Download Ultrastructural Studies of the Development of Nerves in Hydra

AMER. ZOOI.., 14:551-573 (1974).
Ultrastructural Studies of the Development of Nerves in Hydra
LOWELL E. DAVIS
Department of Biology, Syracuse University, Syracuse, New York 13210
SYNOPSIS. Three types of mature epidermal neurons and several of their differentiating
stages aie presented in this ultrastructural study. Each of the three types, neurosensory, p.eurosecretory, and ganglionic cells, is derived from interstitial cells, (i) Mature
neurosensory cells contain elongated nuclei, a well-developed cilium in each cell, and
membrane-bounded neurosecretory droplets (700-1300A in diameter). There may be
two or more neurites in which are numerous microtubules, glycogen particles, ribosoincs and many neurosecretory droplets, (ii) Mature neurosecretory cells closely resemble neurosensory cells, except that no cilium is present. The perikarya contain
small, dens; nuclei, neurosecretory droplets (850-1300A in diameter), mitochondria,
glycogen particles, and microtubules. Active Golgi complexes are present in both cell
types. The nemites are similar to those described for neurosensory cells, (iii) Mature
ganglionic celh are bipolar or multipolar. The small, dense nuclei are surrounded by
a small amount of cytoplasm. The neurites contain mostly microtubules; a few mitochondria, ribosomes, and glycogen particles are also present, but there are no secretory
droplets.
To date, only neuios?nsory and neurosecretory cells have been observed in the gastiodermis. They are structurally indistinguishable from their epideimal counterparts.
A significant finding is that three types of synapsjs—neuroinusciilar, neuronemalocyte, and inicrneuronal—are identified in both the epidermal and gastrodermal
neurons.
INTRODUCTION
The nervous system of Hydra is composed of a gastrodermal and an epidermal
plexus (Hyman, 1940). Little is known of
the gastrodermal plexus except that a few
investigators have referred to its existence
(Hyman, 1940; Burnett and Diehl, 1964;
Lent/., 1966), and recently, two types of
neurons have been described and suggestions made concerning their origin (Davis,
1972). The epidermal nerve plexus is composed of three types of neurons, ganglionic,
neurosensory, and neurosecretory cells
(Burnett and Diehl, 1964; Lent/, and Barrnett, 1965; Davis et al., 1968). Each of
these cells contains two or more neurites
(bi-polar, tri-polar, or multi-polar) and as
such they form a "continuous nerve net"
extending throughout the epidermis. Like
Some of the material presented in this paper was
obtained as a result of a National Science Foundation Grant (CB-27395). The author also acknowledges the technical assistance of Linda M.
Bookman.
all other epidermal cells, neurons are
sloughed off at the extremities (mouth,
tentacle tips, and basal disk) and therefore must be replaced constantly.
Previous ultrastructural studies on the
development of neurons indicate that two
of the three types of neurons—ganglionic
and neurosensory cells—originate from interstitial cells (Lent/., 1965; Davis, 1969,
1971). Some of the crucial morphological
stages of differentiation were presented,
and it was suggested that despite certain
structural similarities during early development, these two types of neurons originate
directly and independently from interstitial
cells. Furthermore, it was suggested that
since neurons have never been observed
in mitosis, the sole source of neurons is
the interstitial cell. It has been assumed
that the third type of neuron—the neurosecretory cell—has a similar independent
origin from interstitial cells, but to date,
there are no reports which demonstrate
the development of this cell type.
This paper will deal exclusively with
the ultrastructural development and mor-
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,532
LOWELL E. DAVIS
FIG. 1. Interstitial cell showing a centrally located
nucleus and prominent nucleolus. The cytoplasm
is filled with numerous ribosomes (ri) , few mitochondria and a few small vesicles. X 19,600.
phology of fully differentiated neurons. It
is emphasized that due to space limitations,
only selected developmental stages and
profiles of mature cells will be presented.
propriate here to enumerate some general considerations. Although neurons are
found throughout the epidermis, they are
concentrated in such regions as the bases
of tentacles, the hypostome, the peduncle,
and the basal disk. Regardless of regional
distribution they are located immediately
adjacent to the longitudinal myonemes at
GENERAL CONSIDERATIONS
Before discussing the development of the
individual types of neurons, it seems ap-
ULTRASTRUCTURE OF HYDRA NERVES
the bases of epithelio-muscular cells. In
such locations, neurons are surrounded
completely or partially by epithelio-muscular cells or partially by cnidoblasts, other
neurons, and interstitial cells. Gastrodermal
neurons are located at the bases of digestive cells, immediately adjacent to the circular myonemes. They are. therefore, surrounded mostly by digestive cells, and in
some cases, may be partly surrounded by
basal reserve cells.
Secondly, neurites are usually tortuous
and branched, and extend into different
planes and directions. Accordingly, it is
exceedingly difficult to obtain serial sections of entire cells. Due to certain structural similarities which will be demonstrated later, it is imperative that for
accurate identification of the individual
neurons, serial sections containing the
nerve cell bodies (perikarya) be examined
carefully. Finally, it should be realized
that these studies represent a logical construction of the differentitive processes,
based on the observation and interpretation of static electron microscopic images,
are are therefore susceptible to some
criticisms.
DIFFERENTIATION OF EPIDERMAL NEURONS
Interstitial cells
Since interstitial cells are the undifferentiated cells from which neurons originate,
it is appropriate to discuss briefly their
ultrastructure so that a more meaningful
comparison may be made between them
and the early stages of neuronal development. They are usually round or oval,
measuring approximately 5 to 6 tx in diameter (Fig. ]). They are located singly or
in groups among epithelio-muscular cells
and cnidoblasts. The nucleus, containing
a conspicuous nucleolus, is centrally located and occupies a considerable portion
of the cell. The cytoplasm is unquestionably embryonic in character, in that it contains numerous ribosomes, few small mitochondria, an inconspicuous Golgi complex,
and few small vesicles, and shows a virtual
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absence of rough endoplasmic reticulum.
These cells undergo frequent mitosis and
differentiate subsequently into neurons and
to other cell types (e.g., cnidoblasts, spermatozoa and ova).
Neurosen.sory cells
Early developmental stages of the neurosensory cell are shown in Figures 2—4. The
earliest recognizable stage reveals an ovalshaped appearance of the original interstitial cell and the presence of two centrioles (arrows in Fig. 2a) at one of the
elongated cytoplasmic regions. From this
point, the ciliary apparatus begins to develop. One of the centrioles (Fig. 2b)
assumes a characteristic alignment immediately beneath the plasma membrane. As
differentiation proceeds, the emerging ciliary bud (Fig. 2fZ) shows a striking resemblance to young ciliary processes of vertebrate neurons (Thornhill, 1967). The
ciliary bud is surrounded completely by
the plasma membrane and contains axial
microtubules, a few dense particles dispersed among the microtubules, and an
amorphous material. It has also been shown
that while the cilium proper is being
elaborated, electron-dense granules, 700 A
in diameter, arranged in a linear fashion,
extend inwardly from the base of the centriole (Davis, 1969). It is believed that
these granules may be the precursor materials for initial rootlet formation shown
in Figure 2c. The role of the Golgi complex and segments of rough endoplasmic
reticulum which are located characteristically at the base of the developing cilium
is unknown.
As the ciliary shaft elongates into the
extracellular space, internal microtubules
extend its entire length (Fig. 3). In transverse sections they reveal the typical 9 + 2
arrangement of cilia, but the number and
location of the tubules vary from one level
to another (see Roth and Shigenaka, 1964;
Satir, 1965; Boquist, 1968; Davis, 1969).
Definite rootlet fibers appear at the base
of the cilium, but they are not striated at
this stage. The electron-dense granules
LOWELL E. DAVIS
FIG. 2. Development of the cilium in neurosensory cells, a, Two centrioles (arrows) oriented at
right angles to each other near the periphery of
the differentiating neurosensory cell. X24.400. b,
Centriole (ce) aligned beneath the cell membrane,
dense fibro-gianular materials (650-700 A in diameter) , and cross-section of niicrotubules (arrows) .
X25.300. c, Fibers (arrows), believed to be precur-
sors of rootlets, extend inward from the base of the
centriole (ce). x 18,600. d, Ciliary bud protruding
from the cell contains axial microtubules within an
amorphous material. The developing cilium is
surrounded completely by the plasma membrane.
Definite rootlet fibers (ro) are observed for the
first time, x25,300.
which extended linearly from the base of
the cilium (considered to be precursor
materials for rootlet formation) have dis-
appeared completely.
The question regarding the number of
cilia in each neurosensory cell remains un-
U LTRASTRUCTURE OF HYDRA NERVES
FIG. 3. Portions of two neurosensory cells (ns) , one
of which contains a ciliiini (ci). Microtubules appear throughout the entire length of the ciliuin.
Rootlets (10) are seen at the base of the cilium but
at this stage they are not striated. Numerous ribosomes, Golgi complex (g) and short segments of
rough endoplasmic reticulura (er) are also observed. X 19,900.
FIG. 4. Golgi complexes in neurosensory cells, a,
One Colgi complex (g) is located characteristically
near the base of the developing ciliuin (ci) ; endoplasmic reticulum (er). b-d, Colgi complexes (g)
in mature cells. Note the membrane-bounded secretory droplets (800-1000 A in diameter), lough
ER (er) and larger irregularly shaped droplets
(d). a, X25.800; b. X27.600; c, X41.300; d, X41.300.
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LOWELL E. DAVIS
resolved. Several investigators, notably McConnell (1932), Burnett and Diehl (1964),
and Bullock and Horridge (1965), have
indicated the presence of one to several
"hairs" (cilia). Electron microscopical
studies by Lentz and Barrnett (1965) and
Davis et al. (1968) suggested only a single
cilium in each neurosensory cell. More recent ultrastructural studies (Davis, 1969)
and our investigations to date demonstrate
that while more than one centriole may
be directed toward ciliary development,
there remains no definite ultrastructural
evidence for bi- or multi-ciliated neurosensory cells.
Prior to the complete maturation of
the cilium, several cytoplasmic changes become evident. In addition to the elongation of the cytoplasm in various regions
during the formation of neurites, certain
organelles, particularly the Golgi complexes, show remarkable development
(Fig. 4) . From the single inconspicuous
Golgi complex of the original interstitial
cell, there may be as many as four Golgi
complexes in these cells. Materials of relatively high density appear within the increased Golgi lamellae. Peripheral vesicles
eventually become distended and separate
from the Golgi membranes. They are first
observed in the perikarya and as the cell
matures, they are transported throughout
the cell body and neurites. These membrane-bounded droplets (700 to 1300 A in
diameter), described as neurosecretory
droplets, reveal a striking morphological
similarity to comparable droplets in known
neurosecretory cells of other organisms
(Bern et al., 1962; Bonga, 1970; Andrews
et al., 1971).
Several other cytoplasmic changes also
occur. The numerous ribosomes of the
original interstitial cell persist throughout
the formation of the cilium. Thereafter,
their number diminishes and when the
neurosensory cell is mature, most ribosomes
have disappeared. Rough endoplasmic reticulum is not a prominent organelle in
developing or differentiated cells. Only
short segments are present and therefore
it has been assumed that this organelle
participates to a minor degree, if at all,
in the synthesis of neurosecretory droplets.
Mitochondria increase in number and usually become reduced in size. In most instances, there is a reduction in the number
of cristae and an increase in the density
of the matrix. Microtubules are first observed at the base of the developing cilium,
and as the cell elongates during the formation of neurites, they extend for long distances, parallel to the long axis of the cell.
The cell body of a mature neurosensory
cell is shown in Figure 5. The nucleus,
elongated or irregularly shaped, contains
scattered dumps of chromatin material.
The nucleolus which was once very conspicuous has fragmented to the point that
it is not easily recognizable. The cytoplasm
contains a characteristic cilium, typical
neurosecretory droplets and larger ovalshaped droplets (up to 1.0 ^ in diameter),
small dense mitochondria, few ribosomes
and a Golgi complex. The cell body may
also reveal synapses with other cells (e.g.,
cnidoblast shown in Figs. 5 and 6). These
will be discussed in more detail later. Another profile of a mature neurosensory cell
is shown in Figure 7. The cilium with its
internal tubules is seen at greater advantage adjacent to the neurite of another
nerve cell.
Ncurosecrelory cells
Morphological data concerning the differentiation of neurosecretory cells are extremely limited. All available evidence,
however, indicates that these cells are also
derived directly and independently from
interstitial cells (Davis, 1971).
An immature neurosecretory cell containing two neurites (top and bottom) is
shown in Figure 8. As the cell elongates
during the formation of neurites, the
nucleus assumes an oval or elongated
shape, and the perikaryon contains only
a small amount of cytoplasm. Ribosomes,
small mitochondria, and microtubules are
observed in the cytoplasm of the cell body.
At the base of the neurites, however, two
or more Golgi complexes develop and are
ULTRASTRUCTURE OF HYDRA NERVES
FIG. f>. Mature neurosensory cell with cilium (ci)
and associated structures, irregularly-shaped nucleus
(nu) , Colgi complex (g) , dense membranehounded droplets (900-1100A in diameter), larger
type droplets (d), and microtubules (mt) aligned
557
parallel to the long axis of the cell. Xote the crosssection of the flagellum and stereocilia of a nematocyst (ne) and points of contact (a and b) to the
neurosensory cell. x21,400.
558
LOWELL E. DAVIS
FIG. 6. Higher magnification of the contact
(arrows) shown at (a) in Figure 5. Note the increased density of the plasma membranes and a
droplet of moderate density in the neuroscnsory
cell. Nematocyst (ne) in a cnidoblast (en).
X 36,900.
FIG. 7. Portion of a mature neurosensory cell
showing elongated vesicles with dense chromatih
material, fully-formed cilium (ci) , rootlet fibers
(ro), small dense mitochondria (mi) , microtulmles
(ml), and secretory droplets (900-1300 A in diameter) of various densities occupying only a portion
of the space enclosed by the membranes. Neurites
(neu) ; myonemes of epithelio-muscular cell (my) .
X 19,000.
aligned parallel to the long axis of the
cell. Oriented in this fashion, some of the
secretory droplets elaborated by the Golgi
complex remain in the perikarya while
others are transported into the neurites.
The neurites at this stage also contain ele-
ments of rough E.R., mitochondria, ribosomes, few microtubules, and large, irregularly-shaped droplets, the nature of which
is still obscure.
The perikaryon of a mature neurosecretory cell is shown in Figure 9. The nucleus
ULTRASTRUCTURE OF HYDRA NERVES
FIG. 8. Immature neurosecretory cell containing
two neurites (only one shown). Golgi complexes
(g) are located at the base o£ the neuiite in which
there are mitochondria (mi) and short segments o£
559
E.R. (ei) . Xote also the few membrane-bounded
secretory droplets (1300 A in diameler) , larger
droplets (d) , ribosoines, and inicrotubules (nil) .
X 22,000.
500
LOWELL E. DAVIS
FIG. 9. Mature neurosecretory cell. The nucleus
contains scattered dumps of chromalin material.
There are several dense membrane-bounded droplets (850-1300 A in diameter), small dense mito-
chondria (mi) , microtubules (nit) , Golgi complex
(S) • glycogcn particles (gl) , and larger dense
droplets, up to 0.6 /i in length (d) . Myonemes
(my) of epithelio-muscular cells (cp) . X26.200.
reveals obvious changes from the immature
nucleus in that there are scattered clumps
of chromatin material and electron-lucent
areas in the nucleoplasm. Typical membrane-bounded neurosecretory droplets are
dispersed throughout the cell body. Their
variation in size (850 to 1300 A in diameter) and density, as well as the presence
of several empty vesicles, suggest that re-
lease of the droplets also occurs in the
perikaryon. T h e cytoplasm of this region
also contains small dense mitochondria,
large dense droplets, glycogen particles,
Golgi complexes, numerous vesicles and
vacuoles of various sizes, and microtubules
which extend from the limits of the nuclear
membrane into the neurites (Fig. 10).
ULTRASTRUCTURE OF HYDRA NERVES
FIG. 10. Portions of two mature neurosecretory
cells (outlined) showing the nucleus (nu) and
neurites (neu) . Xote the Colgi complex (g) at the
base of the neurites, membrane-bounded secretory
561
droplets (800-1500 A in diameter) of various densities, empty vesicles (v), and microtubules (mt) .
X 29,800.
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LOWELL E. DAVIS
FIG. 11. Neurites of neurosensory and/or neurosecretory cells are of various diameters. They contain primarily membrane-bounded droplets (8001300 A in diameter), microtubulcs (nil) , glycogen
particles (gl), and mitochondria (mi) . Note (in
116) the specialized junctions (arrows) between a
neurite and an epithelio-muscular cell (ep).
i\fyonemes of epithelio-muscular cell (my) ; mesoglea (me) . a, X22/I0O; b, X32/I00;
c, x39,800.
2
390
ULTRASTRUCTURE OF HYDRA NERVES
S6B
It has been indicated that neurites o£
neurosensory and neurosecretory cells are
morphologically indistinguishable at the
present lime. Furthermore, accurate identification of the individual neurites is possible only when the respective cell body
is included in the section examined. Figure
11 shows three neurites which may belong
to one or both mature cell types. Although
FIG. 12. Immature ganglionic cell containing two
cytoplasmic extensions (arrows). The mitochondria
appear to accumulate at the bases of or within the
extensions (neurites) and in two other areas which
ma; also give rise to neurites. Except for the
change in cell shape, the nucleus and cytoplasmic
structures are similar to those of I-cells. Epitheliomuscular cell (ep). x 13,600.
Neurites—neiiroscnsory fncurosecrctory
cells
LOWELL E. DAVIS
they differ in terms of their ultimate length
and diameter, they contain essentially
the same components: membrane-bounded
droplets (800 to 1300 A in diameter),
microtubules, mitochondria, glycogen particles, and few ribosomes. Some of these
neurites apparently end freely while others
form specialized junctions (synapses) at
their terminations (Fig. lie).
•
•
*
Ganglionic cells
One of the earliest stages of ganglionic
cell development is shown in Figure 12.
Except for the presence of long cytoplasmic
extensions, this cell type can be easily mistaken for an interstitial cell because of the
obvious similarities. ]n addition, the problem is further compounded by the struc-
<
FIG. 13. Immature multi-polar ganglionic cell
(outlined) with four neurites (arrows). The mi-
clcolus is still recognizable. The cytoplasm contains
numerous ribosomes. x30,500.
ULTRASTRUCTURE OF HYDRA NERVES
FIG. 14. A fully differentiated ganglionic cell (gc) .
Note that the neurite contains mostly microtubules
(mt) . Epithelio-muscular cell (ep) . x 13,900.
tural similarity between these developing
cells and both developing neurosensory
cells and early cnidoblasts. In the case of
the neurosensory cells, this early developmental stage involves the formation of
the ciliary apparatus. On the other hand,
cnidoblasts reveal developed Golgi com-
plexes and the formation of several ves
icles. This early stage of development,
therefore, represents one of the most crucial stages to identify accurately.
A slightly later developmental stage
(outlined) is shown in Figure 13. The
nucleus has now become elongated and
LOWELL E. DAVIS
FIG. 15. Immature gastrodermal neurosensory cell
containing a cilium (ci) and a centriole (arrow).
A cytoplasmic extension of the cell lies between
the bases of digestive cells (dc) and is therefore
immediately adjacent to the mesoglea (me) . Golgi
complex (g); rough E.R. (er); epithelio-muscular
cell (ep). x 22,200.
FIG. 16. Ncurite of gastrodermal neurosensory or
neurosecretory cell containing membrane-bounded
droplets (750-1500A in diameter). Note the intramitochondrial crystal (arrow) . Myonemes (my) of
digestive cell; mesoglea (me); epithelio-muscular
cell (ep). x20,000.
Ul-TRASTRUCTURE OF HYDRA NERVES
567
the nucleolus has begun to be dispersed
throughout the nucleoplasm. The cyto-
plasm, containing numerous ribosomes and
a few mitochondria, extends in four dif-
FIG. 17. Mature gastrodermal
The nucleus, displaced to the
tains dense clumps of scattered
Active Golgi complexes (g),
rough E.R. (er), microtubules (int) , small dense
mitochondria (mi) , secretoiy droplets (1IOO-1600A
in diameter), and larger dense droplets (arrows)
are observed. X22,000.
neurosecrelory cell.
cell periphei), condiromatin material.
short segments of
568
LOWELL E. DAVIS
FIG. 18. Basal reserve cell in the gastroclermis. The
cell resembles an epidermal interstitial cell: centrally located nucleus with a prominent niicleolus,
numerous ribosomes (ri) , few elements of rough
E.R. (er), and few mitochondria (mi). Long, nar-
row cytoplasmic extensions (arrows) suggest that
differentiation may have begun. Digestive cell (dc);
mesoglea (me) ; epithelio-muscular cell (ep).
X 22,200.
I
569
ULTRASTRUCTURE OF HYDRA NERVES
ferent directions (arrows) in the formation of a multipolar cell. From this stage
in development up to the mature cell, one
seldom sees more than two neurites in the
same section.
Further maturation of the cell involves
the continued elongation of the neurites. Microtubules then begin to appear
throughout the neurites, and by the time
the cell is completely mature the neurites
contain mostly microtubules, few mitochondria and ribosomes, and glycogen particles (Fig. 14).
GASTRODERMAL NEURONS
Exceedingly little is known concerning
gastrodermal neurons except for the brief
references as to their existence (Hyman,
1940; Burnett and Diehl, 1964; Lentz,
1966). Recently, however, we described two
types of neurons in the gastrodermis: neurosensory and neurosecretory cells (Davis,
1972). The recognition was based on the
structural criteria used for identification
of epidermal neurons.
An immature gastrodermal neurosensory
cell containing neurites and a developing
cilium is shown in Figure 15. One neurite
has penetrated through the bases of digestive cells and lies immediately adjacent to
the mesoglea. This represents an unusual
observation since epidermal neurons have
not been seen bordering the mesoglea. The
neurites of mature neurosecretory and
neurosensory cells (Fig. 16) contain typical membrane-bounded secretory droplets
(750 to 1500 A in diameter), glycogen particles, and mitochondria.
Neurosecretory cells are the second type
of neuron found in the gastrodermis. A
mature cell is shown in Figure 17. The
nucleus, with dense chromatin material
dispersed throughout the nucleoplasm,
and all the cytoplasmic components (active
Golgi complexes, secretory droplets [110
to 1600 A in diameter], mitochondria,
microtubules, rough E.R.) are identical in
structure to their epidermal counterparts.
From the sparse available data, it appears that gastrodermal neurons originate
from the basal reserve cells. These cells
are located immediately adjacent to the
circular myonemes at the bases of digestive cells or may lie deeper within the
gastrodermis. Accordingly, they are surrounded completely by digestive cells or
partly by digestive cells and neurons. Figure 18 shows a basal reserve cell which
may have begun to differentiate into a
nerve cell. The nucleus is centrally located
and contains a prominent nucleolus. The
cytoplasmic contents resemble those of
interstitial cells in that there are numerous
free ribosomes, few mitochondria, and
short segments of rough E.R. It is noted
that these cells sometimes contain large
droplets ranging up to 0.7 /x in diameter.
This structural similarity with interstitial
cells, together with the fact that both cell
types are capable of division and differentiation into other cell types (Rose and
Burnett, 1968, 1970), allows one to consider the basal reserve cells as embryonic
cells. Accordingly, these cells seem to be
the most likely cell type from which gastrodermal neurons originate.
SYNAPSES
Most investigators concerned with the
morphology of the nervous system of Hydra
have been cautious in stating that synapses
have not been observed in this organism.
Our maceration studies of stained tissues
show repeatedly that there are intimate
associations between neurons, between neurons and epithelio-muscular cells, and between neurons and cnidoblasts. Ultrastructural studies later confirmed the definite
existence of synapses. Westfall et al. (1971)
provided evidence for three types of synapses, interneuronal, neuromuscular, and
neuronematocyte, in the epidermal nerve
net.
We will now present briefly some evidence for synapses. Figure 19a shows two
gastrodermal neurites (a and b) located
adjacent to each other and between the
circular myonemes of digestive cells. They
contain typical membrane-bounded droplets (900 to 1800 A in diameter), glycogen
570
LOWELL E. DAVIS
MG. 19. a, Two neurites (a and b) of gaslrodermal
neurosensory and/or ncurosecretory cells located
near the base of a digestive cell (dc) . The droplets are 990-1800 A in diameter. Note the point of
contact between the two neurites (ins) and between neurite and myonemes (nms) of digestive
cell (dc) . Mesoglea (me); epithelio-muscular cell
(ep). x 16,000. b, Higher magnification of the
neiiromiisrular synapse (nms) and interneuronal
synapse (ins) shown in Figure 19a. Note ihe thickened membranes, vesicles (1100-1500 A in diameter) containing dense droplets (900-1100 A in
diameter) and synaptic clefts (approximately 100 A
wide). Myonemes (my) of digestive cell (dc).
X 51,800.
ULTRASTRUCTURE OF HYDRA NERVES
FIG. 20. Synapses (arrows) in epithelial neurons.
a, Synapse at perikaryon of neurosensory cell (ns)
and epithelio-muscular cell (ep) . Nucleus (nu) .
X 70,000. b, Synapse (arrows) between neurite
(neu) and myoneme (my) of epithelio-rauscular
cell, x 35,700. c, Synapse (arrows) between neurite
571
(neu) and non-myoneme portion of epitheliomuscular cell (ep). Microtubules (mt); glycogen
(gl) . X 26,000. d. Grazing section of two neurites
(a and b) with interneuronal synapse (ins) and
one (a) with neuromuscular synapse (nms) . Microtubules (mt) ; glycogen (gl) . X 35,700.
572
LOWELL E. DAVIS
particles, mitochondria, and microtubules.
Higher magnification (Fig. 196) of the
same neurites shows that one neurite (center) contains a neuromuscular and an
interneuronal synapse. The vesicles at these
junctions still contain dense contents. The
interneuronal synapse also shows thin filaments which bridge the interneuronal gap.
Figure 20 demonstrates the existence of
synapses in the epidermal neurons. They
may occur at the perikaryon of the cell
(Fig. 20a), at neurite terminations (Figs.
lib, 20b) or along the length of the
neurites (Fig. 20c-d). In all instances, the
membranes at the junctions are thickened
and there are vesicles of various sizes and
densities. This evidence is still preliminary,
but we are presently focusing our attention
on a more detailed study of synapse development, location, structure and function.
CONCLUSION
The introduction of electron microscopy
in the study of neurons in Hydra has led
to several significant landmarks: (i) the
reconfirmation of the definite existence of
neurons (despite recent statements to the
contrary); (ii) the discovery of neurosecretion, indicating that Hydra are the most
primitive organisms in which neurosecretion has been demonstrated; and (iii) the
discovery of synapses. These gains, however, have created a spectrum of problems,
all of which are fertile areas of investigation. We need, for example, a better understanding of the ultrastructure of neurons,
including synapses. Other areas for investigation should and must include the chemical nature of the neurosecretory substance,
its synthesis, transport, mechanisms of release and functions; the specific functions
of individual types of neurons; the role of
neurons in integrating the animal's behavior; and the structural and functional
changes during such processes as growth,
regeneration, budding, initiation of sexuality, and aging. These problems indicate
the trend of future research on neurons in
Hydra.
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Bern, H. A., R. S. Nishioka, and E. R. Hagadorn.
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