Anatomical position and connection to vision
The pineal gland is a structure of the diencephalon and is located above the superior colliculi of the corpora quadrigemina (tectum of the midbrain) directly beneath the corpus callosum. The mass of this small gland is about 0.2 grams, which is half the mass of the hypophysis.
The activity of the pineal gland is inextricably linked with light, which is why it is often called the "third eye." In fish and amphibians, this connection is direct: the main cells themselves function as photoreceptors. In mammals, however, there are no photosensitive cells in the gland, and the signal follows a long neural pathway:
- From the retina of the eye via the optic nerves, the impulse travels to the nuclei of the thalamus.
- Then, via descending pathways, it descends to the spinal cord (the center of the sympathetic nervous system).
- It exits through the anterior roots and reaches the superior cervical ganglion, a crucial relay node.
- From there, postganglionic fibers, wrapping around blood vessels, deliver the signal to the pineal gland.
Functions and hormonal profile
The secretion of the pineal gland is subject to circadian (daily) rhythms. Its effect on target organs is exerted both directly and via the hypothalamus or hypophysis.
Hormones of the night (dark phase) These substances have a pronounced antigonadotropic effect:
- Melatonin inhibits the production of gonadotropin-releasing hormone in the hypothalamus, leading to a nighttime decrease in FSH, LH, and prolactin (LTH) in the hypophysis.
- Antigonadotropin acts directly on the hypophysis, suppressing the production of luteinizing hormone (LH).
Daytime hormones and regulation of mineral metabolism Outside the dark phase, the gland secretes substances that affect the hypophysis and other peripheral glands:
- Thyrotropin-releasing hormone (stimulates TSH release in the hypophysis) and luteinizing hormone-releasing hormone (similar to hypothalamic hormones).
- Thyrotropin of pineal origin, analogous to hypophyseal TSH, which directly activates the production of thyroid hormones.
- Calitropin, which regulates mineral metabolism—it increases the level of potassium ($K^+$) in the blood.
Microscopic structure
The histological structure of the pineal gland distinguishes four key components:
- Stroma (connective tissue framework). Represented by an outer capsule from which septa (trabeculae) extend. They divide the parenchyma into separate lobules.
- Pinealocytes. Large neurosecretory cells with vesicular nuclei and numerous processes extending toward capillaries. They are not divided into types based on the hormones they produce (a single cell can synthesize everything). Light (few granules) and dark (many granules) pinealocytes are distinguished—this reflects different stages of a single secretory cycle.
- Interstitial glial cells. These are fibrous astrocytes — small cells with dense nuclei and processes located between pinealocytes, providing structural support for them.
- Capillary network (microcirculation). Ensures the release of hormones into the blood.
Features of blood vessels
The pineal gland is richly vascularized by capillaries that have an important structural feature. Their endothelium lacks pores (fenestrae), and the basement membrane is unique—it splits into outer and inner layers.
Between these layers, an expanded pericapillary space is formed. It is into this space that the processes of pinealocytes release hormones before they enter the general bloodstream.
Briefly: answers to topical questions
From which embryonic source (germ layer) does the pineal gland develop?
The pineal gland develops from the neuroectoderm. In embryogenesis, the organ forms at the 5th–6th week of intrauterine development as an outgrowth of the roof of the diencephalon.
Class and exam questions
Why is the pineal gland called the "third eye"?
Its activity directly depends on illumination. In lower vertebrates (fish, amphibians), the cells of the pineal gland are themselves photoreceptors, whereas in mammals, the signal from the retina reaches the gland via a complex pathway through the sympathetic system.
Which cells synthesize hormones in the pineal gland?
The main secretory cells are pinealocytes. There are no differences between them in terms of the type of synthesized hormone, and the division into "light" and "dark" is solely related to the varying amount of accumulated secretory granules.
What happens to pineal hormones in the dark?
At night, melatonin and antigonadotropin are produced. They suppress the synthesis of hypothalamic gonadotropin-releasing hormone and hypophyseal gonadotropins (FSH, LH, prolactin).
What is the special feature of the capillaries of the pineal gland?
The capillary endothelium lacks fenestrae, and the basement membrane is split into two layers, forming the pericapillary space. Hormones are secreted into it from the processes of pinealocytes.
What else to study on this topic
- Pathway of light impulse transmission to the pineal gland
- Connection of the pineal gland with the hypothalamus and hypophysis
- Mechanism of action of calitropin
- Secretory cycle of pinealocytes
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