The pituitary gland consists of the anterior and posterior lobe, which are controlled by the hypothalamus to secrete hormones. It is also known as the hypophysis (hypo: below, physis: hypothalamus). The hypothalamus is connected to the pituitary gland via the pituitary stalk (infundibulum). This pituitary gland is very small, roughly the size of a pea and lies in the sella turcica/putuitary fossa. This fossa (a depression in the skull) can be approached thru the nasal cavity (via the sphenoid sinus). This approach is called the trans-sphenoidal approach.
Trivia: The anterior pituitary arises embryologically as an invagination of the roof of the pharyx (Rathke’s pouch)
On both sides of the gland, there are sinuses called the Cavernous Venous Sinuses which contains nerves and the internal carotid artery. Medially (which is more to the middle) is the Oculomotor nerve (3rd cranial nerve), abducent nerve (6th cranial nerve) and laterally (more to the side) the ophthalmic & maxillary nerve (both 5th cranial nerve). Therefore we can deduce that when the pituitary gland enlarges, the oculomotor nerve will be compressed 1st.
Infront the gland, situated superiorly, is the optic chiasma. The optic chiasma is when the 2 optic nerves (temporal half, which is the side view, of the visual field) from both eyes cross over each other to enter the optic tract (toward the brain for processing). So what happens is, when there is enlargement of the gland, and if it enlarges to the front, then the optic chiasma will be compressed, causing defect in the temporal 1/2 of the visual field, so the patient wont be able to see the sides. This condition is called Bitemporal hemianopia .
As we know there are 2 lobes:
- Anterior (known as Adeno-hypophysis)
- Pars Distalis (Pars anterior) – Largest section
- Pars Tuberalis – Surrounds the pituitary stalk
- Pars Intermedia – Separated from pars distalis by a cleft (not very prominent)
- Posterior (known as Neuro-hypophysis)
- Pars Nervosa – Largest section
- Median eminence – Upper part
- Infundibular stalk – Connection to the hypothalamus
Starting with the anterior pituitary:
The Pars Anterior consists of cords & clump of endocrine cells (secrete hormone into blood) and large capillary sinusoids (blood). In the pars anterior itself has 2 types of cells,
- Chromophils (phil: like to take up stains): Secretory
- Acidophils: Secrete growth hormones & prolactin
- Basophils: Secrete TSH, ACTH, LH, FSH, ICSH (stimulating hormones that act on other endocrine organs to secrete their respective hormones).
2. Chromophobes (phobe: dont like to take up stains): not secretory
The Pars Intermedia contains cells that secretes MSH (Melanocyte stimulating hormone). These cells are pale cells filled with some kinda jelly called colloid.
Posterior pituitary has another name which is called neuro-hypophysis. Neuro meaning it involves the nervous system/nerves. The posterior pituitary is special in a way that it reacts to nervous stimulation to release it’s hormone which has been produced beforehand and stored. Unlike the anterior pituitary, the anterior pituitary receives releasing hormones from the hypothalamus, then produces the stimulating hormones to be sent to another organs that produces the definitive hormone (the one that acts on the body), except prolactin and growth hormone.
Now, in the posterior pituitary has axonal endings (nerve endings) which comes from the hypothalamic nuclei. Besides that, it also has supporting cells called neuroglial cells/pituicytes. The nerves will send action potential to the nucleis in the posterior pituitary. There are 2 different types of nuclei,
- Paraventricular nucleus: Secrete oxytocin
- Supraoptic nucleus: Secrete ADH (Anti diuretic hormone, prevents loss of fluid thru urination).
In summary, the anterior pituitary is connected to the hypothalamus thru the portal connections (via blood vessels, by hormones) and the posterior pituitary thru the neural connections (via nerve, by neuroendocrine secretions).
(Link to the Hormones of the hypothalamo-hypopheseal axis post)