Heterochronic cultures were formed with E18 neurons
overlain on confluent layers of astrocytes derived from adult
rat cortex of either of two ages: old, 24 mo, at average life
span; young, 3 mo, at maturity. During 5 days of co-culture,
old astrocytes supported 50% less neurite outgrowth (Fig. 1A
and B). Neuronal cell body size (perikaryal area) showed transient
effects of astrocyte age: at day 1, neuronal perikarya in
co-culture with old-derived astrocytes were smaller by 10 ±
1% (P < 0.05; mean ± S.E.M., four experiments), but age effects
did not persist to day 5. Neuronal density did not differ
by astrocyte age (see Section 2).
We also verified the persistence of the age effect on astrocytic
GFAP in astrocyte–neuron co-cultures, because GFAP
expression in neonatal-derived astrocytes is sensitive to the
presence of neurons [29]. In these co-cultures, GFAP protein
per cell was higher in old versus young astrocytes by 78 ±
6% (P < 0.04; mean ± S.E.M., four experiments), consistent
with in vivo aging changes (see Section 1). However, oldastrocyte
cultures did not contain any very large hypertrophic
cells which could be classified as “reactive”, as observed after
lesions in vitro