Invertebrate Myelin Web Page

[updated January 2009] "Glial cells surround the neurons of invertebrates ... but in a loose association that does not form true myelin"

F. Delcomyn: Foundations of Neurobiology W.H. Freeman 1998 [more...]

... so why bother reading what follows?

Myelin

Myelin is a multilayered membranous ensheathment of axons. It speeds nerve impulse conduction, decreases metabolic costs of neural activity and decreases space requirements of the nervous system. These features have clear selective advantage. Myelin is best known and best characterized in vertebrates, where it is a key to subphylum success. Several serious diseases in humans are caused by pathologies involving myelin. Myelin is often considered to be an exclusively vertebrate innovation. However, functionally-equivalent ensheathments of axons have arisen apparently independently at least twice each in annelids and crustaceans (Schweigreiter et al 2006). For a detailed summary of the occurrence of myelin among invertebrates, click here . A list of references to the long scientific literature on invertebrate myelin follows:

Literature on invertebrate myelin

General reviews etc

Friedländer, B. 1889. "Über die markhaltigen Nervenfasern und Neurochorde der Crustaceen und Anneliden." Mitt. Zool. Sta. Neapel 9: 205-265.
Roots, B.I. 1984. "Evolutional aspects of the structure and function of the nodes of Ranvier." In: Zagoren, JC, Fedoroff S (eds) The Node of Ranvier. Academic Press, Orlando, pp 1-29
Waehneldt, T.V. 1990. "Phylogeny of myelin proteins." Ann. N.Y. Acad. Sci. 605 15-28.
Roots, B.I. 1993. "The evolution of myelin." Adv. neural Sci. 1: 187-213
Roots, B.I. 1995. "The evolution of myelinating cells." In A.Vernadakis and B. Roots, (eds) Neuron-Glia Interrelations During Phylogeny: I. Phylogeny and Ontogeny of Glial Cells. Humana Press Inc., Totowa, NJ.
Schweigreiter, R., B.I. Roots, C.E. Bandtlow, and R.M. Gould. 2006. "Understanding myelination through studying its evolution." Int. Rev. Neurobiol. 73: 219-73.
Hartline, D.K. and D. Colman 2007. "Rapid conduction and the evolution of giant axons and myelinated fibers" Curr. Biol. 17: R29-R35 . or [PDF].

Oligochaete myelin

Hama, K. 1959. "Some observations on the fine structure of the giant nerve fibers of the earth worm Eisenia foetida." J. biophys. biochem Cytol. 6: 61-66.
Levi, J.U., R.R. Cowden, and G.H. Collins 1966. "The microscopic anatomy and ultrastructure of the nervous system of the earthworm (Lumbricus sp.) with emphasis on the relationship between glial cells and neurons." J. comp. Neurol. 127: 489-510.
Günther, J. 1976. "Impulse conduction in the myelinated giant fibers of the earthworm. Structure and function of the dorsal nodes in the median giant fiber." J. Comp. Neurol. 168: 505-532.
Roots, B. I., and N. J. Lane. 1983. "Myelinating glia of earthworm giant axons: thermally induced intramembranous changes." Tissue and Cell 15: 695-709.
Pereyra, P.M. and B.I. Roots 1988. "Isolation and initial characterization of myelin-like membrane fractions from the nerve cord of earthworms (Lumbricus terrestris L)." Neurochem. Res. 13: 893-901.
M.J. Zoran, C.D. Drewes, C.R. Fourtner, and A.J. Siegel.1988. "The lateral giant fibers of the tubificid worm, Branchiura sowerbyi: Structural and functional asymmetry in a paired interneuronal system. J. comp. Neurol. 275: 76-86.
Roots, B.I., B. Cardone, and P. Pereyra 1991. "Isolation and characterization of the myelin-like membranes ensheathing giant axons in the earthworm nerve cord." Ann. N. Y. Acad. Sci. 633: 559-561.

Polychaete myelin

Nicol, J.A.C. 1948. "The giant axons of annelids." Quart. Rev. Biol. 23: 291-323. [note: the myelination of polychaete axons identified in light microscopy has failed to be confirmed (so far) by electron microscopy]

Copepod myelin

Lowe E 1935. "On the anatomy of a marine copepod, Calanus finmarchicus (Gunnerus)". Trans. Roy. Soc. Edin. 58: 560-603.
Davis A.D., T.M. Weatherby, D.K. Hartline, P.H. Lenz. 1999 "Myelin-like sheaths in copepod axons." Nature 398: 571
Lenz P.H., D.K. Hartline, A.D. Davis 2000. "The need for speed. I. Myelinated axons and fast reaction times in copepods." J. Comp. Physiol. A submitted
Weatherby, T.M., A.D. Davis, D.K. Hartline and P.H. Lenz 2000. "The need for speed. II. Myelin in calanoid copepods." J. comp. Physiol. A 186: 347-357.

Malacostracan myelin (shrimp etc.)

Penaeid shrimp myelin

Fan, S.F., K. Hsu, F.S. Chen, and B. Hao 1961. "On the high conduction velocity of the giant nerve fiber of shrimp Penaeus orientalis." Kexue Tongbao 12(4): 51-52.
Huang, S-K, Y. Yeh and K. Hsu. 1963. "A microscopic and electron microscopic investigation of the myelin sheath of the nerve fiber of Penaeus orientalis". Acta Physiol. Sinica 26(1): 39-42.
Hama, K. 1966. "The fine structure of the Schwann cell sheath of the nerve fiber in the shrimp (Penaeus japonicus)." J. Cell Biol. 31: 624-632
Hsu, K. and S. Terakawa 1996. "Fenestration in the myelin sheath of nerve fibers of the shrimp: A novel node of excitation for saltatory conduction." J. Neurobiol. 30: 397-409
Kusano, K. 1966. "Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)." J. Cell Physiol. 68: 361-384
Kusano, K. and M.M. LaVail. 1971. "Impulse conduction in the shrimp medullated giant fiber with special reference to the structure of functionally excitable areas." J. comp. Neurol. 142: 481-494.
Nonell, J.G. and A.L.H. Aguilar 1986. "Ultrastructura de los nudos de Ranvier en fibras nerviosas mielinizadas del camaron Penaeus." Rev. Biol. Trop. 34: 115-121.
Terakawa, S. and K. Hsu. 1991. "Ionic currents of the nodal membrane underlying the fastest saltatory conduction in myelinated giant nerve fibers of the shrimp Penaeus japonicus." J. Neurobiol. 22: 342-352.
Xu K and Terakawa S. 1999. "Fenestration nodes and the wide submyelinic space form the basis for the unusually fast impulse conduction of shrimp myelinated axons." J. Exp. Biol. 202: 1979-1989

Caridean shrimp myelin

Holmes, W., R.J. Pumphrey and J.Z. Young 1941. "The structure and conduction velocity of the medullated nerve fibers of prawns." J. exp. Biol. 18: 50-54.
Holmes, W. 1942. "The giant myelinated nerve fibers of the prawn." Phil. Trans. Roy. Soc. Lond. B. 231: 293-314.
Heuser, J. E. and C. F. Doggenweiler. 1966. The fine structural organization of nerve fibers, sheaths and glial cells in the prawn, Palaemonetes vulgaris. J. Cell Biol. 30: 381-403.
Govind CK and J. Pearce 1988. "Remodeling of nerves during claw reversal in adult snapping shrimps." J. Comp. Neurol. 268: 121-130.

Myelin in other malacostraca

*Nageotte, J. 1916. "Notes sur les fibres à myeline et sur les étranglements de Ranvier chez certains crustacés." C.R. Soc. Biol. Paris 79 259-263.
*Retzius, G. 1888. "Ueber myelinhaltige Nervenfasern bei Evertebraten." Biol. Föreningens Förhandlinger I: 58-62.

(* myelination not confirmed by electron microscopy)

More doubters ...

"Invertebrates lack myelin on their axons."
				animalbehavioronline.com accessed January 24, 2009

                                              =================
"No invertebrate has myelin..."
				J. Kiernan Anatomical Foundations of Neuroscience web page 11/12/06

                                              =================

"Invertebrate glia do not generate myelin sheaths, and the Drosophila genome lacks orthologs of most myelin genes
				(Marc R. Freeman, unpublished)"  Freeman and Doherty 2006. TINS 29: 82-90 

                                              =================

"Organisms developed two solutions for enhancing rapid communication between neurons and their effector organs.
In invertebrates, the diameters of axons are enlarged.  In vertebrates, the myelin sheath evolved..."

"... along the invertebrate evolutionary line, the use of bare axons imposes a natural, insurmountable limit
-- a constraint of axonal size -- to increasing the processing capacity of the nervous system." 

"Invertebrate axons or small vertebrate axons are typically unmyelinated, while larger vertebrate axons 
are often myelinated."
				Squire et al. 2008 Fundamentals of Neuroscience Third Edition Academic Press p 47; 48; 121

                                              =================

"Invertebrates, except for some crabs, do not form myelin"  [the "crab" rumor is probably incorrect]
				Dowling 2001 Neurons and Networks 2nd edition, Belknap Press, p 101 [see also p 213]

And a few who have it right ...

"The world speed record is held by myelinated axons of the shrimp, which conduct at speeds in excess of 200 m/s (447 miles/h)"
				Nicholls et al. 2001  From Neuron to Brain 4th edition, Sinauer, p 123.

This material has been assembled and presented as a public service by Dan Hartline, Bekesy Laboratory of Neurobiology, Pacific Biosciences Research Center, University of Hawaii at Manoa (danh at hawaii.edu). Opinions expressed here are those of the author and do not represent the positon or policies of the University or any funding agency.