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How does the anterior pituitary develop?
- Formed by the invagination of the oral ectoderm in the oral cavity during development
- pinches off to form 'Rathke's pouch'
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What is the structure of the anterior pituitary gland?
- aka pars distalis
- non-neural epithelial tissue
- Made up of cords of hormone-secreting (endocrine) epithelial cells that are differentially stained by H&E
- Chromophilic cells and chromophobic cells
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Compare chromophilic and chromophobic cells in the anterior pituitary.
- Chromophobic: stain weakly because they contain few secretory vesicles/granules
- function is unclear, but guessed to be supportive
- capable of secreting paracrine factors
- ie. Folliculostellate cells
- Chromophilic: stain intensely because they contain a high number of secretory vesicles/granules that contain hormones
- acidophils- vesicles bind eosin and appear pink
- basophils- vesicles bind hematoxylin and appear blue
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How does the posterior pituitary develop?
Formed from the ventral extension of the brain - 'neurohyprophyseal bud'
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What is the structure of the posterior pituitary?
- aka pars nervosa
- neuro-ectoderm derived therefore neural tissue
- contains pituicytes as well as neural tissue
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What are pituicytes?
- Cells in the posterior pituitary which are neuroglial in nature.
- They provide physical and nutritive support.
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Describe the neural tissue of the posterior pituitary.
- Terminal neural swellings ('Herring bodies') derived from axons whose nerve cell bodies originate in the nuclei of the hypothalamus.
- Herring bodies are packed with secretory vesicles for the storage and subsequent secretion of peptide hormones (AVP and OT) - hormones are secreted via vesicle fusion
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What is the minor intermediate lobe of the pituitary, and how is it developed?
- same embryonic derivation as anterior pituitary
- rudimentary structure in humans- only <1% of pituitary mass in adults, but larger during fetal stage (~3.5%) and in larger vertebrates
- Secretes melanocyte-stimulating hormone (MSH)
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What is the sella turcica?
A small bony depression under the base off the brain near the optic nerves in which the pituitary sits.
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What is Pituitary Stalk Interruption Syndrome?
- Described in children with growth failure and pituitary hormone deficiencies
- Associated with truncated or absent pituitary stalk and a small sella turcica
- Decreased LH/FSH, GH, and testosterone
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Name the five major types of chromophilic cells in the anterior pituitary, their product, and whether they are acidophilic or basophilic.
- Somatotrope- produce GH - acidophilic
- Lactotrope - produce PRL - acidophilic
- Thyrotrope - produce TSH - basophilic
- Corticotrope - produce ACTH - basophilic
- Gonadotrope - produce FSH/LH - basophilic
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What method(s) must be used in order to determine what is the exact hormonal product of a particular chromophilic cell?
Immunocytochemistry and electron microscopy
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How does the thyroid gland develop?
Derived from endoderm as a downgrowth of the pharynx
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Describe the structure of the thyroid gland.
- consists of functionally distinct units with different modes of secretion and secretion products
- follicular units and parafollicular cell (C cell) clusters
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What is a follicular unit?
- made up of follicular epithelial cells that synthesize, store, secrete, and modify thyroid hormones
- respond to TSH produced in the anterior pituitary
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What is a parafollicular cell cluster?
- small clusters of endocrine epithelial cells located in the loose CT between follicles
- Responds to high plasma Ca levels by secreting calcitonin which lowers plasma Ca levels
- cells release calcitonin basally into the CT. Calcitonin is then picked up by fenestrated capillaries and enters blood stream to be taken to distant sites
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What is calcitonin?
Calcitonin is a peptide hormone that inhibits the release of calcium ions from bone by decreasing osteoclast activity and decreasing calcium absorption in the intestinal epithelium. It effectively lowers plasma Ca levels
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Describe the structure/development of the parathyroid glands.
- 4 glands
- endodermally derived
- buried in the posterior aspect of the thyroid gland, but histologically and functionally distinct from it
- Consists of two cells types haphazardly arranged in clumps and cords - oxyphil cells and chief cells
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What are oxyphil cells?
Inactive, non-secretory cells of the parathyroid glands
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What are Chief cells?
- Secretory cells of the parathyroid glands
- Chromophilic due to secretory vesicles
- Responds to decreases in plasma Ca levels
- Secretes PTH (parathyroid hormone) to increase plasma calcium levels
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What is the function of PTH?
- It is a peptide hormone that acts on target organs.
- In bone- increases the activity of osteoclasts to liberate calcium from the inorganic portion of bone matrix
- In intestines- increases Ca absorption in intestinal epithelium
- In kidneys- increases Ca resorption
- Overall- increases plasma Ca levels
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