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Chapter 136

The thymus gland is one of the more common structures in the anterior mediastinum that requires surgical extirpation. The common indications for thymectomy are thymic neoplasm and treatment of the autoimmune disorder myasthenia gravis (MG). Thymus removal should be a safe, straightforward procedure. The key elements in successful and complete thymectomy depend on a comprehensive knowledge of the anatomic and embryologic characteristics of thymic development and the physiology of thymic disease.

The word thymus comes from a Latin derivation of the Greek thymos because of its resemblance to the flowers of the thyme plant. The thymus is a lymphoepithelial organ that is derived embryologically from the third pair of pharyngeal pouches that descend caudally and medially beginning in the seventh week of life (Fig. 136-1). Through a complex series of migrations, these anlagen continue to descend in a caudal and medial direction. The hollow primordia rapidly become solid epithelial bars, and during the eighth week, the caudad ends of the paired components of the thymus fuse together to form what is generally a four-lobed gland that attaches to the anterior pericardium. This attachment enhances the descent of the thymus into the thorax while the cephalic extremes of the organ become attenuated and generally disappear. However, migration can be incomplete, and remnants (or rests) may be deposited at any point along the excursion of the primordia. In addition to being embedded within the thyroid gland or associated with the parathyroid glands, aberrant thymic rests occur independently along the entire path of thymic descent in as many as 20% of humans.1 The lower lobe capsule tends to be less distinct, and thymic corpuscles, as well as abundant lymphocytes, trail off into the surrounding mediastinal fat and nodal tissue. The thymus gland itself can occupy a cervical position, reaching occasionally as far cephalad as the hyoid bone. The caudad extremes of the gland can extend as far downward as the xiphoid process.

###### Figure 136-1.

The thymus and parathyroid glands descend from the third and fourth pharyngeal pouches during embryologic development.

The thyroid, parathyroid, and thymus share a common origin from the primordial pharynx and its pouches. The inferior parathyroid glands are derived from the dorsal wings of the third pair of pharyngeal pouches. As the thymus migrates downward to its definitive position in the anterior mediastinum, the inferior parathyroid glands are pulled down and left behind at the level of the lower poles of the thyroid. The superior parathyroid glands and the ultimobranchial bodies, the source of parafollicular (calcitonin) cells, develop from the fourth pair of pharyngeal pouches, which also can contribute to the development of the thymus. The parathyroid glands are also “parathymic” glands. In autopsy studies, as many as 20% of inferior parathyroid glands invade the thymic capsule in the neck or mediastinum. Although intrathymic parathyroid is well established, there also ...

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