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Studies on the origin of sheath cells and sympathetic ganglia in the chick.

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The r*csearchcsof Harrison ( '04, '06, '24) have ~ ~ s t a b l i s l i ~ t l
the idpa that tlie neural crest gives rise to sheath cells. Although tliis concept is widely accepted, it i h not in exact RCcwdaiicc wit11 Elar.riso11'R statement of tlir case. He clemonstrated that the ventral roots of amphibian cmhryos clcvclopc~cl
without sheath cells subsequent to the relnoval of the ncnral
crest. H e noted, homevcir, that a few cells migrated out frorri
tlic neural tube into thc ventral root arid clift'crentiatccl iiito
sheath cells at a date later than tile lime of clcvelopineiit of
these (>ellsfrom the neural r r c s t . R c concluded that ii few
sheath cells arc forineci from cells which migrate from the
tieural tube, but that tlie great majority of them arise from
the neural crest. Detwiler ( ' X I has been studying the
question by employing \-ital stains. He liai tlernonstrated to
me some prepa~*wtionsof amphibiaii ernlq-os in which he
has successfully stained tlie ittlnral crest i*ed aiid the iicural
tnbc blue. Tltc sheath cells which develop sii1)sequent to this
btaiiiiug techniqiic carry the red stain, but the blue staiii i b
confined to the spinal c o ~ d He niaintains that the eai.lic
formed sheath cells arc clerivcd f rain the iieural crest, but
adds that the iieuid tube may contribute some at a laku clatc.
Rare11 ( '37) has applied still mother twhnique t o the
problem. I h transplantctl tissue from Aiublystoma mcxi~anui'ril o 'l'riton tarniatus. Thr cells of these two forms tliffcr
sufficiciitly so Ihat thr host cells c a n be ilist,ingnished from
thc transplanted cells and thus the contr.ibutioii of the transplunted tissue to the host can lw followed. He transplantecl
nenral crest and also the median payt of the neural fol(1.
From his data he concluded that all of the sheath cells of
the amphibian originate from cells which m i g ~ a t efroin tlir
iieizral tube. Such is the statns of a f k i m i-egaiding tlic origiii
of sheath cells in the amphibian.
The autlior ( '38) has demmstmted that sheath cells are
abUiidantl~preseiit on the nerve fibcrs of thc vchIitral roots of
7-day chick enib~pusfrom which the neural crest has b e w ITniovcd at 45 hours of iiicuhation. This s h w s that the i i e u ~ a l
crest is not iiecessarp for. the pi*ociudion of sheath cells, bat
leaves one woiideriag about the sequence of evcuts from the
time of reiiio~-alof the neuraul crest lip to the i - d a y period
wben sheath cells a r e present. Iii this p ~ i p e rthe stages of
clcr~lopmcnta r c followed in a sc ries of esperimcntal enibq-os
i l I i d conlpared with stages of (he liormal embryo.
Rudi a
series also throws light upoil the origiii uf sFrnpathctic ganglia
i n the chick. Miiller a i d I u p - a r ('23) h a r e shown that there
a r c 110 sFmpathetic ganglia in 4-day chick embryos from which
t l hours of iucubnt'1011.
the neural c i m t has heen ~ ~ m o ~ate 48
Kuntz ( '22, '26)' I i o ~ w ~ ~dcmonstmted
symputlietic ganglia
in 5-daj- emhq-os, m c l tlic author ( '3)
in i-da? cinhrps,
the neural crest lia\-iiig heen csciscd in each case. Although
these expc~riineiitssemi diarnetriciilly opposed, the followiiig
study of the stagcis in rleveloprncwt folluning the removal of
the ile11l.ill crest n-ill reveal that such is riot thc case.
$1 ATEKIAJ. A N 1)
Thc embryos of tlie experimciitnl serirs were operated npon
at illlout 48 hours of iiiciibation. the^ varied from 10 to
20 somitcs. The operative tc?c,hnique has bceri described in
(letail in it previous p1)lication (.Joiies, 'Xi). The operation
consisted in cutting the cctoderm along either. sicici of the
neural tube am1 thcri cutting off a small portion of thci clorsal
part of the tuhc. l'hc e c t o i h m , 11e111~alcrest a t d dorsal part
of the iieural tube were thus completely removed from the
embryo. Gsually the operation extended through about six
consecutive segments in the most caudal p a r t of the embryo
(the future truiik region) varying more or less depending on
tlie availability of tlie embryo a t tlic operation. Hematoxyliii
arid eosin stains were used. Part of tlie cggs were obtained
f r o m our colony of chickens at Loyola University School of
Xedicine, a i d part tlirough the courtesy of P u r i m Nills
while I was working ;it St. Louis Criirersity in the laboratory
of Dr. illhert Kuntz to t~d101r1T wish to express my appreciation for advice aiid encouragement.
The iiormal sequence of events in the formation of sheath
cells and of sympathetic ganglia is shown in figures 1,2, 3, 7.
lu the earliest stage (fig. 1) the ventral root has beguu to
sprout and a few cells are migrating along it from the spinal
cord. Tlic spiiial ganglion is riot shown i n this photomicrograph, but its ventral extremity has r e a c l i d as far as the
ventral root (7T-hour embryo).
A little later (fig. 2 ) thc ventral root is nioi’e exterisive and
contains a large riumbw of cells. Some of these cells hare
the large vesicular nucleus with c-tarkly stainiug nucleolus aiid
the dark cytoplasm characteristic of the neuroblast. Others
a r e elongated aiid have the light staining cytoplasm arid the
dark nucleus characteristic of sheath cells. The dark cells to
the right of the spinal cord belong to the dorsal root ganglion
which extends down to the ventral root in adjacent sections
(88-hour embryo).
A t the next stage (figs. 3) tlie spiiial ganglion is more differentiated, the veiilral root and spiiial nerve a r e free of
neuroblasts hut a r e provided with sheath cells, and the
sympathetic ganglion is in a. very early stage of development
(91-hour embryo).
In the last stage (fig.7) the characteristic relationships of
the spinal cord, dorsal root ganglion, spinal nerve and sympathetic ganglion have been attained (101-hour embryo).
T E W A S A T O X I C 4 L Ri.CURD, 1 UL. 73. N O 3 A N D SUPI’LEIIENT NO. 3
The figures presented bere wcre chosen from the series in
order to illustrate the orderly progression of development.
The exact number of hours of iiicubatioii is given iii each
case, but it must be remembered that there is considerable
variability in a group of embryos of aiip specific age. Y o
doubt, the same progression could be followed in a series
of embryos all at ages diffcreiit than those presented here.
The progressive development of tlie structures coilsidered is
emphasized and not the particular age at each stage except
in a general comparison with the expcrimciital series.
The experiniental wries consists of a group of embryos
from which the iieural crest and varying amounts of the
dorsal portion of the iieural tube liave been removed f o r
several segments i n the trunk region at stages raiigiiig from
10 to 20 somites. The dorsal roots and spinal ganglia are of
course absent in this series, permitting a study of the development of the ventral roots in the absence of aiiy influence
from the neural crest derivatives.
At first (fig. 4) the ~rc-ntralroot closc1)- resembles that of
the normal cmbrpo a t the samc age (fig. I), although the
spinal cord is open and contains fm-er cells. In this figure,
a few cells arc prcserit in the base of the vcutral root. HOWever, most of the roots in the operated area a t this ad"e are
cell-free (77-liour embryo).
Later (fig. 5) the ventral root is quite extensive hut is completely naked. Tlic striking contrast hetween figurc 2
(normal) and 5 (cxpcriniental) makes a clcscription uunecessary (89-hour embryo).
At a later stage (figs. 6 and 8) the veiitral root is still iiaked
but a group of cells have migrated into the base of tlie nerve.
I n comparison with the normal enibryo of the same age
(fig. 3 ) note that tlic slieatli cells and sympathetic ganglion
arc lacliing (91-hour ernbr3-o).
In the nest stage (figs. 10, 12) the cells in the base of the
ventral root have iiicreasecl in number and sheath cells have
appeared along the iierve fibers. !Phis eiiibryo is older than
the rioraial one sEiomri in figure 7, yet tlie sympathetic ganglion
has not appeared (105-hour embryo).
A t a later period (fig. 13) neuroblasts a r e absent from the
ventral root, sheath cells are present along the nerve and the
sympathetic ganglion has formed (140-hou~e i n b r ~ o ) . 'I'his
stage corresponds quite closcly to tlie normal 101-hour
embryo (fig. 7).
Finally (fig. 9 ) the picture becomes quite nornial except
f o r the ahserice of the spinal ganglion (180-11our embryo).
Comparing the operated with the normal series of embryos
it is seen that the nerve fibers of the ventral root develop at
about the same rate in both. The most conspicuous difIereiice
between the two is the absence of sheath cells 011 the nerve
fibers in the ex1)erimental series. The nerves remain nalicd
nritil around the early part of the fiftli d a of~ incubation when
cells which have migrated into the ventral roots from the
spinal cord differentiate into Schwann cells (fig.10). ,4uother
striking difference is the delayed appearance of tlie sympathetic ganglia. In the normal series thc sympathetic neurohlasts have reached their destiiiatioii a t 91 hours (fig. 3 ) but
in the experimental series the sympathetic ganglia have not
yet appeared at 105 hours (fig. 12). They a r e first visible
in a 114-hour embryo. In normal einbryos, cells begin migration from the lube about the begiiiniiig of the foiwth
day ~7hilein the experimental groul~tliev begin about the close
of the fourth day, aiitl then do not seem t o travel as rapidly
as usual since they have iiot proceeded into the ventral root
very far at 105 hours (fig. 10) anti do not form sympathetic
ganglia until near tlie closc of the fifth day. That those cells
do form sympathetic ganglia lias been previously demonstrated (Jones, '37).
Thns the principal consequeiicm of extirpating the neural
crest may be summarized as foIlows : dorsal roots and ganglia
fail to develop, veritral roots rcmain naked until the fourth
day of incubation, and both tlic migration of cells from the
neural tube and the formation of sympathetic ganglia is clelayed f o r about 1day.
The question immec!iately arises: Is the nakedness of the
nerve fibers clue to the removal of the neiiral crest or is it
due to the clelaycd migration of cells from the neural tube?
I n the normal embryo, as seen in figures 1 and 2, the migration of cells from tlie neural tube begins alniost simultancously with the sprouting of rentral root fibers. The nerve
fibers a r e nearly obscured by numerous cells, some of which
soon differentiate into sheath cells (fig. 2). Thus normally,
the nerve fibers a r c never without their accompanying sheath
I n the operated embryos the nerve fibers remain iiakcd as
long as there a r e no cells within the ventral roots. Soon
after the migration of cells into the roots, sheath cells appear
(fig. 10). The differeiitiatiori of the medullary cells into
sheat,h cells can be followed just as in the normal cmbryo.
This eliminates the possibility that the sheath cclls upon tlie
ventral root fibers of the operated embryos might have been
formed from iiieseiichyme cells under the influence of abnormal conditions, and confirms the prevalent idea that sheath
cells are ectodermal in origin. Sirice the formation of sheath
cells in the operated embryo is similar. to their formation in
normal embryos, except for the delay, it must be assumed that
tlie nakedness of the ventral root fibers is due to the delayed
migration of cells from the neural tube into the veiitral root.
That sheath cells a r e normally formed from cells of medullary origin docs iiot exclude the neural crest as a source of
these cells. There is no evicleiicc in the normal embryo that
cells of mednllary origin migrate into the neural crest. Itloreover, in one experimental emhryo from which the wliolc neural
tube was removed throughout sevci-a1 segments, a portion of
tlic neural crest was accidentally left in place and a spinal
ganglion dewdoped there. Although this ganglion was iiot
connected with tlic spinal cord hy a ventral root, nerve fibers
sprouting from it ~ w r esupplied with sheath cclls (fig. 13).
So in the chick, evidence points to the conclnsion that sheath
cells arise both from the neiiral crest and from the ileural
tube. This seems altogethcr a good ai-rangcnient.
The delayed f vrrriation of sympathetic ganglia following the
extirpation of the neural crest becomes significant when it is
recalled that Miiller and Ingvar ('23) did not find sympatlictic
ganglia in the 4-day chick which had been snbmittcd t o this
operation at 48 hours of incubation. The normal 4-day
embryo has sympathetic ganglia. Four-day embryos from
which the neural crest has been removed do not have sympathetic ganglia. Therefore, they concluded, the neural crest
is the origin of the sympathetic ganglia. The error in this
logic was that the effect of the opcration iqooii the normal developmental processes was not considered. The experimental
series of embryos presented here verifies tlie observation of
Jliiller and Ingvar regarding the 4-day embryo, but negates
their conclusion by showing that the sympathetic ganglia appear at a later date. Thus tlie observations of Miiller and
Ingvar ('23) and of Kuntz ('22, '26) and of the author ( '37)
are not contradictory but are descriptions of different stages
in development. I cannot, however, explain the oft-repeated
and cmphatic statement by Miiller and Ingx7ar that neither in
normal nor experimcntal embryos did they observe cells of
medullary origin in the ventral roots.
Harrison's observations that in the aniphibian embryo the
ventral root fibers develop without sheath cells, subsequent
t o the removal of the neural crest, also holds true for the
chick embryo. However, neither i n the amphibian nor in the
chick is this an adequate basis f o r the conclusion that the
neural crest is normally the sole source of sheath cells.
Harrison's conclusion that sheath cells are ectodermal in
origin is confirmed by the observation that cells from the
spinal cord form thc sheath cells which devclop on tlie naked
ventral roots of the experimental ernlxyos.
Following the removal of the neural crest in chick embryos
incubated f o r about 45 honrs, the nerve fibers of tlic ventral
root develop a t approximatelj- the normal rate. However, the
formation of the sheath cells is delayed so that f o r a time
the ventral root fibers are naked. This delay is due t o the
fact that the operation npon the neural tube retards the
migration of cells from the neural tube along the ventral root.
This retardtition postpones the development of sheath cells
until approximately the beginning of the fifth day of incubation, and also delays the appearance of the sympathetic
ganglia until near the close of tlic fifth day. These observations, together with a study of normal einbryos, lead to the
conclusion that the neural tube iiormally contributes sheath
cclls to the ventral root fibers. The delay i n the appearance
of the sympathetic ganglia explains the differing results obtained by Xiiller and T n p a r , wlio did not firid sympathetic
ganglia in 4-clay chick embryos, and by Knntz and the author
who fouiid sympathetic ganglia in 5- and 7-day embryos respectively, following removal of the neural crest. TTntil a
more detailed accouiit of the role of the neiiral crest in the
formation of the sympathetic ganglia is made available by
further research, the ventral portion of the neural tube must
be considered the source of the cells which form the spmpatlietic ganglia.
There is no evidence that cells from the reiitral part of the
neural tube migrate into the neural crest t o form slicatli cells.
There is evidence that thc neural crest furnishes the sheath
cells to the fibers which grow out from the spinal ganglion.
In brief, the riclurnl crest gives rise to tlie sheath cells of
the dorsal root, the neural tube to the sheath cells of the
ventral root.
That the sheath cells which develop on the iiakcci nerve
fibers arc contributed by the neural tube is convincing coilfirmation of the observation that sheath cells arc ectodermal
in origin.
DETWILER,S . R. 1937 Application of vital dycs to the study of sheath cell
origin. Proc. SOC.E x p Biol. Itled., vnl. 37, pp. 380-388.
Ross 0. 1904 Y e w Vcrfiuche und Beobarhtungen iiber dic Entwicklung der peripher en Nerven der M5rbelticre. Sitzher. niedcrrhem.
Ges. Nat. u. Heilk. Bonn.
_-- 190G Further rvpcrimrnts on the developnient of periphcral nerves.
S m . J. Anat., v o l . 6 , pp. 721-131.
1924 NPuroblast versus sheath cell in thc development of pcriplieral
nerves. J. Comp. Seur., vol. 37, pp. 133-194.
JONES, DAVIDS. 1937 The origin of thc sympatlietic trunks in the ehiek
e m b q o . Anat. RPC., vol. TO, pp. 45-62.
1938 The origin of sheath cells in the chick. Anat. Rer., vol. 70
(Suppl. no. 3 ) , p. 42.
XUNTZ.A. 1922 Experimental studies on the histngcnesis of the sympathetic
nervous s)stem. J. Comp. Neur., vol. 34, pp. 1-26.
_ _ _ _ 1926 The role of erlls of medullary origin in the development of
the sympathetic trunks. J. Comp. Neur., 1.01. 40, pp. 389408.
M~LLER,E., AND S. JNGVAR 1923 TJeber den Ursprung des Ryinpathicus beim
Hiinchen. Arch. f. mikr. Anat., Bd. 99, S. 650-671.
R ~ V E N CHR.
P. 1937 Experiments on t h e origin of the sheath cells a n d sprnpathetic neuroblasts in Amphibia. J. Comp. Xeur., ~ o l 67,
. pp. 281-280.
1 Spinal cord (5.c.) and ventral root (v.) of a 77-hour chick embryo. Note
cells (c.) in the base of the ventral root. X 657.
2 Spinal cord (5.c.) and rentral root (v.) of 88-hour chick embryo. The
cells in the ventral root h a l e begun differentiating into sheath cells (s.) and
neuroblasts (n.). X 567.
3 Spinal cord (KC.), spinal ganglion (s.g.), ientral root
and sympathetic
ganglion (syni.) in a 9l-liour chick embryo. Kote the sheath cells ( 8 . ) along
thc nerve trunk. X 485.
4 Spinal cord (s.c.) aiid ventral root (v.) of ii-hour experimcntal chick
embryo. X 579.
5 Spinal cord (s.c.) and ventral r o o t (v.) of R9 hour expcrirncntal rhick
embrj-o. Note absence of sheath cells and iieuroblssts in the ventral root as
compared with the normal embryo (fig. 5 ) at 88 hours. X 375.
6 Spinal cwrd (R.c.) and ventral root (v.) of 91 hour Pipenmental chick
embryo. Kotc cells in t h r base of the vrritral root. X 293.
7 Normal 101-hour chick cmlhrjo. sx., spinal cord; s.g., spinal ganglion:
s.n., spinnl nerre; sym., ~ympathetieganglion; s., sheath erll. X 286.
8 Higher iiiagnifiration of figure 6. SJ., spinal cord; I-., ventral r o o t ; e.,
cells in base of rentrnl root. X 668.
9 Experimental chick embryo a t 180 hours. Y.c., spinal cord ; v., rentral root ;
s)-m , sjnipathetic ganglion. X 90.
10 Spinal cord ( R . c . ) and v-eiitrnl root, (T.) of 10.7-lruur experiiiirnt:il Chicvk
ombrgo. Note slicath cells (s.) along tlic! 1-entral root fihrrs. X 524.
11 Exp~~riiiicntalehick eiribrpo at 1-10 hour.;. s.c., sl~inad port1 ; v., rviitral
root; sym., s p p a t h c t i r ganglion. X 298.
12 Lower power o f figure 10. Note atiscure of synqmthctic gnn
13 I2xperimc?ntnl chic.1: enihrj-o of i cl;iys. l’lic spinal gang
riot eonuoCted with the spinal cord in this rrnhrgo yet, the nerve t r u n k (n.t.)
sprouting f r o m t h e ganglion .is provided with sliratli cc\lls (s.).
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sheath, chick, origin, ganglia, sympathetic, studies, cells
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