Sustentacular Cell Enwrapment of Olfactory Receptor Neuronal Dendrites: An Update

Abstract

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.

Anosmia in COVID-19: Underlying Mechanisms and Assessment of an Olfactory Route to Brain Infection

“…However, at closer look, there are three major inconsistencies with this scenario: the time course of cellular regeneration versus clinical recovery, the lack of expression of viral entry proteins, and the absence of the virus within olfactory neurons. When olfactory receptor neurons die, their replacement requires 8 to 10 days ( Brann and Firestein 2014 ; Schwob 2002 ; Schwob and others 1995 ), plus about 5 days for cilia maturation ( Liang 2020 ), but the time course of smell recovery in COVID-19 often is less than one week ( Dell’Era and others 2020 ; Kaye and others 2020 ; Lee and others 2020 ; Printza and Constantinidis 2020 ; Sayin and Yazici 2020 ; Sedaghat and others 2020 ; Vaira and others 2020a ; von Bartheld and others 2020 ). Thus, functional recovery after anosmia often is faster than the time it takes for neuron replacement, cilia maturation, and the growth of the new axons from the olfactory epithelium through the cribriform plate to form synapses in the olfactory bulb ( Bryche and others 2020 ; Liang 2020 ; Schwob 2002 ; Soler and others 2020 ; Fig.…”

“…The cilia began to recover within 7 to 10 days after infection ( Bryche and others 2020 ). This suggests that the odorants would fail to bind to their cognate odorant receptors until cilia are structurally and functionally restored ( Liang 2020 ). Sustentacular cells appear to be essential for the maintenance and normal function of the cilia extending from the knobs ( Fig.…”

A Comparison of the Primary Sensory Neurons Used in Olfaction and Vision

“…Mature OSN dendrites appear to be fully enveloped by a single support cell while immature neurons extend dendrites between support cells (Nomura et al, 2004;Liang, 2018). The function of this envelopment remains unknown but the similarities between support cells and the myelinating Schwann cells and oligodendrocytes raises the possibility that these cells function as a ''pseudo-myelin sheath'' around the OSN dendrite (Liang, 2020). Support cells further assist the OSN with functions typically assigned to glia such as regulation of ion homeostasis and metabolic coupling that provides the glucose needed to power odorant detection in the OSN cilia (Suzuki et al, 1996;Menco et al, 1998;Hegg et al, 2009;Tang et al, 2009;Nunez-Parra et al, 2011;Villar et al, 2017).…”

The Cellular basis of loss of smell in 2019-nCoV-infected individuals

“…In addition to these, the olfactory epithelium also possesses several other cell types for the maintenance of tissue architecture and homeostasis. SUS and MVC subtypes are responsible for providing metabolic as well as physical support to the olfactory epithelium [ 29 ]. GBCs and HBCs collectively constitute the basal stem cell population and mainly reside near the basal lamina [ 30 ].…”