Chapter 8 - Patterning of the Diencephalon

Abstract

The anatomic complexity of the diencephalon (considered here as defined within the prosomeric model; Puelles L and Rubenstein JLR (2003) Forebrain gene expression domains and the evolving prosomeric model. Trends in Neurosciences 9: 469–476; Puelles L, Martinez-de-la-Torre M, Paxinos G, Watson C, and Martinez S (2007) The Chick Brain in Stereotaxic Coordinates: an Atlas featuring Neuromeric Subdivisions and Mammalian Homologies . San Diego: Elsevier/Academic Press; Puelles L, Martinez-de-la-Torre M, Bardet S, and Rubenstein JLR (2012) Hypothalamus. In: Watson C, Paxinos G, and Puelles L (eds.) The Mouse Nervous System , pp. 221–312. London: Academic Press (Elsevier); Puelles L, Martinez-de-la-Torre M, Bardet S, and Rubenstein JLR (2012) Diencephalon. In: Watson C, Paxinos G, and Puelles L (eds.) The Mouse Nervous System , pp. 313–336. London: Academic Press (Elsevier) is the result of various molecular and cellular regulative patterning mechanisms orchestrated by signaling from different morphogenetic organizers during early development, at both neural plate and neural tube stages ( Martinez S, Puelles E, Puelles L, and Echevarria D (2012) Molecular regionalization of the developing neural tube. In: Watson C, Paxinos G, and Puelles L (eds.) The Mouse Nervous System , pp. 2–18. London: Academic Press (Elsevier)). In the diencephalon, like that in the rest of the brain, a variety of graded signals modulate positional information along the dorsoventral and anteroposterior (AP) dimensions, leading to areal neuroepithelial regionalization, that is, emergence of a differential molecular identity of characteristic areal patches of progenitor cells. Dorsalizing molecular signals such as retinoic acid, Fgf8, bone morphogenetic proteins, and Wnts are postulated to diffuse ventralward from the diencephalic roof plate. They interact antagonistically with Shh protein and other ventralizing signals produced first at the notochord and later also at the floor and basal plates of the diencephalon, which diffuse upward across the alar–basal boundary. Both effects participate in alar–basal patterning, which includes formation of an intermediate liminal territory with peculiar characteristics. The diencephalon is also regionalized along the AP dimension, after separating from the hypothalamus and telencephalon, rostrally, and from the midbrain, caudally. An intrinsic diencephalic secondary organizer, the zona limitans intrathalamica (ZLI), also known as mid-diencephalic organizer, develops transversally after neurulation in a central part of the diencephalic alar plate, and releases supplementary Shh along the AP axis. The consequent rostralward and caudalward signaling increases regional complexity by establishing added differential anteroposterior positional codes within the alar plate. ZLI signaling emerges as a singular mechanism that cooperates with earlier diencephalic conditions to pattern the three neuromeric developmental units of the diencephalon, defined as prosomeres 1–3 (p1–p3) in the prosomeric model ( Martinez S, Puelles E, Puelles L, and Echevarria D (2012) Molecular regionalization of the developing neural tube. In: Watson C, Paxinos G, and Puelles L (eds.) The Mouse Nervous System , pp. 2–18. London: Academic Press (Elsevier); Puelles L and Rubenstein JLR (2003) Forebrain gene expression domains and the evolving prosomeric model. Trends in Neurosciences 9: 469–476). These encompass alar pretectal, thalamic, and prethalamic territories, respectively, as well as corresponding basal (tegmental) territories.