Histology of endocrine glands Flashcards
What are the three locations of endocrine tissue?
- Major endocrine organs with sole endocrine function e.g. thyroid, adrenal gland
- Endocrine clusters within solid organs e.g. gonads, kidney, pancreas
- Diffuse neuroendocrine system: neuroendocrine glands scattered throughout parenchyma of most organs, acting in a paracrine fashion i.e. locally on the organ they are scattered through e.g. lungs
Define endocrine glands
Endocrine glands are organised clusters of secretory epithelial cells. They:
- synthesise and secrete hormones into interstitium and then into capillaries and general circulation
- are associated with a rich capillary network
- empty inside of body i.e. do not have ducts
Compare and contrast endocrine and exocrine glands
Endocrine glands secrete hormones into the interstitium and blood, from clumps of endocrine cells.
Exocrine glands, by contrast, secrete hormones via ducts into specific spaces e.g. salivary gland secretes saliva into mouth via ducts.
The secretory unit, either an acinus or tubule, is lined by cuboidal cells.
Describe the location and general features of the pituitary gland
Location and general features
The pituitary is located at the base of the brain.
It is connected to the hypothalamus by a stalk or infundibulum.
It is 1 cm in diameter.
It sits in a bony cavity at the base of the skull known as the sella turcica.
It weighs between 600 to 800 mg.
Describe the macroscopic appearance of the pituitary gland and its parts
The pituitary gland can be broadly divided into three parts:
- the anterior pituitary or pars distalis or adenohypophysis
- the posterior pituitary or neurohypophysis or pars nervosa
- the stalk or stem or infundibulum
The anterior pituitary is larger than posterior posterior pituitary.
Different hormones are secreted in different areas of the anterior pituitary.
- central “mucoid” wedge contains basophils
- lateral wedge contains acidophils
- note also: dispersed chromophobe cells
Note also: pars intermedia (of anterior pituitary) which consists of small cystic structures lined by a cuboidal epithelium, known as Rathke’s pouch remnants.
Distinguish between the anterior and posterior pituitary
The anterior is glandular, larger and darker staining, as it largely contains epithelial cells.
The posterior is smaller, stains lighter as it mainly consists of largely unmyelinated axons.
The posterior pituitary is contiguous with the infundibulum or ‘stalk’.
Hormones secreted by the anterior pituitary: GH, PRL, ACTH, TRH, FSH and LSH.
Hormones secreted by the posterior pituitary: ADH and oxytocin.
Discuss the cell types of the anterior pituitary
There are three cell types of the anterior pituitary. They appear differently on H and E staining due to their differing affinities, and are named thusly:
- Acidophils: appear pink and secrete GH and PRL
- Basophils: appear purple and secrete ACTH, TRH, FSH and LH
- Chromophobes: stain pale, and contain few cytoplasmic granules. May also contain resting cells, or degranulated cells.
Note that individual hormone secreting cells cannot be distinguished by typical H&E staining,
Instead specific cell types are identified by immunohistochemical staining.
Secretory granules can be identified on EM
Note also that the distribution of cell types in the anterior pituitary is not even:
- somatotrophs comprise 50%
- lactotrophs: 15-20%
- corticotrophs: 15-20%
- gonadotrophs: 10%
- thyrotrophs: 5%
Describe the posterior pituitary and its components
The posterior is smaller, stains lighter as it mainly consists of largely unmyelinated axons.
The posterior pituitary is contiguous with the infundibulum or ‘stalk’.
The non-myelinated axons arise from neurosecretory cells located in the hypothalamus containing ADH and oxytocin. They appear as pink and fluffy, with no nuclei ^[cell bodies up in hypothalamus].
It also contains pituicytes, seen as nuclei. They serve as the glial cells of the posterior pituitary and support neuronal cells.
Describe the pars tuberalis and pars intermedia
The pars tuberalis surrounds the infundibulum and mainly consist of gonadotrophs.
The pars intermedia is rudimentary in humans and is made up of colloid-containing follicles. It releases MSH.
Describe pituitary diseases
Pituitary adenomas are a common disease of the gland.
Histological sections appear more homogenous, as all cells have arisen from the same progenitor.
Chromatin has a “salt and pepper” appearance.
It is also has a granular appearance due to the presence of secretory granules.
Describe the location, macroscopic and microscopic appearance of the thyroid
Location
The thyroid gland is located anterior to the upper part of the trachea.
Macroscopic appearance
The thyroid gland is butterfly shaped.
It weighs about 20g normally.
It produces T3, T4 and calcitonin.
Microscopic appearance
At low power: circular structures can be seen. These are follicles.
Follicles are filled with uniformly pink staining substance. This is colloid. Colloid contains thyroglobulin - which is the storage form of T3/T4.
At high power: a layer of thyroid epithelial cells can be seen lining each follicle.
In the interstitium are parafollicular or C cells - which cannot be seen on H and E and require IHC to be visualised. They secrete calcitonin.
Discuss the function of the thyroid gland
Thyroid hormones T3 and T4 are produced from the follicles.
TSH stimulates thyroid epithelial cells, which take up colloid, leaving reabsorption droplets behind in colloid.
The epithelial cell cleaves T3 and T4 off thyroglobulin, and thus T3 and T4 are secreted. ^[can tell how active follicle is by number of reabsorption droplets on H&E]
T3 and T4 serve to increase the basal metabolic rate.
Parafollicular cells secrete calcitonin in response to increases in serum calcium concentration.
Together with PTH it regulates blood calcium levels, by inhibiting the osteoclastic resorption of bone to decrease serum calcium levels.
Describe thryoid diseases
Follicular adenoma.
Well circumscribed, encapsulated lesion.
Describe the location, microscopy and macroscopy of parathyroid glands
Location
Typically 4 glands located on the posterior aspect of thyroid gland.
Note that there can be more than four spread in the mediastinum as low as the heart – which can be hard to find in removal surgery.
Macroscopy
- About 6 mm diameter
- 0.2g in females, 0.3 g in males
Microscopy
At low power, endocrine epithelial cells and adipose tissue can be seen. It is encased by a thin capsule ^[comp?]. Endocrine cells are scattered in clusters throughout adipose.
Endocrine epithelial cells can be divided into active chief cells and inactive oxyphilic cells.
Chief cells are the most common cell type in the PT gland. The nuclei are round and centrally located.
The cytoplasm is homogeneous.
The chief cells are arranged in cords or plates, and surrounded by capillaries.
The function of chief cells is to secrete PTH.
Oxyphil cells present as groups or nests of cells.
They are larger than chief cells, and have an eosinophilic cytoplasm.
The function of oxyphils is not known.
Describe chief cells and oxyphil cells
Chief cells are the most common cell type in the PT gland. The nuclei are round and centrally located.
The cytoplasm is homogeneous.
The chief cells are arranged in cords or plates, and surrounded by capillaries.
The function of chief cells is to secrete PTH.
Oxyphil cells present as groups or nests of cells.
They are larger than chief cells, and have an eosinophilic cytoplasm.
The function of oxyphils is not known.
