Unstudied molecules that may be related to these functions. These findings support the contention that absence of the AR in SC has major implications for the pubertal and postpubertal events of tubular restructuring that are essential to allow normal initiation and progression of spermatogenesis. The cognitive deficits in patients with DS have been associated with structural changes in the architecture and alterations in the number of dendritic spines. Morphological abnormalities such as unusually long spines, shorter spines, and reduced number of spines have been documented in the cortex of DS fetuses and newborns. Similar alterations were observed in the hippocampal formation, and additional reductions in spine number in adult DS patients have been linked to the 4-(Benzyloxy)phenol development of Alzheimer’s disease pathology. Spine pathology is also present in the Ts65Dn mouse model of DS, which shows decreased spine and synaptic density, and aberrant spine morphology including enlarged spines, irregular spine heads, and globular spine shapes. Since dendritic spines are the primary sites of excitatory synapses, defects in spine structure and function can result in synaptic and circuit alterations leading to cognitive impairment and the progression of AD pathology in DS patients. Unfortunately, there is little information available on the cellular and molecular mechanisms involved in DS spine malformation. In recent years, a number of studies indicate that astrocytes regulate the stability, dynamics and maturation of dendritic spines. In addition, astrocytes participate in the regulation of synaptic plasticity and synaptic transmission. Astrocytes modulate the establishment and maintenance of synaptic contacts through the release of soluble factors such as cholesterol or thrombospondins, or by direct physical interaction with neuronal cells. Our previous research indicates the presence of mitochondrial dysfunction and energy deficits in DS astrocytes leading to abnormal amyloid precursor protein processing and secretion, and to intracellular accumulation of amyloid b. To investigate the role of astrocytes in DS spine pathology, we established a coculture system in which rat hippocampal Diperodon neurons were plated on top of normal or DS astrocyte monolayers. Using this experimental paradigm, we found abnormal spine development and reduced synaptic density and activity in neurons growing on top of DS astrocytes, and identified thrombospondin 1 as a critical astrocytesecreted factor that modulates spine number and morphology. TSP-1 levels were markedly reduced in DS astrocytes and brain homogenates, and restoration of TSP-1 levels prevented spine and synaptic alterations. These results underscore the potential therapeutic use of TSP-1 to treat spine and synaptic pathology in DS and other neurodevelopmental or neurodegenerative conditions. Previous reports have established the critical role of astrocytes in synapse formation and regulation of dendritic spines. However the role of astrocytes in DS spine pathology has not been investigated. Our previous results indicate the presence of mitochondrial dysfunction in DS astrocytes leading to alterations in protein secretion and intracellular Ab accumulation. To establish whether deficits in DS astrocyte function could be involved in spine pathology, rat hippocampal neurons were cultured on top of NL or DS astrocyte monolayers. Under these conditions, rat hippocampal neurons survived well for extended periods of time and developed fast-growing axons and dendrites. No differences in neuronal survival were observed between regular rat hippocampal cultures and rat hippocampal/human astrocyte cocultures. After 21 DIV, neurons developed numerous spines exhibiting characteristic shapes including stubby-, mushroom-, thin- and filopodium-like spines. Stubby spines were especially abundant. In contrast, human cortical neurons exhibited poor spine development in culture, precluding their use for this study. Thus, to assess the capacity of DS astrocytes to sustain spine and synapse formation we utilized human cortical astrocyte/rat hippocampal neuron cocultures.