Pineal Gland

Anatomic Position and Connection to Vision

The pineal gland is a diencephalic structure located above the superior colliculi (tectum of the midbrain) directly beneath the corpus callosum. This small gland weighs approximately 0.2 grams, which is about half the weight of the pituitary gland.

The function of the pineal gland is inextricably linked to light, earning it the moniker of the "third eye." In fish and amphibians, this connection is direct: the main cells function as photoreceptors themselves. In mammals, however, the gland lacks photosensitive cells, and the signal travels a long neural pathway:

1. From the retina via the optic nerves, impulses travel to the thalamic nuclei.
2. Signals then descend via descending pathways to the spinal cord (the center of the sympathetic nervous system).
3. They exit through the anterior roots and reach the superior cervical ganglion, a crucial relay station.
4. From there, postganglionic fibers wrap around blood vessels to deliver the signal to the pineal gland.

Functions and Hormonal Profile

Pineal secretion is governed by circadian (daily) rhythms. It influences target organs both directly and via the hypothalamus or pituitary gland.

Nocturnal Hormones (Dark Phase) These substances exhibit a pronounced antigonadotropic effect:

• Melatonin inhibits the production of gonadotropin-releasing hormone (GnRH) in the hypothalamus, leading to a nocturnal decrease in FSH, LH, and prolactin (LTH) in the pituitary gland.
• Antigonadotropin acts directly on the pituitary gland, suppressing the production of luteinizing hormone (LH).

Diurnal Hormones and Mineral Metabolism Regulation Outside of the dark phase, the gland secretes substances that affect the pituitary and other peripheral glands:

• Thyrotropin-releasing hormone (TRH) (stimulates TSH release in the pituitary) and gonadotropin-releasing hormone (GnRH) (similar to hypothalamic hormones).
• Pineal thyrotropin, analogous to pituitary TSH, which directly activates thyroid hormone production.
• Calitropin, which regulates mineral metabolism by increasing blood potassium ($K^+$) levels.

Microscopic Structure

The histological structure of the pineal gland features four key components:

1. Stroma (Connective Tissue Framework). Represented by an outer capsule from which septa (trabeculae) extend, dividing the parenchyma into distinct lobules.
2. Pinealocytes. Large neurosecretory cells with vesicular nuclei and numerous processes extending toward capillaries. They are not divided into types based on the hormones produced (a single cell can synthesize all of them). Light pinealocytes (few granules) and dark pinealocytes (many granules) are distinguished, reflecting different stages of a single secretory cycle.
3. Interstitial Glial Cells. These are fibrous astrocytes—small cells with dense nuclei and processes located between pinealocytes, providing structural support.
4. Capillary Network (Microcirculation). Ensures the release of hormones into the bloodstream.

Blood Vessel Characteristics

The pineal gland is richly vascularized by capillaries with an important structural feature: their endothelium lacks pores (fenestrations), and the basement membrane is unique, splitting into outer and inner layers.

An expanded perivascular space is formed between these layers. Pinealocyte processes discharge their hormones into this space before the hormones enter the general circulation.