Early Steps in the Development of the Forebrain

Abstract

The tremendous complexity of the adult forebrain makes it a challenging task to elucidate how this structure forms during embryonic development. Nevertheless, we are beginning to understand how a simple epithelial sheet of ectoderm gives rise to the labyrinthine network of cells that constitutes the functional forebrain. Here, we discuss early events in forebrain development--those that lead to the establishment of the anterior neural plate and the regional subdivision of this territory into the different domains of the prospective forebrain.

The ups and downs of Wnt signaling in prevalent neurological disorders

“…It is well established that Wnt signaling is essential for normal embryonic development of the central nervous system (reviewed by Wilson and Houart (2004), Ciani and Salinas (2005)). Indeed, mounting evidence gathered from functional studies of either Wnt ligands or their downstream signaling components, including the complex of receptors, have shown that in the developing brain Wnt signaling participates in patterning the midbrain-hindbrain boundary, which later on gives rise to the brainstem and the cerebellum ( McMahon and Bradley, 1990; Thomas and Capecchi, 1990;Thomas et al, 1991;McMahon et al, 1992;Hall et al, 2000), and forebrain derivatives such as the cerebral cortex, hippocampal formation and the amygdala (Grove et al, 1998;Lako et al, 1998;Lee et al, 2000;Houart et al, 2002;Maretto et al, 2003;Abu-Khalil et al, 2004;Zhou et al, 2004).…”

21st Century neontology and the comparative development of the vertebrate skull

“…Analyses of the development of the early vertebrate epiblast have typically been concerned either with (1) when and how the distinction between neural and non-neural ectoderm is established or (2) how is positional information, i.e., anterior (head) and posterior (trunk), within the neural plate established Stern, 2002Stern, , 2005Wilson and Houart, 2004), and have lead to several questions, including: Is the epiblast ubiquitously competent to become either neurectodermal or surface ectodermal, and if so, is the specification and eventual commitment to neural and surface ectodermal achieved simultaneously, or does one precede the other? Is there is a default state, either neural or surface ectodermal (epidermal in general) in character, in the early epiblast, and if so, does the nondefault state come about by means of an active induction, a repression, or a de-repression?…”

“…With regard to anterior positional identity and neural induction, does one precede the other or does the neural plate come about with a default anterior character? Although these issues have been detailed and reviewed elsewhere, particularly in relation to neural induction (Niehrs, 1999;Foley and Stern, 2001;Stern, 2002Stern, , 2005Wilson and Houart, 2004), with regard to the formation of jaws it is expedient to make here note of several points that follow from them.…”

Sp5l is a mediator of Fgf signals in anteroposterior patterning of the neuroectoderm in zebrafish embryo

“…It has been proposed that during specification of the neural plate in vertebrate embryos, the initial neural induction leads to acquisition of the anterior neuroectodermal fate, and later locally produced posteriorizing signals, including Fgf and Wnt signals, specify the posterior neuroectodermal fate (Foley and Stern, 2001;Wilson and Houart, 2004 ulating the expression of anterior and posterior neuroectodermal genes. This raises a possibility that Sp5l may act as a transcriptional activator for posterior genes, but as a transcriptional repressor for anterior genes.…”

“…Subsequent regionalization of the neuroectoderm along the anteroposterior (AP) axis will result in the formation of the forebrain, the midbrain, the hindbrain, and the spinal cord. Many studies suggest that early-induced neuroectodermal cells acquire a default anterior neural identity (forebrain) and then caudally localized posteriorizing signals allow local cells to commit to the posterior neural fates (Foley and Stern, 2001;Wilson and Houart, 2004), which will give rise to the posterior midbrain, the hindbrain, and the spinal cord.…”