Time of origin of neurons of thalamus ventralis, thalamus dorsalis, and epithalamus was determined autoradiographically at three transverse levels in adults which received thymidine-H3 once during gestation. A ventrodorsul gradient in the entire region demonstrates a wave of neuron origin in the forebrain, probably rostrocaudal originally but revectored by rearrangement of neuromeres 1-111. A caudorostrul gradient in dorsal thalamus similarly may represent redirection of a n earlier ventrodorsal one. A lateromedial gradient in dorsal thalamus and epithalamus (opposite that i n cortex) may reflect lateral displacement of neuroblasts (or their nuclei) by younger elements. Large neurons generally arise before small ones, as seen in epithalamus; the two habenular nuclei illustrate programmed origin of large and small neurons and glia consecutively, with progressively restricted potentiality by the primitive ependyma. Phylogenetically, slight differences in duration of proliferation might radically alter the numbers of small neurons. Although dorsal thalamic nuclei "appear" in external germinal and mantle layers, their neuroblasts originate in primitive ependyma; time of origin accords with subsequent recognition of nuclei and appearance of thalamocortical connections. Massive proliferation (from the 10th to 15th days of gestation) and the gradients overshadow nuclear differences and demonstrate a developmental unity in the dorsal thalamus. The findings further validate Herrick's diencephalic zones and suggest some orderly mechanism, probably at the molecular level, underlying generation of the neurons in this complex brain region.The improved resolution brought to morphogenetic studies by autoradiography with titiated thymidine has initiated a systematic regional analysis of brain development with this technique. It is now possible to trace the history of individual neurons from proliferative through migratory stages to ultimate disposition in the mature brain. A label affixed premitotically to a neuron precursor is p e fmunent; once a neuroblast Ieaves its germinal center, it loses the capacity to divide. Moreover, a rigorous schedule of proliferative events within the matrix layer of the neuraxis facilitates selective labeling by appropriate timing of radioactive thymidine injections. Thus, the identities of neurons originating as end products of germinal cell division at a given developmental stage may be ascertained by noting their positions in the adult brain. Neighboring neurons, originating earlier or later, remain nonradioactive.Exploiting this technique, Angevine ('65), Taber Pierce ('66, '67) and Hinds ('67, '68a,b) and elaborate timetables of neuronal birthdates for the components of the hippocampal formation, various nuclear complexes of the brain stem, and olfactory bulb, respectively. For the latter two regions, moreover, examination of serial embryonic stages following the stage at which thymidine-H3 was administered permitted isolation and day-by-day tracing of the paths of young neurons.The...
The layers and cells of the early developing central nervous system lack direct counterparts in the adult and must be designated by a special terminology. The inconsistent and inaccurate langua,ze now in use leads to misunderstanding and a revision is proposed in which the four fundamental zones are termed the ventricular, subventricular, intemediate, and marginal zones. Each is defined according to the form, behavior, and fate of its constituent cells. All neurons and macroglia of the central nervous system can be derived from these developmental zones.
EIGHT FIQURESThe vast growth of literature on the mitotic effects of colchicine (Eigsti and Dustin, '47, '49, '55) overshadows a number of interesting writings, often incidental to other studies, revealing that the drug exerts profound effects upon the nervous system. And buried here and there one can find allusions, albeit scant, to observable morphological changes in nervous tissues following colchicine treatment. Recently, studying effects of the drug on limb regeneration in salamanders, Singer, Flinker, and Sidman ( '56) observed that colchicine destroys the peripheral nerve. This effect, unknown previously, is extended herewith to the mammal, where the nature of the nerve fiber destruction, the time relations, and the extent of damage are detailed.In the colchicine literature are many reports, even of the ancients, of the distressing symptoms-vomiting, diarrhea, grave lethargy, and respiratory crisis-which attend colchicine intoxication and several workers have stressed the fact that these disturbances are neurogenic : Jacobj (1890) noted the delayed onset of the symptoms, postulated excitation of The paper is a condensation of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Zoology a t Cornell University.
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