Microanatomy Of Endocrine System Flashcards
Describe the main histological features, connections and embryological origins of the pituitary gland
Pituitary gland (hypophysis)
- Suspended from the floor of the hypothalamus by the infundibulum.
- Sits in a depression of the sphenoid bone called the sella turcica.
- Composed of the anterior and posterior pituitary.
- The anterior pituitary arises from a pouch that grows upward from the embryonic pharynx. The posterior pituitary arises as a bud growing downward from the brain.
Anterior pituitary (adenohypophysis)
- Constitutes 3/4 of the pituitary.
- Has no nervous connection to the hypothalamus but is linked to it by a complex of blood vessels called the hypophyseal portal system.
- Contains two types of cells called chromophils and and chromophobes.
- The chromophils are divided into two subtypes called acidophils and basophils which are brightly stained, glandular cells in nests surrounded by numerous capillaries.
- Chromophobes are pale staining and quiescent chromophils.
- Chromophils and chromophobes can be classified according to their secretory product which can be determined by immunohistochemicak staining:
Somatotrophs (majority) = growth hormone (GH)
Mammotrophs = prolactin
Corticotrophs = adrenocorticotrophic hormone (ACTH)
Thyrotrophs = thyroid-stimulating hormone (TSH)
Gonadotrophs = follicle-stimulating hormone (FSH) and luteinising hormone (LH)
Posterior pituitary
- Pale staining.
- Composed of unmyelinated axons of neurosecretory cell bodies located in the supraoptic and paraventricular nuclei of the hypothalamus.
- Axons from supraoptic nuclei secrete antidiuretic hormone and axons from from paraventricular nuclei secrete oxytocin.
- Their axons pass down the infundibulum as a bundle called the hypothalamo-hypophyseal tract and end in the posterior lobe where they are stored until a nerve signal coming down the same axons triggers their release.
- Herring bodies present which are dilations of axons for the storage of hormones.
- Support cells present are called pituicytes.
- Highly vascular.
Relate the histological features to the hypothalamo-pituitary neuroendocrine link.
- The posterior pituitary is controlled by the neuroendocrine reflex (release of hormones in response to nerve signals).
For example:
- Dehydration raises the osmolarity of the blood which is detected by hypothalamic neurones called osmoreceptors.
- Osmoreceptors trigger the release of antidiuretic hormone which promotes water conservation.
- In contrast, excessive blood pressure stimulates stretch receptors in the heart and certain arteries.
- By another neuroendocrine reflex, this inhibits ADH release, increases urine output, and brings blood volume and pressure back to normal.
Describe the histological features of the thyroid gland and relate these to its function
- Located in the anterior neck, spanning between the C5 and T1 vertebrae.
- It is divided into two lobes separated by an isthmus.
- Lies sternohyoid and sternothryoid muscles, wrapping around the cricoid cartilage and superior tracheal rings. It is inferior to the thyroid cartilage of the larynx.
- Bilobed - located on anterior and lateral surface of upper trachea.
Histology
- The thyroid is composed mostly of sacs called thyroid follicles. Each is filled with a protein-rich colloid and lined by a simple cuboidal epithelium of follicular cells.
- The follicular cells secrete iodine-containing hormones called triiodothyronine (T3) and thyroxine (T4).
- To secrete the hormone, the hormone is reabsorbed from the cavity, and then released into the surround interstitial spaces.
- Colloid is an inactive precursor of T3 and T4. It is made up of a glycoprotein called thyroglobulin.
- The thyroid gland regulates basal metabolic rate, and it is regulated by the pituitary hormone thyroid stimulating hormone (TSH).
- Also secretes calcitonin which causes blood calcium levels to drop. Its secretion is directly dependent on blood calcium levels.
Describe the histological characteristics of the adrenal glands
- Adrenal gland layers (outermost to innermost): capsule, adrenal cortex, adrenal medulla.
- The adrenal cortex of each adrenal gland consists of three concentric zones:
Superficial = zona glomerulosa (ball of cells) has round clusters of cells producing mineralocorticoids such as aldosterone which stimulates the kidneys to retain sodium. Falling blood pressure leads to increased aldosterone secretion by a mechanism called the
renin-angiotensin-aldosterone system.
Middle = zona fasciculata (bundle of sticks) has elongated strands of cells producing glucocorticoids such as cortisol, which stmimulatefat and protein catabolism, gluconeogenesis, and the release of fatty acids and glucose into the blood. This helps the body adapt to stress and repair damaged tissues. Also have an anti-inflammatory effect.
Innermost = zona reticularis (net) has a network of cells making the weak androgen dehydroepiandrosterone (DHEA) that is converted to testosterone in men and women. - The adrenal medulla contains neural crest-derived chromaffin cells that synthesise either epinephrine or norepinephrine which regulate the stress response.
What is the endocrine portion of the pancreas
Islets of Langerhans
What system does the endocrine portion of the pancreas form part of
gastro-entero-pancreatic (GEP) system
What are the four cell subtypes found in the GEP system and what do each of them secrete
- Alpha cells (glucagon)
- Beta cells (insulin)
- Delta (somatostatin)
- F cells (pancreatic polypeptide)
Outline the GEP system
- Enteroendocrine cells are specialised cells found within the gastrointestinal tract, stomach and pancreas. They produce and release hormones in response to a number of stimuli.
- Hormones may be released directly into the bloodstream to generate systemic effects or may be distributed as local messengers. May also stimulate a nervous response.
- Secrete hormones such as gastrin (stimulates gastric acid release), motilin (controls smooth muscle contraction in upper GI tract), and secretin (regulates water homeostasis).
