pineal body.

pineal region tumor A brain tumor on or near the pineal gland. There are multiple types of pineal gland tumors, most of which are not cancerous but can nonetheless cause extreme distress. Diagnosis is made via biopsy of affected tissue. Benign pineal tumors are treated with surgery; malignant tumors may be treated with surgery, radiation…

pinealoblastoma See pineoblastoma.

pinealocytoma See pinealoma.

photodynamic therapy A form of treatment that uses a photosensitizing agent, administered by mouth or intravenously, which concentrates selectively in certain cells, followed by exposure of the involved tissue to a special light (such as laser or ultraviolet light), in order to destroy as much of the abnormal tissue as possible. For example, photodynamic therapy…