A 13-year-old boy is brought to the clinic due to progressive difficulty with breathing, hoarseness, and dysphagia. The symptoms began insidiously 6 months ago but have rapidly become worse in the last 4 weeks. The patient has no associated skin rash, pharyngeal pain, fever, weight loss, or loss of appetite. Medical history is unremarkable and the patient takes no medications. He is up to date on scheduled vaccinations. The patient's parents and siblings have no history of significant health problems. Vital signs are normal, and he is at 70th percentile for height and weight. Oropharyngeal examination reveals a large, red lingual mass. The patient subsequently undergoes excision of the mass; histopathologic preparation of the recovered tissue is shown in the image below.
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Failure of which of the following embryologic processes is most likely responsible for this patient's lesion?
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This patient's pathologic specimen shows normal thyroid follicles containing colloid, consistent with a lingual thyroid. The thyroid gland is formed from an outpouching (evagination) of the pharyngeal epithelium and subsequently descends to the lower neck anterior to the upper trachea and larynx. The lowest part of the evagination forms the thyroid gland and the remaining portion forms the thyroglossal duct, which extends from the foramen cecum on the dorsal surface of the tongue to the superior border of the thyroid isthmus.
Due to failure of migration, the thyroid can reside anywhere along the thyroglossal duct's usual path, including the tongue (lingual thyroid). Enlargement of a lingual thyroid can lead to obstructive symptoms (eg, dysphagia, dysphonia, dyspnea), typically during times of heightened thyroid stimulation (eg, puberty, pregnancy). Sometimes, this lingual thyroid is the only thyroid tissue in the body, so hypothyroidism can occur if it is removed.
(Choice A) Apoptosis is the process of programmed cell death and is important in regulating the turnover of cells. Failure of apoptosis during fetal development can result in conditions such as duodenal atresia and syndactyly (ie, fusion of adjacent digits).
(Choice B) Differentiation is the formation of mature cells from undifferentiated precursor cells (eg, formation of red blood cells, white blood cells, and platelets from bone marrow stem cells). Although thyroid cells do differentiate after their descent from the pharynx, defective differentiation would result in impaired thyroid follicle formation.
(Choice C) Fusion is the joining of two opposing tissues to form one continuous structure. Disruption of this process leads to a variety of birth defects, including cleft palate, atrioventricular septal defects, pancreatic divisum, and neural tube defects.
(Choice E) Proliferation denotes an increase in the number of cells via mitosis. Failure of proliferation during fetal development can result in organ hypoplasia (eg, thymus abnormalities in DiGeorge syndrome).
Educational objective:
The thyroid gland is formed from evagination of the pharyngeal epithelium and descends to the lower neck. Due to failure of migration, the thyroid can reside anywhere along the thyroglossal duct's usual path, including the tongue (lingual thyroid).