University of Iowa, Department of Biology, 143 BB, Iowa City, IA 52242, USA.
Hear Res. 2011 May;275(1-2):66-80. doi: 10.1016/j.heares.2010.12.002. Epub 2010 Dec 10.
Atonal homolog1 (Atoh1, formerly Math1) is a crucial bHLH transcription factor for inner ear hair cell differentiation. Its absence in embryos results in complete absence of mature hair cells at birth and its misexpression can generate extra hair cells. Thus Atoh1 may be both necessary and sufficient for hair cell differentiation in the ear. Atoh1 null mice die at birth and have some undifferentiated cells in sensory epithelia carrying Atoh1 markers. The fate of these undifferentiated cells in neonates is unknown due to lethality. We use Tg(Pax2-Cre) to delete floxed Atoh1 in the inner ear. This generates viable conditional knockout (CKO) mice for studying the postnatal development of the inner ear without differentiated hair cells. Using in situ hybridization we find that Tg(Pax2-Cre) recombines the floxed Atoh1 prior to detectable Atoh1 expression. Only the posterior canal crista has Atoh1 expressing hair cells due to incomplete recombination. Most of the organ of Corti cells are lost in CKO mice via late embryonic cell death. Marker genes indicate that the organ of Corti is reduced to two rows of cells wedged between flanking markers of the organ of Corti (Fgf10 and Bmp4). These two rows of cells (instead of five rows of supporting cells) are positive for Prox1 in neonates. By postnatal day 14 (P14), the remaining cells of the organ of Corti are transformed into a flat epithelium with no distinction of any specific cell type. However, some of the remaining organ of Corti cells express Myo7a at late postnatal stages and are innervated by remaining afferent fibers. Initial growth of afferents and efferents in embryos shows no difference between control mice and Tg(Pax2-Cre)::Atoh1 CKO mice. Most afferents and efferents are lost in the CKO mutant before birth, except for the apex and few fibers in the base. Afferents focus their projections on patches that express the prosensory specifying gene, Sox2. This pattern of innervation by sensory neurons is maintained at least until P14, but fibers target the few Myo7a positive cells found in later stages.
Atonal homolog1（Atoh1，以前称为 Math1）是内耳毛细胞分化的关键 bHLH 转录因子。胚胎中该基因的缺失会导致出生时成熟毛细胞完全缺失，而其异常表达会产生额外的毛细胞。因此，Atoh1 可能是内耳毛细胞分化所必需且充分的。Atoh1 缺失的小鼠在出生时死亡，在携带 Atoh1 标记的感觉上皮中存在一些未分化的细胞。由于致死性，新生儿中这些未分化细胞的命运尚不清楚。我们使用 Tg(Pax2-Cre)在内耳中删除 floxed Atoh1。这产生了可行的条件性敲除（CKO）小鼠，用于研究没有分化的毛细胞的内耳的出生后发育。通过原位杂交，我们发现 Tg(Pax2-Cre)在可检测到 Atoh1 表达之前重组了 floxed Atoh1。由于不完全重组，只有后管嵴具有表达 Atoh1 的毛细胞。大多数 CKO 小鼠的耳蜗器官因晚期胚胎细胞死亡而丢失。标记基因表明，耳蜗器官减少到两排细胞，夹在耳蜗标记物（Fgf10 和 Bmp4）之间。这些两排细胞（而不是五排支持细胞）在新生儿中 Prox1 呈阳性。在出生后第 14 天（P14），耳蜗器官的剩余细胞转变为扁平上皮，没有任何特定细胞类型的区别。然而，在晚期出生后阶段，一些剩余的耳蜗器官细胞表达 Myo7a 并被剩余的传入纤维支配。在对照小鼠和 Tg(Pax2-Cre)::Atoh1 CKO 小鼠之间，胚胎中传入和传出纤维的初始生长没有差异。在出生前，除了尖端和基部的少数纤维外，大多数传入和传出纤维在 CKO 突变体中丢失。传入纤维将其投射集中在表达前感觉指定基因 Sox2 的斑块上。这种感觉神经元的支配模式至少在 P14 之前保持不变，但纤维靶向后期发现的少数 Myo7a 阳性细胞。
