FIGURE 9.

Updated prosomeric model of Puelles et al. (2012a) and Puelles and Rubenstein (2015) (A) and acroterminal domain (B). (A) In this schema, which resolves the uncertainties we had before, the whole forebrain is now held to be epichordal, as a result of recognizing that the notochordal tip reaches the mamillary pouch, thus inducing there the rostral end of the floor plate (a hypothalamic floor plate). This means as a big conceptual change that there is nothing more rostral than the mamillary pouch (M), although the preoptic area (POA) is equally rostral, as are all acroterminal entities (see also Puelles, 2018). This allows the definition of two complete hypothalamo-telencephalic prosomeres, called hp1 and hp2, which reach independently the commissural roof plate. Note they are organized dorsoventrally into floor, basal, alar, and roof longitudinal domains, like all other neuromeres. The alar plate regions are divided between alar hypothalamus and telencephalon (the later is evaginated in hp1, but represents the non-evaginated preoptic area in hp2; POA). The adjustment of the floor domains pushes the tuberal region (Tu) with the neurohypophysis (NH) out of the floor and into the rostral median end of the brain, the basal part of the acroterminal domain (see B). (B) Sagittal midsagittal section through an E13.5 mouse embryo, labeled with a Dlk1 riboprobe that identifies selectively the acroterminal area of the rostral hypothalamo-telencephalic prosomere. Note this rostromedian domain encompasses alar and basal portions, reflecting its participation in standard dorsoventral patterning due to antagonistic floor ventralizing and roof dorsalizing morphogens (note also alar and basal hypothalamic partitions reflecting dorsoventral patterning). The acroterminal area also receives strong signals from the prechordal plate, whose cells migrate ventrodorsally in front of it. The prechordal tissue is held not to form ‘rostral’ to the notochord, but ‘dorsal’ to its rostral end beneath the floor plate, and extends progressively dorsalward toward the roof plate (see Puelles, 2017; Diaz and Puelles, 2020).