An analysis of the time of origin of neurons in the entorhinal and subicular cortices of the cat

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Abstract
The [3H]thymidine autoradiographic method was used to determine the birth dates of neurons in the cat parahippocampal gyrus. Cat fetuses were exposed to a single pulse of the radioactive marker between the 20th and 55th embryonic days. All animals were delivered normally and allowed to survive for 2–6 months postnatal. The resulting autoradiographs demonstrate three spatiotemporal gradients of cell birth in the entorhinal and subicular cortices. First, an inside-out gradient is apparent; i.e., neurons in the deeper layers are born earlier than those in the more superficial layers. Second, a rhino to dentate gradient exists. Accordingly, cells closer to the lateral entorhinal region tend to be generated earlier than those further away. Third, a temporal to septal gradient is present. Neurons close to the anterior pole of the temporal lobe are born earlier than those more caudally located. Whereas the first two gradients have been observed in other species, the latter gradient has not been reported consistently. Three exceptions to these overall gradients exist. First, neurons near the layer I/II border are born earlier than the majority of the layer II neurons. Second, neurons near the transition zone between two adjacent regions are born earlier than neurons located in the middle of each region. Third, the prosubiculum and subiculum do not exhibit a clear inside-out or temporal to septal gradient.