JTW's Evolutionary Origins - Author: Hall, Brian K.
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The Neural Crest as a Fourth Germ Layer
"... neural crest cells are not a homogenous population of cells, but rather a collective of populations of cells. Although they overlap in the neural crest, some of these cell populations have been separate since the origin of the vertebrates almost 500 million years ago."(Brian K. Hall, 2000b)
"Neural crest cells arise from neural ectoderm during early development of the neural tube. Initially epithchal (i.e., a sheet of cells) neural crest cells transform to mesenchyinal cells and migrate away from the developing brain and spinal cord, migrating as discrete populations of cells. Similar populations of cells can be observed across the vertebrates, i.e.. they have been conserved throughout vertebrate evolution. The neural crest gives rise to so many different cell types - more types than arise from mesoderm - that the neural crest can be regarded as a fourth germ layer, one that is unique to vertebrates and that allowed many distinctive vertebrate tissues to arise."
(Hall, 2000a)
"Any approach to understanding the neural crest ... [in development] must be undertaken against the background of neural crest cells as a collection of independent cell populations arising within a single embryonic region, the neural crest."
(Hall, 2000a)
The Mesoderm and Neural Crest as Secondary Germ Layers
"Mesoderm is neither present at the onset of development nor a primary germ layer. Mesoderm arises from "ectoderm" or at the ectoderm-endoderm boundary at the equator of the blastula as a result of inductive interactions initiated by the endoderm. Therefore, with respect to one another, ectoderm is primary and mesoderm secondary. As a consequence of further inductive interactions, ectoderm is partitioned into neural and epidermal ectoderm, which are therefore secondary with respect to ectoderm. Neural crest arises from neural ectoderm as a consequence of primary neural induction. Therefore, with respect to one another, ectoderm is primary and neural crest is secondary."(Hall, 1998)
"Neural crest appears at the boundary between neural and epidermal ectoderm whether that boundary is at the normal site in vivo or created when a neural tube is induced ectopically within the epidermal ectoderm. This is important. It indicates that a specific neural crest inducer need not be sought and that neural crest, like mesoderm, arises secondarily from another germ layer." (Hall, 1998)
"I have argued that the neural crest is a fourth germ layer. This brings the neural crest a long way from the days when the mere suggestion that mesenchymal tissues could arise from ectoderm was biological heresy and professional suicide. Rather than flying in the face of a germ-layer concept, the existence of the neural crest confirms the basic layers as fundamental features of metazoan development. It also recommends a much more dynamic approach to germ layers than taken in the past. Germ layers are not immutable building blocks that constrain development. They are dynamic entities, capable of fundamental evolutionary change and modification. The evolutionary origins of secondary germ layers is a dramatic illustration of evolutionary change in the most fundamental mechanisms. Like mesoderm, neural crest arises from a primary germ layer after secondary induction, i.e., neural crest is a secondary germ layer. The neural crest is also a chordate synapomorphy. Therefore, chordates are quadro- not triploblastic."
(Hall, 1998)
- Hall, Brian K.
- Germ Layers and the Germ-Layer Theory Revisited: Primary and Secondary Germ Layers, Neural Crest as a Fourth Germ Layer, Homology, and Demise of the Germ-Layer Theory
- Evolutionary Biology: Vol. 30, pp. 121-186
- QH 366.A1.E9 v.30 1998
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- The Neural Crest as a Fourth Germ Layer and Vertebrates as Quadroblastic Not Triploblastic
- Evolution and Development: Vol. 2, No.1, pp. 3-5
- January-February 2000a
- [Pubmed]
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- Populations of Neural Crest Cells and Craniofacial Development
- Current and Basic Clinical Research in Craniofacial Development and Disorders Syposium
- December 7, 2000b
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