Thryoid and Parathyroid Histology Flashcards
Th thyroid diverticulum grows inferiorly, often between the skeletal elements of the ____ and _____ pharyngeal arches. Then migrates to a position _____ to upper portion of the developing trachea
second and third
Anterior
The thyroid gland responds to TSH about week ___ in the fetus. The congenital absence of thyroid gland causes irreversible ___________ in the infant
22
neurolgic damage
The thyroid gland consists of ___ lobes connected by a narrow band of thyroid tissue called the _______.
2
ishtmus
The thyroid gland is located below the ________ and the lobes rest on the sides of the ________
larynx
trachea
The thyroid gland is surrounded by what?
double connective tissue capsule
What are located on the posterior surface of the thyroid gland, between or outside the two capsules?
Tho pairs of parathyroid glands
Each thyroid lobe consists of follicles filled with ______
colloid
What are the structural and functional units of the thyroid lobe?
follicles
Describe the thyroid follicle
single layer of epithelium (follicular)
Varies from cuboidal to columnar with activity
Surrounds central lumen of colloid
Thyroglobulin produced in follicular cells
What does the binding of TSH to its receptor in follicle cells of the thyroid gland cause the cell to do?
actively transport aa and iodide ions across their cell membrane from bloodstream into cytosol
The concentration of iodide ions trapped in follicular cells is many times higher than concentration in bloodstream
What happens to iodide ions once trapped in follicular cells?
Move to the lumen that border the colloid where they undergo oxidation
What does the oxidation of iodide ions results in?
two iodide ions results in iodine which passes through the follicle cell membrane into the colloid
Addition of iodine to tyrosine residues of TGB by ________ _________, released into the lumen by exocytosis
tyrosine peroxidase
What is thyroid peroxidase activity and the iodination process inhibited by?
propylthiouracil and methyl mercaptoimidazole (MMI)
used to inhibit the production of thyroid hormones by hyperactive glands
How are thyroid hormones stored extracellularly ?
as TGB
How does endocrine function of the thyroid gland occur?
TSH stimulus causes endocytosis and digestion of colloid
Colloid droplets fuse with lysosomes
Digestive enzymes breakdown TGB, releasing T3, T4 and iodine
T3 and T4 diffuse through membrane into capillary
What is capillary transport of T3 and T4 facilitated by?
thyroxine-binding protein
What thyroid hormone is secreted my by the thyroid?
T4, but T3 is 4x more potent
Active thyroid gland has a _____ edge
scallped
In the bloodstream, ____ of the circulating T3 and T remains unbound
> 1%
What can free T3 and T4 in the bloodstream do?
can cross the lipid bilayer of cell membranes and be taken up by cells
What is the bound T3 and T4 hormones bound to?
99% bound to specialized thyroxine-binding globulins (TBG), to albumin, or to other plasma proteins. The packaging prevents their free diffusion into body cells
What happens when blood levels of T. and T4 begin to decline ?
bound T3 and T4 are released from plasma proteins and readily cross the membrane of target cells
Describe T3 to T
Shorter half-life of 18hrs, more potent, and less abundant than T4
The half-life of T4 is 5-7 days and reps about 90% of secreted thyroid hormones
What are the effects of T3 and T4 release?
increased basal metabolic rate of body cells
Rise in body temperature (calorigenic effect)
Negative feedback: elevated levels inhibit releast of TRH and TSH
What are the functions of thyroid hormones?
Stimulates basic metabolic rate
augments thermogeneisis
augments glucose production
required for normal development of CNS
What are the symptoms of hyperthyroidism?
due to excessive production of TSH, there is increased metabolic rate, weight loss, hyperactivity and heat intolerance
What are the common causes of hyperthyroidism
Excessive stimulation by adenohypophysis
Loss of feedback control by thyroid gland (Graves’ disease)
Ingestion of T4 (used for weight loss)
What is Graves’ Disease caused by ?
autoimmune disorder that produces antibodies to the receptors for TSH on the follicular epithelium
Antibodiess bind to the receptor and chronically stimulate it
Inflammatory response with cytokine production
Too much circulating thryoid hormone
What are the symptoms of Graves’ Disease?
enlargement of thyroid gland (goiter), bulging of the eyes (exophthalmos), tachycardia, warm skin, and fine finger tremors are typical clinical features
What are the Tx for Graves’ disease?
Surgical removal
Radioactive iodine
Post-Tx regimen requires supplementation of thyroid hormones
What is hypothyroidism caused by?
insufficient production of thyroid hormone
Decreased iodine intake, loss of pituitary stimulation, post-therapeutic or destruction of the thyroid by the immune system
What are the symptoms of hypothyroidism ?
low metabolic rate, feeling of being cold, weight gain (some patients)
In adult: manifested by coarse skin with a puffy appearance due to the accumulation of proteoglycans and retention of fluid in the dermis of the skin (myxedema) and muscle
What is Hashimoto’s Disease?
An autoimmune disease associated with hypofunction of the thyroid gland caused by autoantibodies (antimicrosomal antibodies) to thyroid peroxidase and thyroglobulin
What is the Tx for hypothyroidism?
Oral thyroid medication
Describe the size of Goiter
Vary considerably in size, and enlargement may be diffuse throughout the whole gland or irregular and affecting part or all of one lobe
What are many nodules in the thyroid called?
Multinodular goiter
Goiters are either ____ or _______
endemic or sporadic
What is endemic goiter caused by?
iodine deficiency
Pituitary releases more TSH but the gland cannot respond
What is the treatment for goiter?
Surgery
What is congenital hypothyroidism or FKA cretinism?
Condition of severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones
Can be endemic, genetic, or sporadic
What do sporadic and genetic forms of congenital hypothyroidism result from?
abnormal development or function of the fetal thyroid gland. This type has been almost completely eliminated in developed countries with early diagnosis by newborn screening programs followed by lifelong thyroid hormone treatment
Describe the defects that come with congenital hypothyroidism?
poor length growth
Bone maturation and puberty severely delayed
Ovulation impeded and infertility common
Neurological impairment may be mild
Reduced muscle tone and coordination or no muscle tone
The follicular epithelium also contains about 10% of scattered ________.
Parafollicular cells, also called C cells
What are C cells?
derived from neural crests
contain small cytoplasmic granules representing the stored hormone calcitonin
What is the role of calcitonin ?
acts to decrease Ca concentration by inhibiting bone resorption
Binds to receptor on osteoclast
Hypercalcemia - stimulates calcitonin secretion
What does the parathyroid glands originate from?
Interaction of the endoderm of the third and fourth pouch
Describe the formation of the parathyroid glands
The position of the glands reverses during development. The pair of glands which is ultimately inferior develops from the third pouch and the pair of glands which is ultimately superior develops from fourth pouch
What repositions the parathyroid 3 glands to more inferior positions?
Migration of the thymus (also pouch 3)
What two cell populations are supplied by sinusoidal capillaries in the parathyroid glands?
The more numerous chief or principal cell
The oxyphil or acidophilic cell
How are the cells in parathyroid gland arranged?
in cordlike or follicular-like clusters
What do the Chief (principal) cell secrete ?
Parathyroid hormone
What is the function of oxyphil cells?
function unknown, transition chief cell?
What happens when Ca binds to extracellular region of the CaSR in chief cells?
G protein is associated with CaSR and this triggers the release of intracellular signals suppressing the secretion of parathyroid hormone, with the consequent decrease in serum Ca concentration.
What is stimulated when serum Ca concentration decreases?
secretion of parathyroid hormone is stimulated, resulting in an increase in serum Ca
PTH act on _______ to promote osteoclast activity to increase circulating ___ levels
osteoblasts
Ca
PTH acts on ______ to stimulate resorption of Ca
renal tubules
PTH controls rate of Ca uptake in GI tract by regulating production of __________ in kidneys?
vit D
Vitamin D stimulates cells of intestinal mucosa to absorb Ca and synthesizing _________
calbindin
What are the clinical manifestations of Hypoparathyroidism?
Deficiency in secretion of PTH
Blood Ca is low but bone Ca is not released
Ca deficiency results in spontaneous depolarization of neurons and muscle fibers resulting in tetany
What are the clinical manifestations of hyperparathyroidism
High blood Ca levels
Bone loss-result in osteomalacia and osteitis fibrosa cystica
Causes abnormal Ca deposition in arteries and kidneys
What does a deficiency of Vit D cause in children?
rickets: bone remodeling is defective
Ends of bones bulge, and poor calcification of the long bones causes bending
What does Vit D deficiency cause in adults
osteomalacia,
calcification of the bone matrix osteoid is deficient in both conditions
Pain, partial bone fractures, an muscular weaknesses are typical in the adult
Chronic _______ or a congenital disorder-resulting in the lack of _______ can also cause rickets or osteomalacia
renal failure
1a-hydroxylase
How does the thyroid diverticulum form?
the endodermal mass of cells invaginates into the underlying mesoderm, creating the thyroid diverticulum that is positioned between the anterior two-thirds and posterior one-third of the developing tongue
