Causes of Rathke's Cleft Cyst
What we refer to as the "pituitary gland" is actually made up of two glands: the anterior (adenohypophysis) and posterior (neurohypophysis) glands. As a fetus develops:

The cells that form the anterior gland migrate from the roof of the mouth (stomadeum).

The cells that form the posterior gland migrate down from the brain.

The place where the anterior and posterior glands meet is called Rathke's pouch. This pouch normally closes early in fetal development, but a remnant often persists as a cleft. Occasionally, this remnant gives rise to a large cyst called the Rathke's cleft cyst (RCC).

Rathke's cleft cysts that cause symptoms are relatively uncommon lesions, accounting for less than one percent of all primary masses within the brain. They can occur at any age, although most are identified in adults. Rathke's cleft cysts sometimes occur together with pituitary adenomas.