Thymic aplasia is also called DiGeorge syndrome, named after the pediatric endocrinologist Angelo DiGeorge.
This disease is caused by a 22q11 deletion on chromosome 22, which is thought to cause defects in the migration of neural crest cells.
The 22q11 deletion is thought to affect the migration of neural crest cells, particularly affecting the development of the third and fourth pharyngeal pouches. The third pouch develops into the inferior parathyroids and thymus, while the fourth pouch develops into the superior parathyroids.
Due to abnormal development of the third and fourth pharyngeal pouches, the thymus fails to develop properly. The thymus gland is an organ in the mediastinum that plays an important role in the differentiation and induction of tolerance in T cells.
Because the thymus plays an important role in the differentiation and induction of tolerance in T cells, thymic aplasia causes a severe T cell deficiency.
T cell deficiency caused by thymic aplasia makes individuals susceptible to recurrent viral and fungal infections due to poor cellular immunity.
The third pharyngeal pouch develops into the inferior parathyroids while the fourth develops into the superior parathyroids. Failure of these pouches to develop properly leads to improper development of the parathyroids, leading to hypocalcemia.
Hypocalcemia is characterized by low calcium levels in the serum and is caused by hypoparathyroidism, which plays an essential role in calcium homeostasis.
This is a medical sign consisting of the involuntary contraction of muscles, typically seen in the hands. Low blood calcium levels increase the permeability of neuronal membranes to sodium ions that cause a progressive depolarization, which leads to increased possibility of action potentials.
Improper neural crest migration also causes aortic arch defects in many individuals, because neural crest migration is responsible for the truncal and bulbar ridges that spiral and fuse to form the aorticopulmonary septum. Common defects include an interrupted tetratology of Fallot, interrupted aortic arch, and persistent truncus arteriosus.
Improper neural crest migration also causes congenital heart disease in about 40% of individuals with DiGeorge syndrome. Common defects include tetralogy of Fallot and ventricular septal defects.
On chest X-ray, patients with DiGeorge syndrome will have an absent thymic shadow due to thymic aplasia.
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