Describe the ultimobranchial glands.

The ultimobranchial glands are small, paired endocrine glands located near the thyroid gland in the neck of vertebrates. These glands are responsible for producing and secreting calcitonin, a hormone that helps regulate calcium levels in the blood. The ultimobranchial glands develop from the embryonic ultimobranchial bodies, which are derived from the fourth pharyngeal pouch during embryonic development. In some species, such as birds and reptiles, the ultimobranchial glands also play a role in the production of thyroid hormones.

Ultimobranchial Glands

Location:

- Located in the posterior pharyngeal region, adjacent to the thyroid gland

Embryology:

- Develop from the sixth branchial pouch
- Fuse and descend caudally to lie adjacent to the thyroid

Structure:

- Composed of small, round to oval glands
- Histologically similar to the C cells of the thyroid gland

Function:

- Calcitonin production: Ultimobranchial C cells secrete calcitonin, a hormone that lowers blood calcium levels
- Calcium homeostasis: Calcitonin acts on osteoclasts to inhibit bone resorption, reducing calcium release into the bloodstream

Clinical Significance:

- C-cell hyperplasia: Enlargement of the ultimobranchial C cells, which can lead to increased calcitonin production and hypocalcemia
- Medullary thyroid carcinoma: A neuroendocrine tumor that originates from the C cells of the thyroid and the ultimobranchial glands