Pattern formation in rat forebrain

Vincent A Mazzarella vamg6792 at uxa.cso.uiuc.edu
Sat May 18 12:21:17 EST 1991

The following is a one page synopsis of a recent one-hour seminar.
The impressions are my own and do not reflect accuracy of the facts
contained in the presentation. Corrections and discussion are welcomed.

Using a marker for opiate receptors, different compartments of
the rat forebrain are distinguished: superficial (layers 2,3)
cortex vs. deep (layers 5,6) cortex and "patch" areas of striatum
vs. "matrix" areas of striatum. The deep cortex and patch
striatum label heavily for opiate receptors, the other areas do
not. Other markers, such as for cholinesterase, for example, may
show a complementary pattern, but compartmentalization is
maintained nevertheless. Such compartmentalization of opiate
receptors appears to exist only in mammalian species, not
invertebrates. Furthermore, the number of striatal patch areas in
different mammalian species is constant, but the size of the
patches increase with increased cell number.
   Neurons divide in the peri-ventricular area and then migrate
outward toward the cortex. The "birthdate" of a neuron is the
time when it becomes post-mitotic. If a 3H-thymidine label is
injected just before a cell's birthdate, that cell will retain
much label in its DNA, whereas dividing cells will dilute label
taken up as they divide. Using this method of determining
birthdate, the cells of the striatal patches and those of the
deep cortex are seen to have early birthdates of about E13.
Matrix striatal neurons and superficial cortical neurons have
later birthdates, migrating around E19-20. Retrograde fluorescent
label injected into substantia nigra shows projections from the
striatum at E19 are primarily from patch neurons; only post-
natally do matrix striatal neurons send their axons to the sub.
nigra. It is suggested that as the late birthdate matrix neurons
migrate to the striatum, they push apart the patch neurons, kind
of like swiss cheese. Cultured, separated patch cells will re-
aggregate in vitro, indicating a natural adhesiveness, but matrix
cells do not. However, if an explant of the striatum is placed in
the retina and allowed to develop, the patch/matrix division does
not occur, suggesting some time of input may be necessary to
ensure compartmentalization; patch cell adhesiveness is not
sufficient. (It should be noted that striatal patch neurons will
aggregate with deep cortex neurons, and that both sets of neurons
have the same birthdate. Thus birthdate may be directly related
to adhesiveness.)
   To try to determine what sort of input might be important in
compartmentalization, the effect of a unilateral sub. nigra
lesion at day E19 on patches was examined on the ipsilateral side
and comparted to contralateral control. (It had been shown that
striatal patch cells are labelled for tyrosine hydoxylase,
indicating DAergic input from nigral cells.) The experiment was
inconclusive, showing only a reduction in opiate receptor on the
affected side, but not on number or viability of patch cells
involved. Because sub nigra <--> striatum connections start to
form at about E14, these lesions may be too late to be
   Is compartmentalization accounted for by simple clonality of a
progenitor cell? Retroviruses with beta-galactosidase marker were
injected into forebrain lateral ventricle in concentration
sufficient to only infect 1-3 cells. Progeny of cells that
incorporate the retrovirus into their genome show the beta-gal
marker. It was found that clones were restricted to respective
compartments. However, when chimeric animals were made from
blastocysts of different mice species, the cells of each parent
from a different species appeared in each compartment, therefore
clonality alone does not cause compartmentalization.

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