Embryology

Question 1

Question

Answer 1

E. Third Pharyngeal pouch

A 22q11.2 microdeletion underlies the spectrum of physical features seen in DiGeorge syndrome (DGS). Embryologically, the neural crest fails to migrate into the derivatives of the third (affects inferior parathyroid and thymus) and fourth (affects superior parathyroid) pharyngeal/branchial pouches. The consequent parathyroid and thymic hypoplasia results in hypocalcemia and T cell deficiency, respectively.

The hypocalcemia in DGS leads to increased neuromuscular excitability, which manifests as tetany, carpopedal spasms, or seizures. Tapping on the facial nerve usually elicits twitching of the nose and lips (Chvostek sign), and inflation of the blood pressure cuff leads to carpal spasm (Trousseau sign). Normally, the thymic shadow is visible on a newborn’s x-ray. Its absence reflects thymic hypoplasia, which leads to T cell dysfunction and results in recurrent viral, fungal, and protozoan infections.

Conotruncal cardiac anomalies (eg, interrupted aortic arch, truncus arteriosus) may also result from failure of neural crest migration. If there is additional involvement of the first and second pharyngeal/branchial pouches, patients with DGS may have features such as hypertelorism, short palpebral fissures, micrognathia, bifid uvula, and cleft palate.

(Choice A) Failure of the anterior neuropore to close leads to neural tube defects such as anencephaly. Neural tube defects can be prevented with maternal folic acid supplementation.

(Choice B) The foramen cecum is a depression on the tongue that represents the embryological remnant of the superior end of the obliterated thyroglossal duct. A persistent duct results in a thyroglossal cyst that presents as a midline neck mass.

(Choice C) Rathke pouch represents an invaginated oral ectoderm that eventually develops into the anterior pituitary. Craniopharyngiomas arise from remnants of this pouch and can result in hypopituitarism, hydrocephalus, and diabetes insipidus, but not hypocalcemia.

(Choice D) The second branchial cleft (ie, second pharyngeal groove) is normally obliterated in utero. Persistence leads to a branchial cleft cyst between the angle of the mandible and sternocleidomastoid muscle.

Educational objective:
DiGeorge syndrome results from maldevelopment of the third (inferior parathyroid and thymus) and fourth (superior parathyroid) pharyngeal/branchial pouches. Subsequent parathyroid and thymic hypoplasia results in hypocalcemia and T cell deficiency.

Question 2

Rathke pouch deriveatives

Answer 2

Rathke pouch represents an invaginated oral ectoderm that eventually develops into the anterior pituitary. Craniopharyngiomas arise from remnants of this pouch and can result in hypopituitarism, hydrocephalus, and diabetes insipidus, but not hypocalcemia.

Question 3

Foramen Cecum

Answer 3

The foramen cecum is a depression on the tongue that represents the embryological remnant of the superior end of the obliterated thyroglossal duct. A persistent duct results in a thyroglossal cyst that presents as a midline neck mass.

Question 4

The second branchial cleft derivitives

Answer 4

The second branchial cleft (ie, second pharyngeal groove) is normally obliterated in utero. Persistence leads to a branchial cleft cyst between the angle of the mandible and sternocleidomastoid muscle.

Question 5

1st pharyngial pouch derivitives

Answer 5

primary tympanic cavity

auditory tube

Ext. aduitory meatus

Question 6

2nd pharyngial pouch derivitives

Answer 6

Palatine tonsil

Question 7

3rd pharyngial pouch derivitives

Answer 7

Parathyroid gland inferior

thymus

Question 8

4th pharyngial pouch derivitives

Answer 8

parathyrodoid gland superior

ultimo breanchial body