Department of Anatomy, Yong Loo Lin School of Medicine, National University of Singapore, 4 Medical Drive, Singapore 117594, Singapore.
Genes (Basel). 2020 Apr 30;11(5):493. doi: 10.3390/genes11050493.
The pseudostratified olfactory epithelium (OE) may histologically appear relatively simple, but the cytological relations among its cell types, especially those between olfactory receptor neurons (ORNs) and olfactory sustentacular cells (OSCs), prove more complex and variable than previously believed. Adding to the complexity is the short lifespan, persistent neurogenesis, and continuous rewiring of the ORNs. Contrary to the common belief that ORN dendrites are mostly positioned between OSCs, recent findings indicate a sustentacular cell enwrapped configuration for a majority of mature ORN dendrites at the superficial layer of the OE. After vertically sprouting out from the borderlines between OSCs, most of the immature ORN dendrites undergo a process of sideways migration and terminal maturation to become completely invaginated into and enwrapped by OSCs. Trailing the course of the dendritic sideways migration is the mesodendrite (mesentery of the enwrapped dendrite) made of closely apposed, cell junction connected plasma membrane layers of neighboring folds of the host sustentacular cell. Only a minority of the mature ORN dendrites at the OE apical surface are found at the borderlines between OSCs (unwrapped). Below I give a brief update on the cytoarchitectonic relations between the ORNs and OSCs of the OE. Emphasis is placed on the enwrapment of ORN dendrites by OSCs, on the sideways migration of immature ORN dendrites after emerging from the OE surface, and on the terminal maturation of the ORNs. Functional implications of ORN dendrite enwrapment and a comparison with myelination or Remak's bundling of axons or axodendrites in the central and peripheral nervous system are also discussed.
假复层嗅上皮（OE）在组织学上可能看起来相对简单，但细胞类型之间的细胞学关系，尤其是嗅感觉神经元（ORNs）和嗅支持细胞（OSCs）之间的关系，比以前认为的更为复杂和多变。增加复杂性的是 ORN 的短寿命、持续的神经发生和不断的再连接。与 ORN 树突主要位于 OSCs 之间的普遍观点相反，最近的发现表明，在 OE 的浅层，大多数成熟的 ORN 树突呈支持细胞包裹的形态。大多数不成熟的 ORN 树突从 OSCs 的边界垂直伸出后，经历侧向迁移和末端成熟的过程，完全内陷并被 OSCs 包裹。尾随树突侧向迁移的是中树突（包裹树突的系膜），由相邻 OSCs 宿主支持细胞折叠的紧密贴合、细胞连接连接的质膜层组成。只有少数成熟的 ORN 树突位于 OE 顶端表面的 OSCs 边界处（未包裹）。下面我简要介绍一下 OE 中 ORNs 和 OSCs 的细胞构筑关系。重点介绍 ORN 树突被 OSCs 包裹、不成熟的 ORN 树突从 OE 表面伸出后的侧向迁移以及 ORNs 的末端成熟。还讨论了 ORN 树突包裹的功能意义，并与中枢和周围神经系统中轴突或轴突树突的髓鞘形成或 Remak 束进行了比较。
