Expression of thyroid transcription factor-1 (TTF-1) gene in the ventral forebrain and endostyle of the agnathan vertebrate, Lampetra japonica.

The Thyroid transcription factor-1 (TTF-1) gene belongs to the Nkx-2.1 subfamily and encodes a transcription factor containing an NK-2-type homeodomain. In our study, we isolated and characterized cDNA clones for the TTF-1/Nkx-2.1 orthologue (LjTTF-1) from the agnathan vertebrate Lampetra japonica. Spatial and temporal expression patterns assessed by in situ hybridization revealed the expression of LjTTF-1 in the anterior nerve cord and anteroventral region of the pharynx. The neural expression was subsequently restricted to the ventral diencephalon. The pharyngeal expression, on the other hand, extended posteriorly to the fourth pharyngeal-pouch level and was finally localized in the endostyle anlage. In the differentiated endostyle of ammocoete larvae, the expression of LjTTF-1 was chiefly detected in type 2a, 2b, and 2c cells, which develop adjacent to glandular cells. These expression patterns of LjTTF-1 support the idea that this gene family plays an important role in the development of the rostral brain and endostyle equivalent organs. Furthermore, histological comparisons between TTF-1/Nkx-2.1 expression in the endostyles of ammocoetes and ascidians suggested the possibility that the organogenetic architecture of the endostyle is conserved among chordates.