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|One of the four basic characteristics that define chordates is a dorsal, hollow, tubular nerve cord. At the cranial end of this cord, a brain forms. The adults of all craniates share the same basic organization of the brain: a cerebrum, diencephalon, midbrain, cerebellum, and a midbrain. In mammals, the growth of the cerebrum and cerebellum causes the prominence of an additional region of the brainstem, the pons. In primitive vertebrates, the regions of the brain exist as a series of linear expansions of the cranial neural tube. This pattern is also observed in the embryos of humans and other higher vertebrates despite the subsequent modifications of this simple ancestral plan.|
Some of this organization
may actually have evolved in the ancestors of coelomates. Both vertebrates and Drosophila share a brain
divided into a forebrain, midbrain, and hindbrain with an important midbrain/hindbrain
boundary characterized by the expression patterns of Otx, Hox, and Pax2/5/8
genes (Hirth, 2003).
Despite the differences between the brains of adult placental mammals and the brains of more primitive vertebrates, the brain of placental embryos show a number of similarities with the more primitive condition. The brain develops as a series of linear regions which slowly fold into the adult configuration. The midbrain is prominent and exposed in early embryonic development and its channel for cerebrospinal fluid is much more prominent than the condition in adults.
|Cat fetal Brains|
|In reptiles and marsupials, there is no corpus callosum uniting the cerebral hemispheres. The absence of a corpus callosum can also be observed in the fetal brain.|
An estimated 1/250 human conceptions suffer from an abnormality known as holoprosencephaly (although the incidence at birth is far lower (1/16,000) due to the lethality of the condition). In the most severe condition, the forebrain is not divided into hemispheres and possesses only a single ventricle (Aruga, 2004).
Chick Embryonic brain
Molecularly, the boundaries between major
brain regions express specific genes. For example, the midbrain-hindbrain
boundary is marked by the expression of by Otx-2, Wnt-1, FGF8, En (2,5,8), Pax(2,5,8) genes (Hall, 1999). Microanatomy and analysis of gene expression
suggest that Amphioxus possesses
homologs of the vertebrate forebrain (the diencephalon rather than the
telencephalon), anterior midbrain, and hindbrain (
The hindbrain begins its development by forming a series of rhombomeres with distinct patterns of gene expression. For example, the various genes of the Hox clusters whose expression establishes a positional map in various regions of the body, are only expressed in specific rhombomeres. For example, Hoxb-1 is produced only in rhombomere 4 while Hoxa3 is expressed at the boundary between rhombomeres 4 and 5 (Hall, 1999). Hoxa2 and Hoxb2 are more highly expressed in r3 and r5; Hoxa3 and Hoxb3 are more highly expressed in r5. Hoxd4, Hoxb4, and Hoxa4 are expressed in r7. Hoxa1 is expressed from r4 through r7 and Hoxd3 from r6 to r7 (Iulianella, 2003).
The development of the cerebrum and other regions of the brain depends on the ability of neurons to migrate from their place of origin. The migration of neurons is apparently an ancient mechanism in animal development. Many developing neurons are known to migrate in invertebrates as diverse as cnidarians, mollusks, and arthropods. Although the cells of the developing pig brain (in the following images) develop along the lumen of the neural tube, they migrate in order to reach their final destinations.