pinealocytes

Pinealocytes are the principal secretory cells of the pineal gland, a small endocrine organ in the brain’s epithalamus. They are responsible for the synthesis and secretion of melatonin, a hormone that helps regulate circadian and seasonal rhythms in response to environmental light.

Morphology and organization: Pinealocytes are polygonal to fusiform cells with large rounded nuclei and abundant cytoplasm. Histologically, they form clusters or cords around a dense network of fenestrated capillaries. The cytoplasm contains smooth endoplasmic reticulum, mitochondria, and Golgi apparatus, and often holds secretory vesicles carrying melatonin precursors. Interstitial glial-like cells are present, and with aging calcified deposits known as corpora arenacea may accumulate within the gland.

Biosynthesis and regulation: Melatonin is produced from serotonin through the enzymes arylalkylamine N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT). Melatonin production increases during darkness and decreases with light exposure. Sympathetic innervation from the superior cervical ganglion releases norepinephrine onto pinealocytes, activating cAMP signaling and upregulating the melatonin-synthesizing enzymes.

Function and significance: Melatonin is released into the bloodstream and cerebrospinal fluid, helping to regulate sleep-wake cycles, circadian timing, and, in some species, seasonal reproduction. It also functions as an antioxidant and may modulate immune and neural processes.

Development and evolution: Pinealocytes originate from neuroectoderm of the diencephalon. The pineal gland exhibits phylogenetic variation in photoreceptive properties, being more directly photosensitive in some non-mammalian vertebrates than in mammals.