Postnatal fate of the ultimobranchial remnants in the rat thyroid gland

Abstract

The ultimobranchial follicles (UBFs) are considered embryonic remnants from the ultimobranchial body (UBB). They are follicular structures that vary in size and appearance depending on the age of the rat. The main objective of this article was to study the progressive changes in shape, size, and frequency of the UBFs in the postnatal rat, from birth to old-age. To accomplish that objective, a systematic morphometric and incidental study of the UBF has been carried out in 110 Wistar rats of different ages and both sexes, divided into three groups: 1) young rats (5-90-day-old); 2) adult rats (6-15-month-old), and 3) old rats (18-24-month-old). The glands were serially sectioned and immunostained for calcitonin at five equidistant levels. According to our results, UBFs were observed in all thyroid glands but a more exhaustive sampling was occasionally necessary in male rats. In young rats, immature UBFs predominantly appeared whereas in adult rats, mature UBFs with cystic appearance and variable luminal content prevailed. We frequently found spontaneous anomalous UBFs in old rats, which we have termed as "ultimobranchial cystadenomata." Additionally, in young rats, UBF areas significantly increased with age and they were larger when compared to that of normal thyroid follicles. Likewise, in adult rats, UBFs were significantly larger than normal thyroid follicles but only in female rats. In general, UBFs in females were also significantly larger than those found in male rats. Finally, all these differences related to UBFs together with a higher incidence in females of UB cystadenomata suggest a sexual dimorphism in regard to the destiny of these embryonic remnants during postnatal thyroid development.

Immunohistochemical profiling of the ultimobranchial remnants in the rat postnatal thyroid gland

“…The study material consisted of 60 formalin‐fixed, paraffin‐embedded, and serially sectioned thyroid glands of Wistar rats, of both sexes and different ages, in which we previously described the appearance of different morphological patterns that adopt the UB‐remnants throughout rat postnatal life (Vazquez‐Roman et al, ). Specifically, 25 cases of “immature UBFs” (detected in young rats), 25 cases of “mature UBFs” (detected in adult rats), and 10 cases of UB‐cystadenomata (detected in old rats) were analyzed.…”

“…Nevertheless, despite this common embryonic origin, there are differences regarding the appearance and evolution of these UB‐remnants during postnatal life in both species. Specifically, in rats, these structures appear in all the thyroid glands, although they adopt different morphological patterns throughout postnatal life (Vazquez‐Roman et al, ). Thus, in young rats (0–180 days), they evolve from narrow cellular nests to tubular forms, the so‐called “immature UBFs.” In addition, “mixed follicles,” which are partially formed by UBFs fusing to usual thyroid follicles, may also be observed in young rats.…”

“…In contrast, in adult rats (6–15 months) and old rats (18–24 months), mature cystic forms predominate, the so‐called “mature UBFs,” which are rather rounded, with stratified flattened cells in the wall, and cellular debris in the lumen. Moreover, in adult and old rats, an unusual progression of the forms described above can be found that resembles enormous onion‐like structures that we have termed “UB cystoadenomata.” With the exception of UB cystoadenomata, there are difficulties in the microscopic identification of UB remnants from the surrounding parenchyma due to either relative small size or their uncharacteristic form (Vazquez‐Roman et al, ).…”

“…In rats, no IHC studies on UBFs have been reported, with the exception of two articles published by us in which the occasional presence of CT and thyroglobulin (Tg) in the wall was analyzed (Conde, Moreno, Martin‐Lacave, Fernandez, & Galera, ; Vazquez‐Roman et al, ). No published data describe a precise staining method to specifically identify UBFs in the rat independently of their morphological pattern and magnitude.…”

Anatomy and Development of the Thyroid

“…These structures show a notable interspecies difference. 36 In humans, ultimobranchial body remnants are known as solid cell nests (SCN). 37 SNC are frequently present in the thyroid gland, preferentially located in the middle and upper third of the lobes.…”