Spatial Patterning

Spatial Patterning in Vertebrates HOX CODE – Anterior/Posterior Patterning Drosophila and Mammalian HOX Gene Clusters Expression is sensitive to retinoic acid (anterior end of cluster is more RA sensitivity sensitive and therefore more easily activated by RA) 1. 4 clusters in mammals, 1 in flies 2. expressed in overlapping domains ALONG the A/P axis 3. loss of function  homeotic/anterior transformation 4. gain of function  posterior transformation Hindbrain: discrete HOX gene expression boundaries along borders of Hindbrian RHOMBOMERES Rhombomeres Hensen’s Node: secretes RA Hensen’s Node Increases RA secretion as it moves anterior  posterior **this activates the posterior genes appropriately** SHH INDUCTION – Dorsal/Ventral Patterning Neural Patterning: Shh Notochord expresses shh  neural tube floorplate induced Later begins expressing shh (high levels of shh induce floor plate, low levels: motor neurons) Shh induces ventral neuronal cell types based on conc. GRADIENT Somite Patterning: Neural tube floorplate releases shh Neural tube roofplate releases WNT-1 Wnt-1 Ectodermal BMP4/7 induces roofplate and wnt expression BMP 4/7 **Shh and Wnt act in opposition to influence ventral and dorsal patterns, respectively** Spatial Patterning in Vertebrates LEFT-RIGHT PATTERNING Embryonic left-right differences: 1. heart loops to right 2. tail to right 3. gut derivatives are sided 3 types of asymmetry mutants situs inversus totalis 1. situs inversus totalis – everything on opposite side rare and not harmful heterotaxia 2. heterotaxia – heart and viscera orient independently