Endocrine System Part 2 PDF

Endocrine System (Part 2) Dr. Hayder Naji Sameer Msc in Clinical Pharmacy The Thyroid Gland It is located in the front of the neck, anterior to the larynx and trachea. It releases two hormones: 1. Thyroid hormone (Thyroxin) this hormo...

Endocrine System (Part 2) Dr. Hayder Naji Sameer Msc in Clinical Pharmacy The Thyroid Gland It is located in the front of the neck, anterior to the larynx and trachea. It releases two hormones: 1. Thyroid hormone (Thyroxin) this hormone regulates metabolism, growth and development. 2. Calcitonin hormone lower calcium level in the blood by decrease bone resorption. Thyroid Gland Feedback Mechanism  Thyroid gland Histology Stroma Parenchyma Structure of thyroid gland 1. Stroma A double capsule CT septa which dividing the gland into incomplete lobes and lobules Reticular fibers that form a network supporting the thyroid parenchyma 2. Parnechyma The parenchyma of the thyroid is composed of millions of rounded epithelial structures called follicles. Each follicle consists of a simple epithelium and a central lumen filled with a gelatinous substance called colloid. The thyroid is the only endocrine gland in which a large quantity of secretory product is stored. In humans there is sufficient hormone in follicles to supply the body for up to three months with no additional synthesis. Thyroid colloid contains the large glycoprotein thyroglobulin, the precursor for the active thyroid hormones.  Follicular cells( Principal cells) Follicular cells range in shape from squamous to low columnar. The size and cellular features of follicles vary with their functional activity. Active glands have more follicles of low columnar epithelium; glands with mostly squamous follicular cells are considered hypoactive.  Parafollicular cells Another endocrine cell type, the parafollicular, or clear cell (C cell), is also found inside the basal lamina of the follicular epithelium or as isolated clusters between follicles. Parafollicular cells are somewhat larger than follicular cells and stain less intensely. They have a smaller amount of rough ER, large Golgi complexes, and numerous small granules containing polypeptide hormone. Parafollicular cells These cells synthesize and secrete calcitonin, one function of which is to suppress bone resorption by osteoclasts. Calcitonin secretion is triggered by elevated blood Ca2+ levels  The Parathyroid glands The parathyroids are 4 small glands located behind thyroid gland. Despite their small size, these glands play a crucial role in regulating the body's calcium and phosphorus levels through the secretion of parathyroid hormone (PTH).  Functions of the parathyroid glands 1. Calcium Regulation: The primary function of the parathyroid glands is to regulate calcium levels in the blood. When blood calcium levels drop below normal, the parathyroid glands release parathyroid hormone (PTH) into the bloodstream. 2. Bone Maintenance: PTH acts on the bones to release calcium stored in bone tissue, which helps to maintain normal levels of calcium in the blood. 3. Kidney Function: PTH also acts on the kidneys to increase the reabsorption of calcium from the urine back into the bloodstream. Additionally, PTH stimulates the kidneys to convert vitamin D into its active form, Calcitriol, which helps enhance the absorption of calcium from the intestines. 4. Phosphorus Regulation: In addition to regulating calcium levels, PTH helps to regulate phosphorus levels in the blood. PTH decreases phosphorus reabsorption in the kidneys, leading to increased excretion of phosphorus in the urine. Parathyroid gland Histology Stroma Parenchyma  Parathyroid gland histology 1. Stroma Each gland is surrounded by a thin capsule CT septa Network of reticular fibers that support the parenchyma cells 2. Parenchyma It consists of cords of polygonal cells surrounded by a rich network of fenestrated blood capillaries The cells are of two types: Chief and Oxyphil cells.  Chief cells Function: It secrets of parathyroid hormone Shape: Small polygonal cells Nucleus: Large nuclei Cytoplasm: pale due to glycogen granules and lipid droplets) Cytoplasm contains moderate amount of organelles (rER, Golgi complex, mitochondria and secretory granules).  Oxyphil cells Function: unknown Number: few Shape: large polygonal Nucleus: small and dense Cytoplasm: deep eosinophilic and contain many mitochondria With increasing age: The chief cells are replaced by fat cells which may constitute more than 50% of the gland.  Chief cell Oxyphil cell Size Smaller Larger Number Numerous Few Cytoplasm Pale Deep acidophilic Nucleus Large Small and dense rER More Less Mitochondria Less Numerous Function PTH secretion Unknown Summary Gland Endocrine cells Major Major functions hormones Thyroid Follicular Thyroid Increase metabolic glands cells hormones rate  C cells  Calcitonin  Lowers blood Ca2+ levels by inhibiting osteoclast activity Parathyroid Chief cells Parathyroid Raises blood Ca2+ glands hormone levels by stimulating (PTH) osteoclast activity Thank You