Parathyroid Embryology and Developmental Abnormalities

The inferior parathyroid glands arise from the dorsal part of the P III. The thymus arises from the ventral portion of the same pouch. This common origin justifies labeling P III and thymus as parathymus. The dorsal part of the PIV gives rise to the superior parathyroids. The fate of the ventral portion of the P IV is little understood in humans. Gilmour4 regarded it as the origin of the thymus IV (rudimentary thymus IV), which rapidly undergoes involution. The fatty lobules sometimes found at the site of the upper parathyroid (P IV) may well constitute the vestigial remnants of this thymus IV.

At the 13- to 14-mm stage, the P HI and P IV migrate together with the thymus and ultimobranchial bodies, respectively. The P III-thymus complex separates from the pharyngeal wall and moves toward the caudal and medial regions. Because of the extension of the cervical spine and the descent of the heart and great vessels, the thymus and the P III are drawn toward the superior mediastinum. At the 20-mm stage, the cephalic regression of the thymus brings about its separation from the P III, which are thus abandoned at the level of the anterior or posterolateral region of the inferior poles of the thyroid lobes or at the level of thyrothymic ligaments, vestigial structures indicative of their former connections. This embryologic migration results in an extensive area of dispersal of the normal P III. In 61% of cases, they are situated at the level of the inferior poles of the thyroid lobes on the posterior, lateral, or anterior aspects. In 26% of cases, they are situated in the thyrothymic ligaments or on the upper cervical portion of the thymus. More rarely, in 7% of cases, they are situated higher up, at the level of the middle third of the posterior aspect of the thyroid lobes, and may then be confused with P IV (Fig. 2-2).

The P IV follow the thyroid migration of the ultimobranchial bodies, which travel toward the lateral part of the main median thyroid rudiment. Their descent in the neck is thus relatively limited. They remain in contact with the posterior part of the middle third of the thyroid lobes. The short course of embryonic migration of P IV explains why they remain relatively stable in their topography when they are not pathologic. Thus, in 85% of cases, they are grouped at the posterior aspect of the thyroid lobes, in an area 2 cm in diameter, whose center is situated about 1 cm above the

FIGURE 2-2. The embryonic migration of the third branchial pouch (P III)-thymus complex results in an extensive area of the normal P III from the angle of the mandible to pericardium.

FIGURE 2-3. The short course of embryonic migration of the fourth branchial pouches explains why they remain relatively stable in the topography when they are not pathologic. In 85% of cases, they are grouped at the junction of the middle and superior thirds of the posterior aspect of the thyroid lobe.

FIGURE 2-3. The short course of embryonic migration of the fourth branchial pouches explains why they remain relatively stable in the topography when they are not pathologic. In 85% of cases, they are grouped at the junction of the middle and superior thirds of the posterior aspect of the thyroid lobe.

crossing of the inferior thyroid artery and the recurrent nerve (Fig. 2-3).4"6 Thus, the P IV are crossed by the P III during the descent of the parathymus. This embryonic crossing of P III and P IV explains why their grouping at the level of the inferior thyroid artery, at the junction of the middle and inferior thirds of the thyroid lobe, is more or less close, depending on the migration of P III.

Because the area of dispersal of the P IV is limited by their short migratory course, a congenital ectopic position of P IV is unusual. In 12% to 13% of cases, the glands are on the posterior aspect of the superior pole of the thyroid lobe in a laterocricoid, lateropharyngeal, or intercricothyroid position, and, exceptionally, in less than 1% of cases, they are above the upper pole of the lobe. In 1% to 4% of cases, they are frankly posterior behind the pharynx or esophagus.

Because the embryonic descent of the thymus extends from the angle of the mandible to the pericardium, anomalies of migration of the parathymus, whether excessive or defective, are responsible for high or low ectopias of P III. The incidence of high ectopias, along the carotid sheath, from the angle of the mandible to the lower pole of the thyroid, does not seem to exceed 1% to 2%.5"8 Conversely, if their separation from the thymus is delayed, the P III may be dragged down into the anterior mediastinum to a varying degree. They are then usually in the thymus, at the posterior aspect of its capsule, or still in contact with the great mediastinal vessels. These low ectopias are found in 3.9% to 5% of cases.6,8 Parathyroid glands found in the posterosuperior mediastinum are usually tumoral P IV that have migrated subsequently because of gravity.9

The strictly intrathyroid localization of some parathyroids is explicable only on embryologic grounds. According to Wang,10 the PIV may become included within the thyroid at the time of fusion of the ultimobranchial bodies with the median thyroid rudiment. Although the P III do not arise from the P IV, undeniable cases of a normal or pathologic P III included in the lower poles of the thyroid lobes have been reported.411 According to Gilmour,12 intrathyroid inclusion of parathyroid tissue may be found with the same incidence as inclusions of thymic tissue. Overall, the incidence of intrathyroid ectopias that seem to involve both P III and P IV is between 0.5% and 3.5%.4'811

Other embryologic cervical or mediastinal ectopic glands are more rare and usually related to supernumerary glands.1314 These develop from accessory parathyroid debris arising from fragmentation of the pharyngotracheal duct when the pharyngeal pouches separate from the pharynx. The incidence of these supernumerary glands is relatively high at 13%.5 Akerstrom and colleagues5 distinguish between accessory parathyroid glands containing simple tissue debris and weighing less than 5 mg, found very close to the main glands, and true supernumerary glands weighing more than 5 mg (average weight, 24 mg) situated apart from the other glands.

Ectopic or supernumerary parathyroids may also be situated in quite exceptional positions. They are then revealed by tumoral formations developing from them and are responsible for hyperparathyroidism: in the middle mediastinum (0.3%) at the level of the aortopulmonary window,15 lateral to the jugulocarotid axis.16 The migration of pathologic parathyroid tissue seems highly improbable in such cases. In both cases, the embryologic hypotheses suggest a precocious fragmentation of P IV.1516

Parathyroid tissue17 and parathyroid adenomas18 have also been described within the vagus nerve. In the latter case, it has been hypothesized that parathyroid tissue arises from the P III, which is closely related to the vagus nerve during embryogenesis.17 A case of a parathyroid located in the mucosa of the piriform sinus has even been reported.19

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