ERS02 Development And Structure Of Pituitary Flashcards
Pituitary gland and Hypothalamus
Pituitary gland / Hypophysis:
- where Nervous system and Endocrine systems interact
- Master gland of endocrine system
- 0.5g - 1.5g (significantly enlarged in women given birth to >=2 children ∵ lactotropic cells hyperplasia + hypertrophy)
- within Sella turcica (Hypophyseal fossa) of Sphenoid bone
- posterior to Optic groove / chiasm (prone to compression by pituitary adenoma)
- sits outside of BBB
2 functional components from different embryologic origins:
- Anterior pituitary (Adenohypophysis):
- ***Ectoderm of oropharynx
- produce 9 major hormones
—> regulate body functions
—> regulate secretions of other endocrine glands
- 3 areas with indistinct boundaries:
—> Pars distalis
—> Pars tuberalis
—> Pars intermedia - Posterior pituitary (Neurohypophysis):
- **Neuroectoderm, from 3rd ventricle (Diencephalon)
- extension of Hypothalamus
- extension of Nervous system via **Infundibulum (from Median eminence)
—> secrete ***Neurohormones (e.g. ADH, oxytocin)
Hypothalamus:
- regulates secretion of Anterior pituitary
Development of pituitary gland
- Anterior pituitary (Adenohypophysis)
- **Oropharynx Ectoderm
- out-pocket from roof of primitive mouth
—> **Rathke’s pouch (Hypophyseal diverticulum)
—> migrate superiorly + breaking off from oral cavity
—> Anterior wall grow rapidly —> Pars Distalis (Anterior lobe of pituitary) + Pars Tuberalis (funnel-shaped region wrap around infundibulum)
—> Dorsal wall —> Pars Intermedia (no significance)
- Posterior pituitary (Neurohypophysis)
- Pars Nervosa (posterior lobe) + Infundibulum (attach gland to Hypothalamus)
- ***Neuroectoderm (Neurohypophyseal diverticulum)
—> evaginates from floor of Diencephalon (from 3rd ventricle)
—> grows caudally
—> distal end of Infundibulum becomes solid as neuroepithelial cell proliferate
—> gives rise to Pars Nervosa - Pars Nervosa: composed of neuroglial cells + **fibres from hypothalamus (Paraventricular nucleus + **Supraoptic nucleus)
Histology of pituitary gland
Anterior lobe (hormone secreting cells) stains much darker than Posterior lobe (nerve fibres + neuroglial cells)
Pituitary hormones
Anterior pituitary hormones (1-4: ***Tropic hormones —> stimulate release of hormones from other endocrine organs):
- TSH
- ACTH
- LH
- FSH
- Growth hormone (GH) / Somatotropin
- Prolactin
- MSH (Melanocyte-stimulating hormone)
- Beta-endorphin
- Lipotropin
Posterior pituitary hormones (***Neuropeptides):
- ADH
- Oxytocin
Pars distalis - Adenohypophysis
Contains most of hormone secreting cell in Anterior pituitary
—> organised in cords
—> epithelial cells interspersed with capillaries
Hormone secreting cells have different staining property:
- Chromophobes (do not stain intensely)
- Chromophils
- Acidophils (red in H+E, orange yellow in OFG)
- Basophils (blue in H+E, magenta in OFG)
Secretory cells (記: Growth in Breast: Acidophilic):
- Somatotrope (acidophilic)
- GH / Somatotropin
- stimulated by GHRH (from Hypothalamus: Neurohormone)
- inhibited by Somatostatin (from Hypothalamus: Neurohormone) - Lactotrope (acidophilic)
- Prolactin (PRL)
- stimulated by PRH
- inhibited by PIH (e.g. Dopamine) - Gonadotrope (basophilic)
- FSH, LH
- stimulated by GnRH - Thyrotrope (basophilic)
- TSH / Thyrotropin
- stimulated by TRH - Corticotrope (basophilic)
- ACTH / Corticotropin
- stimulated by CRH
Pars tuberalis, Pars intermedia - Adenohypophysis
Pars tuberalis:
- secrete ***Gonadotrophins (FSH, LH) (most cells) (+ ACTH: immunoactivity)
- arranged in cords with blood vessels
- highly vascular
Pars intermedia:
- cords + follicles of weakly basophilic cells containing basophilic granules
- characteristics feature: series of small cysts / follicles filled with colloid (residual lumen of Rathke’s pouch)
- unknown function
Neurohypophysis
Pars Nervosa: major hormone secreting region
Composed of
1. **Unmyelinated fibres (innervation from Hypothalamus: **Paraventricular nucleus + **Supraoptic nucleus)
2. Pituicyte
3. Fibroblast
4. Mast cells
—> **Hypothalamo-Hypophyseal tract
—> Neurons terminate on target cells (rather than on other neurons)
—> Close proximity to ***fenestrated capillary of Pars Nervosa
Pituicyte:
- primary cell in posterior lobe, associate with fenestrated capillary
- specific glial cell: ***supporting role
- regulate release and storage of neurohormones
Neurosecretions accumulate at the end of the fibres —> **Herring bodies
- large axon swellings in neurosecretory cell in Hypothalamo-Hypophyseal tract
- contain **Neurohormones (ADH, oxytocin)
—> stored as membrane-bound granules in axon endings
Function:
Secrete ADH (Vasopressin) + Oxytocin
—> Each hormone bound to a specific binding protein —> ***Neurophysin
Blood supply of Pituitary gland
Arterial blood supply:
- ***Superior hypophyseal artery (ICA)
- Median eminence
- Infundibulum
- Pars Tuberalis - Inferior hypophyseal artery (ICA)
- Pars Nervosa
- Infundibulum (little)
***NO direct arterial blood supply to Anterior lobe (Pars Distalis, Pars Intermedia)
Venous drainage:
***Efferent hypophyseal vein
—> Cavernous sinus (base of Diencephalon)
—> systemic circulation
Regulation of Hypothalamus on Anterior pituitary
Hypothalamic neurons involved:
1. **Dorsal medial (DM)
2. **Ventral medial (VM)
3. Infundibular nuclei
4. **Paraventricular nuclei (Parvocellular neurosecretory neurons)
(Posterior pituitary: **Magnocellular neurosecretory neurons)
Stimuli from nervous system
—> Hypothalamic neurons in Median eminence secrete Releasing / Inhibiting hormones
—> Hypophyseal portal system
—> **Primary capillary plexus in Median eminence and Infundibulum (from R/L Superior hypophyseal arteries; supply Median eminence + Infundibulum)
—> rejoin to form Hypophyseal portal veins
—> **Secondary capillary plexus in Pars Distalis (Neuropeptides入)
—> Releasing / Inhibiting hormones stimulate / inhibit Anterior pituitary to release stimulatory hormones (Direct influence on secretory activity)
—> **Secondary capillary plexus (Glycoprotein / Protein hormones出)
—> **Hypophyseal veins
—> Target tissue / ***Other endocrine gland
Capillary plexus: Fenestrated
- allow passage of small molecules / proteins (e.g. neurohormones)
- pores may be closed by thin diaphragm
Regulation of Hypothalamus on Posterior pituitary
Hypothalamic neurons involved:
1. Supraoptic nuclei (Magnocellular neurosecretory neurons)
2. Paraventricular nuclei (Magnocellular neurosecretory neurons)
(Anterior pituitary: ***Parvocellular neurosecretory neurons)
Stimuli from nervous system
—> Hypothalamic neurons secrete neurohormones (ADH, Oxytocin)
—> ***Hypothalamo-hypophyseal tract (nerve fibres directly derived from Hypothalamic neurons)
—> stored / released by Posterior pituitary (hormones NOT made by Posterior pituitary!!! Posterior pituitary only store / release!!!)
—> Fenestrated capillary system
—> Target tissue
Median eminence
- Arise from inferior border of Hypothalamus
- Enlarged area in wall of Infundibulum
- Neuro-Haemal region where ***Releasing / Inhibiting hormones (neurohormones from Hypothalamus) pass into capillaries
- surrounded by extended perivascular connective tissue spaces
- ***Axon endings of neuro-hormonal nerve cells open and release hormones into perivascular spaces
—> neurohormones then pass into blood through portal veins (Hypothalamo-hypophyseal portal system)
—> Anterior pituitary
Hypophyseal portal system
ICA
—> ***Superior hypophyseal arteries (L+R)
—> Primary capillary plexus in Median eminence and Infundibulum
—> Hypophyseal portal veins
—> Secondary capillary plexus in Pars Distalis
—> Hypophyseal veins
Summary: Hormone production in 3 sites of Hypothalamo-Hypophyseal system
- Neuropeptides by neurons in Supraoptic + Paraventricular nuclei (Hypothalamus)
—> transported along axons
—> accumulated at end of axons in Neurohypophysis (Posterior pituitary)
—> released in blood capillaries
—> systemic circulation - Neuropeptides by neurons in DM + VM + Infundibular nuclei + Paraventricular nuclei (Hypothalamus)
—> transported along axons
—> ending in Median eminence (stored and secreted)
—> enter Primary capillary plexus of Median eminence
—> transported to Adenohypophysis (Anterior pituitary) via Hypophyseal portal system
—> control secretion from Anterior pituitary - Proteins, Glycoproteins in Pars Distalis
—> released into Secondary capillary plexus of Hypophyseal portal system
—> distributed to general circulation
