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Fine structure of scale development in the teleost Brachydanio rerio.

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Fine Structure of Scale Development in the
Teleost, Brachydanio rerio
ROBERT EARLE WATERMAN
Department of Anatomy, Harvard Medical School,
Boston, Massachusetts 02115
ABSTRACT
Mature and embryonic scales of the zebrafish, Brachydanio rerio, were
examined by light and electron microscopy. Each scale consists of a mineralized
“osseous layer” superficially and a deeper, non-mineralized, “fibrillary plate.” The
mineralizing matrix contains randomly oriented filaments in decalcified sections.
whereas the fibrillary plate is composed of orthogonally arranged lamellae of banded
collagen fibrils embedded i n electron dense material.
Scale papillae and small scales first appear in the midbody region of fry between
0.95 and 1.14 cm long. The matrix of the osseous layer is produced prior to the fibrils
of the fibrillary plate. Foci of mineral deposition appear in this matrix soon after its
production, and increase gradually in number and extent. Cells surrounding the
periphery of the scale are continuous with two layers of cells beneath the inner
surface and with two layers extending a variable distance over the superficial surface.
These “scale-associated” cells are separated from the dermal collagen by other investing cell processes. The probable roles of these cells in scale formation are discussed
and the need for further investigation of the fish scale as a mineralizing system is
stressed.
Early histological examination of fish
scales was motivated by the belief that
they were valuable in classifying the fishes
(Agassiz, 1833-1845; Williamson, 1849;
Goodrich, ’07; Hase, ’07). The basis for
interest in this subject shifted slightly during the early years of this century with
revival of the proposal that surface features of teleost scales might be a useful
index of age and record of the life-history
of a fish (Hoffbauer, ’00; Cockerell, ’11;
Bhatia, ’31). These early studies have
shown a “typical” teleost scale to be a two
layered structure, having an external “osseous” portion, and an inner, non-mineralized, “fibrillary plate” composed of closely
apposed fibrous lamellae. Each scale occupies a dermal “scale pocket” and exhibits
characteristic surface patterns of proven
practical value to fisheries biologists
(Bertin, ’58).
Electron microscopical examination of
adult scales in two teleost species (Cooke,
’67; Brown and Wellings, ’69) revealed in
the mineralized portion of the scale small
crystals similar to those of hydroxyapatite
in other calcified tissues. Collagen fibers
with a major period about 600 A were also
described within these scales, but their
relationship to the mineralized portion differed in the species examined.
ANAT. REC.,168: 361-380.
Histological studies of normal scale development (reviewed by Thomson, ’04;
Maret-Tims, ’05; Hase, ’07; Nusbaum, ’07;
Goetsch, ’15; Fach, ’36; Neave, ’36a, ’40;
Wallin, ’57) have established that teleost
scales are formed within cellular accumulations (“papillae”) which develop between
the epidermis and compact dermis. These
appear first in the region of the lateral line
(“primary papillae”) (Hase, ’11; Everhart,
’49; Ward and Leonard, ’54; McCrimmon
and Swee, ‘67; et al.), and subsequent
generations of papillae are formed at intervals along the path of cells which migrate
away from each primary papilla( Paget,
’20; Neave, ’36a,b, ’40; Weiss, ’59). Each
papilla is thought to give rise to a single
scale which increases in size by addition
of “osteoid” material at its periphery and
of collagen fibers to the fibrillary plate.
Current concepts of normal scale differentiation also depend in part on histological
examination of regenerating scales in adult
animals (Nardi, ’35; Neave, ’40; Wallin,
’57).
Development of the surface features of
the adult scale is poorly understood and
no ultrastructural study of scale development has been reported heretofore. Questions regarding the origin and fate of the
Received Apr. 10, ’70.Accepted June 4, ’70.
361
362
ROBERT E. WATERMAN
cells of the scale papillae, their role in the
production of the scale and their possible
role in the mineralization process also remain unresolved.
This investigation was undertaken,
therefore, to elucidate certain fine structural details of the development of a “typical” teleost scale. It is hoped that such a
descriptive study may serve as a basis for
future investigation of scale differentiation
and mineralization. Portions of this work
have been previously reported (Waterman,
’70).
MATERIALS AND METHODS
The zebrafish, Brachydanio rerio, a Cyprinid, was used throughout. Eggs collected by means of a dip-tube (RoosenKunge, ’38) were maintained in incubator
tanks at 26-27’ C. Larvae and fry were
periodically removed from the tank, measured by means of a dissecting microscope
fitted with a n ocular micrometer, and returned to the tank. Length was measured
from snout to tip of tail fin. Older fry
were anesthetized with a small amount of
MS222 (Tricaine methanesulfonate, Sandoz Pharmaceuticals).
Scales were previously shown to appear
in the interval between several days after
hatching and the time the fry are 1.3 cni
long (Waterman, ’69). Developing scales
and dermis from the niidbody region of
intermediate stages were examined in this
investigation.
For electron microscopy of adult scales,
skin was dissected from the midbody region and sandwiched between two pieces
of fine-mesh metal screen to prevent curling. This was trimmed into smaller pieces
prior to post-fixation. Fry were anesthetized
with MS222 prior to fixation by immersion. Tissues fixed for one hour in buffered
picric
acid-paraformaldehyde
solution
(Stefanini et al., ’67) to which 3% glutaraldehyde was added (PAFG) were washed
in either 0.2 M phosphate or cacodylate
buffer, post-fixed in phosphate-buffered
osmium tetroxide and embedded in Epon
(Luft, ’61).
Adult scales were decalcified according
to a modification of the method of Baird
e t al. (’67). Following fixation for 7.5
hours in cold PAFG, scale-containing skin
was placed for seven days in 0.1 M di-
sodium EDTA (Ethylenediamine tetraacetate) containing 5% glutaraldehyde,
washed in phosphate buffer (Warshawsky
and Moore, ’67) for 23 hours, post-fixed
in osmium for 3.5 hours, dehydrated in
ethanol and embedded in Epon (Luft, ’61).
Thin sections cut with a diammd knife
were stained with uranyl acetate and lead
citrate and examined with a n RCA-3F
electron microscope.
OBSERVATIONS
Adult. There is considerable overlapping of scales within the skin. The anterior portion of each scale extends into the
dermis, while most of the posterior third
is covered only by epidermis on both inner
and outer surfaces. Characteristic features
of a large cycloid scale from the midbody
region of a mature zebrafish are shown i n
figure 2. The outer surface of the scale
may be divided into four approximately
triangular fields conventionally termed anterior, posterior, dorsal and ventral with
reference to the normal orientation of the
scale within the skin (Oosten, ’57). The
apices of these fields lie at the focus, which
represents the original growth center of
the scale and which is located anterior to
the geometrical center of the scale in this
species. The serrations are linear thickenings of the outer surface of the scale. They
form parallel ridges which are closely
spaced in the anterior field, more widely
separated in the dorsal and ventral fields,
and are absent from the posterior field.
The circuli seen in the posterior field result
from undulations in the surface contour.
They roughly parallel the posterior margin
of the scale, and mark successive stages
in growth of the scale. Unlike the serrations and circuli which indicate variations
in thickness of the surface layer of the
scale, the radii extending across the posterior field in the anterior-posterior direction are grooves in the outer surface of
the scale.
Each scale consists of a mineralized
(“osseous”) layer underlain by a nonmineralized “fibrillary plate” (fig. 3). The
mineralized layer is approximately 1.52.0 p thick, tapering near the periphery.
The matrix of this region contains filaments about 200-250 A in diameter in
decalcified sections (fig. 4 ) . The fibrillary
TELEOST SCALE DEVELOPMENT
plate may contain 10 to 15 lamellae of
variable thickness. Each lamella consists
of closely packed collagen fibrils surrounded by a dense matrix, and the orientation of fibrils is orthogonal with respect
to fibrils of adjacent lamellae. The dianieter of the fibrils is greatest near the
mineralized portion of the scale.
A rim of cells, including melanophores,
surrounds the periphery of the scale. These
cells are continuous with two layers of extremely thin cells covering the inner surface of the scale (fig. 3 ) , and with similar
layers of flattened cells which cover the
outer surface except in the posterior field
where the mineralized portion lies immediately subjacent to the basal cells of
the epidermis.
Sections cut tangential to the posterior
margin reveal periodic thickenings of the
osseous layer which presumably correspond
to the circuli seen in surface view. The
interface between successive thickenings
displays interdigitating irregularities which
may serve to anchor newly produced matrix to the older (fig. 4).
Developmental stages. Larvae increased
in length from approximately 0.35 cm at
hatching to approximately 1.5 cm at 110
days after hatching. Variations in body
length were observed at any given chronological age, and the appearance of scales
seems to be more closely related to body
length than to time after hatching.
Scales first appear in the midbody region
when the fry are between 0.95 cm and
1.14 cm, and a complete layer of scales
is present around the entire midbody region at 1.3 cm. This corresponded to approximately 35 days under the temperature
and feeding conditions of this investigation.
In the 0.95 cm fry, prior to scale formation, the non-cellular basal (basement)
lamella (Garrault, ’37; Nadol et al., ’69)
beneath the epidermis is approximately
1.3 thick. It is composed of 10-12 laminae, each consisting of two to three layers
of small collagen fibrils (fig. 5). A single
epithelial layer lies immediately subj acent
to the deep surface of this “primitive
dermis” (Hay, ’64). These cells are connected by desmosomes and contain numerous cisternae of the rough endoplasmic
reticulum. Melanophores, guanidophores,
363
and processes of other cell types are interposed between this simple epithelium and
the superficial muscle cells of the lateral
musculature.
In the 1.14 cm fry, the basal lamella in
regions not containing scales is composed
of 25-30 laminae of collagen fibrils (fig. 6).
The average diameter of these fibrils is
greatest in the middle laminae and decreases slightly toward the epidermal basement lamina and toward the deep surface
of the basal lamella. Cells are present at
this stage among the collagen fibrils at
various levels, and a conspicuous cellular
layer is seen near the epidermis. Collagen
fibrils associated with some cellular profiles extend transversely across the basal
lamella from its medial surface. These
fibrils oriented perpendicular to the body
axis may indicate that such cells are migrating into the basal lamella from its
medial surface. Following the appearance
of cells within the basal lamella, this formerly acellular region might more properly
be termed the “dermis.”
Along the side of the body at the level
of the horizontal septum, two or three rows
of small scales are seen beneath the epidermis. These initial scales are not oriented
obliquely relative to the dermis, but lie
flat against the deep surface of the epidermis. There is little or no overlapping
of adjacent scales. Several small cellular
aggregations between the epidermis and
the collagenous laminae dorsal and ventral
to these scales presumably represent
“secondary papillae .”
In the 1.3 cm fry, the entire surface of
the midbody region contains scales similar
in appearance to those of the 1.14 cm
stage (fig. 1). The structural organization
of the skin in the 1.3 cm stage is seen in
figure 7. The epidermis consists of two to
four layers of flattened cells with an underlying basement lamina. Several layers
of pigment cells and other cell types are
interposed between the compact dermis
and the superficial muscle cells. Fibroblasts
are present between the layers of dermal
collagen, and occasional small nerves also
occur within this region.
The anterior ends of the scales are now
slightly inclined into the dermis while the
posterior borders lie directly under the
epidermis. Each scale consists of a min-
364
ROBERT E. WATERMAN
Fig. 1 A diagram of the dermis from the midbody region of a 1.3 cm zebrafish fry summarizing
features of the newly forming scales discussed in the text. A, cell of the episquama proximal to a developing serration; B, mineralizing (“osseous”) layer of scale; C , fibrillary plate of scale; D, innermost
cellular layer of hyposquama; E, peripheral cell.
eralizing portion approximately 1.5 p thick,
which tapers rapidly at the periphery, and
a thin region of collagen fibrils on its inner
surface. The mineralizing matrix contains
filaments of at least two diameters (fig. 8).
Curvilinear densities are sparsely distributed within this matrix at its periphery
(fig. 9 ) , but increase greatly in number
toward the central (i.e., older) part of the
scale. They are approximately 80 A in
diameter (fig. 10) and of unknown length
since they leave the plane of section. Many
of these slender dense profiles appear
curved, particularly in more heavily minPeralized regions (figs. 13, 14). The similarity between the appearance of these
densities and that of hydroxyapatite
crystals in vertebrate bone and other calcified tissues (Glimcher, ’59; Jackson, ’60;
Matukas and Krikos, ’68; Bonucci, ’69;
Anderson, ’69; Paegle, ’69) suggests that
they may represent foci of mineralization.
The foci are randomly oriented within the
matrix, and are associated with regions of
slight condensation of the matrix (fig. 10).
The fibrillary plate does not extend to
the margin of the scale. Small fibrils are
first encountered a short distance proximal to the margin (i.e., toward the focus)
(fig. 11). The number of fibrils increases
toward the center of the scale where the
fibrillary plate consists of two to four ortho gonally arranged 1amell ae.
The entire margin of the scale is bounded
by a rim of cells (fig. 9 ) . These are connected by desmosomes with a n epithelium
two or three cells thick, which underlies
the entire inner surface of the scale, and
365
TELEOST SCALE DEVELOPMENT
with a similar epithelium which extends
for a variable distance over the outer surface where i t is interposed between the
mineralizing matrix and the epidermal
basement lamina. A similar distribution
of cellular layers has been described in
other species and appears to be a general
feature of teleost scales. The diverse nomenclature (reviewed by Neave, ’36a)
previously applied to these epithelia, or
regions thereof, were based either on cell
shape (e.g., “polygonal cells”) or on their
supposed function (“scleroblasts;” “osteoblasts”). However, because the precise
function of these cells has not yet been
demonstrated, adoption of the following
terminology based on the topographical
relationship of the cells to the scale is
suggested : episquama, epithelial layers intimately covering the superficial surf ace of
the scale; hyposquama, the epithelial layers
underlying the deep surface of the scale;
and peripheral cells, the cells which form
a rim around the periphery of the scale
and are attached to cells of both the episquama and hyposquama. The margin of
the scale thus fits into the space (scale
groove) between the episquama and hyposquama bounded by the peripheral cells.
The episquama, hyposquama, and peripheral cells together comprise the “scaleassociated” cell population. The surrounding dermal collagen and several layers of
flattened cells which separate the scaleassociated cells from the dermis constitute
the scale-pocket.
A short distance proximal to the scale
margin the cells of the hyposquama are
flattened and elongated. Cells closest to
the scale measure approximately 1.3
thick and contain an extensive rough endoplasmic reticulum with occasional localized areas of smooth surfaced membranes and vesicles (figs. 7, 13). Cells
further from the scaIe are more attenuated
and do not contain an extensive rough
endoplasmic reticulum. All cells of the
hyposquama are connected by desmosomes
and by more extensive regions of cell to
cell junction.
Cells of the episquama become extremely
flattened a short distance proximal to the
scale margin. They cover most of the anterior portion of the scale, but are absent
over much of the posterior field. Near the
point where the episquama ends, the epidermal basement lamina is also discontinuous and the mineralized matrix abuts directly against the basal epidermal cells
(fig. 12).
The serrations of the scale surface appear in cross-section as barb-like thickenings of the mineralized portion which project at intervals between certain cells of
the episquama in the anterior, dorsal and
ventral fields. Nuclei of the episquamal
cells tend to lie in the troughs between
serrations. The majority of the episquamal
cells and those of the peripheral rim do
not contain a well-developed rough endoplasmic reticulum. Ribosome studded cisternae are prominent, however, in those
cells immediately proximal to newly forming serrations (fig. 11).
The radii are discontinuities in the mineralized portion of the scale which extend
across the posterior field. Each radius
originates as a defect in the mineralizing
matrix at the margin of the growing scale
which is propagated in a linear manner
as the scale enlarges. The mechanism of
this linear propagation remains unknown,
but a proposed sequence of stages in the
formation of a radius is seen in figures
15-17. Sites of future radii are revealed
at intervals along the posterior margin at
points where contact between the episquama and hyposquama is maintained by
cells which extend through the mineralizing matrix and are connected by desmosomes. Production of matrix and its subsequent mineralization occurs on both sides
of these cellular bridges. Contact between
the episquama and hyposquama is later
lost leaving a cell attached to the episquama (radius cell) lying in a gap within
the mineralizing matrix (the radius). Formation of a continuous intervening fibrillary plate beneath the mineralizing layer
and radius then proceeds. The radius cell
may either retain its attachment to the
episquama or become “trapped’ in a groove
on the basal surface of the epidermis. The
cytoplasm and nucleus of these cells become dense and stain intensely, but their
ultimate fate remains unknown.
DISCUSSION
A distinct ultrastructural difference between the matrix of the mineralizing and
366
ROEERT E. WATERMAN
non-mineralizing portions observed in the (mineralizing) portion is produced prior
zebrafish scale has also been reported in to the non-mineralizing fibrillary plate
E'undulus rnajalis (Cooke, '67). I n Fundu- (see Neave, '40). The particular cells reZus, however, the mineralized region is sponsible for production of these extrainterposed between non-lamellar regions of cellular materials is not known, however.
collagen fibers. I n Hippoglossoides elasso- Three main possibilities have been prodon (Brown and Wellings, '69) large col- posed: (1) that both the osseous and fibrillagen fibers of similar appearance were lary plate are produced by the same cells
described throughout the scale, with min- (i.e., at the periphery) (Wallin, '57); ( 2 )
eral deposited between and within those of that the osseous layer and fibrillary plate
the osseous layer. The ultrastructural dif- are produced respectively by the outer and
ferences indicated by the few data avail- inner cells surrounding the scale (Setna,
able, and the reported variation in the '34; Pevsner, '26; Ussow, 1897; Kyle, '27);
organic and inorganic composition of and ( 3 ) that the osseous layer is produced
scales (Green and Tower, '02; Block et al., by the peripheral cells ("osteoblasts")
'49; Wallin, '56; Seshaiya et al., '63; Moss while the majority of the fibrillary plate
et al., '64), suggest caution i n generalizing arises by incorporation of layers of dermal
between species and stress the need for collagen (Klaatsch, 1890; Neave, '36a, '40).
correlative ultrastructural and chemical
In the zebrafish the fibrillary plate apstudies in attempting to understand the pears to be produced entirely by the cells
fish scale as a mineralizing biological on the inner surface of the scale. This is
system.
suggested by the observation that the disKnowledge of the inorganic components tribution of small collagen fibrils of the
involved and their relationship to the or- plate is coextensive with cells of the hyganic components of the scale is funda- posquama which contain large amounts
mental to such a n understanding, but is of rough endoplasmic reticulum, and these
as yet incomplete. Scales of many species cells continue to exhibit the cytological
are known to contain calcium phosphate features of protein secreting cells while
and calcium carbonate (Oosten, '57), but the fibrillary plate is rapidly growing. The
large variations in percentage composition fact that the fibrils of the fibrillary plate
of mineral residues have been reported. It increase i n diameter from near the innerhas been assumed that electron-dense most layer of hyposquamal cells toward
structures seen within the mineralized por- the mineralized layer is also consistent
tion of the scale i n electron micrographs with this proposal. The hyposquama is also
represent sites of mineral deposition, but present beneath the adult scale, but the
their exact nature cannot be stated. Pre- cells are extremely thin and by this time
vious histological studies indicated that contain little rough endoplasmic reticulum.
the osseous portion of the developing scale These latter observations are in accord
is not completely miner31ized at its pe- with light microscopic findings by others
riphery, and a region of unmineralized of a loss o i basophilia and an apparent
matrix was thought to exist at the margin. disappearance of these cells (Neave, '40).
Observations of developing zebrafish scales
That production of the mineralizing
with the electron microscope reveal that matrix is limited to the cells surrounding
small foci of curvilinear densities, assumed the periphery of the scale is suggested by
to represent mineral deposition, occur in the observations that the mineralized layer
the mineralizing matrix out to the extreme is of nearly uniform thickness throughout
margin, indicating that mineralization of the scale, except at the periphery, and that
the osseous layer is a gradual and con- the thickness of the mineralized layer is
tinuous process beginning soon after ma- approximately equal in both newly formed
trix production.
and adult scales. Fibrils of the fibrillary
With few exceptions (Paget, '20; Pevs- plate are interposed between the mineralizner, '26; Petrov and Petruschewsky, '29; ing matrix and the hyposquama a short
Setna, '34), most investigators of teleost distance proximal to the scale margin
scale development agree that the osseous making production of this matrix by the
TELEOST SCALE DEVELOPMENT
hyposquamal cells beneath the fibrillary
plate unlikely, and the episquama is not
present over a large portion of the superficial surface of the scale. It is not clear
from ultrastructural observations alone
whether there is a separation of function
among the peripheral cells.
It is accepted by most investigators that
teleost scales are entirely of mesodermal
origin (Oosten, ’57; Bertin, ’ 5 8 ) , although
a possible role of the ectoderm in scale
formation has recently been revived (Moss
et al., ’64; Kresja, ’67; Moss, ’68). The
absence of an intervening basement lamina between the epidermis and portions of
the underlying mineralizing matrix of developing zebrafish scales (fig. 12) indicates
that a close structural relationship exists
between the epidermis and the scale, but
active participation of the epidermis in
scale production remains to be determined.
Changes in shape of the scale-associated
cells from the periphery toward the older
portions of the developing scale (fig. 1)
suggest that as the periphery of the scale
enlarges by addition of new mineralizing
matrix and the peripheral cells move away
from the center, some of these cells are
left behind, becoming “stretched out” to
form the episquama and hyposquama. Cells
of the hyposquama maintain strong attachments to older cells of this layer, and those
nearest the scale differentiate to produce
the fibrillary plate. Cells of the episquama
do not remain as a continuous epithelium
over the entire mineralized layer, but become extremely flattened a short distance
from the periphery, and may perhaps be
“pulled after” the outwardly moving peripheral cells.
The number of scale-associated cells no
doubt increases as the scale grows, but the
source of this increase remains uncertain.
Recruitment of cells from the surrounding
connective tissue has been proposed
(Klatsch, 1890; Pevsner, ’26). Others
have suggested that the number of scaleassociated cells increases solely by mitosis
of the cells of the orginal scale papilla
(Paget, ’ 2 0 ; Setna, ’34; Dietrich, ’ 5 3 ) .
Neave (’36a) reported frequent mitotic
figures in cells of the outer layer in brown
trout. In the zebrafish, the innermost cells
of the hyposquama appear to arise from
the peripheral cells and are not incorpo-
367
rated from the dermis as suggested by
Neave (‘36a). However, one cannot exclude
the possibility that the several layers of flattened cells between the scale-associated
cells and the dermal collagen may become
incorporated into the scale-associated cell
assemblage. The fact that mitotic figures
were not observed in any scale-associated
celIs during this investigation does not
rule out their existence, and more study
will be required to understand the population dynamics of the scale-associated cells.
ACKNOWLEDGMENTS
The author wishes to thank Dr. Don W.
Fawcett for his critical review of the manuscript and acknowledges the artistic aid of
Mrs. Sylvia Colard Keene. This investigation was supported by Public Health Service Training grant GM-00406.
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TELEOST SCALE DEVELOPMENT
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PLATE 1
EXPLANATION OF FIGURES
370
2
Photomicrograph of a typical cycloid scale from the midbody region
of a mature ( 4 c m ) zebrafish. The epidermis has been removed from
the posterior field to reveal the characteristic surface features. Unfixed.
x 39.
3
Electron micrograph of a cross-section through a decalcified mature
zebrafish scale. The mineralizing matrix of the osseous layer and the
orthogonally arranged lamellae of collagen fibers comprising the nonmineralizing fibrillary plate of the scale are seen. Electron-dense material surrounds the collagen fibers of the fibrillary plate. Two or three
layers of attenuated cells (arrow) underlie the inner surface of the
scale (inset). Decalcified. Uranyl acetate and lead stain. x 8,600.
Inset: electron micrograph of cells in region indicated. Decalcified.
Uranyl acetate and lead stain. x 35,500.
4
Electron micrograph of a tangential section near the posterior margin
of a decalcified mature zebrafish scale. Extensive interdigitations ( * )
occur along the interface between adjacent regions of mineralized
matrix. Decalcified. Uranyl acetate and lead stain. x 14,500. Inset:
Fibers (arrow) within a portion of the mineralizing matrix are shown
at higher magnification. Decalcified. Uranyl acetate and lead stain.
x 33,000.
TELEOST SCALE DEVELOPMENT
Robert Earle Waterman
PLATE 1
371
PLATE 2
EXPLANATION O F FIGURES
5
Electron micrograph of the developing dermis i n a 0.95 cm zebrafish
fry. The basal lamella (BL) consists of 9-12 thin fibrous laminae. A
single layer of cells ( * ) with prominent rough endoplasmic reticulum lies immediately subjacent to the basal lamella. Several other
layers of flattened cells are also present between the basal lamella
and the lateral musculature. PAFG fixation. Uranyl acetate and lead
stain. x 13,500.
6
Electron inicrograph of a region of dermis from a 1.14 cm zebrafish
fry not containing scales. Cellular processes are found among the
25-30 laminae of dermal collagen. Fibrils oriented perpendicular to
the body axis are frequently associated with some cell processes and
cells near the lateral muscle (arrows). PAFG fixation. Uranyl acetate
and lead stain. x 7,400.
7 A portion of a newly formed scale is seen laterally within the dermis
of a 1.3 cm zebrafish fry. The scale consists of a mineralized osseou5
layer superficially and a non-mineralized fibrillary plate medially .
The fibrillary plate is composed of several collagenous laminae. Flattened cells (episquama) cover the osseous layer and two to three
layers of cells (hyposquama) lie beneath the fibrillary plate. The cells
immediately subjacent to the fibrillary plate contain prominent rough
endoplasmic cisternae anci smooth surfaced vesicles. Numerous thin
cellular processes separate the scale and its associated cells from the
epidermis and surrounding dermal collagen. PAFG fixation. Uranyl
acetate and lead stain. x 10,300.
372
TELEOST SCALE DEVELOPMENT
Robert Earle Waterman
PLATE 2
373
PLATE 3
EXPLANATION O F FIGURES
8 A portion of the mineralizing matrix near the periphery of a newly
formed scale from a 1.3 cm zebrafish fry. Bundles of larger filaments
(::< are frequently observed. PAFG fixation. Uranyl acetate and lead
stain. x 57,200.
1
9 The margin of a young scale and a portion of a n overlapping scale
are seen i n a 1.3 cm zebrafish fry. The rim of cells surrounding the
peripheral margin of the scale are continuous with the episquama
and hyposquama. Presumed foci of mineral deposition (see fig. 10)
are present at the extreme margin of the mineralizing matrix and
gradually increase i n number and extent toward the older portions of
the scale. A developing serration is also shown. PAFG fixation. Uranyl
acetate and lead stain. x 13,200.
10
374
Curvilinear densities within the mineralizing matrix near the periphery of a scale from a 1.3 cm zebrafish fry are shown. These dense
rods approximately 80 A i n diameter which are associated with regions
of slightly condensed matrix are thought to represent sites of mineral
deposition. PAFG fixation. Uranyl acetate and lead stain. x 111,000.
'I'ELEOST SCALE DEVELOPMENT
Robert Earle Waterman
PLATE 3
375
PLATE 4
EXPLANATION O F FIGURES
11
Cross-section near the periphery of a scale from a 1.3 cm zebrafish
fry. Small fibrils of the fibrillary plate ( F P ) are first encountered a
short distance proximal to the periphery. The subjacent cells of the
hyposquama contain numerous ribosome studded cisternae of the endoplasmic reticulum ( "). Rough endoplasmic reticulum cisternae are
also prominent i n cells of the episquama proximal to developing serrations. PAFG fixation. Uranyl acetate and lead stain. x 20,300.
12
Cross-section near the periphery in the posterior field of a young scale
i n a 1.14 c m zebrafish fry. Near the point at which the episquama
ends, the epidermal basement lamina ( B L ) is also discontinuous and
the osseous layer of the scale lies directly beneath the basal epidermal
cells (
PAFG fixation. Uranyl acetate and lead stain. x 29,200.
").
13 Cross-section through the posterior field of a scale from a 1.3 cm
zebrafish fry. The osseous layer contains numerous curvilinear densities approximately 80 A in diameter and of unknown length. Collagen fibrils of the fibrillary plate are surrounded by a n electron-dense
matrix near the osseous layer. Cells of the hyposquama are attached
by desmosomes and longer regions of cell junction ( * ) , PAFG fixation. Uranyl acetate and lead stain. x 33,200.
376
TELEOST SCALE DEVELOPMENT
Robert Earle Waterman
PLATE 4
377
PLATE 5
EXPLANATION OF FIGURES
14 A portion of the mineralizing matrix near the center of a developing
scale from a 1.3 cm zebrafish fry is seen. Many of the slender, dense
profiles within this region appear curved (arrows). PAFG fixation.
Uranyl acetate and lead stain. x 100,000.
Figs. 15-17 represent stages in the process of radius formation as observed i n developing scales of 1.3 cm zebrafish fry.
15 Areas of cellular contact ( * ) between the episquama and hyposquama
occurring a t intervals in the anterior, dorsal and ventral fields of
developing scales represent sites of future radii. Production and subsequent mineralization of the osseous layer matrix proceeds on both
sides of these cellular bridges. PAFG fixation. Uranyl acetate and lead
stain. x 14,400.
16 Contact between the episquama and hyposquama is lost as development proceeds, and a “radius cell” ( * ) remains in a gap within the
mineralizing matrix. Production of a continuous intervening fibrillary
plate occurs beneath the mineralizing matrix. PAFG fixation. Uranyl
acetate and lead stain. x 12,700.
17 The “radius cell” ( * ) decreases in size as formation of the fibrillary
plate proceeds. PAFG fixation. Uranyl acetate and lead stain.
378
x
11,200.
TELEOST SCALE DEVELOPMENT
Robert Earle Waterman
PLATE 5
379
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