The pineal organ is a small gland in the centre of our brain that regulates the daily and annual rhythms underlying sleep, food intake and reproduction through secretion of melatonin, the hormone of the night. Since Descartes postulated in his The Treatise of Man, that the pineal gland is the ‘seat of the soul’, it has attracted considerable attention from both scientists and mystics. More recently, a number of cellular and genetic similarities with the eye have led to the theory that the pineal organ is a rudimentary ‘third eye’.
Our knowledge of the embryonic origin of the pineal organ has remained vague. Classical studies from the late 1980s and early 1990s suggested that the entire pineal organ forms as an outpocketing from the caudal forebrain (diencephalon). In a paper that is now published in Development, Florence Giger, Triona Fielding, Clemens Kiecker, Corinne Houart and colleagues have re-investigated this, using zebrafish and chick embryos as experimental model organisms, and have found that a substantial amount of pineal cells originates outside of the embryonic nervous system, in an area that is called the pre-placodal region (PPR) and that gives rise to a number of sensory and secretory structures of the vertebrate head.
This study reveals a similarity between the pineal organ and the eye, as the eye also has a dual origin: the retina is derived from embryonic neural tissue whereas the lens forms from the PPR. Moreover, the pituitary organ, another endocrine gland that regulates a number of reproductive and metabolic processes has a similar dual origin, forming from both an outpocketing of the embryonic forebrain and an area within the PPR. Thus, the pineal organ joins a club of sensorial and secretory structures of the vertebrate brain that have a mixed descendance and form at the interface between neural and non-neural tissue.
Nicole Staudt, Florence A Giger, Triona Fielding, James A Hutt, Isabelle Foucher, Vicky Snowden, Agathe Hellich, Clemens Kiecker & Corinne Houart (2019). Pineal progenitors originate from a non-neural territory limited by FGF signaling. Accepted for publication in Development 146. doi: 10.1242/dev171405.