Our conscious thoughts, our emotions, and many of our memories reside within the forebrain, and indeed it is this region of our CNS that confers many uniquely human attributes. Despite this, the general organization of the forebrain is conserved in all vertebrates. What makes the brain of each species unique is not the initial presence or absence of different subdomains of the CNS; rather, it is the extent to which these domains are elaborated as they form the various structures that comprise the mature brain. Early steps in CNS patterning are largely conserved, and studies primarily undertaken in chick, fish, frog, and mouse are beginning to unravel the mechanisms by which the forebrain is induced and patterned.
The forebrain arises from anterior neuroectoderm during gastrulation, and by the end of somitogenesis it comprises the dorsally positioned telencephalon and eyes, the ventrally positioned hypothalamus, and the more caudally located diencephalon (). The diencephalon contains, from rostral to caudal, the prethalamus (or ventral thalamus), the thalamus (or dorsal thalamus), and the pretectum. Sitting between the prethalmus and thalamus is a prominent boundary region termed the zona limitans intrathalamica (zli), a structure that may constitute a separate subdivision of the diencephalon (; Zeltser et al., 2001
). In recent years, the prosomeric model of forebrain organization has provided a framework for understanding many studies of forebrain development, and a thorough discussion of the latest incarnation of this model can be found elsewhere (Puelles and Rubenstein, 2003
Organization of the Rostral Neural Plate and Forebrain in Embryonic Zebrafish
Despite its complexity, the forebrain derives from a simple sheet of neuroepithelial cells. However, morphogenesis is more complex than in other regions of the CNS. In consequence, from looking at the mature forebrain, the derivation and topological relationships of its component parts are not immediately obvious. In this regard, fate-mapping studies, although incomplete in all model species, have helped to reveal the neural plate origins of the cells that comprise the different regions of the forebrain. An unexpected finding of these studies is that the dorsal and ventral forebrain have different origins within the gastrula ectoderm. In fish, cells destined to contribute to the hypothalamus are located caudal to prospective dorsal forebrain tissue (telencephalon and eye field) and are close to the organizer where they are intermingled with prospective floorplate cells (; Mathieu et al., 2002
; Woo and Fraser, 1995
). From this location, prospective hypothalamic cells move rostrally within the neural plate, displacing more dorsal forebrain tissue laterally (; Varga et al., 1999
). In chick, the anlage of the entire forebrain moves rostrally (; Foley and Stern, 2001
), but, as in fish, movements may occur to a greater extent in prospective ventral than in more dorsal neural tissue (e.g., Patten et al., 2003
Cell Movements Separate the Prospective Forebrain from Sources of Caudalizing Factors
One consequence of the rostral movement of ventral forebrain cells is the lateral displacement of eye field cells from an initially unitary field spanning the midline toward the prospective left and right optic vesicles (; Varga et al., 1999
). Subsequent to this, the optic vesicles evaginate from the lateral wall of the forebrain and the most distal cells invaginate to form the neural retinal layer of the optic cup. The cells that line the back of the optic cup differentiate as pigmented epithelium and the cells that connect the optic cup to the rest of the forebrain form the optic stalk and differentiate as glial cells of the optic nerve ().
Telencephalic precursors are located rostral and lateral to the eye field, adjacent to the anterior margin of the neural plate (; Cobos et al., 2001
; Eagleson et al., 1995
; Fernandez-Garre et al., 2002
; Inoue et al., 2000
; Rubenstein et al., 1998
; Varga et al., 1999
; Whitlock and Westerfield, 2000
). Prospective subpallial (ventral) telencephalic cells are located rostrally, directly in front of the eye field, whereas prospective pallial precursors are positioned more caudally along the neural plate margin, in continuity with the dorsal diencephalon (Cobos et al., 2001
; Whitlock and Westerfield, 2000
). Diencephalic precursor cells are located caudal to the eye field (), but it is not known if the prethalamic, thalamic, and pretectal subdivisions are fully elaborated at the neural plate stage.