Micro of Endocrine Glands - Thyroid Gland

Question 1

Gross Anatomy of Thyroid Gland

Answer 1

1. Just caudal to larynx
2. Left and Right lobes that are connected by narrow region called the isthmus
3. Isthmus in dog, cat, horse is fibrous connective tissue
4. Isthmus in ruminants is glandular
5. Fibrous capsule covers thyroid gland

Question 2

Embryology of Thyroid Gland

Answer 2

1. Follicular cells - originate from endoderm cells of pharynx at the root of the tongue. Primodrium of cells grow caudally to its permanent location, just caudal to the larynx
   
   • Thyroglossal duct - collection of cells that grows caudallyl
2. Parafollicular cells - dervied from the embryonic neural crest

Question 3

Parenchyma of Thyroid Gland

Answer 3

• Thyroid gland is composed of fluid filled spaces called follicles (NOT drained by duct), hollow space is surrounded by follicular cells
• Follicles
  • Follicular cells –> squamous to columnar
  • Colloid fills the follicles, its major constituent is thryoglobulin
• Parafollicular cells (C cells)
  
  • Between the follicles
  • Cytoplasm stains clear

Question 4

Stroma of thyroid gland

Answer 4

• Reticular fibers
• Perifollicular blood and lymph capillary networks

Question 5

Function of follicular cells

Answer 5

• To produce thyroid hormones –> stored in colloid
  
  • Triiodothyronine (T3)
  • Thyroxine (T4)

Question 6

Effects of T3 and T4

Answer 6

• Increase basal metabolic rate
• Increase CHO absorption from intestine
• Regulate lipid metabolism
• Influence body growth and fetal CNS development

Question 7

Function of parafollicular (C) cells

Answer 7

• Produce calcitonin (-thryocalcitonin)
  
  • Calcitonin - minor regulatory of blood calcium levels
  • Polypeptide
  • Lowers blood calcium by inhibiting osteclastic bone resorption