The paper examines how the different types of neurons and the layered structure of the dorsal horn of the mouse spinal cord are formed during development.[1][3] Using single-cell RNA sequencing, the authors show that neurons in the dorsal part of the spinal cord arise in six successive waves of neurogenesis, with each wave producing one excitatory and one inhibitory "family" of neurons.[1][3] They found that the main principle of organization is temporal diversity: excitatory neurons born in different time waves arrange themselves into adjacent laminae, forming a chronotopic map of the dorsal horn.[1][3] This chronotopic organization of excitatory neurons is also necessary for the correct spatial arrangement of inhibitory neurons and descending sensory axons.[1][3] At a finer level of cell types, the paper shows that a dorsoventral gradient of Zic transcription factors in progenitor cells divides each family into multiple specific neuron types and is required for proper cell composition in the dorsal horn.[1][3] The work provides a detailed "catalogue" of dorsal spinal neuron types, including their origin, time of generation, gene profiles, and spatial distribution, and describes the basic temporal and spatial rules that enable the formation of structures that control sensorimotor behavior.[1][2][3]