Third Pharyngeal Pouch

Thyroid Factor

The Natural Thyroid Diet

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The third and fourth pouches are characterized at their distal extremity by a dorsal and a ventral wing (Fig. 15.10). In the fifth week, epithelium of the dorsal wing of the third pouch differentiates into the inferior parathyroid gland, while the ventral wing forms the thymus (Fig. 15.10). Both gland primordia lose their connection with the pharyngeal wall, and the thymus then migrates in a caudal and a medial direction, pulling the inferior parathyroid with it (Fig. 15.11). Although the main portion of the thymus moves rapidly to its final position in the anterior part of the thorax, where it fuses with its counterpart from the opposite side, its tail portion sometimes persists either embedded in the thyroid gland or as isolated thymic nests.

Growth and development of the thymus continue until puberty. In the young child, the thymus occupies considerable space in the thorax and lies behind the sternum and anterior to the pericardium and great vessels. In older

Auditory tube

Superior parathyroid g (from 4th pouch)

Inferior parathyroid (from 3rd pouc

Ultimobranchia

Palatine tonsi

Primitive tympani cavity

External auditory meatus

Auditory tube

Superior parathyroid g (from 4th pouch)

Inferior parathyroid (from 3rd pouc

Ultimobranchia

Palatine tonsi

Primitive tympani cavity

External auditory meatus

Superior Parathyroid Gland

Foregut

Thyroid gland

Figure 15.11 Migration of the thymus, parathyroid glands, and ultimobranchial body. The thyroid gland originates in the midline at the level of the foramen cecum and descends to the level of the first tracheal rings.

Ventral side of pharynx

Foramen cecum

Foregut

Thyroid gland

Figure 15.11 Migration of the thymus, parathyroid glands, and ultimobranchial body. The thyroid gland originates in the midline at the level of the foramen cecum and descends to the level of the first tracheal rings.

persons, it is difficult to recognize, since it is atrophied and replaced by fatty tissue.

The parathyroid tissue of the third pouch finally comes to rest on the dorsal surface of the thyroid gland and forms the inferior parathyroid gland (Fig. 15.11).

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