Pituitary Tutorial—Anatomy and Imaging
We will first deal with the pituitary gland itself,
then deal with its neighborhood, the normal sella turcica.
The pituitary gland...
...is pretty small for its importance. It is roughly the size of a kidney bean lying on its side (for you non-vegetarians, that is 12 mm in lateral dimension and about 8 mm in AP and superior/inferior dimension). Typical weights are 500-700 mg in the non-pregnant individual, easily doubling in pregnancy.
The pituitary is divided into an anterior lobe and a posterior lobe, each with its unique and colorful history and function.
The anterior lobe produces
- Prolactin (PRL)
- Growth hormone (GH)
- Adrenocorticotrophin (ACTH)
- Thyroid stimulating hormone (TSH)
- Gonadotrophins (FSH and LH)
Each hormone is regulated by releasing and inhibiting factors coursing through the venous portal system which connects the hypothalamus to the anterior pituitary via the infundibular stalk.
The anterior pituitary lobe develops from primitive mouth tissue invaginating upwards. Rests of this tissue (i.e. clumps of fetal cells which haven't completed their migration and differentiation) probably cause craniopharyngiomas and other tumors in this region, but we digress.
The posterior lobe, a direct extension of brain tissue, produces oxytocin and vasopressin. This area has neurons, glia, and all the trappings of real brain.
The Normal Sella Turcica...
... is a box that contains the pituitary gland. The box has a top, a bottom, a front wall, a back wall, and two side walls. Each contains important structures. If you would know your pituitary, know its environs.
General comments:
The pituitary gland is beneath the dura, but outside the arachnoid (i.e. there is no CSF surrounding the pituitary gland). The sella turcica is lined by dura.
For quick overview, jump ahead to imaging of the pituitary gland. But don't forget to jump back, now...
Top of the box
The top of the box (i.e. the superior aspect of the sella turcica) is bounded by the diaphragma sellae, a leaf of dura which is perforated by the hole that the infundibular stalk uses to communicate the hypothalamus and the pituitary. The diaphragma sellae varies in size and strength, and the hole can be bigger or smaller, causing variations in patterns of expansion of pituitary tumors. For example, a large hole will allow the tumor to balloon upward pretty much in the shape of an egg, whereas a thick diaphragma sellae with a small hole will "pinch" a significant waist in the tumor as it expands upwards, making transsphenoidal resection of the suprasellar portion that much more difficult.
Lying just superior to the diaphragma sellae, in the suprasellar cistern, is the optic chiasm. Recall that the most medial fibres of each optic nerve cross in the chiasm. Recall further that these medial retinal fibres, thanks to the lens, receive photons from the LATERAL visual fields. Thus, compression of the optic chiasm will stretch the medial, crossing fibres first, causing the classical bitemporal hemianopia associated with pituitary tumors. If this is confusing, draw it out (a good general principle, by the way).
Bottom of the box
The bottom of the box (i.e. the inferior aspect of the sella turcica) is bounded by the sphenoid bone. Anatomical variations include a sphenoid sinus which extends underneath the sella turcica, making transsphenoidal approaches easy, and solid bone underneath the sella turcica, making transsphenoidal approaches difficult.
Front wall of the box
The front wall of the box (i.e. the anterior aspect of the sella turcica) is bounded by the bone which constitutes the back (posterior) wall of the sphenoid air sinus. Typically, the sphenoid septum inserts somewhere on the anterior face of the sella turcica, and a fine landmark it is during surgery. Anatomic variations include a small or absent sphenoid sinus, requiring massive quarrying to get to the sella for a transsphenoidal approach, as well as carotid arteries (cf. side walls) pushing anteriorly into the sphenoid sinus, requiring some care in doing the anterior sellar wall resection during transsphenoidal surgery.
Back wall of the box
The back wall of the box (i.e. the posterior aspect of the sella turcica) is bounded by the bone of the clivus. It generally is intact, and generally keeps you from poking a curette through the basilar artery and into the brainstem. Generally. So it behooves the surgeon to inspect this aspect of the sellar wall carefully to see what his margin of safety is.
Side walls of the box
The side walls of the box (i.e. the lateral aspect of the sella turcica) are two, and they are made up of a layer of dura, then the contents of the cavernous sinus:
- Vascular structures in the cavernous sinus include the sigmoid segment of the carotid arteries and the plexus of veins constituting the cavernous sinus.
- Nerves in the cavernous sinus include the 3rd, 4th, and 6th cranial nerves, which control eye movement, as well as the top two divisions of the 5th cranial nerve, supplying sensation to the face
The contents of these side walls are very, very important. Surgeons are very careful when curetting tumor out laterally, in order to avoid turning this operation into a vascular procedure
Imaging of the sella and its contents...
...is best done with MR scanning. Even though 99% of us couldn't explain the principles behind MR scanning if our lives depended upon it, we use it routinely simply because it is so good.
Look and marvel:
Now go back to the box analogy and see if you can put it together again, using the scans.
