The Apparent 'Corneal Specificity' of
Sensory Neurons
by GEORGE SZEKELY1
From the Department of Anatomy, University of Pecs, Hungary
WITH ONE PLATE
INTRODUCTION
SINCE the first fundamental papers of Paul Weiss (1924), a considerable body of
evidence has been accumulated in favour of the assumption that different tissues
or organs transplanted heterotopically exert some specific modulatory influence
on neurons, the processes of which happened to grow into the grafts; the grafts
in turn provide these neurons with capacities present in the neurons normally
supplying these tissues or organs. These observations led to the formulation of
a 'resonance' hypothesis, but later Weiss himself elaborated a somewhat more
cautious concept of 'specific modulation' (see Weiss, 1955), which was corroborated and extended in masterly manner in different fields of neurogenesis by
Sperry (1951) and his collaborators. Strong support was given to this concept
by an experiment of Weiss (1942) in which, by tactile stimulation of the cornea
of a supernumerary eye implanted into the region of the nostrils or the ear
capsule, an ordinary corneal reflex response could be elicited in the ipsilateral
eye of the host. The cornea of the implanted eye being innervated by the trigeminus like that of the normal eye, this experiment was not completely convincing. But when Kollros (1943) implanted supernumerary eyes between the
ear capsule and the base of the gill, a region innervated by the vagal nerve,
stimulation of the cornea yielded also corneal reflexes of the host's ipsilateral
eye. Thus the assumption of a 'corneal specificity' appeared to be well justified.
Working for other purposes with limbs of urodele larvae transplanted to the
base of the gill, it happened that the distal part of an implanted limb was bitten
off by another animal. Mechanical stimulation of the regeneration blastema
evoked a corneal reflex of the ipsilateral eye, an effect never met with when an
intact implanted limb was stimulated. This observation being inconsistent with
the assumption of a 'corneal specificity' the matter was more closely investigated,
and the results will be presented here.
1
Author's address: Department of Anatomy, University Medical School, Dischka u. 5, Pe"cs,
Hungary.
[ J. Embryol. exp. Morph. Vol. 7, Part 3, pp. 375-9, September 1959]
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G. SZF.KELY—'CORNEAL SPECIFICITY' OF NEURONS
METHODS
The experiments reported were performed on larvae of Pleurodeles waltlii
and Triturus vulgaris. The eye excised previously from young larvae was transplanted into the immediate neighbourhood of the base of the gills in larvae of
similar age. In a second group a forelimb was transplanted into the same locus.
A fortnight after the transplantation the grafts were tested by touch stimuli to
discover whether they had received their sensory supply. After the mid-larval
stage animals were kept in aquaria and fed maximally in order to promote
optimum growth. They were investigated weekly for reflexes to touch stimulation, applied to the grafts by means of a fine glass needle. All stimulations and
observations were performed under the low power of a stereoscopic microscope.
Stimulation of larvae was made under water and of adults in air. A few days
before the onset of metamorphosis the foot of the limb grafts was cut off and the
animals were repeatedly tested as to the reflexes evoked by tactile stimulation of
the resulting blastemata.
RESULTS
Responses from the cornea of supernumerary eyeballs
Seven larvae of Pleurodeles and three of Triturus had a supernumerary eye
grafted homoplastically just at the base of the gills. The grafts developed
normally; they were as large as the host's own eye (Plate,figs.A, B). The corneae
remained perfectly transparent and they were reinnervated within one or two
weeks of transplantation. This was determined by touch stimuli, the response to
which was either the depression of the gills or an escaping movement. The gill
depression response was already noticed in the first half of larval life, i.e. in the
period in which this response can be elicited exclusively by stimulation of the
vagal sensory area (Szekely, 1959). At the onset of metamorphosis the orderly
appearance of the corneal reflex to the stimulation of the graft developed in
exactly the way described by Kollros (1942). Six Pleurodeles and one Triturus
regularly yielded corneal reflexes from the grafted cornea until one year after
metamorphosis. None of these animals showed any sign of a corneal response to
stimulation of the skin around the grafts or of the corresponding cutaneous
regions on the opposite side of the head.
Responses from the limb grafts
Eleven Pleurodeles and four Triturus had a forelimb graft transplanted homoplastically quite near to the base of the gills. The grafts developed normally and
were reinnervated both by sensory and motor fibres of the 10th nerve. Touch
stimulation applied to the limb grafts resulted in depression of the gills and a
synchronous movement of the graft. Stronger stimuli caused either a head-jerk
or an escaping movement, but never any specific responses of the host limbs.
Any responses of the host limbs turned out to be either vestibular effects due to
G. SZF.KELY—'CORNEAL SPECIFICITY' OF NEURONS
377
tilting of the animal during stimulation, or stepping movements preliminary to
escaping reactions. Shortly before the onset of gill reduction the foot was cut off.
Within a week a blastema developed on the tip of the grafts (Plate, fig. C) and
at the same time the corneal reflex to mechanical stimulation of the host eye
appeared. Touching the regeneration blastema of the foot very lightly with a fine
glass needle, we were able to record in this period a complete corneal reflex in
nine Pleurodeles and in one Triturus. The stimulation of the limb skin immediately next to the blastema proved completely ineffective in evoking any
corneal reflex. Similarly, the stimulation of the skin of the head on the opposite
side did not evoke any lid-closure reflex. The further the regeneration of the foot
advanced, the higher became the threshold of the blastema for the corneal reflex.
After complete regeneration of the foot, i.e. when all the four digits had appeared
and the pigment pattern of the skin had become normal (Plate,fig.D), the corneal
reflex disappeared. Even the strongest stimuli were ineffective, resulting only in
the generally observed head-jerk or in escaping movements, but never in a
corneal reflex or in an isolated response of the host's limbs. After reamputation
of the foot and stimulation of the resulting blastema, the corneal reflex reappeared. The reamputation was performed in this case one month after metamorphosis. The development of the blastema and also the regeneration was
delayed, and the threshold appeared to be somewhat higher than in the first case.
One animal even displayed a corneal reflex from the blastema after the third
amputation. From the fourth month after metamorphosis no corneal reflex from
the resulting blastema was ever seen, and it may be mentioned that the regeneration of the foot also either failed or was incomplete after this period.
DISCUSSION
These experiments show clearly that tactile stimulation of a grafted cornea
and of a limb regeneration blastema equally yield retraction of the host's eye on
the same side. Cornea and limb blastema evidently cannot have any common
'specific biochemical properties', and consequently cannot exert the same effect
of 'specific modulation' on vagal sensory neurons in the sense of Weiss's concept. But consideration of the histological type of innervation, both of the
corneae and of the blastemata, may perhaps suggest an explanation of the foregoing results from another point of view. The sensory fibres of the cornea are
known to form a meshwork-like plexus in the connective tissue of the corneal
substantia propria, from which the free beaded terminals supplying the epithelium arborize. This is a nerve-ending of a low grade of differentiation subserving
pain alone (Nafe & Wagoner, 1937) or, at most, pain and touch (Jalavisto, Orma,
& Tawast, 1951). The sensory fibres in the blastema first form very similar
terminal patterns of freely arborizing fibres, from which the characteristically
more elaborate, partly encapsulated, and more highly differentiated receptors
only gradually emerge during regeneration. Since the capacity of vagal sensory
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G. SZ£KELY— 'CORNEAL SPECIFICITY' OF N E U R O N S
fibres supplying a limb regeneration blastema to bring about corneal reflexes
lasts for as long as their nerve-terminals are of a low grade of differentiation,
similar to that of the cornea, and is lost when the regeneration approaches the
restitution of an ordinary limb, the explanation is close at hand: that certain little
differentiated sensory terminal patterns of the head region may have the fundamental capacity to elicit corneal reflexes. A simple injury effect can be ruled out
in view of the fact that from the stump of a foot recently cut off no similar
reflexes can be provoked.
This explanation presupposes the existence of certain cues in the central
nervous system, which—in spite of our familiar concepts concerning the function of fixed neuronal pathways—would be capable of interpreting the different
peripheral messages. We may think here of the frequency code, so lucidly discussed in Granit's (1956) monograph: e.g. different conduction velocities and
relative spike sizes due to different fibre diameters (Gasser, 1946); the sensitivity
optima and the pattern of firing of thermoreceptors at different temperatures
(Dodt & Zotterman, 1952); the so-called spike frequency/time differential of the
retina elements which varies in specific fashion with wavelength (Donner, 1950).
All these provide examples of the central interpretation of frequency codes. On
the basis of modern neurophysiological knowledge of sensory discrimination
and integration, it can be well imagined that the central nervous system has the
capacity to analyse the different impulse patterns and to dispatch them towards
different channels. The change of a given receptive field may alter the respective
impulse pattern resulting in changed reflex relations. We cannot, however, completely neglect another more simple alternative, that sensory inputs arriving
from nerve-terminals of primitive type could have the tendency to become
generalized in the newt medulla, with the final result that the effector mechanism
of the corneal reflex may be brought into action as an especially sensitive system
for generalized states of excitation. This, however, would be only a marginal
case of the previously discussed discrimination mechanism.
The strange fact, that stimulation of the grafted limbs innervated by the vagal
nerve do not yield specific reflexes of the host's limb, while limbs grafted into
the back of tadpoles (Miner, 1951) or into the dorsal fin of newts (Szekely &
Szentagothai, 1959)—even in non-limb segments—readily elicit such responses,
may also call in question the current concepts of 'limb specificity'.
SUMMARY
1. Tactile stimulation of a forelimb grafted into the gill region in urodeles
(Pleurodeles waltlii and Triturus vulgaris) yields no corneal reflex.
2. If the foot of such a grafted limb has been cut off, stimulation of the resulting regeneration blastema evokes a corneal reflex in the same way as does
stimulation of the cornea of an eye transplanted into this region.
3. After complete regeneration of the foot the corneal reflex disappears, but
J. Embryol. exp. Morph.
Vol. 7, Part 3
G. SZ^KELY
G. SZEKELY—'CORNEAL SPECIFICITY' OF N E U R O N S
379
from a new regeneration blastema developing after a second and in some cases
even after a third amputation the corneal reflex can again be elicited.
4. These results are considered to be inconsistent with the 'modulation theory'
of Weiss.
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•
(1955). Nervous System (neurogenesis). In Analysis of Development, edited by B. H.
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E X P L A N A T I O N OF PLATE
Pleurodeles waltlii at the onset of metamorphosis.
FIGS. A and B. Eye-grafts in the posterior head region (at the base of the gills).
FIG. C. Forelimb grafted into gill region, the foot having previously been cut off. Stimulation
of the regeneration blastema yields an ipsilateral corneal reflex.
FIG. D. The same animal one month later. No corneal reflex evoked by stimulation of the
regenerated foot.
(Manuscript received 2: Hi: 59)