title: Critical periods in the development of motoneurons.
abstract: We define "critical periods" in motoneuron development as periods when neurons can express a particular aspect of their sequence of differentiation only within a limited time interval. Embryonic rat motoneurons were axotomized and their survival and capacity to regenerate their axons studied at later times. A critical period of absolute target dependence extended from embryonic day 14 (E14) until E17. All motoneurons axotomized during this interval died, while cells axotomized earlier or later could survive. During the period E18- postnatal day 14 (PN14) regenerating motoneurons accurately reinnervated motor unit territories of normal size, and intact motoneurons did not exhibit collateral sprouting in response to denervation of adjacent muscle fibres. After PN21, muscle fibres were reinnervated in a spatially indiscriminate manner, and collateral sprouting occurred in response to partial denervation of muscles. We conclude that adult motoneurons have a powerful capacity to reinnervate denervated muscles, even in an inappropriate manner, in contrast to motoneurons in neonates which can recapitulate embryogenesis and accurately reinnervate their proper motor unit territories.
authors.affiliation: Department of Physiology, University of Otago, Dunedin, New Zealand.
authors.forename: A J
authors.initials: AJ
authors.lastname: Harris
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009416:Nerve Regeneration; D051381:Rats