Lab 3 - Thyroid, parathyroid and pituitary glands Flashcards
What parts make up the pituitary gland?
- adenohypophysis
- neurohypophysis
How does the adenohypophysis develop?
from a dorsal outgrowth from the wall of the embryonic pharynx
How does the neurohypophysis develop?
as a ventrally orientated down growth from the
diencephalon
How can the adenohypophysis and the neurohypophysis be distinguished histologically
H&E stained section
- the adenohypophysis is more darkly stained.
What surrounds the pituitary gland?
thin fibro-elastic capsule
What tissue does the capsule of the pituitary gland develop from?
meninges - continuous with dura
What makes up the adenopophysis?
unequal populations of cells forming irregular cords of secretory cells
Includes:
- chromophobes
- chromophils
Describe the staining of the cells within the adenopophysis
H& E stained section
chromophobes are palely stained –
phobos =hating
chromophils are strongly stained- chromophil =
colour loving
What is the difference functionally between chromophobes and chromophils?
Chromophobes are exhausted secretory cells, chromophils are active
secretory cells.
What are the 5 distinct cell types found in the adenohypophysis
- somatotrophs
- lactotrophs
- corticotrophs
- thyrotrophs
- gonadotrophs
What do somatotrophs secrete?
Growth hormone (GH)
What do lactotrophs secrete
prolactin (PL)
What do corticotrophs secrete?
- adenocorticotrophic hormone (ACTH)
- beta-lipotropin (beta-LPH)
What do thyrotrophs secrete
thyroid stimulating hormone (TSH)
What do gonadotrophs secrete?
- follicle stimulating hormone (FSH)
- luteinising hormone (LH).
What is the target organ for somatotropin?
- Body tissues
- hepatocytes
What is the target organ for prolactin?
Breast
What is the target organ for TSH?
Thyroid gland - thyroid follicular cells
What is the target organ for FSH/LH
Gonads
What is the target organ for ACTH/lipotropin/melanocyte stimulating hormone
Adrenal cortex
What effect does somatotrophin have?
- Metabolic actions - growth
- Increased protein synthesis
- Increased growth of cartilage
- Increased fatty acid production
- Increased insulin resistance
What effect does prolactin have?
- Lactogenesis
- renal sodium/water reabsorption
- steroidogenesis
- stimulates T cells
What effect does thyroid stimulating hormone have?
- Thyroxine (T4) minimal effect
- Triiodothyronine (T3) – increased metabolism, growth and development, catecholamine effect
What effect does FSH/LH have?
- Early growth of follicle
- Maturation of follicle
- oestrogen secretion
- formation of corpus luteum
- Stimulates seminiferous epithelium
- Leydig cells — testosterone
What effect does ACTH/lipotropin/melanocyte stimulating hormone have?
Synthesis and release of cortisol principally from zona fasciculata
What is the basic chemistry of somatotrophin
protein
What is the basic chemistry of prolactin
protein
What is the basic chemistry of thyroid stimulating hormone
glycoprotein
What is the basic chemistry of FSH
glycoprotein
What is the basic chemistry of LH
glycoprotein
What is the basic chemistry of ACTH?
a polypeptide
What is the basic chemistry of lipotropin?
a polypeptide
What is the basic chemistry of melanocyte stimulating hormone
a polypeptide
Which cells in the pituitary gland are acidophils?
- somatotrophs
- lactotrophs
Which cells in the pituitary gland are basophils
- thyrotrophs
- gonadotrophs
- corticostrophs
What is the importance of fenestrated capillaries in the adenohypophysis?
facilitates rapid passage of secretory hormones to their target organs
How does the thyroid gland develop?
as an endodermal downgrowth (the ‘thyroglossal duct’) from the
floor of the developing pharynx
What landmark is the site of downgrowth
The foramen caecum at the back of the tongue marks the
site of the downgrowth.
Are there any remnants of downgrowth?
Cyst-like remnants of the original downgrowth may persist in the
adult.
Describe the gross structure of the thyroid
two lobes connected by a midline isthmus
What are the thyroid lobes made of histologically?
follicles – flask-shaped structural units that are epithelial structures
Describe the epithelium of inactive follicles in the thyroid gland
simple low cuboidal or squamous
epithelium
Describe the epithelium of active follicles in the thyroid gland
the cells become columnar when they are actively
secreting.
What is the function of thyroid epithelium?
concentrate iodide from the blood and synthesize two hormones: thyroxine (T4) and tri-iodothyronine (T3).
What hormone regulates the activity of the thyroid gland and where is it produced?
THYROID STIMULATING HORMONE FROM ADENOHYPOPHYSIS
What stages are required for thyroid hormone synthesis?
exocrine and an endocrine stage
What is the exocrine stage of thyroid hormone synthesis?
Firstly, colloid is produced. This iodinated glycoprotein, rich in thyroglobulin, is a storage form of thyroxine and is secreted by exocrine secretion to the follicle lumen.
Which cell organelle produces the protein part of thyroglobulin?
Rough endoplasmic reticulum
Where is the sugar component of thyroglobulin added?
Golgi apparatus
Which enzyme enables iodide to be converted to iodine?
Thyroid peroxidase
Which cell synthesises thyroid peroxidase?
the follicle cell
How is thyroglobulin re-uptaken by the follicle epithelium?
involves pseudopodial extensions of cytoplasm which can enclose droplets of thyroglobulin and bring them into the cell when
thyroxine and tri-iodothyronine are required.
Describe the endocrine stage of thyroid hormone synthesis
storage form is broken down by lysosomes;
T4 and T3 are released across the basal lamina of the epithelium and enter capillaries – i.e.
by endocrine secretion.
What antibodies are produced in Hashimoto’s?
because antibodies to thyroid
peroxidase (antimicrosomal antibodies) are produced
What connective tissue changes occur in Hashimoto’s?
prominent fibrosis
What are the follicle changes in Hashimoto’s?
few normal follicles
What is the colloid and where is it found?
The colloid within the follicles is an inactive, storage form of the thyroid hormones T3 and T4.
What is the major component of the colloid
thyroglobulin, an iodinated glycoprotein
Define glycoprotein
any of a class of proteins which have carbohydrate groups attached to the polypeptide chain.
What is the epithelium lining the follicles called?
the follicular epithelium.
What are the three hormones produced and secreted by the follicular epithelium, and which cells are they secreted by?
triiodothyronine (T3),
tetraiodothyronine (T4) (thyroxine) released by follicular cells and calcitonin released by C
cells.
Describe the structure of T3 and T4
T3 and T4 hormones are iodinated derivatives of tyrosine.
What controls the synthesis and breakdown of thyroglobulin
controlled by the hypothalamus and
adenohypophysis through the action of TSH (thyrotropinin).
How does TSH work?
TSH reaching the thyroid gland via the circulation binds to its receptor (Thyrotropin receptor (TSHr), a glycoprotein hormone
receptor) stimulates the cAMP pathway and results in hormone synthesis, differentiation and enhanced growth.
What happens when there is low levels of thyroxine in the blood?
stimulate the hypothalamus to produce TRH
thyrotopin-releasing hormone
How does TRH work?
TRH stimulates the adenohypophysis to produce thyroid stimulating hormone (TSH).
What happens when TSH receptor is activated?
Activation of TSHr stimulates thyroid follicular cells to synthesise and breakdown thyroglobulin from thyroid follicles and result in an increased release of thyroxine into the thyroid capillaries.
What are the functions of T3 and T4?
regulate cell and tissue basal metabolism, may stimulate production of heat and influence body growth and development.
What is a sign of low calcium levels in the blood?
twitching of muscles of face and arms and results in an
increased level of phosphate in the blood (normal range 0.8-1.4 mmol/L).
What are signs of hyperphosphataemia?
severe itching of the skin and weakening of bones.
What can result from hypophosphataemia?
linked to increased insulin
resistance.
What hormone do C cells synthesise and release?
Calcitonin
What is the function of calcitonin
lowers serum calcium levels - inhibits osteoclast activity and reduces renal calcium reabsorption
What is the functional significance of the close packing of the small tubes and the secretory
epithelium of the follicles?
facilitates endocrine secretion of hormone directly into the blood stream
What produces parathyroid hormone?
the four parathyroid glands which are located
posterior to the thyroid gland or sometimes embedded within the thyroid gland
What kind of cells are found in the parathyroid glands?
- chief cells (basophilic)
- oxyphil cells (acidophilic)
Does a parathyroid gland possess a CT capsule?
variable - it is often within the thyroid capsule. If it is found at a separate site, a fine fibrous capsule is usually seen
