The morphology of microglial cells is often closely related to their functions. The mechanisms that regulate microglial ramification are not well understood. Here we reveal the biological mechanisms by which astrocytes regulate microglial ramification. Morphological variation in mouse microglial cultures was measured in terms of cell area as well as branch number and length. Effects on microglial ramification were analyzed after microinjecting the toxin L-alpha-aminoadipic acid (L-AAA) in the mouse cortex or hippocampus to ablate astrocytes, and after culturing microglia on their own in an astrocyte-conditioned medium (ACM) or together with astrocytes in coculture. TGF-β expression was determined by Western blotting, immunohistochemistry, and ELISA. The TGF-β signaling pathway was blocked by the TGF-β antibody to assess the role of TGF-β on microglial ramification. The results showed that microglia had more and longer branches and smaller cell bodies in brain areas where astrocytes were abundant. In the mouse cortex and hippocampus, ablation of astrocytes by L-AAA decreased number and length of microglial branches and increased the size of cell bodies. Similar results were obtained with isolated microglia in culture. However, isolated microglia were able to maintain their multibranched structure for a long time when cultured on astrocyte monolayers. Ameboid microglia isolated from P0 to P3 mice showed increased ramification when cultured in ACM or on astrocyte monolayers. Microglia cultured on astrocyte monolayers showed more complex branching structures than those cultured in ACM. Blocking astrocyte-derived TGF-β decreased microglial ramification. Astrocytes induced the formation of protuberances on branches of microglia by forming glial fibers that increased traction. These experiments in mice suggest that astrocytes promote microglial ramification by forming glial fibers to create traction and by secreting soluble factors into the surroundings. For example, astrocyte-secreted TGF-β promotes microglia to generate primitive branches, whose ramification is refined by glial fibers.

小胶质细胞的形态通常与其功能密切相关。调节小胶质细胞分枝的机制尚不十分清楚。在这里，我们揭示了星形胶质细胞调节小胶质细胞分支的生物学机制。小鼠小胶质细胞培养物中的形态学变化根据细胞面积以及分支数目和长度进行测量。在小鼠皮层或海马中微注射毒素L-α-氨基己二酸（L-AAA）以消融星形胶质细胞，并在星形胶质细胞条件培养基（ACM）或与之一起培养小胶质细胞后，分析了对小胶质分枝的影响共培养中的星形胶质细胞。通过Western印迹，免疫组织化学和ELISA确定TGF-β的表达。TGF-β抗体阻断了TGF-β信号通路，以评估TGF-β在小胶质细胞分支中的作用。结果表明，在星形胶质细胞丰富的脑区域，小胶质细胞的分支越来越长，细胞体也较小。在小鼠皮质和海马中，L-AAA消融星形胶质细胞减少了小胶质细胞分支的数量和长度，并增加了细胞体的大小。在培养物中分离的小胶质细胞获得了相似的结果。但是，分离的小胶质细胞在星形胶质细胞单层上培养时能够长时间维持其多分支结构。当从A0或星形胶质细胞单层培养时，从P0到P3小鼠分离的变形虫小胶质细胞显示出增加的分枝。在星形胶质细胞单层上培养的小胶质细胞比在ACM中培养的小胶质细胞显示更复杂的分支结构。阻断星形胶质细胞来源的TGF-β可降低小胶质细胞的分支作用。星形胶质细胞通过形成增加牵引力的神经胶质纤维来诱导小胶质细胞分支上的突起的形成。这些在小鼠中进行的实验表明，星形胶质细胞通过形成神经胶质纤维以产生牵引力并通过向周围环境分泌可溶性因子来促进小胶质细胞分支。例如，星形胶质细胞分泌的TGF-β促进小胶质细胞生成原始分支，其分支通过神经胶质纤维得以改善。这些在小鼠中进行的实验表明，星形胶质细胞通过形成神经胶质纤维以产生牵引力并通过向周围环境分泌可溶性因子来促进小胶质细胞分支。例如，星形胶质细胞分泌的TGF-β促进小胶质细胞生成原始分支，其分支通过神经胶质纤维得以改善。这些在小鼠中进行的实验表明，星形胶质细胞通过形成神经胶质纤维以产生牵引力并通过向周围环境分泌可溶性因子来促进小胶质细胞分支。例如，星形胶质细胞分泌的TGF-β促进小胶质细胞生成原始分支，其分支通过神经胶质纤维得以改善。