Kl. Earle et J. Mitrofanis, IDENTIFICATION OF TRANSIENT MICROGLIAL CELL COLONIES IN THE FOREBRAINWHITE-MATTER OF DEVELOPING RATS, Journal of comparative neurology, 387(3), 1997, pp. 371-384
Herein, we describe the existence of distinct colonies of transient mi
croglial cells that reside in well-defined zones of the forebrain whit
e matter. Rats, aged at postnatal day (P) 0, P2, P5, P7, P10, P15 or a
dult, were anaesthetised with halothane gas, and various neural centre
s were injected unilaterally with the tracer biotinylated Dextran. The
neural centres injected were cingulate or sensorimotor cortices, vent
ral nuclei of the dorsal thalamus, and the pontine reticular formation
of the brainstem. Rats were allowed to survive to various stages, fro
m 4 hours to 21 days, after the injection. They were then anaesthetise
d with sodium pentobarbitone, and their brains were aldehyde-fixed and
processed by using standard methods. The following is a description o
f what is seen after injections at PO, P2, P5, P7, P10; we saw no labe
lled cells (described below) in the rats injected at P15 or adult. Fro
m 2 to 21 days after an injection of dextran into the above-mentioned
centres, labelled microglial cell colonies, identified by using double
-labelling with anti-OX-6 or Griffonia simplicifolia (Bandeiraea; isol
ectin B4), were seen in small isolated zones in the forebrain white ma
tter. These colonies were in the corpus callosum, the dorsal and ventr
al regions of the external capsule, and the internal capsule. A striki
ng feature of these labelled microglial cell colonies was that they we
re seen on both sides of the brain. Thus, regardless of the location o
f the injection site in either the cortex, thalamus, or brainstem, the
same microglial cell colonies were labelled with dextran in the foreb
rain white matter. After injections of different coloured fluorescent
dextrans into the cortex and into the brainstem of the same animal, ma
ny double-labelled cells in each of the colonies were seen. From our s
hort-term survival cases (4 hours to I day), a rather strict sequence
or progression of labelling of the colonies across the white matter fr
om tile injection site was seen; in general, the microglial cell colon
ies closest to the injection site became labelled well before (about a
day) those further away These results lead us to suggest that the mic
roglial cells in each colony become labelled after a slow diffusion of
the tracer through the extracellular space from the injection site. (
C) 1997 Wiley-Liss, Inc.