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IMS: Microanatomy > Hypothalamus and pituitary > Flashcards

Hypothalamus and pituitary Flashcards

1
Q

Hypothalamus

Direct interaction in homeostasis

A

Neuronal axons from the hypothalamus project directly into the posterior pituitary and upon stimulation they secrete hormones into the blood that affect the target organs.

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2
Q

Hypothalamus

Indirect interaction in homeostasis

A

Neurons from the hypothalamus secrete stimulatory and inhibitory hormones that travel through a vascular portal system to the anterior pituitary and there they regulate the production of other hormones by the adenohypophysis (anterior pituitary).

These adenohypophyseal hormones then affect target organs.

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3
Q

Hypothalamus

A
  • Magnocellular neuroscretory system
    • Supraoptic and paraventricular nuclei
    • Contains neurons whose unmyelinated axons are located in the neurohypophysis
    • Secretes oxytocin and antidiuretic hormone (vasopressin) into the capillaries of the neurohypophysis
  • Parvicellular system
    • Contains neurons that secrete into the portal system to control the adenohypophysis
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4
Q

Anatomy of the Hypophysis

(Pituitary)

A
  • Located at the base of the third ventricle in a depression of the sphenoid bone called the sella turcica (Turkish saddle)
  • 0.5 grams and 1-1.5 cm
    • Larger size seen in women with 2 or more children
  • Located within the CT coverings of the brain but not protected by the blood brain barrier due to fenestrated capillaries
  • Composed of the adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary)
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5
Q

Adenohypophysis

Blood supply

A

Supplied by the hypothalamo-hypophyseal venous portal system.

Both plexuses have highly permeable fenestrated capillaries which lie outside of the blood-brain barrier.

  • The superior hypophyseal artery is the main blood supply for the primary plexus located in the median eminence and infundibular process.
    • Primary plexus is the site where neurosecretory neurons from hypothalamus secrete their products.
  • The hypophyseal portal veins connect the primary and secondary plexuses.
    • Carry blood and hormones from hypothalamus to anterior pituitary.
  • The secondary plexus is located mainly in the pars distalis.
    • Here the hypothalamus hormones reach their target cells in the anterior pituitary.
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6
Q

Neurohypophysis

Blood supply

A
  • Supplied mainly by the inferior hypohyseal artery
  • Formes a capillary plexus that receives hormones (oxytocin and vasopressin) secreted in the neurohypophysis from axons of neurons whose cell bodies are in the hypothalamus
  • Blood vessels carry these hormones to target organs
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7
Q

Hypophyseal Development

A
  • Adenohypophysis develops from an outpocketing of oral ectoderm forming Rathke’s pouch which pushes through an opening in the sphenoid bone during development
  • Neurohypophysis develops from neuroectoderm in the floor of the diencephalon (CNS)
    • Maintains connection to it via a stalk
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8
Q

Adenohypophysis

(Anterior pituitary)

Basic anatomy

A
  • Receives no direct innervation from the hypothalamus
  • Receives hypothalamic releasing/inhibitory hormones via portal system which regulates production of hormones by adenohypophyseal cells
  • More cellular and glandular in appearance than neurohypohysis, therefore, darker staining with H&E
  • Contains three parts:
    1. Pars distalis: receives regulatory signals from portal system.
    2. Pars tuberalis: collar-like around the infudibulum.
    3. Pars intermedia: between the pars distalis and pars nervosa; characterized by colloid-filled cysts (Rathke’s cysts).
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9
Q

Pars distalis

A
  • Makes up most of the adenohypophysis
  • Glandular cells in cords and clumps
  • Extensive system of fenestrated capillaries makes up the secondary capillary plexus
  • Contains chromophils (acidophils and basophils) and chromophobes
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10
Q

Pars intermedia

A
  • Located between the pars distalis and pars nervosa
  • Characterized by multiple small colloid-containing cysts (Rathke’s cysts) lined by cuboidal cells
  • Contains basophils and chromophobes
  • Rodents: have large intermedia containing basophils which are melanotropes
  • Fish and amphibians: melanotropes also have a role in pigmentation.
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11
Q

Pars tuberalis

A
  • Identified mainly by location
  • Forms a sleeve around the infundibular stalk
  • Highly vascularized by major arteries and portal system
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12
Q

Adenohypophysis

Cell Types

A
  • Chromophils: refers to staining properties of the cytoplasmic granules
    • Acidophils (2 types):
      • Stain with eosin
    • Basophils (3 major types):
      • Stain with hematoxylin
      • Best distinguished with PAS which stains their glycoprotein secretory products dark pink
  • Chromophobes: do not take up stains well and appears pale
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13
Q

Chromophil Types

A
  • Acidophils: (PAS negative)
    • Somatotropes
      • Produces growth hormone (GH)
    • Lactotrophes
      • Produces prolactin (PRL)
  • Basophils: (PAS positive)
    • Corticotropes
      • Produces adrenocorticotropic hormone (ACTH), β-lipotropin, and MSH
        • Both derived from pro-opiomelanocortin (POMC)
        • POMC can be differently processed in fetal life and pregnancy to produce different hormones
    • Gonadotropes
      • Produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
    • Thyrotropes
      • Produces thyrotropic hormone (TSH)

Immunocytochemistry using antibodies specific for a particular hormone is the most reliable method for distinguishing between the types of chromophils.

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14
Q

Chromophobes

A
  • Small cells with very little cytoplasm and few if any granules
  • Usually occur in clusters
  • Probably represent a heterogenous cell population including:
    • Chromophils which quickly released granules
    • Undifferentiated stem cells
    • Folliculostellate cells
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15
Q

Folliculostellate cells

A
  • Chromophobes in anterior pituitary
  • Have long processes
  • Form gap junctions between other folliculostellate cells and hormone-producing cells in pars distalis
  • May be involved in signal transmission between pars tuberalis and pars distalis
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16
Q

Adenohypophysis hormones

A

FLAT PEG

  • Trophic hormones (hormones that act on other endocrine organs)
    • Follicle-stimulating hormone (FSH)
    • Luteinizing hormone (LH)
    • Adrenocorticotrophic hormone (ACTH)
    • Thyrotropic hormone (TSH)
  • Direct hormones (hormones that act directly on some other, non-endocrine, part of the body)
    • Prolactin (PRL)
    • Endorphins
    • Growth hormone (GH)
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17
Q

Follicle-stimulating hormone

(FSH)

A
  • Produced by gonadotrope cells.
  • Stimulates development of ovarian follicles in females.
  • Stimulates spermatogenesis in testes in males.
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18
Q

Luteinizing hormone

(LH)

A
  • Produced by gonadotrope cells.
  • In females, causes final maturation of ovarian follicles.
  • In males, stimulates androgen secretion by testes.
19
Q

Adrenocorticotropic hormone

(ACTH)

A
  • Produced by corticotrope cells.
  • Stimulates synthesis of glucocorticoids and gonadocorticoids by the adrenal cortex.
20
Q

Thyrotropic hormone

(TSH)

A
  • Produced by thyrotrope cells.
  • Stimulates synthesis of thyroid hormones (T3 & T4)
21
Q

Prolactin

(PRL)

A
  • Produced by lactrotrope cells.
    • Secretory granules remaining after weaning are destroyed by lysosomal enzymes via crinophagy.
  • Stimulates mammary gland development and lactation.
22
Q

Growth Hormone

(GH)

A
  • Made by somatotrophin cells.
  • Stimulates production of insulin-like growth factor 1 (IGF-1) by the liver.
  • IGF-1 targets muscle and bone to produce growth.
23
Q

Adenohypophysis

Methods of Regulation

A
  1. Hypothalamic neurons produce releasing hormones and inhibiting hormones that reach the adenohypophysis via the portal system and directly affects horone production by acidophils and basophils.
  2. Negative feedback by hormones from target organs inhibits hormone production by acidophils and basophils.
24
Q

Hypothalamus

Releasing & Inhibiting Hormones

Affecting the Adenohypophysis

A
25
Q

Growth hormone-releasing hormone

(GHRH)

aka

Somatotropin

A
  • Stimulates secretion and gene expression of growth hormone by somatotropes
  • Two forms in humans
    • 40 and 44 amino acid polypeptides
26
Q

Somatostatin

A
  • Inhibits secretion of growth hormone by somatotropes
  • 14 amino acid polypeptide
27
Q

Dopamine

A
  • Inhibits secretion of prolactin by lactotropes
  • Is a catecholamine (amino acid derivative)
28
Q

Corticotrophin-releasing hormone

(CRH)

A
  • Stimulates secretion of adrenocorticotropic hormone (ACTH) by corticotropes.
  • 41 amino acid polypeptide
29
Q

Gonadotrophin-releasing hormone

(GnRH)

A
  • Stimulates secretion of LH and FSH by gonadotropes.
  • 10 amino acid polypeptide
30
Q

Thyrotropin-releasing hormone

(TRH)

A
  • Stimulates secretion and gene expression of TSH by thyrotropes.
  • Also stimulates prolactin (PRL) secretion
  • 3 amino acid polypeptide
31
Q

Ghrelin

A
  • Produced by ghrelinergic cells in the gastrointestinal tract
  • Stimulates growth hormone (GH) secretion by the anterior pituitary
32
Q

Vasoactive inhibitory peptide

(VIP)

A

Stimulates prolactin secretion.

33
Q

Thyroid

Negative Feedback Control

A

Hypothalamus → TRH → thyrotropes of adenohypophysis → TSH → thyroid → T3 and T4 → negative feed back effects on pituitary and hypothalamus

34
Q

Neurohypophysis

(Posterior pituitary)

Basic anatomy

A
  • Pale-staining in H&E
  • Has three parts
    1. Pars nervosa
    2. Infundibulum composed of the:
      • Infundibular process
      • Median eminence
  • Receives direct innervation from hypothalamus (supraoptic and paraventricular nuclei) via many unmyelinated axons
  • Produces no hormones of its own
  • Site where oxytocin and vasopressin (made in the hypothalamus) are released from the axons of hypothalamic neurons which extend into the neurohypophysis
    • Secretory granules accumulate in Herring bodies
      • ​Herring bodies: dilations found along the length of the neuronal axon and at the nerve terminus.
  • Contains pituicytes
35
Q

Pituicytes

A
  • Non-neuronal cells of the neurohypophysis that do not produce hormones
  • Most of the nuclei visible in the neurophypophysis belong to pituicytes
  • Sometimes surround axons and axon terminals like glial cells
  • Have gap junctions between processes
  • Have trophic and support role
36
Q

Hormones of the neurohypophysis

A
  • Antidiuretic hormone and oxytocin
  • Both are cyclic polypeptide hormones of 9 amino acids (differing by only 2 residuals)
  • Both hormones associated with neurophysins in granules
  • Hormone and its neurophysin are cleavage products of a larger precursor protein
37
Q

Oxytocin

A
  • Causes contraction of smooth muscle cells in myometrium of pregnant uterus
  • Causes contraction of myoepithelial cells in the lactating mammary gland (part of the milk ejection reflex)
38
Q

Milk ejection reflex

A
  • Neuronal afferent limb
    • Suckling stimulates afferent nerve endings in the nipple
    • Nerve impulses reach the hypothalamus via cortex
  • Hormonal efferent limb
    • Axons of the hypothalamic neurons release oxytocin in the neurohypophysis
    • Oxytocin carried to the mammary gland in the blood where it causes myoepithelial cells to contract → enhancing milk ejection
39
Q

Antidiuretic hormone (ADH)

aka

Vasopressin

A
  • Acts on vascular smooth muscle causing higher blood pressure
  • Affects the permeability of collecting ducts in the kidney to promote fluid resorption → producing small volumes of more concentrated urine and conserving body fluid
40
Q

Diabetes insipidus

A
  • Caused by lesions of the hypothalamus that destroy ADH producing cells leading to insufficiency or absence of ADH.
  • Loss of renal ability to concentrate urine.
  • Results in constant/intense thirst and constant/heavy urination.
41
Q

Excess of somatotrophin (aka GHRH)

A
  • If hormone overproduction occurs before closure of bone epiphyses can result in gigantism:
    • A normally proportioned but unusually tall person.
  • If after epiphyseal closure causes acromegaly:
    • Thickening of bones resulting in enlarged hands and feels and coarsening of facial features.
42
Q

Pituitary tumors

A
  • Most pituitary tumors are benign but cause dramatic clinical symptoms because they continue to secrete hormones.
  • Hormone production is not controlled by negative feedback.
  • Clinical symptoms are due to the activities of the excess hormones.
43
Q

Graves disease

A
  • Autoimmune disease
  • Auto-antibodies to the TSH receptor on thyroid follicular cells bind to the receptor and mimic the stimulatory effects of TSH.
  • Causes thyroid follicular cells to produce inappropriately high levels of T3 and T4.
  • Treated with pharmacological inhibition vs thyroidectomy and subsequent hormone replacement therapy.

IMS: Microanatomy (16 decks)

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