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Synaptogenesis in the primary visual cortex of the tree shrew ( Tupaia belangeri )
Author(s) -
Ungersböck Alfred,
Kretz Robert,
Rager Günter
Publication year - 1991
Publication title -
journal of comparative neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.855
H-Index - 209
eISSN - 1096-9861
pISSN - 0021-9967
DOI - 10.1002/cne.903080313
Subject(s) - neuropil , biology , anatomy , lateral geniculate nucleus , visual cortex , ocular dominance column , diencephalon , synaptogenesis , neuroscience , central nervous system , ocular dominance
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 μm 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 μm 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 μm 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 intensity of type I synapses is continuous and does not show any sign of overproduction. The density of type II synapses rapidly reaches its final value and then remains constant. Possibly there is a slight overproduction during the period of eyelid opening. The fact that an overproduction of synapses could not be observed may be related to the specific pattern of ingrowth of geniculate afferents which seem to go to their appropriate laminae without any detectable sign of segregation.