Research article
Pyramidal lobe of the thyroid gland and the thyroglossal duct remnant: A study using human fetal sections

https://doi.org/10.1016/j.aanat.2014.09.001Get rights and content

Abstract

To investigate developmental changes in the thyroglossal duct, we observed serial sagittal sections of eight embryos (crown-rump length (CRL) 6–12 mm; approximately 5–6 weeks of gestation) as well as serial horizontal or cross-sections of 70 embryos and fetuses (CRL 15–110 mm; 6–15 weeks). In the sagittal sections, the thyroglossal duct was identified as a small sheet or mass of relatively large cells with vacuolization anterior, superior or inferior to the fourth pharyngeal arch artery. However, we found no continuous duct-like structure that reached the thyroid gland. Thus, previous classical schemes might have overestimated the continuity of the duct. Among cross-sections of 70 specimens, we found the thyroglossal duct remnant in only two specimens (CRL 15 mm and 100 mm), in contrast to the pyramidal lobe, which was seen in one-third of the specimens. The duct remnant ran downward along the lateral edge of the hyoid body to reach the anterior aspect of the thyroid cartilage. However, the connection between the pyramidal lobe and the duct remnant was interrupted by the anterior cervical muscles. Therefore, it was unlikely that the thyroglossal duct remnant would more frequently be evident in fetuses than in adults. The highly tortuous course of the duct along the lingual aspect of the hyoid body, which has been reported previously, appeared to become established near term. Descent of the thyroid gland was not evident after the CRL 20 mm stage (6 weeks): the gland appeared to retain its position at the level of the third–sixth cervical vertebrae.

Introduction

Presence of the pyramidal lobe of the thyroid gland is a common anatomic variation with a reported incidence of over 50% (Marshall, 1895, Sittkou, 1923). The pyramidal lobe may arise when the caudal end of the thyroglossal duct persists. Thyroglossal duct cyst is a common anomaly with an estimated incidence of 7% in adults (Ellis and van Nostrand, 1977). At the beginning of the present study, we hypothesized that the pyramidal lobe as well as the thyroglossal duct remnant would be found more frequently in fetuses than in adults because these structures are likely to be obliterated or to degenerate due to mechanical stress from surrounding structures such as the anterior cervical muscles and the hyoid bone. Therefore, our primary aim was to confirm whether the duct remnant and the pyramidal lobe were present in a large study sample of human fetuses.

Descent of the thyroid gland anlage starts at 5 weeks of gestation and it reaches its final position inferior to the cricoid cartilage at 7 weeks (Chou et al., 2013). A textbook by Keibel and Mall (1912) contains elegant sequential diagrams showing the stage-dependent descent: the thyroid gland anlage does not form a “duct-like structure” but rather an island of cells during its descent. In fact, according to the textbook, the thyroglossal duct is isolated from the lingual epithelium by the crown-rump length (CRL) 7.5 mm stage (approximately 4 weeks of gestation). If so, the presence of a duct-like remnant extending from the foramen cecum of the tongue to the thyroid gland proper would be illogical or very rare, in contrast to the likely anomaly of a “cyst”. However, to our knowledge, no photographic representations of the process are available. Accordingly, the second aim of the present study was to clarify the early process of descent of the thyroid gland anlage.

There is a considerable amount of morphometric information on the human fetal thyroid gland (Bocian-Sobkowska et al., 1992, Bocian-Sobkowska et al., 1997, Achiron et al., 1998), and growth curves based on length, width and volume have been postulated. However, to our knowledge, there is little information on changes in location in terms of the vertebral level. Does the thyroid anlage really descend along the vertebral column? If so, the vertebral level would change during the process of descent. Therefore, the third aim of this study was to clarify the vertebral level of the fetal thyroid gland to evaluate any changes in this level during development. Consequently, using the extensive collection of fetal specimens kept at Universidad Complutense, Madrid, we observed serial sagittal sections of eight embryos (CRL 6–12 mm; approximately 5–6 weeks of gestation) as well as serial horizontal or cross-sections of 70 embryos and fetuses (CRL 15–110 mm; 6–15 weeks) in order to gain a better understanding of developmental changes in the thyroglossal duct or thyroid anlage.

Section snippets

Materials and methods

The study was performed in accordance with the provisions of the Declaration of Helsinki 1995 (as revised in Edinburgh 2000). Horizontal serial paraffin sections stained with hematoxylin and eosin had been obtained from 78 human embryos and fetuses with a CRL of 6–110 mm (approximately 6–15 weeks of gestation) for observations of the pyramidal lobe as well as the thyroglossal duct remnant. A major proportion of them (70 specimens) had been used for horizontal sections, while sagittal sections

Observations of sagittal sections of eight embryos

The thyroglossal duct was not identified as a continuous duct-like structure but as a small sheet or mass of relatively large cells (Fig. 1A, C and E). This structure contained multiple lumen-like spaces (vacuolization or partial degeneration), but these did not correspond to the duct or the thyroid follicle because they were not lined by cuboidal epithelium and because the lining was not continuous (Fig. 1B and D). Thus, the structure was more akin to a “thyroid gland anlage” than to a duct.

Discussion

The present observations demonstrated that (1) the thyroglossal duct was almost always obliterated, fragmented or disappeared immediately after migration of the anlage; (2) the pyramidal lobe morphology seemed to remain unchanged once the lobe had developed in the early stage; (3) the position of the thyroid gland (i.e., its vertebral level) seemed to remain unchanged during the significant growth of the body during 7–15 weeks of gestation. We hypothesized that the duct remnant and the

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