Florence S L, Casagrande V A
Department of Cell Biology, Vanderbilt University, Nashville, TN 37232.
Vis Neurosci. 1990 Sep;5(3):291-309. doi: 10.1017/s0952523800000365.
The main objective of the present study was to describe the postnatal development of magnocellular and parvocellular LGN axons within the primate striate cortex. For this purpose, we bulk labeled axons in neonatal prosimians (galagos) in vivo or in vitro at regular intervals from birth (PO) to 12 weeks after birth by injecting horseradish peroxidase (HRP) into white matter anterior to the striate cortex. Filled axons within layer IV were reconstructed, quantitatively analyzed, and compared to a population of adult axons described previously (Florence & Casagrande, 1987). Our results show that although axons are morphologically immature at birth, they are restricted to the upper (IV alpha) and lower (IV beta) tiers of layer IV of the striate cortex as in adults. In adults, we referred to the presumed magnocellular LGN axons terminating in IV alpha as type I and the presumed parvocellular axons terminating in IV beta as type II. We used the same convention for developing axons. From birth to 3 weeks postnatal, type I and II axon classes are more variable in appearance than adult counterparts, and are not morphologically class distinct. As axons mature, parent axon shafts increase in caliber, arbors become smaller and more radial, and other immature features (e.g. spikes, protrusions, growth cones) are less evident. Both arbor classes mature slowly and some still exhibit immature features (e.g. growth cones) as late as 12 weeks postnatally. Although arbors do not show class-distinctive features until late in development, each class does show some unique maturational trends. Type I arbors are only slightly larger than adult counterparts at birth, whereas type II arbors are dramatically larger. Type I arbors increase in branch complexity with age, whereas type II arbors simply show a shift in complexity toward the center of the arbor with decreasing size over time. These growth trends suggest that magnocellular and parvocellular pathways to cortex could be differentially vulnerable to the manipulation of postnatal visual experience.
本研究的主要目的是描述灵长类动物纹状体内大细胞和小细胞外侧膝状体轴突的出生后发育情况。为此，我们在新生原猴（婴猴）出生时（出生后第0天）至出生后12周期间，定期通过将辣根过氧化物酶（HRP）注入纹状皮质前方的白质，在体内或体外对轴突进行大量标记。对IV层内充满标记物的轴突进行重建、定量分析，并与先前描述的成年轴突群体进行比较（Florence & Casagrande，1987）。我们的结果表明，尽管轴突在出生时形态上不成熟，但它们如在成年动物中一样，局限于纹状皮质IV层的上部（IVα）和下部（IVβ）层。在成年动物中，我们将终止于IVα层的推测大细胞外侧膝状体轴突称为I型，将终止于IVβ层的推测小细胞轴突称为II型。对于发育中的轴突，我们采用相同的分类方法。从出生到出生后3周，I型和II型轴突类别的外观比成年对应物更具变异性，且在形态上没有明显的类别区分。随着轴突成熟，母轴突干的直径增大，树突分支变小且更呈放射状，其他不成熟特征（如棘突、突起、生长锥）则不太明显。两类树突分支成熟缓慢，有些在出生后12周时仍表现出不成熟特征（如生长锥）。尽管树突分支在发育后期才表现出类别特异性特征，但每一类都显示出一些独特的成熟趋势。I型树突分支在出生时仅比成年对应物略大，而II型树突分支则明显更大。I型树突分支的分支复杂性随年龄增加，而II型树突分支只是随着时间推移尺寸减小，复杂性向树突中心转移。这些生长趋势表明，通向皮质的大细胞和小细胞通路可能对出生后视觉经验的操纵有不同的易损性。
