The pars intermedia is a part of the pituitary gland that is found in the brains of vertebrates. Melanocyte-stimulating hormone (MSH), which is produced in this region, determines the skin pigmentation in human fetuses. MSH also affects the ability of fish and amphibians to darken their color for camouflage. Malfunctions in the pars intermedia causes a common disease in horses called equine pituitary pars intermedia dysfunction (PPID).
During the development of a vertebrate embryo, a depression forms in the roof of the mouth. This depression, called Rathke's pouch, detaches from the roof at a later stage of development. The anterior portion of the pouch then forms the anterior section of the pituitary gland, and the posterior portion forms the pars intermedia. In humans, the pars intermedia shrinks with age and can be entirely absent in adults.
The pars intermedia forms a boundary between the anterior and posterior sections of the pituitary. Its primary function is to produce and excrete MSH, but other hormones also are present. Adrenocorticotropic hormone (ACTH) pertains to growth and nutrition. Corticotropin-like intermediate lobe peptide (CLIP) is another hormone of the region, and its importance is to act as a precursor to ACTH.
MSH has an effect on melanocyte skin cells, which contain melanin. The reaction of the hormone and the skin pigment can produce freckles and moles in human fetuses. In fish and amphibians, MSH can branch out from the center of the melanocyte cells to create a darker skin tone. This is done as protective camouflage when the creature crosses a darker background.
PPID, also called equine Cushing's disease, occurs when the pars intermedia enlarges and creates an overabundance of hormones in the body of the horse. This swelling can press into the surrounding areas of the pituitary and impede their function. Horses that are more than 15 years old are most likely to get PPID. Symptoms include excessive thirst and development of a curly coat that doesn't shed. Treatment typically involves medication and changes in diet, because the condition might also cause insulin resistance in the horse.
In humans, an improper degradation of the Rathke's pouch can cause a pars intermedia cyst. These colloid-filled cysts are common, particularly in children, in whom the area is better defined, but they often cause no symptoms because they are small in size. Symptomatic cysts can cause headaches and growth malfunctions that can mimic thyroid problems.