Ungersböck A, Kretz R, Rager G
Institute of Anatomy, University of Fribourg, Switzerland.
J Comp Neurol. 1991 Jun 15;308(3):491-504. doi: 10.1002/cne.903080313.
The primary visual cortex of the tree shrew is characterized by the lack of ocular dominance columns. The two eyes are represented in sublayers of laminae 3 and 4. In an earlier study using the transneuronal transport we observed that the geniculate afferents from the two eyes do not initially overlap and then segregate into their appropriate sublaminae. The final distribution pattern can already be observed during the early postnatal period. Since segregation and elimination of afferent terminal branches do not seem to take place, we wanted to investigate whether or not an overproduction of synapses can be observed as in several other animals. We examined layers 3B, 3C, 4A, and 4B, which receive afferents from the retina via the lateral geniculate nucleus, from P5 to maturity by using the electron microscope. The brain tissue was excised in the region where the central vision is represented in adult animals. Then we determined the density of synapses per 100 microns 2 neuropil for each of the four sublayers at the ages P5, P15, P19, P23, P31, and P42 and in the adult animal (AD). In determining the neuropil we measured the size of two additional compartments, i.e., the compartments consisting of perikarya and of blood vessels. At a higher resolution we determined the fraction of Gray type I and type II synapses in each sublamina and in each developmental stage. The size of the neuropil increases from 57% at P5 to 81% in AD whereas the compartment of perikarya decreases from 42% to 15% and the compartment of blood vessels increases from 1.3% to 3.9%. The synaptic density starts with very low values (3.5/100 microns 2) at P5. Then it increases rapidly and attains a maximal rate of increase during the period of eyelid opening. After this period the increase is slowed down and approaches the adult value (12.5/100 microns 2) slowly. An overproduction of synapses could not be observed. The percentage of type I and type II synapses also changes during this period. The fraction of type I synapses amounts to 73% at P5 and increases to 92% in AD. The increase in density of type I synapses is continuous and does not show any sign of overproduction. The density of type II synapses rapidly reaches it final value and then remains constant. Possibly there is a slight overproduction during the period of eyelid opening.(ABSTRACT TRUNCATED AT 400 WORDS)
树鼩的初级视觉皮层的特征是缺乏眼优势柱。双眼在第3和第4层的亚层中有所代表。在一项早期使用跨神经元运输的研究中，我们观察到来自双眼的膝状体传入纤维最初并不重叠，然后分离到各自合适的亚层中。在出生后早期就能观察到最终的分布模式。由于传入终末分支的分离和消除似乎并未发生，我们想研究是否能像在其他几种动物中那样观察到突触的过度产生。我们通过电子显微镜检查了从P5到成熟阶段的3B、3C、4A和4B层，这些层通过外侧膝状体核接收来自视网膜的传入纤维。在成年动物代表中央视觉的区域切除脑组织。然后我们确定了在P5、P15、P19、P23、P31、P42年龄以及成年动物（AD）时，四个亚层中每100平方微米神经毡的突触密度。在确定神经毡时，我们测量了另外两个区域的大小，即由胞体和血管组成的区域。在更高分辨率下，我们确定了每个亚层和每个发育阶段中I型和II型突触的比例。神经毡的大小从P5时的57%增加到成年时的81%，而胞体区域从42%减少到15%，血管区域从1.3%增加到3.9%。突触密度在P5时开始时非常低（3.5/100平方微米）。然后它迅速增加，并在眼睑张开期间达到最大增加速率。在此之后，增加速度放缓，并缓慢接近成年值（12.5/100平方微米）。未观察到突触的过度产生。在此期间I型和II型突触的比例也发生变化。I型突触的比例在P5时为73%，在成年时增加到92%。I型突触密度的增加是连续的，没有任何过度产生的迹象。II型突触密度迅速达到最终值，然后保持不变。在眼睑张开期间可能有轻微的过度产生。（摘要截断于400字）
