Thalamus (alar plate).
This large nuclear complex can be subdivided into 5 main histogenetic areas (Redies et al.,2000; Puelles,2001b; Martínez-de-la-Torre et al.,2002; García-López et al.,2004). These areas are the habenular/subhabenular complex or epithalamus, the dorsal tier group, the intermediate tier group, the ventral tier group, and the anteroventral group (Redies et al.,2000; Puelles,2001b; Martinez-de-la-Torre et al.,2002; Puelles et al.,2007).
In the habenular/subhabenular complex, Sulf1 was strongly expressed in the subhabenular ependymal region, the lateral and medial subhabenular nuclei (LSHb, MSHb), as well as in the periventricular stratum and lateral habenular nucleus (LHb; Figs. 3A–C, 4A–C, 5A,B).
Ventral to the subhabenular nuclei appears the dorsal tier domain, which produces a voluminous mass of derivates in its mantle layer that represent the largest components of the avian thalamus (Redies et al.,2000; Puelles,2001b; Puelles et al.,2007). High levels of Sulf1 expression was detected in several of the nuclear components in the dorsal tier: dorsomedial anterior (DMA; Figs. 3A, 4A–C, 5A), dorsal intermediate anterior and posterior nuclei (DIA, DIP; Figs. 3A,B, 4A–C, 5A,B) and in the dorsolateral anterior nuclei (DLA; Figs. 3A, 4A,B, 5A). Superficially, a continuous stream of Sulf1-expressing cells was observed from the subhabenular nuclei to the superficial parvicellular nucleus (SPC; Figs. 3A–C, 4A–C, 5B) and in the superficial microcellular nucleus (SMi; Figs. 3A,B, 4A,B, 5A,B).
The intermediate tier is larger caudally, where it makes extensive contact with the pretectum (Redies et al.,2000; Puelles,2001b; Martínez-de-la-Torre et al.,2002; Puelles et al.,2007). In this tier, Sulf1 expression was restricted to the largest part of this complex, the rotundus nucleus (Rot), which is pushed outward ventrolaterally by the disproportionate growth of the dorsal tier group. Faint Sulf1 expression in the scattered rotundic cell was detected at stage E12 (HH38, data not shown). At E14 (HH40), the low level of Sulf1 expression was apparent (Fig. 4C–G) but at E18 (HH44), we observed high expression levels compared with previous stages (Fig. 5B–E).
Ventral to the intermediate tier is a smaller histogenetic domain called the ventral tier. This domain contains the avian medial geniculate nucleus (MG, Puelles et al.,2007), formerly called “ovoidal nucleus” (Papez,1935,1936). It is surrounded by inner and outer perigeniculate formations: medial perigeniculate nucleus (MPG) and semilunar perigeniculate nucleus (SLPG). To better locate Sulf1 expression in the intermediate and ventral thalamic tiers, we carried out double in situ hybridization experiments for Sulf1 (labeled in blue) and Gbx2 (labeled in red). Gbx2 was strongly expressed in the ventral tier derivatives (i.e., the MG) and in the subrotundus nucleus (SRot; Puelles et al.,2007), a derivative of the anteroventral area (Martinez-de-la-Torre et al.,2002). We observed that the MG and SRot, stained with the Gbx2 probe, were negative for Sulf1 (positive for Gbx2, Fig. 4F) while MPG and SLPG were strongly positive (Figs. 3B–D, 4D–F, 5B,C). In addition, the diffuse perigenulate area also expressed Sulf1 (DPG; Figs. 3D, 4G, 5D,E).
In the chicken, cells originating in the boomerang-shaped anteroventral histogenetic zone of the dorsal thalamus migrate to the surface, partially filling the area of origin in depth and partially dispersing tangential-caudally in the superficial strata of p2, to finally cover the most of the dorsal, intermediate, and ventral tiers (Rendahl,1924; Puelles et al.,1991; Yoon et al.,2000; Puelles et al.,2007). The known derivatives of this region (subrotundus nucleus, perirotundic area and intergeniculate leaflet) differentiate in close spatial relation to the rotundus nucleus. The expression of Sulf1 was restricted at the perirotundic area (PRot), pirotundic nucleus (ERot; Puelles et al.,1991), and intergeniculate leaflet (IGL; Figs. 4B, 5A,D). The complex formed by IGL plus PRot corresponds to the “n.superficialis magnocellularis” of Rendahl (1924). This was then renamed “n.interstitialis tractus opticus” by Puelles et al. (1991) and Mart´inez et al. (1991). Also, we observed expression in the posteroventral and posterocaudal thalamic nuclei (PVTh, PCTh; Figs. 4H, 5F).