Postnatal Fate of the Ultimobranchial Remnants in the Rat Thyroid Gland

The ultimobranchial follicles (UBFs) areconsidered embryonic remnants from the ultimobran-chial body (UBB). They are follicular structures thatvary in size and appearance depending on the age of therat. The main objective of this article was to study theprogressive changes in shape, size, and frequency of theUBFs in the postnatal rat, from birth to old-age. Toaccomplish that objective, a systematic morphometricand incidental study of the UBF has been carried out in110 Wistar rats of different ages and both sexes, dividedinto three groups: 1) young rats (5–90-day-old); 2) adultrats (6–15-month-old), and 3) old rats (18–24-month-old).The glands were serially sectioned and immunostainedfor calcitonin at five equidistant levels. According to ourresults, UBFs were observed in all thyroid glands but amore exhaustive sampling was occasionally necessary inmale rats. In young rats, immature UBFs predominantlyappeared whereas in adult rats, mature UBFs withcystic appearance and variable luminal content pre-vailed. We frequently found spontaneous anomalousUBFs in old rats, which we have termed as ‘‘ultimobran-chial cystadenomata.’’ Additionally, in young rats, UBFareas significantly increased with age and they werelarger when compared to that of normal thyroid follicles.Likewise, in adult rats, UBFs were significantly largerthan normal thyroid follicles but only in female rats. Ingeneral, UBFs in females were also significantly largerthan those found in male rats. Finally, all these differen-ces related to UBFs together with a higher incidence infemales of UB cystadenomata suggest a sexual dimor-phism in regard to the destiny of these embryonic rem-nants during postnatal thyroid development. ; Junta de Andalucía, España, Consejeria de Innovacion; Ciencia y Empresa CTS-439/2009 ; Junta de Andalucía, España, Consejeria de Innovacion; Ciencia y Empresa P08-CVI-03598