Principles of Endocrinology Flashcards
The pituitary gland receives inputs from . . .
- The brain (via the hypothalamus)
- The blood stream (via mediators released from other endocrine glands and metabolically active sites)
Why is the pituitary gland also called the “master gland?”
Because it releases hormones that control the activity of many other glands in the body. Think TSH for the thyroid, ACTH for the adrenal gland, leutenizing hormone for reproductive glands, etc.
Location of the pituitary gland
Located centrally in the cranium in a small fossa (depression) called the sella turcica (aka Turkish saddle).
Sellar diaphragm
Infold of the dura mater which separates the pituitary gland from the main cranial cavity and protects it. A “pituitary stalk” goes through the sellar diaphragm connecting the pituitary gland to the hypothalamus.
Location of pituitary within skull floor
Important structures adjacent to the pituitary gland
- Optic chiasm (superoanterior to the gland)
-
Cavernous sinuses, laterally, containing:
- CN IV, V, and VI (ocular movements)
- CN V1 and V2 (trigeminal, facial sensation)
- Cavernous segment of the internal carotid artery
- Sphenoid sinus inferior to the gland, just above the nasopharynx
Most pituitary surgeries are now done via. . .
. . . the sphenoid sinus by way of the nasopharynx using long tools, and are accompanied by dopplar ultrasound to avoid the internal carotid artery.
Nettergram demonstrating anatomical relationships of the pituitary gland and other important structures
Radiologic examination of the pituitary
Pituitary MRI
This involves use of gadolinium contrast.
The pituitary is surrounded by bony structures, making X-ray difficult. CT is also an option, but has poor resolution for the pituitary.
Standard pituitary radiology. What are the structures?
A - optic chiasm
B - Hypothalamus
C - Pituitary stalk
D - Posterior pituitary (shows a characteristic “bright spot” due to presence of ADH vesicles)
E - Anterior pituitary
F - Suprasellar cistern
Pituitary embryology
The anterior and posterior have separate origins!
The anterior is derived from the surface ectoderm, and forms form an upward outpouch of primitive oral cavity tissue called Rathke’s pouch. Meanwhile, the posterior is derived from the neuroectoderm, and forms from a downward outpouch of brain called the infundibulum via an intermediate structure called the pars nervosa.
Rathke’s pouch cyst
In adulthood, a small slit (Rathke’s cleft) can sometimes be seen in the pituitary which is the remnant of the fetal pouch – in some cases, this slit can fill with fluid and exert local mass effects with deleterious effects.
Hypothalamo-hypophyseal portal system
Portal venous system supplying the anterior pituitary gland. Blood in this system comes from the internal carotid artery, but passes first through the hypothalamic capillaries. The resulting milieu is low-pressure and rich in hypothalamic signaling molecules.
GnRH neurons
These neurons originate at the olfactory placode, where the neurons responsible for the sense of smell also originate. They migrate to the hypothalamus under the control of a range of transcription factors. A failure to migrate can lead to absent GnRH and therefore absent sexual development, a condition called idiopathic hypogonadotropic hypogonadism.
If the sense of smell is also absent, it is called Kallman syndrome.
Production and storage of posterior pituitary hormones
Hormone-producing cells of the anterior pituitary gland
- Somatotropes : GH-secreting
- Lactotropes : Prolactin-secreting
- Corticotropes : ACTH-secreting
- Gonadotropes : FSH and LH-secreting
- Thyrotropes : TSH-secreting
The posterior pituitary gland consists of. . .
. . . axons and glial cells
The nerve cell bodies for these axons are located in the hypothalamus, primarily in the paraventricular nucleus (responsible for oxytocin) and supraoptic nucleus (responsible for ADH)
Pars Nervosa histology
c - capillaries
p - pituicytes, a type of glial cell, most numerous cell present
NB - neurosecretory bodies from which oxytocin or vasopressin is released upon stimulation
ADH / Vasopressin
Reduces urinary volume by increasing water permeability of the renal collecting ducts
Stimulus: Increased plasma osmolality, decreased plasma volume
Oxytocin
Stimulates contraction of mammary gland myoepithelial cells (important in lactation) and uterine smooth muscle
Stimulis: Suckling / nipple stimulation, Baby’s cry, Mechanical stimulation of cervix
Regulation of ADH
The overall purpose of this hormone is to promote water retention.
If the person drinks a lot of water, the hormone level is low, allowing most of the water to be quickly excreted by the kidney.
If the person drinks very little water, the hormone level is high to ensure that the water is being retained in the body and not excreted.
Mechanism of ADH regulation and action
- Inputs:
- Pressure sensors (aortic arch, cardiac atria, carotid sinus)
- Osmolality sensors
- Osmolality response more sensitive than pressure response
- ADH acts on V1 receptors in smooth muscle, promoting contraction and increased SVR
- ADH acts on V2 receptors in collecting duct of kidney to increase permeability (insertion of vesicle-sequestered AQP2)
Diabetes insipidus
Deficiency of ADH resulting in the production of large quantities of dilute urine
Hormones secreted by anterior pituitary
- Growth hormone
- FSH
- LH
- ACTH
- TSH
- Prolactin
- MSH
- Endorphins
Classes of anterior pituitary hormones
Summary of anterior pituitary function