Early Allocation and Progenitor Expansion Model (or Prespecification Model)
Cells are specified for each segment in the early limb bud and this population of cells expand out as the limb bud grows. This model is consistent with the following observations. Cell division is seen throughout the limb bud. Cell death occurs within a 200 μm zone subjacent to the AER when it is removed; cell death forecloses some patterning. FGF-releasing beads are able to rescue limb development when the AER is removed by preventing this cell death.
Experimental evidence:
- Labeled cells in different position of an early limb bud were restricted to single segments of the limb.
- Limbs lacking expression of required FGF4 & FGF8 showed all structures of the limb and not just the proximal parts.
More recently, however, the investigators primarily responsible for both the Progress Zone and Prespecification models have acknowledged that neither of these models accounts adequately for the available experimental data.
Read more about this topic: Limb Development, Axial Patterning and Related Issues, Proximal/distal Patterning
Famous quotes containing the words early, expansion and/or model:
“For the writer, there is nothing quite like having someone say that he or she understands, that you have reached them and affected them with what you have written. It is the feeling early humans must have experienced when the firelight first overcame the darkness of the cave. It is the communal cooking pot, the Street, all over again. It is our need to know we are not alone.”
—Virginia Hamilton (b. 1936)
“We are caught up Mr. Perry on a great wave whether we will or no, a great wave of expansion and progress. All these mechanical inventions—telephones, electricity, steel bridges, horseless vehicles—they are all leading somewhere. It’s up to us to be on the inside in the forefront of progress.”
—John Dos Passos (1896–1970)
“...that absolutely everything beloved and cherished of the bourgeoisie, the conservative, the cowardly, and the impotent—the State, family life, secular art and science—was consciously or unconsciously hostile to the religious idea, to the Church, whose innate tendency and permanent aim was the dissolution of all existing worldly orders, and the reconstitution of society after the model of the ideal, the communistic City of God.”
—Thomas Mann (1875–1955)