Thyroid, Parathyroid, Pancreas Dr. Al Lecture 30
Terms in this set (51)
What are the functions of the epithelial lining of the thyroid follicle?
- They store thyroglobulin
What is a parafollicular cell?
What hormone is released by this cell?
- AKA: C cell
- Comes from the 4th pharyngeal pouch when the ultimo pharyngeal body fuses with the thyroid gland.
3rd Pharyngeal Pouch
- Inferior parathyroid gland
- Caudal parathyroid glands
- thymus, descends into the mediastinum
4th Pharyngeal Pouch
- Cranial parathyroid gland
- C cells (parafollicular cells) differentiate from neural crest cells in this pouch
What is the 1st endocrine gland to develop in the embryo?
What structure provides the thyroid primordial cells?
- The pharynx floor
- The thyroid primordiums then descend into the neck
The primordial thyroid gland is connected to the tongue via the
- Thyroglossal duct
- Connects the primordial thyroid gland to the tongue.
- It normally disappears
- The opening of the thyroglossal duct normally persists though as the foramen cecum in the tongue
- The ducts remnants form cysts and sinuses in the tongue and in the neck (inferior to the hyoid bone). Cysts may contain thyroid tissue.
Remnants of the thyroglossal duct form:
1. The duct opening persists as the foramen cecum in the tongue.
2. Cysts and sinuses in the tongue
3. Cysts and sinuses in the neck (inferior to the hyoid bone)
- These cysts may contain some thyroid tissue
Ultimopharyngeal body (pouch IV)
The ultimo pharyngeal body fuses with the thyroid which gives rise to parafollicular cells (C cells).
Ectopic thyroid gland
AKA: Accessory thyroid gland
- Extra thyroid tissue left behind in the path of thyroid descent.
- Lingual thyroid tissue= most common
- Sublingual thryroid gland
- Thyroid tissue may appear in the thymus (may be functional based on its size)
- Accessory thyroid tissue comes from the thyroglassal ducts.
- Agenesis of thyroid gland
- Absence of thyroid gland lobes or just one lobe.
How many lobes does a thyroid gland have? Where are they located?
How are they connected?
- 2 lobes
- Lateral to the trachea, by the larynx
- Connected by an isthmus in cattle (and sometimes in dogs and cats)
What is the origin of the thyroid gland tissue?
- Parenchyma= Endoderm
- Stroma= Mesoderm
What hormones does the thyroid gland secrete?
1. Thyroxin (T4)
2. Tri-iodothyronine (T3)
3. Calcitonin (secreted by the c cell/ parafollicular cell)
What controls T4 and T3 secretions?
(Thyroid Stimulating Hormone)
- Released by C cells (Parafollicular cells) in the thyroid gland.
- Decreases blood calcium level
- Facilitates calcium uptake/storage in bones
- Inhibits bone resorption by osteoclasts to lower the blood calcium level (inhibits osteoclasts from breaking down bone/freeing up calcium)
The thyroid capsule is derived from ----?
Deep cervical fascia
Capsular septa in the thyroid gland:
- Subdivide the gland into lobules
- Cary blood, lymph and nerve supply
Basement membrane of the thyroid gland separates 3 things:
1. Blood vessels
3. Parafollicular cells (C cells)
What is embedded in the capsule of the thyroid gland?
What is the thyroid glands functional unit?
- Lumen stores secretory substance
- Lined by simple cuboidal epithelium
- Store colloid as thyroglobulin (which has several weeks supply of hormone)
- T3 and T4 hormones bound to glycoproteins are stored as thyroglobulin (not functional)
AKA: Principle cells
- Columnar, cuboidal or squamous (functional state)
- Round to oval nucleus with nucleoli
- Underlined by a basement membrane
- Apical microvilli dip into the colloid they surround and store.
- Contain vesicle which contain thyroglobulin
- In hormone demand, these cells extend their psuedopods to absorb colloidal substance inside the follicle to process it into thyroxin (T4).
- A glycoprotein (colloid) stored in thyroid follicles.
- Essential for the synthesis of the thyroid hormones (T3 and T4)
- T3 is the active form of the hormone
AKA: C Cells, Clear Cells
- They DO NOT reach the follicle lumen
- They migrate from the neural crest cells (4th pouch)
- Lie singly or in clusters next to the follicular cells
- Largeer than follicular cells
- Round nucleus, they're APUD cells
- They make Calcitonin (thyrocalcitonin).
The synthesis of thyroid hormones is regulated y 2 things:
1. Iodine blood levels
Synthesis of thyroid hormones.
1. Thyroglobulin is made in the RER and golgi
2. The glycoprotein is then packaged in the trans golgi network
3. Vesicles that contain the thyroglobulin move apically where their contents are released into the colloid
4. In the colloid, it is iodinated thyroglobulin
5. TSH is released from pars distalis basophils
6. TSH binds TSH receptors on the follicular cells
7. Endocytosis of colloid
8. Vesicles that contain colloid fuse with endosomes and iodinated residues are cleaved from thyroglobulin
9. Transferred into the cytosol as free T3 and T4
10. T3 and T4 are released from the follicular cell basal plasmalemma into the connective tissue of the thyroid gland and then diffuse into the bloodstream
- An enlarged thyroid gland caused by an individual not getting enough iodine in their diet.
- Condition is treated with dietary iodine (eating more iodine)
Hypothyroidism is characterized by:
1. Fatique, long sleeping hours
2. Muscular sluggishness
3. Slow HR
4. Decreased cardiac output
5. Decreased blood volume
7. Loss of hair growth
Cretinism is due to---.
extreme hypothyroidism in fetal life
- Increased thyroid hormone production
- Decreased body weight
- Increased HR. metabolism, respiration, appetite
Excessive thyroid hormone production causes:
- Excessive menstrual bleeding
How many parathyroid glands are there?
What is their origin?
- 4 small ovoid structures
- Origin: Endoderm
They are embedded in the thyroid gland capsule on the cranial and caudal poles.
Each parathyroid gland has its own connective tissue capsule.
Parenchyma of the parathyroid glands:
2 cell types:
- Consists of cords or clusters of epithelial cells.
- 2 cell types:
1. Chief cells
2. Oxyphilic cells
- Principal cells in the parathyroid glands
- They make PTH (Parathyroid hormone)
- In the parenchyma of the parathyroid glands
- Present in horse, man and cow.
- It is the inactive phase of a chief cell
- Unknown function
- Dark eosinophilic cytoplasm
Parathyroid Hormone (PTH)
- Made by chief cells
- Maintain proper concentration of calcium (increases blood calcium levels when they're low)
- Opposite effect of calcitonin
- Bind receptors on osteoblasts (bone forming cells) to increase the secretion of osteoclast- stimulating factor (to make osteoclasts break down bone and release more calcium into the blood stream)
- In the kidneys, PTH prevents calcium loss in the urine
Islet of Langerhans
- Regions of the pancreas that contain endocrine (hormone-producing) cells.
- These regions are scattered between the pancreatic acin
- Very vascularized regions. Each islet is surrounded by a capillary network.
- A group of 3,000 endocrine cells
- One million islets are scattered throughout the pancreas
The pancreas produces which 2 hormones?
- Produced by the pancreas and released into the intercellular spaces when blood glucose levels increase (like after consuming carbohydrate-rich food)
- Insulin binds to receptors on the plasma membrane of skeletal muscle, liver and adipose cells.
- Activates these cells to take up glucose to decrease the blood glucose levels.
5 cells types in the Islets of Langerhans
4. PP cell
5. G cell
- Polygonal in shape
- Blue cytoplasmic granules
- Makes and stores insulin
- Makes up 98% of sheep Islet cells
- Polygonal in shape (like the beta cells)
- Red cytoplasmic granules
- Makes and stores Glucagon
- Makes and stores somatostatin (SST)
Can routine histological stains differentiate between various Islet cells?
- Cell differentiation is done by immunocytochemical procedures.
- Electron microscope can identify cell by size and density of their granules
- Produced by alpha cells in the Islet of Langerhans in the pancreas
- Released when blood glucose levels are low
- Acts on hepatocytes and causes their enzymes to break down glycogen (into glucose) which is then released into the blood stream and raises blood glucose levels
- Made by delta cells in the pancreas
- Effects smooth muscle cells of the alimentary tract and gallbladder by reducing the motility of these organs
- It inhibits alpha and beta cells from releasing hormones (glucagon and insulin)
- It is released in response to increased levels of blood glucose after a meal.
- Released by G cells in the pancreas
- Stimulates gastric release of HCl
Diabetes Mellitus is ---- disorder that results from ----.
Diabetes Mellitus is a hyperglycemic metabolic disorder that results from :
1. Lack of insulin production by beta cels
2. Defective insulin receptors on target cells
2 Major forms of Diabetes Mellitus
1. Type I
2. Type II (non-insulin dependent)- most common type and affects people over 40 years old.
When untreated, both types of Diabetes Mellitus show these 5 symptoms/consequences:
1. Circulatory disorders
2. Renal failure
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