In this note, we will discuss the anatomy of the pineal gland (corpus pineale) also known as the epiphysis cerebri.
The pineal gland is a neuroendocrine organ that is part of the epithalamus,
one of the three divisions of the diencephalon (alongside the thalamus and hypothalamus). The pineal gland regulates circadian rhythms in the body through the secretion of special hormones, which we will discuss in more detail later.
The pineal gland develops as a small protrusion from the posterior wall of the third ventricle and is located between the superior colliculi of the quadrigeminal plate.
The pineal gland is connected to the rest of the brain by a small pineal stalk, which splits into two laminae, superior and inferior. The space between these laminae is filled with an extension of the third ventricle, known as the pineal recess (recessus pinealis).
The pineal gland is externally covered by the dura mater, which forms septa, which divide the organ into lobes. To better understand the physiology of the pineal gland, let’s look at its histological structure. There are two types of cells:
- Neuroglial cells (astrocytes)
Pinealocytes are essentially modified neurons which produce the hormone melatonin and release it into the surrounding capillary network. Astrocytes, on the other hand, are neuroglial cells, serving as supporting cells.
Another component of the pineal gland is what’s known as “brain sand” (corpora arenacea). It consists of deposits of calcium, phosphates, and other inorganic substances, forming inclusions within the parenchyma. These inclusions accumulate with age and are primarily a byproduct of the pineal gland’s secretory activity.
Let’s delve deeper into the function of the pineal gland. As mentioned earlier, it secretes the hormone melatonin, which is a derivative of serotonin. This hormone regulates the so-called circadian (or cyclic) rhythms in our body, in particular the sleep-wake cycles.
The main factor affecting melatonin production is the amount of light that reaches the retina. Melatonin production increases in low light or darkness, while bright light suppresses its secretion. This means that its concentration in the blood rises during the night and decreases during the day.
Information about the change of day and night, as well as their duration, is transmitted via the so-called retinohypothalamic tract (tractus retinohypothalamicus).
Light reaches the retina, and the generated action potential is transmitted along the retina’s fibers to the suprachiasmatic nucleus of the hypothalamus.
From there, the information travels to the paraventricular nucleus, which then sends efferent fibers to the intermediolateral nucleus of the spinal cord. Then, through the cervical sympathetic fibers, the impulse finally reaches the pineal gland.
Due to its connection with the eyes and ability to react to light, the pineal gland was historically referred to as the “third eye”. Moreover, there is some evidence suggesting that the pineal gland may suppress the secretion of certain pituitary hormones – the follicle-stimulating hormone (FSH) and luteinizing hormone (LH). It is believed that in this way, the pineal gland plays a role in regulating the onset of puberty by suppressing the premature development of reproductive organs.
Blood supply to the pineal gland is provided by branches of the middle and posterior cerebral arteries. Venous blood drains into the dural sinuses of the brain.
- pineal gland / epiphysis cerebri
- corpus pineale / epiphysis cerebri
- superior colliculus
- colliculus superior
- pineal recess
- recessus pinealis
- brain sand
- corpora arenacea
- retinohypothalamic tract
- tractus retinohypothalamicus