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'
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
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
How does the posterior pituitary develop?
Formed from the ventral extension of the brain - 'neurohyprophyseal bud'
What is the structure of the posterior pituitary?
- aka pars nervosa
- neuro-ectoderm derived therefore neural tissue
- contains pituicytes as well as neural tissue
What are pituicytes?
- Cells in the posterior pituitary which are neuroglial in nature.
- They provide physical and nutritive support.
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
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)
What is the sella turcica?
A small bony depression under the base off the brain near the optic nerves in which the pituitary sits.
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
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
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
How does the thyroid gland develop?
Derived from endoderm as a downgrowth of the pharynx
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
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
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
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
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
What are oxyphil cells?
Inactive, non-secretory cells of the parathyroid glands
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
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