here are two papers dealing with the dolphin brain as a
model for investigating the occurrence of "adult
neurogenesis" in mammals devoid of olfaction. Adult
neurogenesis, namely the continuous genesis of new neurons
through adulthood, in laboratory rodents is linked to
specific functions such as olfaction and, for this reason,
is highly reduced in humans. The first paper shows that
neurogenic processes are completely absent in dolphins
(since birth); the second one is a review article
discussing the high reduction of adult neurogenesis in
humans and the evolutionary aspects of such reduction. The
occurrence/absence of neurogenesis in humans is at present
a very hot topic in Neurosciences and the study of the
dolphin brain greatly contributes to the general debate.
Abstract
Pre-clinical research is carried out on
animal models, mostly laboratory rodents, with the
ultimate aim of translating the acquired knowledge to
humans. In the last decades, adult neurogenesis (AN) has
been intensively studied since it is viewed as a tool
for fostering brain plasticity, possibly repair. Yet,
occurrence, location, and rate of AN vary among mammals:
the capability for constitutive neuronal production is
substantially reduced when comparing small-brained,
short living (laboratory rodents) and large-brained,
long-living species (humans, dolphins). Several
difficulties concerning scarce availability of fresh
tissues, technical limits and ethical concerns did
contribute in delaying and diverting the achievement of
the picture of neurogenic plasticity in large-brained
mammals. Some reports appeared in the last few years,
starting to shed more light on this issue. Despite
technical limits, data from recent studies mostly
converge to indicate that neurogenesis is vestigial, or
possibly absent, in regions of the adult human brain
where in rodents neuronal addition continues into adult
life. Analyses carried out in dolphins, mammals devoid
of olfaction, but descendant of ancestors provided with
olfaction, has shown disappearance of neurogenesis in
both neonatal and adult individuals. Heterogeneity in
mammalian structural plasticity remains largely
underestimated by scientists focusing their research in
rodents. Comparative studies are the key to understand
the function of AN and the possible translational
significance of neuronal replacement in humans. Here, we
summarize comparative studies on AN and discuss the
evolutionary implications of variations on the
recruitment of new neurons in different regions and
different species.
Keywords: adult neurogenesis, brain plasticity and
aging, comparative anatomy, doublecortin, immature
neurons
Parolisi R, Cozzi B, Bonfanti L (2017)
Non-neurogenic
SVZ-like niche in dolphins, mammals devoid of olfaction.
Brain Structure and Function 222:2625-2639. doi:
10.1007/s00429-016-1361-3
Brain Struct Funct link:
https://rdcu.be/2Cyu
Abstract
Adult neurogenesis has been implicated in brain
plasticity and brain repair. In mammals, it is
mostly restricted to specific brain regions and
specific physiological functions. The function and
evolutionary history of mammalian adult neurogenesis
has been elusive so far. The largest neurogenic site
in mammals (subventricular zone, SVZ) generates
neurons destined to populate the olfactory bulb. The
SVZ neurogenic activity appears to be related to the
dependence of the species on olfaction since it
occurs at high rates throughout life in animals
strongly dependent on this function for their
survival. Indeed, it dramatically decreases in
humans, who do not depend so much on it. This study
investigates whether the SVZ neurogenic site exists
in mammals devoid of olfaction and olfactory brain
structures, such as dolphins. Our results demonstate
that a small SVZ-like region persists in these
aquatic mammals. However, this region seems to have
lost its neurogenic capabilities since neonatal
stages. In addition, instead of the typical newly
generated neuroblasts, some mature neurons were
observed in the dolphin SVZ. Since cetaceans evolved
from terrestrial ancestors, non-neurogenic SVZ may
indicate extinction of adult neurogenesis in the
absence of olfactory function, with the retention of
an SVZ-like anatomical region either vestigial or of
still unknown role.
Keywords: Adult neurogenesis; Brain
plasticity; Cetaceans; Doublecortin; Evolution;
Olfactory bulb; Subventricular zone
Luca Bonfanti & Bruno Cozzi