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Hypophysis Anatomy Flashcards

1
Q

What is the hypophysis?

A

Pituitary gland

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2
Q

Where does the pituitary develop from?

A

Adenohypophysis – from roof of mouth.

Neurohypophysis – from hypothalamus.

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3
Q

Is the pituitary larger in males or females?

A

It is slightly larger in females and Larger in pregnancy.

Relatively smaller in larger animals.

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4
Q

Where does the pituitary gland sit?

A

The hypophysis is ventral to hypothalamus in a bony recess in the basisphenoid (cavity is called the Hypophyseal fossa)

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5
Q

What is the turkish chair?

A

Sella turcica (“Turkish chair”) refers to the form of the fossa and the surrounding processes

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6
Q

The hypophysis pokes through a sheet of dura as the dura forms the…

A

The dura forms the diaphragma sellae, through which the hypophyseal stalk connects between the hypothalamus and the hypophysis.

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7
Q

Why is there no subarachnoid space around the hypophysis?

A

The hypophysis needs to be outside the BBB

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8
Q

What is the glandular portion of the pituitary and where does it lie?

A

Adenohypophysis - the anterior lobe

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9
Q

What is the Adenohypophysis made up of?

A

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.

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10
Q

Which species lacks the cleft of the hypophysis?

A

horses

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11
Q

What is the neural part of the pituitary and where does it lie?

A

Neurohypophysis - the posterior lobe

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12
Q

What does the Neurohypophysis consist of?

A

Infundibulum – attached to tuber cinereum of hypothalamus.

Neural lobe – oxytocin and ADH produced here, this is essentially an out pouch of the brain

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13
Q

The adenohypophysis is hormonally linked to the hypothalamus via…

A

Via a capillary portal system.

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14
Q

The neurohypophysis is neurologically linked to the hypothalamus via…

A

via cell bodies in the hypothalamus as Axons extend down the stalk and end of axons (telodendria) in the neurohypophysis.

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15
Q

What is the pars intermedia formed from?

A

It is formed from Remnants of Rathke’s pouch

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16
Q

What is the function of the pars intermedia?

A

It produces melanocyte stimulating hormone (MSH) in the fetus.

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17
Q

Adenomas of the pars intermedia (PPID in horses) produces what?

A

Produce both MSH and ACTH (because they share the same precursor: pro-opiomelanocortin aka POMC).

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18
Q

How is the adenohypophysis formed?

A

Upgrowth of ectoderm of roof of the developing mouth (stomodeum) aka Rathke’s pouch
Cups around developing neurohypophysis
Connection with mouth lost

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19
Q

If Rathke’s pouch does not differentiate into glandular tissue then what happens?

A

no adenohypophysis is formed, instead pituitary cysts result.
This results in pituitary dwarfism (GSDs).

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20
Q

How is the neurohypophysis formed?

A

Down growth of ectoderm of forebrain from diencephalon/future hypothalamus.
Retains connection with brain via infundibulum.

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21
Q

The hypothalamus contains GnRH cells, where do there migrate from?

A

GnRH releasing cells start life in the olfactory bud and migrate to their final position within the hypothalamus.

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22
Q

What is special about the capillaries of the pituitary?

A

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

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23
Q

How are hormones such as ACTH and TSH released?

A

Releasing Factors produced within hypothalamus.
Circulate via the “hypothalamo-hypophyseal portal system” to the adenohypophysis.
Cause endocrine secretion.

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24
Q

How are neurohormones such as oxytocin and ADH (vasopressin) released?

A

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.

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25
What hormones re released by the anterior pituitary (pars distalis)?
1. Thyroid-stimulating hormone (TSH/thyrotropin) 2. Adrenocorticotropin hormone (ACTH / corticotropin) 3. Growth hormone (GH / somatotropin) 4. Prolactin (PRL) 5. Follicle-stimulating hormone (FSH) 6. Luteinizing hormone (LH)
26
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) 3. Corticotropes (ACTH) 4. Thyrotropes (TSH) 5. Gonadotropes (FSH and LH)
27
What hormones regulate growth hormone release?
1. GH-releasing hormone (GHRH) 2. Somatostatin (also called GHIH) 3. Ghrelin (hormone from the stomach) secrete GH in response to food
28
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
29
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.
30
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)
31
ACTH is cleaved from POMC by what hormone?
POMC to ACTH is catalyzed by Prohormone convertase I in the Pars distalis and Pars intermedia
32
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
33
Is ACTH secretion regulated by tonic or episodic stimulation?
More CRH increases ACTH secretion Less CRH decreases ACTH secretion Negative feedback loop
34
How much of the pars distalis is dedicated to secreting gonadotropes FSH and LH?
10%
35
What does FSH do?
Female: Follicle development (ovary) and estradiol (an estrogen) production Male: Support spermatogenesis by Sertoli cells
36
What does LH do?
Female: Stimulates ovulation, corpus luteum development and progesterone Male: Maintenance of Leydig cells and therefore testosterone production
37
How much of the pars distalis is dedictaed to TSH synthesis?
5%
38
What does TSH do?
Acts on follicular cells of the thyroid gland which produce thyroglobulin that will become triiodothyronine (T3) and thyroxine (T4)
39
Is TSH regulated by tonic or episodic stimulation?
Tonic stimulation
40
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
41
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
42
What systems don't depend on the hypothalamus and pituitary for regulation?
1. Glucose regulation: Insulin, glucagon, epinephrine 2. Calcium regulation: Parathyroid hormone (PTH), calcitonin, and active vitamin D 3. Sodium and potassium regulation 4. Gastrointestinal enzyme secretion
43
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
44
What breed is most susceptible to pituitary dwarfism?
Most common in German Shepherd dogs
45
What causes gigantism?
Caused by hyperactivity or neoplasia before adolescence | All tissues grow rapidly including bones
46
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)
47
What does the anterior (neural) hypophysis produce?
1. Anti-diuretic hormone (ADH) Also called vasopressin Acts on the renal tubules 2.Oxytocin Acts on the mammary gland Acts on the uterus
48
How can we assess the concentration of urine?
Use a Refractometer to measure specific gravity of urine
49
Define osmolarity
Osmolarity = number of dissolved particles per unit volume, the more concentrated the urine the higher its osmolarity.
50
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.
51
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 ```
52
Define Diuresis
Diuresis refers to increased urine output | Anti-diuresis causes a decrease in urine
53
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.
54
What causes diabetes insipids?
1. Neurogenic diabetes insipidus Inadequate ADH release from pituitary 2. Nephrogenic diabetes insipidus Inadequate response within the kidney
55
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
56
How does prolactin stimulate milk synthesis?
1. Stimulates mRNA for milk protein synthesis 2. 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
57
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
58
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
59
What is the effect of oxytocin on the udder?
Increases pressure within alveoli Reduces resistance in excretory ducts Reduces resistance in teat canal
60
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
61
Estrogen/progesterone ratio is lower in pregnancy to ................ oxytocin and higher at birth to stimulate oxytocin
inhibit
62
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

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