Axonal guidance of adenosine deaminase immunoreactive primary afferent fibers in developing mouse spinal cord

This study examined the precision of central fiber growth in a subpopulation of dorsal root ganglion neurons in developing mouse spinal cord. Immunohistochemical techniques using a monospecific, polyclonal antiserum to mouse adenosine deaminase (ADA) were utilized to label a population of primary sensory afferents that have been found to exclusively innervate laminae I and II of the dorsal horn in adult mice. Initial growth of ADA-immunoreactive (ADA-IR) primary afferents occurred very early in development, embryonic day 10 (E10), a time coincident with the earliest settling time of dorsal root ganglion neurons. Adenosine deaminase immunoreactive primary afferents were observed throughout the cross-sectional area of the primordial dorsal funiculus (DF) as early as E10. Immunostained fibers remained quiescent in the DF during its growth and separation into the tract of Lissauer and dorsal column pathway. By E15, the two pathways had formed and ADA-IR fibers were observed exclusively in the tract of Lissauer. This segregation of fibers remained throughout development and reflected the adult pattern. Growth was reinitiated at E16 when the fibers advanced into the dorsal horn and proceeded directly to laminae I and II mimicking their adult distribution. Exuberant fiber growth was not detected throughout their development. These results strongly suggest that ADA-IR fibers exhibit precise fiber guidance to a preferred pathway, the tract of Lissauer, and accurate laminar innervation of the dorsal horn.