Expression of the homeobox genes OTX2 and OTX1 in the early developing human brain

Abstract

In rodents, the Otx2 gene is expressed in the diencephalon, mesencephalon, and cerebellum and is crucial for the development of these brain regions. Together with Otx1, Otx2 is known to cooperate with other genes to develop the caudal forebrain and, further, Otx1 is also involved in differentiation of young neurons of the deeper cortical layers. We have studied the spatial and temporal expression of the two homeobox genes OTX2 and OTX1 in human fetal brains from 7 to 14 weeks postconception by in situ hybridization and immunohistochemistry. OTX2 was expressed in the diencephalon, mesencephalon, and choroid plexus, with a minor expression in the basal telencephalon. The expression of OTX2 in the hippocampal anlage was strong, with no expression in the adjacent neocortex. Contrarily, the OTX1 expression was predominantly located in the proliferative zones of the neocortex. At later stages, the OTX2 protein was found in the subcommissural organ, pineal gland, and cerebellum. The early expression of OTX2 and OTX1 in proliferative cell layers of the human fetal brain supports the concept that these homeobox genes are important in neuronal cell development and differentiation: OTX1 primarily in the neocortex, and OTX2 in the archicortex, diencephalon, rostral brain stem, and cerebellum.

Figure 1

Expression of OTX2 in the human fetal brain at 7 fetal weeks. (A) A Nissl-stained sagittal section of the brain. (B) X-ray image of the in situ signal of OTX2 in the same brain showing expression in the tectum, basal telencephalon, hippocampal anlage, choroid plexus, and walls of the diencephalic and mesencephalic superventricle. Squares marked in B highlight the areas of the positive immunohistochemical reactions of the OTX2 protein in C–E. (C–E) Photomicrographs of immunohistochemical reactions for the OTX2 protein in the hippocampal anlage and choroid plexus (C), diencephalic wall (D), and the caudal limit of OTX2 expression in the mesencephalon (E). Tel, telencephalon; Di, diencephalon; Mes, mesencephalon; Met, metencephalon. Bars: A,B = 1 mm; C–E = 100 μm.

Figure 2

Expression of OTX2 and OTX1 mRNA in the human fetal forebrain at the age of 9 fetal weeks. (A) X-ray image of the in situ signal of OTX2 in the human fetal forebrain showing strong expression in the choroid plexus, hippocampus, and ganglionic eminence. (B) Photomicrograph of a Nissl-stained parallel section of the same brain. (C) X-ray image of the in situ signal of OTX1 in the same brain. Expression of OTX1 is most notable in the proliferative zones of the neocortex. (D) Photomicrograph of a Nissl staining of the hippocampus in an additional fetus. (E) X-ray image of the hippocampal in situ signal of OTX2 in this fetus. GE, ganglionic eminence; Hippo, hippocampus; Neo, neocortex; CP, cortical plate; VZ, ventricular zone. Bar = 1 mm.

Figure 3

Expression of the OTX2 protein in the human forebrain, age 9 weeks. (A) Photomicrograph of an immunohistochemical reaction for the OTX2 protein in the hippocampus. (B) Photomicrograph of the OTX2 protein in the thalamus. (C) Photomicrograph of the OTX2 protein in the choroid plexus. Bar = 100 μm.

Figure 4

Expression of the OTX2 protein in the human fetal brain at 13–14 fetal weeks. (A) Photomicrograph of a Nissl-stained paraffin-embedded section from a fetus, age 13 weeks. (B) Photomicrograph of a parallel section immunohistochemically reacted for the OTX2 protein. A very strong immunoreactivity is seen in the periventricular cells of the pineal recess and in the pineal gland. (C) Photomicrograph of a Nissl-stained paraffin-embedded section from a 13-week-old fetus. (D) Photomicrograph of the OTX2 protein in the cerebellar rhombic lip (cerebellum) and choroid plexus in a parallel section. (E) Photomicrograph of a Nissl-stained paraffin-embedded section from a fetus, age 13 weeks. (F) Photomicrograph of a parallel section with a very strong OTX2 immunoreactivity in the internal layer of the median eminence. (G) Photomicrograph of a Nissl-stained paraffin-embedded section from a 14-week-old fetus. (H) Photomicrograph of the OTX2 protein in the pineal gland. Bars: A,B,D = 200 μm; C = 100 μm.