The pineal gland is a tiny, reddish-gray structure roughly the size and shape of a grain of rice, tucked deep in the center of the brain. It measures about 7 to 8 mm long, 7 mm wide, and just 2.5 mm thick. Its name comes from the Latin word “pinea,” meaning pinecone, because the ancient physician Galen noted that its tapered, slightly bumpy surface resembles a small pinecone.
Shape, Size, and Color
A healthy pineal gland has a soft, reddish-gray appearance with a vaguely conical or flattened oval shape. At under a centimeter in every dimension, it’s one of the smallest structures in the brain, yet it has an unusually rich blood supply fed by branches of the posterior cerebral artery. That heavy blood flow gives the tissue its reddish tint and reflects just how metabolically active this little gland is relative to its size.
Where It Sits in the Brain
The pineal gland grows outward from the back wall of the third ventricle, one of the fluid-filled cavities near the brain’s midline. It nestles in a groove between two rounded bumps on the brainstem called the superior colliculi, with the two halves of the thalamus flanking it on either side. A thick band of nerve fibers called the corpus callosum arches just above it. Because it sits right at the geometric center of the brain, the pineal gland often serves as a useful landmark on brain scans.
What It Looks Like Under a Microscope
Zoom in and the gland’s internal architecture becomes visible. About 95% of its cells are pinealocytes, the hormone-producing cells responsible for making melatonin. Each pinealocyte has a large, round-to-oval nucleus with a prominent central dot (nucleolus) and sends out branching arms that narrow into fine strands with distinctive club-shaped tips. These cells are organized into cords and lobules, clusters that look a bit like bunches of grapes when stained and viewed under magnification.
The remaining 5% of cells are glial support cells, mostly a type called fibrous astrocytes. These form a dense web that wraps around the pinealocytes, fills the spaces between them, and lines the blood vessels running through the tissue. The overall effect is a tightly packed but well-organized glandular structure.
Calcification and “Brain Sand”
One of the most striking visual features of the pineal gland, especially in adults, is calcification. Tiny mineral deposits build up inside the gland over time, forming layered, sand-like particles historically called “corpora arenacea” or “brain sand.” These deposits are so common that they show up on routine CT scans as a bright white spot in the center of the brain.
Calcification increases sharply with age. Only about 2% of children under 10 show visible pineal calcification on imaging, but that number jumps to roughly 32% in teenagers, 53% in people in their twenties, and over 80% in adults past age 30. Large population studies from Turkey, Iran, Ethiopia, and the United States have all found rates around 70% or higher in adults. The deposits themselves are layered like tree rings: the older you are, the more layers (lamellae) accumulate, which tells researchers the process is organized and ongoing rather than random.
Modern imaging techniques like susceptibility-weighted MRI and PET scans can now detect even tiny concretions that older technology would have missed, which means the true prevalence of at least some calcification is likely even higher than those numbers suggest.
How It Changes With Age
Beyond calcification, the gland’s internal structure shifts as you get older. The supportive astrocyte network gradually becomes denser, a pattern seen almost exclusively in elderly individuals. Some lobules lose their organized architecture, and the functional tissue can shrink as glial cells proliferate to fill in where pinealocytes have died off. In some cases, small fluid-filled cysts form at the center of these replacement patches where blood supply has been cut off. These cysts are usually harmless and asymptomatic, but they reduce the volume of active, melatonin-producing tissue.
That said, aging doesn’t follow a single predictable path. Researchers examining pineal glands across age groups found that intact, partially intact, and disrupted lobular structures can all appear at any age. Some older adults maintain surprisingly well-preserved glands, while some younger adults already show early structural changes.
An Evolutionary Echo of a Third Eye
The pineal gland’s appearance makes more sense when you consider its evolutionary history. In some living reptiles, frogs, and the tuatara (a lizard-like reptile from New Zealand), a version of the pineal organ still sits near the top of the skull as a literal third eye, complete with a lens-like structure and photoreceptor cells similar to those in regular eyes. This “parietal eye” detects changes in daylight and sends that information to the brain to regulate body temperature, reproduction, and biological rhythms.
Genetic studies show that this pineal eye and the lateral eyes we see with share a common developmental origin. As mammals evolved, the opening in the skull that housed the photoreceptive organ closed over. Without direct light exposure, the pineal lost its eye-like structure and took on a purely hormonal role, secreting melatonin in response to light signals relayed through the regular visual system instead. The small, gland-like organ we carry today is what remains of that ancient light-sensing apparatus.