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
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