Hox complexes have a retinoic acid receptor response element (RARE) in the DNA before paralogue 1. This DNA enhancer element controls expression of many genes in the complex. In retinoic acid teratogenesis, Hox gene expression borders move into more anterior regions.
外胚层被诱导形成神经板
The nervous system is induced during gastrulation. The ventral tissue has not been determined at the time of transplantation, and the neural tissue is induced during gastrulation.
神经系统是背方外胚层由邻近组织，特别是组织者 (organizer)区域发出的诱导形成的。
体节是沿前后轴按照预定顺序形成的
Growth factor FGF and retinoic acid (RA) gradients help to pattern the anteroposterior axis in the mouse embryos
组织者的作用及神经诱导：genes in the organizer
A similar pattern of gene activity in the 2 animals, with homologous genes being expressed.
Goosecoid, is an early marker of the organizer that is expressed in the cells giving rise to foregut, pre-chordal plate and notochord; mimic almost all the properties of the organizer.
特点：外胚层 细胞下陷进入胚胎 形成实心细胞索， 然后在细胞索中心 产生空洞形成中空 的神经管。
鸟类、哺乳类 、两栖类动物胚胎 的后部神经管及鱼 类胚胎的全部神经 管的形成采取此种 方式。 斑马鱼神经管的形成
神经诱导作用
Organizer mesoderm 诱导神经管的形成 两栖动物胚胎胚孔背唇诱导第二胚轴形成的作用叫做primary embryonic induction
Temporal and spatial colinearity: order of Hox genes in DNA follows the antero-posterior body axis.
Why have Hox genes stayed together in a complex?
Fig. 19
In the mouse, 4 Hox complexes, designated Hoxa, b, c, d
果蝇HOM-C的表达使每一个体 节被进一步特化， The homeobox genes are a widespread conservation of developmental genes in animals
敲除或过表达Hox 基因均可造成体轴模式建成的改变
Hoxa3 -/- : structural defects
in the regions of the head and thorax; but more posterior axial structures, show no evident defects.
敲除或过表达Hox 基因均可造成体轴模式建成的改变
Homeotic transformation: loss of Hox gene function results in the conversion of one body part into another. e.g.,
Hoxa8-/- mice, the first lumbar
Hox基因在小鼠体节中的表达
The patterns of Hox gene expression could specify the identity of the tissues at different position
For example, the pattern of expression is quite different in anterior and posterior regions of the body axis
Patterns of Hox gene expression in the mesoderm of chick and mouse embryos, and their relation to regionalization
Hoxd9 and Hoxd10 are expressed at the transition between lumbar and sacral regions, as Hoxc5/6 at the cervical and thoracic vertebral transition in both chick and mouse.
神经管形成的起始：来自背部中胚层的信号诱导预置神经板 边缘的细胞的背测收缩，而预置的表皮细胞向中线移动，使表皮 与神经板交接处凸起形成神经褶。
Primary neurulation的过程
神经管形成的扫描电镜图
神经管与相邻外胚层细胞分离可能与细胞粘连分子有关
神经管沿A－P 轴线依次闭合， 完成形成过程。
An antero-posterior gradient of FGF develops in the embryo, with its high point at the node
The somites are formed when the FGF levels drops to some low threshold.
vertebra is transformed into a rib-bearing thoracic vertebra.
microRNA may be involved in post-transcriptional regulation of Hox gene expression.
Hox基因缺陷导致肢体相应部位的缺失
人的多指症也可能
与Hox基因的异位 表达有关
神经管的形成
神经管(neural tube)是中枢神经 系统的原基，其形成称为 neurulation。其方式分primary neurulation和secondary neurulation两种。
1. Primary neurulation: 由外胚层细胞增殖、内陷并最终 离开外胚层表面而形成中空的神经管 。绝大多数脊椎动物前部神经管的形 成采用此种方式。
Retinoic acid secreted by the somites forms an opposing gradient that antagonizes FGF.
体节是沿前后轴按照预定顺序形成的
Somites differentiate into particular axial structures depending on their position along the antero-posterior axis: anterior-most – skull, posterior-cervical vertebrae, more posterior-thoracic vertebrae with ribs. Position specification
前后轴上不同位置的体节的特性是由Hox基因特化的
• HOM-C基因的结构十分复杂，有的基因有多个启动 子和多个转录起始位点。其另一个重要特征是都含有 一段的保守序列，称为同源异型框(homeobox)。含 有同源异型框的基因统称为同源异型框基因 (homeobox gene)。
• 由同源异型框编码的同源异型结构域 (homeodomain) 可形成与DNA特异性结合的螺旋转角-螺旋结构 (helix-turn-helix) 。
HOM-C同源异 型框形成与 DNA特异结合 的螺旋-转角螺旋结构。
Hale Waihona Puke 后轴上不同位置的体节的特性是由Hox基因特化的
Patterning along anteroposterior axis involves the expression of a set of genes that specify positional identity or positional value along the axis. The Hox genes.
脊椎动物体节与早期神经系统的模式建成
在原肠运动进行的过程中及之后一段时间内，脊椎动 物的模式建成沿身体的前后轴和背腹轴进行。
Key morphogenetic events are the formation of the somites, which give rise to muscles, skeleton and dermis. 轴旁中胚层由紧邻脊索增殖较 快的中胚层细胞形成纵列的细胞索，并很快横裂为左右 成对的体节。体节共42-44对，分化为真皮、中轴骨骼 及骨骼肌。
Posterior dominance or posterior prevalence : more posteriorly expressed Hox genes tend to inhibit the action of the Hox genes normally expressed anterior to them.