Adult neurogenesis was an incidental discovery during the process of perfecting the complex 3H-thymidine autoradiographic technique. Adult-generated neurons were found sparsely scattered in the neocortex (Altman Science 1962.pdf). In the 1990s adult neurogenesis was “rediscovered,” and those studies often did not give credit to the original Altman - Bayer work. Adult-generated neurons are routinely found in the granular layer of the dentate gyrus in the hippocampus and in the granular layer the olfactory bulb (see the olfactory development page). Here are our studies on the adult-generated hippocampal dentate granule cells; the new literature on hippocampal adult neurogenesis has expanded on what we presented many years ago.
Altman Anat Rec 1963.pdf
The arrow indicates a labeled hippocampal granule cell in a young adult rat that was injected two times intraperitoneally with 3H-thymidine at 4 months (one week interval between injections) and survived to 5 months (counterstained autoradiogram). This is Figure 13 in the above paper and is the first illustration (among others in the paper) of a hippocampal neuron generated during the adult period.
These data are volumetric counts of the total number of granule cell nuclei throughout the entire hippocampus in rats at ages 30, 120, 200, and 365 days. The important finding is that the granule cell population substantially increases its numbers with age. The adult-generated neurons are adding to the population.
Initially, 3H-thymadine is picked up by these cells during the S-phase of the cell cycle, and some of their progeny become neurons that differentiate into mature granule cells. The graph in figure 4 shows that when animals survive longer after injections, there is a higher proportion of labeled mature granule cells(black bars) compared to immature cells (clear bars). See also fig. 5 in Altman&Das JCN 1965.pdf
On the left is Figure 2 in Altman&Bayer hippo dev 1975.pdf that shows the heavy 3H-thymidine label uptake by presumptive granule cell precursors in the dispersed subgranular zone in the hilus (HI) of the dentate gyrus. As rats mature, the subgranular zone becomes smaller and is only composed of scattered small cells in adults (Fig. 3 above). Many of the postnatally-generated and all of the adult-generated dentate granule cells arise in this germinal matrix. (AH, Ammon’s horn; GL, dentate granular layer; MO, dentate molecular layer)