Meninges that surround the CNS consist of an outer fibrous sheet of dura mater (pachymeninx) that is also the inner periosteum of the skull. Underlying the dura are the arachnoid and pia mater (leptomeninges) that form the boundaries of the subarachnoid space. In this review we (1) examine the development of leptomeninges and their role as barriers and facilitators in the foetal CNS. There are two separate CSF systems during early foetal life, inner CSF in the ventricles and outer CSF in the subarachnoid space. As the foramina of Magendi and Luschka develop, one continuous CSF system evolves. Due to the lack of arachnoid granulations during foetal life, it is most likely that CSF is eliminated by lymphatic drainage pathways passing through the cribriform plate and nasal submucosa. (2) We then review the fine structure of the adult human and rodent leptomeninges to establish their roles as barriers and facilitators for the movement of fluid, cells and pathogens. Leptomeningeal cells line CSF spaces, including arachnoid granulations and lymphatic drainage pathways, and separate elements of extracellular matrix from the CSF. The leptomeningeal lining facilitates the traffic of inflammatory cells within CSF but also allows attachment of bacteria such as Neisseria meningitidis and of tumour cells as CSF metastases. Single layers of leptomeningeal cells extend into the brain closely associated with the walls of arteries so that there are no perivascular spaces around arteries in the cerebral cortex. Perivascular spaces surrounding arteries in the white matter and basal ganglia relate to their two encompassing layers of leptomeninges. (3) Finally we examine the roles of ligands expressed by leptomeningeal cells for the attachment of inflammatory cells, bacteria and tumour cells as understanding these roles may aid the design of therapeutic strategies to manage developmental, autoimmune, infectious and neoplastic diseases relating to the CSF, the leptomeninges and the associated CNS.

围绕中枢神经系统的脑膜由硬脑膜外纤维膜（pachymeninx）组成，也是颅骨的内部骨膜。硬脑膜下面是蛛网膜和软脑膜（软脑膜），它们形成了蛛网膜下腔的边界。在这篇综述中，我们（1）研究了软脑膜的发展及其在胎儿中枢神经系统中作为屏障和促进剂的作用。胎儿早期有两个独立的CSF系统，脑室内的CSF和蛛网膜下腔的外部CSF。随着Magendi和Luschka平台的发展，一个连续的CSF系统不断发展。由于胎儿生命中缺乏蛛网膜颗粒，因此最有可能通过穿过筛状板和鼻粘膜下层的淋巴引流途径消除了脑脊液。（2）然后，我们回顾了成年人类和啮齿类动物软脑膜虫的精细结构，以确立它们作为体液，细胞和病原体运动的障碍和促进者的作用。薄脑膜细胞在脑脊液空间中排列，包括蛛网膜颗粒和淋巴排泄途径，以及从脑脊液中分离出细胞外基质的元素。软脑膜衬里促进脑脊液内炎性细胞的运输，但也允许细菌附着，例如脑膜炎奈瑟氏球菌和作为CSF转移的肿瘤细胞。单层的软脑膜细胞延伸到与动脉壁紧密相关的大脑中，因此大脑皮层中的动脉周围没有血管周围的空间。白质和基底神经节中动脉周围的血管周围空间与它们的两个软脑膜包围层有关。（3）最后，我们检查了由脑膜小脑细胞表达的配体在炎性细胞，细菌和肿瘤细胞附着中的作用，因为了解这些作用可能有助于设计治疗策略，以管理与CSF相关的发育，自身免疫，传染性和肿瘤性疾病，软脑膜及相关的中枢神经系统。