Baker N E, Zitron A E
Department of Molecular Genetics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
Mech Dev. 1995 Feb;49(3):173-89. doi: 10.1016/0925-4773(94)00314-d.
Loss of function mutations of scabrous and conditional alleles of Notch and Delta affect the pattern of morphogenetic furrow development. By studying differentiation of R8 cells, the first photoreceptor neuron subtype to differentiate, we show that all furrow cells pass through an R8-competent stage. Function of Notch and scabrous is necessary if most of these cells are to attain other cell fates. The scabrous gene confers a regular pattern on the morphogenetic furrow, restricting R8 differentiation to alternating groups of cells. Notch and Delta function to restrict the R8 fate to a single cell in each group. Without scabrous gene function, action of Notch and Delta on the entire morphogenetic furrow results in a disorganised pattern of ommatidia arising from a disorganised array of single R8 cells. Aspects of the scabrous mutant phenotype also suggest a secondary role in selecting a single R8 cell from competent clusters. We show that scabrous expression preceeds changes in the apical profiles of morphogenetic furrow cells that identify ommatidial precurf1p4cells, and also preceeds changes in levels of Notch and Delta expression. The pattern of initiation of sca expression depends on sca gene function, indicating that patterning of the morphogenetic furrow depends on the pattern of posterior columns. Our results suggest that in the eye, Notch and Delta amplify and refine a morphogenetic landscape generated by scabrous. Cell determination in other tissues and organisms might also be molded in a two-step process where initial inhomogeneities determined by one protein provide a context for subsequent development.
糙面基因的功能丧失突变以及Notch和Delta的条件等位基因会影响形态发生沟的发育模式。通过研究最早分化的光感受器神经元亚型R8细胞的分化过程，我们发现所有沟细胞都会经历一个具备R8细胞能力的阶段。如果这些细胞中的大多数要获得其他细胞命运，Notch和糙面基因的功能是必需的。糙面基因赋予形态发生沟一种规则的模式，将R8细胞的分化限制在交替的细胞群中。Notch和Delta的功能是将R8细胞命运限制在每组中的单个细胞。没有糙面基因的功能，Notch和Delta对整个形态发生沟的作用会导致由单个R8细胞的无序排列产生的小眼排列紊乱。糙面突变体表型的一些方面还表明，在从有能力的细胞簇中选择单个R8细胞方面，糙面基因具有次要作用。我们发现糙面基因的表达先于形态发生沟细胞顶端轮廓的变化，这些变化确定了小眼的前体细胞，也先于Notch和Delta表达水平的变化。糙面基因表达起始的模式依赖于糙面基因的功能，这表明形态发生沟的模式形成依赖于后柱的模式。我们的结果表明，在眼睛中，Notch和Delta放大并细化了由糙面基因产生的形态发生格局。其他组织和生物体中的细胞决定也可能通过两步过程形成，其中由一种蛋白质决定的初始不均匀性为后续发育提供了背景。
