Hypophysis Anatomy Flashcards
What is the hypophysis?
Pituitary gland
Where does the pituitary develop from?
Adenohypophysis – from roof of mouth.
Neurohypophysis – from hypothalamus.
Is the pituitary larger in males or females?
It is slightly larger in females and Larger in pregnancy.
Relatively smaller in larger animals.
Where does the pituitary gland sit?
The hypophysis is ventral to hypothalamus in a bony recess in the basisphenoid (cavity is called the Hypophyseal fossa)
What is the turkish chair?
Sella turcica (“Turkish chair”) refers to the form of the fossa and the surrounding processes
The hypophysis pokes through a sheet of dura as the dura forms the…
The dura forms the diaphragma sellae, through which the hypophyseal stalk connects between the hypothalamus and the hypophysis.
Why is there no subarachnoid space around the hypophysis?
The hypophysis needs to be outside the BBB
What is the glandular portion of the pituitary and where does it lie?
Adenohypophysis - the anterior lobe
What is the Adenohypophysis made up of?
Pars tuberalis – monitors melatonin levels.
Pars intermedia – surrounds neurohypophysis in equidae and carnivores.
Pars distalis – main secretory region. Collection of cells, each secrete a different hormone.
Which species lacks the cleft of the hypophysis?
horses
What is the neural part of the pituitary and where does it lie?
Neurohypophysis - the posterior lobe
What does the Neurohypophysis consist of?
Infundibulum – attached to tuber cinereum of hypothalamus.
Neural lobe – oxytocin and ADH produced here, this is essentially an out pouch of the brain
The adenohypophysis is hormonally linked to the hypothalamus via…
Via a capillary portal system.
The neurohypophysis is neurologically linked to the hypothalamus via…
via cell bodies in the hypothalamus as Axons extend down the stalk and end of axons (telodendria) in the neurohypophysis.
What is the pars intermedia formed from?
It is formed from Remnants of Rathke’s pouch
What is the function of the pars intermedia?
It produces melanocyte stimulating hormone (MSH) in the fetus.
Adenomas of the pars intermedia (PPID in horses) produces what?
Produce both MSH and ACTH (because they share the same precursor: pro-opiomelanocortin aka POMC).
How is the adenohypophysis formed?
Upgrowth of ectoderm of roof of the developing mouth (stomodeum) aka Rathke’s pouch
Cups around developing neurohypophysis
Connection with mouth lost
If Rathke’s pouch does not differentiate into glandular tissue then what happens?
no adenohypophysis is formed, instead pituitary cysts result.
This results in pituitary dwarfism (GSDs).
How is the neurohypophysis formed?
Down growth of ectoderm of forebrain from diencephalon/future hypothalamus.
Retains connection with brain via infundibulum.
The hypothalamus contains GnRH cells, where do there migrate from?
GnRH releasing cells start life in the olfactory bud and migrate to their final position within the hypothalamus.
What is special about the capillaries of the pituitary?
The various hormones are released into the interstitial tissues of the hypothalamus and hypophysis.
The capillaries are of the fenestrated type to allow free flow of molecules into the circulation.
Lack tight junctions.
Therefore the blood-brain barrier is absent from these parts of the hypothalamus and the hypophysis
How are hormones such as ACTH and TSH released?
Releasing Factors produced within hypothalamus.
Circulate via the “hypothalamo-hypophyseal portal system” to the adenohypophysis.
Cause endocrine secretion.
How are neurohormones such as oxytocin and ADH (vasopressin) released?
Axons from supraoptic and paraventricular nuclei in hypothalamus form supraopticohypophyseal and paraventriculohypophyseal tracts.
Descend via the infundibulum to the neural lobe.
Terminations of axons release antidiuretic hormone (ADH) and oxytocin.
What hormones re released by the anterior pituitary (pars distalis)?
- Thyroid-stimulating hormone (TSH/thyrotropin)
- Adrenocorticotropin hormone (ACTH / corticotropin)
- Growth hormone (GH / somatotropin)
- Prolactin (PRL)
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
How many cell types are in the anterior pituitary?
Chromophobes stain poorly
Chromophils stain well
Acidophils: Acid dyes: eosin(red)
1. Somatropes (GH)
2. Lactotropes/mammotropes/luteotropes (PRL)
Basophils: basic dyes: Hematoxylin (blue)
- Corticotropes (ACTH)
- Thyrotropes (TSH)
- Gonadotropes (FSH and LH)
What hormones regulate growth hormone release?
- GH-releasing hormone (GHRH)
- Somatostatin (also called GHIH)
- Ghrelin (hormone from the stomach) secrete GH in response to food
What do growth hormones stimulate?
Growth hormones stimulates the secretion of IGF-1 from the liver which stimulates protein synthesis (which has anabolic effects), lipolysis (which mobilises fatty acids and glycerol) and inhibits glucose utilization which inhibits insulin
The cells that store and secret PRL are ………………. when storing PRL and ………………. once it has been secreted.
Chromophil (acidophil) during storage
Chromophobe (poor staining) once released.
What cleaves proopiomelanocortin? What is it cleaved into?
Proteolytic hormones cleave POMC into:
Adrenocorticotropin hormone (ACTH) which travels through the blood to act on the adrenal cortex.
Melanocyte-stimulating hormone (MSH)
β-lipotrophic hormone
β-endorphin
Enkephalin (type of endorphin)
Corticotropin-like intermediate lobe peptide (CLIP)
ACTH is cleaved from POMC by what hormone?
POMC to ACTH is catalyzed by Prohormone convertase I in the Pars distalis and Pars intermedia
ACTH is cleaved into CLIP and aMSH by what hormone?
ACTH to CLIP and aMSH is catalyzed by Prohormone convertase II in the Pars intermedia only
Is ACTH secretion regulated by tonic or episodic stimulation?
More CRH increases ACTH secretion
Less CRH decreases ACTH secretion
Negative feedback loop
How much of the pars distalis is dedicated to secreting gonadotropes FSH and LH?
10%
What does FSH do?
Female: Follicle development (ovary) and estradiol (an estrogen) production
Male: Support spermatogenesis by Sertoli cells
What does LH do?
Female: Stimulates ovulation, corpus luteum development and progesterone
Male: Maintenance of Leydig cells and therefore testosterone production
How much of the pars distalis is dedictaed to TSH synthesis?
5%
What does TSH do?
Acts on follicular cells of the thyroid gland which produce thyroglobulin that will become triiodothyronine (T3) and thyroxine (T4)
Is TSH regulated by tonic or episodic stimulation?
Tonic stimulation
What is the pars intermedia made up of?
Series of small cystic cavities and follicles filled with colloid
Residual lumen of Rathke’s pouch
Cells: Basophils and chromophobes
What is the difference between open and closed loop?
Closed loop: Target cells secrete hormones that affect their own secretion
Open loop: No direct feedback , Stopped by removal of stimulus or depletion of biochemicals
What systems don’t depend on the hypothalamus and pituitary for regulation?
- Glucose regulation: Insulin, glucagon, epinephrine
- Calcium regulation: Parathyroid hormone (PTH), calcitonin, and active vitamin D
- Sodium and potassium regulation
- Gastrointestinal enzyme secretion
What causes pituitary dwarfism?
Cyst instead of adenohypophysis (genetic defect and test is now available)
So no GH gives you a little dog
No ACTH so it has hypoadrenocorticism
No TSH so it also has hypothyroidism
What breed is most susceptible to pituitary dwarfism?
Most common in German Shepherd dogs
What causes gigantism?
Caused by hyperactivity or neoplasia before adolescence
All tissues grow rapidly including bones
What causes acromegaly?
Caused by pituitary disorders after adolescence when the epiphyses of bones have fused
Bones of feet and hands (paws) grow
Membranous bones of face grow because their growth continues after adolescence
Eventual organ enlargement
Kyphosis of the vertebrae (hunchback)
What does the anterior (neural) hypophysis produce?
- Anti-diuretic hormone (ADH)
Also called vasopressin
Acts on the renal tubules
2.Oxytocin
Acts on the mammary gland
Acts on the uterus
How can we assess the concentration of urine?
Use a Refractometer to measure specific gravity of urine
Define osmolarity
Osmolarity = number of dissolved particles per unit volume, the more concentrated the urine the higher its osmolarity.
Besides hypothalamic detection of blood osmolarity, how else is dehydration detected?
Dehydration can also be detected by stretch receptors in the atria and veins and respond to large changes in blood volume (more than 10%), they send nerve impulses to the hypothalamus and more ADH is released.
What are the actions of ADH?
Acts on distal tubule and collecting duct Increases permeability to water More water moves out of tubule, resorbed Urine volume decreases Urine concentration increases
Define Diuresis
Diuresis refers to increased urine output
Anti-diuresis causes a decrease in urine
What is the effect of ADH on aquaporins?
Aquaporins are always present in baso-lateral membranes
Only present in apical membranes when ADH present
When ADH binds to receptors there is activation of adenyl cyclase and production of cAMP. This causes the vesicles containing Aquaporins to move to the cell surface which are inserted via exocytosis.
What causes diabetes insipids?
- Neurogenic diabetes insipidus
Inadequate ADH release from pituitary - Nephrogenic diabetes insipidus
Inadequate response within the kidney
How can we diagnose diabetes insipidus?
Injection of exogenous ADH then measure urine
Concentrated urine: neurogenic ie compensates for failure to produce ADH
Dilute urine: nephrogenic ie kidney fails to respond to extra ADH
How does prolactin stimulate milk synthesis?
- Stimulates mRNA for milk protein synthesis
- Ensures adequate α-lactalbumin (aLA) for lactose production.
aLA is a whey protein that is produced by mammary epithelial cells and forms a complex with galatosyltransferase to form lactose synthase.
Lactose draws water into the gland and is also an easily digestible food
What stimulates milk let down?
But Oxytocin stimulates milk let down, It acts on smooth muscle cells for example uterine muscle during parturition and induces contraction of alveoli of mammary glands
How does suckling cause oxytocin release?
High density of sensory nerve fibers in teats detect suckling or preparation for milking
This causes impulses to be transmitted via superficial sensory pathways and inguinal nerve
These impulses travel via afferent sensory neurons and enter the lumbar segments of the spinal cord
They ascend the spinal cord sensory tracts to the thalmus where they influence cell bodies of neuroendocrine cells
Secretion of oxytocin from nerve endings in posterior pituitary gland into circulation
What is the effect of oxytocin on the udder?
Increases pressure within alveoli
Reduces resistance in excretory ducts
Reduces resistance in teat canal
When oxytocin reaches the mammary gland what receptor does it bind to?
Binds receptors on myoepithelial cells, Myoepithelial cells surround alveoli and walls of small secretory ducts
Oriented along long axis of secretory duct
Estrogen/progesterone ratio is lower in pregnancy to ……………. oxytocin and higher at birth to stimulate oxytocin
inhibit
How does the fetus decrease progesterone?
An increase in ACTH from anterior pituitary gland of the fetus stimulates enzymes that convert progesterone to estrogens in placenta.
This causes an increase in PGF2α (prostaglandin) which Increases intracellular calcium within myometrial cells (increase contractility) and causes lysis of the corpus luteum (CL) and thus a reduction of P4
