1. THYROID GLAND

2. THYROID GLAND Located in the anterior neck region adjacent to the larynx and trachea Consists of two lateral lobes united by an isthmus Develops from the endodermal lining of the floor of the primitive pharynx Composed of connective tissue septa and follicles

4. Tri – iodothyronine (T3)

7. STROMA A low-power micrograph of thyroid gland shows the thin capsule (C), from which septa (S) with the larger blood vessels, lymphatics, and nerves enter the gland

8. FOLLICULAR EPITHELIUM Surrounds each follicle Composed of reticular fibers and network of capillaries Can be simple squamous, cuboidal, or low columnar epithelium

10. Roughly spherical cyst-like compartment

11. Wall formed by follicular epithelium

12. Central cavity contains a gelatinous substance called colloid

15. FOLLICULAR CELLS Principal cells Most numerous cell type Cuboidal in shape with spherical nucleus Periods of increased activity – cells are more columnar and colloid is less abundant

17. Synthesize, release, and store their product outside their cytoplasm

19. PARAFOLLICULAR CELLS C (clear) cells Larger than follicular cells Contain secretory granules Can occur either alone or in clumps Synthesize and secrete calcitonin

20. PARAFOLLICULAR CELLS RED– Follicle with colloid BLUE – Parafollicular cells PINK – Retracted or distorted colloid

21. FOLLICULAR AND C CELLS C cells may be part of the follicular epithelium or present singly or in groups outside of follicles. Follicular cells (F) can usually be distinguished from C cells (C) by the smaller size and darker staining properties. Unlike follicular cells, C cells seldom vary in their size or pale staining characteristics.

22. FOLLICULAR AND C CELLS

23. FOLLICULAR AND C CELLS

24. ULTRASTRUCTURE (Follicular and C cells)

25. ULTRASTRUCTURE (Follicular and C cells)

27. Tri – iodothyronine (T3)

29. CALCITONIN Lower blood calcium levels through inhibition of bone resorption Secretion is triggered by elevation of blood calcium concentration

30. HORMONES