Pineal Glands (Epiphysis)
Endocrine System of Class 11
Pineal Glands (Epiphysis)
Pineal lies under the corpus callosum between the two cerebral hemispheres of brain at the tip of a short pineal stalk arising from roof of diencephalon.
It is very small, reddish-grey, vascular, conical and solid body.
It is also called epiphysis or epiphysis cerebri or third eye or seat of soul.
Earlier it was considered as a vestigial structure in brain but in 1926 Axelrod got Noble prize for discovering melatonin as secretion of pineal gland.
It is a biological clock of body
Structure of pineal :- It is composed of modified nerve cells called pinealocytes. These are embedded in fibrous connective tissue called stroma which contains collagen and elastin fibres, histiocytes lymphocytes & fibroblast cells.
Status of pineal in lower animals
Pineal is well developed in lower animals (particularly reptiles).
In lower animals pineal has the ability of photoreception ( i.e. light perception )
In Sphenodon, [ it is a reptile, also called Tuatara, or Hatteria or Living tossil, found only in New Zealand], pineal is well developed and
located in the centre of forehead. It has lens, retina as well as nerve supply (Third eye).
Status of pineal in humans
In human pineal is prominent during early childhood. At the age of 7 years, it starts degenerating and in adults, it is largely a fibrous tissue.
In the adult, pineal gland can often be seen in X-rays of brain because of accumulation of radio opaque calcium carbonate and calcium
phosphate into small granules called pineal sand or brain sand or cervuli.
Pineal secrete melatonin, serotonin and adrenoglomerulotropin hormone.
Melatonin and serotonin are two biogenic amine hormones formed from amino acid tryptophan.
Pineal gland secretes these two hormones in direct response to nervous activity.
In the course of evolution pineal has lost the ability of direct perception of light i.e. in case of humans pineal can not perceive light directly
(It receives light through eyes ).
Pineal is also called Biological clock because it shows a rhythm in hormone secretion.
Level of melatonin is more during dark phase (scotophase) or during night time where as level of serotonin is more during light phase
(photophase) or during day time.
Biosynthesis of Melatonin & Serotonin
Serotonin (5-hydroxy tryptamine) is an intermediate metabolic product in biosynthesis of melatonin.
Functions of pineal hormones
Adrenoglomerulotropin :- It stimulates zona glomerulosa layer of adrenal cortex gland to secrete aldosterone.
It is a powerful vasoconstrictor and increases blood pressure.
It is also acts as a neurotransmitter.
It regulates skin colour in lower animals (fishes, amphibians). It is antagonistic to MSH of pars intermedia of pituitary. It acts on
melanophores and aggregates melanin pigment in centre of melanophore and makes the skin colour pale. [ MSH disperses melanin pigment & makes skin colour dark]. In humans it has no such role.
It regulates sleep/wake cycle, (controls circadian behaviour) mood & behaviour also.
In humans it is antigonadal in action i.e. it delays the onset of puberty (sexual maturity). As long as level of melatonin is high in plasma,
puberty is delayed. [Remember in humans pineal starts degenerating at the age of 7 years and level of melatonin goes down which inturn stimulates onset of puberty].
Congenitally (by birth) blind girls attain puberty at an early age because in them eyes can’t perceive light and pineal is not stimulated to produce serotonin. If serotonin is not there then melatonin is not formed. Absence of melatonin results in precocious (early) puberty. Congenitally, blind boys also attain puberty at an early age but in boys it is not clearly evident because in boys puberty is a slow process and there is no clear cut symptom of puberty attainment. Where as in girls onset of menstrual cycle is a clearcut symptom of puberty attainment.