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Sheep vs. Human Pineal Gland: Comparative Anatomy and Melatonin Production
The pineal gland (epiphysis cerebri) regulates circadian rhythms via melatonin in all mammals.
Similarities
- Midline, posterior to third ventricle, attached by stalk.
- Pinealocytes produce melatonin from serotonin (AANAT/HIOMT enzymes).
- Regulated by suprachiasmatic nucleus (SCN) via sympathetic pathway (light → retina → SCN → PVN → spinal cord → superior cervical ganglion → pineal).
Differences
- Size: Sheep ~150–200 mg; Human ~100–180 mg (highly variable).
- Calcification: Human pineal calcifies extensively with age ("brain sand"); sheep less so.
- Melatonin Amplitude: Sheep show higher nocturnal peaks (seasonal breeder – photoperiodic).
- Seasonality: Sheep are short-day breeders; melatonin duration encodes season. Humans lost strong photoperiodic reproductive control.
- Vascularization: Sheep pineal has richer blood supply relative to size.
Research Value
Sheep model human pineal physiology (size allows sampling) but differ in seasonal regulation.
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