(A) Embryonic brain slice highlighting the path of developing thalamocortical axons (green lines) and corticothalamic axons (blue lines) at E14.5. A thalamocortical axon reporter mice line (TCA^mGFP) was generated by crossing a Cre-reporter line containing a foxed ‘stop transcription’ sequence in front of membrane-anchored green fuorescent protein (mGFP) followed by an IRES-NLS-lacZ gene inserted into exon 2 of the Tau locus with RORα-Cre mice. (B) Thalamocortical axons extending toward the cortex are GFP labeled in TCA^mGFP reporter mice. (C) Using the TCA^mGFP mice, CB₁R is demonstrated to be localized to corticothalamic, but not thalamocortical, axons during brain development. (D–I) DAGLβ is localized mainly to GFP-labeled thalamocortical axons (dashed arrow in [F]), while MAGL is localized to both CB₁R-containing corticothalamic (arrow in I) and GFP-labeled thalamocortical axons (dashed arrow in [I]). (F & I) higher magnifcation of squared areas in (E & H). CB₁R: CB₁ cannabinoid receptor; cp: Cortical plate; DAGLβ: Diacylglycerol lipase β; ge: Ganglionic eminence; GFP: Green fuorescent protein; hc: Hippocampus; lv: Lateral ventricle; MAGL: Monoglyceride lipase; RORα: Retinoic acid-receptor-related orphan receptor α; st: Striatum; TCA: Thalamocortical axon; th: Thalamus.

The earliest born neurons form the PP, which is later split into the superfcial MZ and the deeper layer SP. Neuroprogenitors residing in the VZ and SVZ in mice produce pyramidal projection neurons in an ‘inside-out’ fashion, migrating radially towards the surface to populate the cp. The cp develops inbetween the MZ and SP and gives rise to the multilayered pattern, while the progenitor zones progressively reduce in size. The relationship between different populations of cells is depicted at E16.5. Radial glial cells divide symmetrically in the VZ to produce additional radial glial cells. A fraction of these depart from the VZ and migrate radially towards the pial surface, giving rise to different types of projection neurons. Interneurons migrate tangentially from the ganglionic eminence and enter the neocortex around E17. cp: Cortical plate; ge: Ganglionic eminence; hc: Hippocampus; lv: Lateral ventricle; MZ: Marginal zone; PP: Preplate; SP: subplate; st: Striatum; SVZ: Subventricular zone; th: Thalamus; VZ: Ventricular zone.

DAGLα and MAGL are expressed starting at midgestation. Expression levels of DAGLα decrease from E18.5 onwards, while there is a transient decrease in MAGL expression levels around birth. CB₁R is expressed at relatively constant levels throughout brain development and, prenatally, is primarily localized to the corticofugal tract. The expression pattern undergoes a dramatic change after birth, with the adult-like distribution pattern in the cortex is apparent after P1. CB₁R: CB₁ cannabinoid receptor; DAGLα: Diacylglycerol lipase α; MAGL: Monoglyceride lipase; P1: Postnatal day 1.
Based on data from [103,114].