Proc. Arthropod. Embryol. Soc. Jpn. 47, 21–28 (2012)Ⓒ 2012 Arthropodan Embryological Society of Japan
ISSN 1341–1527

OVARIAN STRUCTURES IN CENTIPEDES21[ORIGINAL PAPER]

Morphological Study of Ovarian Structures in Scolopendromorph
Centipedes (Myriapoda: Chilopoda) with Special Reference to the
Position of Oocyte Growth
Yui MIYACHI and Kensuke YAHATA
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305–8572, Japan
E-mail: yui.miyachi@gmail.com (YM)

Abstract
The ovarian structures of scolopendromorph centipedes, Scolopocryptops rubiginosus and S. sexspinosus, are describedbased on histological and ultrastructural observations. Particular attention was paid to the position of growing germ cells. In
the Scolopocryptops species examined, the ovary was a single longitudinal sac-like organ that consisted of a layer of ovarianepithelium. Oogonia and young oocytes were sparsely distributed or closely clustered among ovarian epithelial cells all along
the ventral region of the ovary. Early previtellogenic oocytes surrounded by a single layer of vitelline membrane were alsopresented among ovarian epithelial cells and irregularly distributed along the ventral region of the ovary. In contrast, lateprevitellogenic and vitellogenic oocytes surrounded by two layers of vitelline membrane occupied their own individual
follicular pouches. Each follicular pouch appeared as a dent in the ovary and was formed by a continuous layer of the ovarianepithelium accompanied by a continuous lining of basement membrane. Therefore, oogenetic growth of these folliculated
oocytes proceeded in hemocoelic spaces rather than in the ovarian lumen. Oocyte growth within hemocoelic spaces settled
in the follicular pouches, i. e. growth outside of the ovary, could be regarded as a shared derived feature of myriapods, and thistrait is never observed in other arthropods.

Introduction
Several differences in ovarian structure have been
represented in the two major arthropod subphyla, theChelicerata and the Mandibulata (Makioka, 1988). One of
the most noticeable differences is the position of oocyte
growth within the ovary (Fig. 1). In the type found in mostchelicerates (the chelicerate-type; Fig. 1A), the early
oocytes leave the germarium, not to enter the ovarian
lumen, but to ride on the outer surface of the ovarian wall,
where they grow sandwiched between the ovarianepithelium and its basement membrane during theoogenetic period. The ripe oocytes are then ovulated into
the ovarian lumen through the ovarian wall epithelium.
The other type is found in many mandibulates (themandibulate-type; Fig. 1B). In this type of ovary, theoocytes leave the germarium to enter the ovarian lumen,
where they grow during the oogenetic period.
This histological understanding of arthropod ovaries
was, however, challenged by an ultrastructural study of
diplopod ovaries. Kubrakiewicz (1991a) observed theultrastructure of ovaries in the julid diplopod, Ophyiulus
pilosus, and reported that the oocytes underwent oogenetic
growth in a hemocoelic space outside of and immediately

adjacent to the ovary. Oocyte-growth in a hemocoelic
space is never known in the other mandibulates (also inthe chelicelates). In the ovary of this julid diplopod, eachgrowing oocyte was surrounded by a follicular epithelial
layer that was continuous with the ovarian epithelium andaccompanied by the basement membrane. Therefore, theinner space of the follicular pouch was hemocoelic. Suchan ovarian structure is quite different from those of theboth mandibulate- and chelicelate-type of ovary. Until now,
only a few ultrastructural studies have been done in othermyriapod species: a lithobiomorph chilopod (Herbaut,1974), a symphylan (Biliński, 1979) and some diplopods(Kubrakiewicz, 1991b, c). However, they provided very
few details on the position of germ cells at different
developmental stages within and/or around the ovary.
Therefore, it remains to be determined whether oocyte
growth occurs in the hemocoel in some myriapods andwhether it is common among myriapod groups.
The growing oocytes are commonly connected withthe ovarian epithelium via their own follicle layers in all offour myriapod groups (e. g., in Chilopoda: Jangi, 1957;
Herbaut, 1974; Knoll, 1974; in Symphyla: Tiegs, 1940; in
Pauropoda: Tiegs, 1947; in Diplopoda: Sareen and Adiyodi,

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1983; Yahata and Makioka, 1994). Such a follicularconnections between oocytes and the ovarian epithelium
is unique among arthropod groups, and may be a shareddistinctive feature of the all myriapod groups.
Recently, Miyachi and Yahata (2011) suggested thepossibility that oocyte growth occurs in the hemocoel insome centipedes. The present study histologically andultrastructurally describes the ovaries in two
scolopendromorph centipede species, Scolopocryptops
rubiginosus and S. sexspinosus. Special attention was paid
to the position of female germ cells at different
developmental stages to compare with the findings of
Kubrakiewicz (1991a).
Materials and Methods
Adult specimens of Scolopocryptops rubiginosus (L.
Koch, 1878) and S. sexspinosus (Say, 1821) were collectedfrom leaf litter, decayed tree, and under stones in the
Ibaraki, Fukuoka, Miyazaki and Shizuoka Prefectures ofJapan.
For light microscopy, dissected ovaries or wholebodies of which the legs were removed out were fixed with
Bouinʼs solution. Some fixed specimens were dehydrated
in a graded ethanol-acetone series, embedded inmethacrylate resin, and were cut into serial sections in 1 or
2 µm thick. Other specimens were dehydrated in a gradedethanol-n-butanol series, embedded in paraffin and were
cut into serial sections of 5 or 8 µm thick. Both the paraffin
and the resin sections were stained with Mayerʼshematoxylin and eosin, alcian blue-periodic acid SCHIFF(PAS)-hematoxylin, or azocarmine G-aniline blue-orange
G.
For transmission electron microscopy, dissectedovaries were pre-fixed with Karnovskyʼs fixative bufferedwith 0.2 M HCl-sodium cacodylate in a few hours. After
post-fixation with 1% osmium tetroxide in two hours,

Fig. 1

specimens were dehydrated in a graded acetone series,
embedded in Quetol 651 resin, and cut into ultrathin
sections with an ultramicrotome (Leica ULTRACUT-S or
RMC MT-7000). Sections were un-stained or double
stained with uranyl acetate and lead citrate and wereobserved under a transmission electron microscope(Hitachi H-7650, JEOL JEM-1010 or TOPCON LEM-2000)
at 80 kV.
Results
Gross anatomy of female reproductive organ
The reproductive systems of adult female
Scolopocryptops rubiginosus and S. sexspinosus were very
similar in morphology. In both species, the adult ovary wasa single, long, sac-like organ located dorsaly to thealimentary canal along the median axis (Fig. 2). An oviductextended posteriorly from the most posterior end of theovary. The ovarian wall, which surrounded an ovarian
lumen (Figs. 2, 3), consisted of a layer of ovarian epithelial
cells ranging from 0.5–6 µm in height (Figs. 3, 4). Theseovarian epithelial cells were connected to each others bydesmosome junctions (Fig. 5). On the outer surface of theovary, the ovarian epithelium was lined with a thickbasement membrane from 350–1000 nm in thickness (Fig.
4). The basement membrane was distinctive in that it was
unusually thick and it had a homogeneous mono-lamina
structure that lacked distinct lamina lucida, lamina densa,
and zona reticularis layers (Figs. 4, 5).
Oogonia, young oocytes, and early previtellogenic
oocytes were sparsely distributed or closely clusteredamong ovarian epithelial cells throughout the ventral wallof the ovary (Fig. 2). In contrast, late previtellogenic andvitellogenetic oocytes were separated into their own
follicular pouches (Fig. 3). The follicular pouch was anextended inward hollow formed by a continuous layer ofovarian epithelium, which was lined with a continuous

Schematic drawings of two types of arthropodan ovaries (modified from Makioka, 1988). A) Chelicerate-type; oocytes grow between ovarianepithelial layer and its basement membrane. B) Mandibulate-type; oocytes grow within ovarian lumen. bm: basement membrane, oc: oocyte,
oe: ovarian epithelium, og: oogonia, ol: ovarian lumen.

OVARIAN STRUCTURES IN CENTIPEDES

Fig. 2

Fig. 3Fig. 4

Fig. 5

Fig. 6

Cross-section of adult ovary in Scolopocryptops sexspinosus. Paraffin. Alcian blue-PAS-hematoxylin. Dorsal side is oriented towards figure top.
Previtellogenic (po) and vitellogenic oocytes (vo) within the ovary at ventral region of the ovary. ac: alimentary canal, ol: ovarian lumen. Scale =500 µm.
Cross-section of adult ovary in Scolopocryptops rubiginosus. Paraffin. Azocarmine G-aniline blue-orange G. Dorsal side is oriented towards figure
top. Ovarian lumen (ol) surrounded by ovarian epithelium (oe), and late previtellogenic oocyte (lpo) growing in hemocoelic space withinfollicular pouch. Note the presense of hemocyte (hc) between the late previtellogenic oocyte (lpo) and follicle epithelium (fe). Asterisk indicatesthe hemocoelic space. bm: basement membrane, fp: follicle pore. Scale = 100 µm.
Ovary in Scolopocryptops rubiginosus. TEM. Un-stained. Ovarian and follicle epithelial layers accompanied by their own basement membrane(bm), and narrow ovarian lumen (ol) between the both layers. Note that the late previtellogenic oocyte (lpo) with developed microvilli (mv) is
enveloped by the outer (ovm) and inner layer of vitelline membrane (ivm), and the enveloped oocyte is directly surrounded by the layer offollicle epithelial cells (fec) and its basement membrane (bm). Asterisk indicates the hemocoelic space. Inset: Extended part of the inner surface
of follicular pouch. m: muscle, oec: ovarian epithelial cell. Scales = 5 µm in main and 1 µm in insetted figure.
Ovarian epithelial cells in Scolopocryptops rubiginosus. TEM. Un-stained. Arrows indicate the desmosome junctions between adjacent ovarianepithelial cells (oec). Arrowheads indicate the hemidesmosomes by which the epithelial cells are anchored with the basement membrane (bm).
Asterisk indicates the hemocoelic space. ol: ovarian lumen. Scale = 1 µm.
Ovary around the fold of ovarian and follicular epithelial layer in Scolopocryptops rubiginosus. TEM. Un-stained. Continuous construction offollicle epithelium with the ovarian epithelium. Asterisks indicate the hemocoelic space. bm: basement membrane, fec: follicular epithelial cell,
ivm: inner layer of vitelline membrane, lpo: late previtellogenic oocyte, m: muscle, oec: ovarian epithelial cell, ovm: outer layer of vitelline
membrane. Scale = 10 µm.

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Y. MIYACHI AND K. YAHATA

basement membrane (Fig. 6). Such construction of follicle
epithelium proves the hemocoelic nature for the innerspace of follicular pouch. Each follicular pouch had a follicle
pore that opened to the hemocoel (Fig. 3). Based on thehistological analysis of whole-body sections, the follicle
pores opened towards the alimentary canal all along theovary (Fig. 2).
Female germ cells in ovarian epithelium
Oogonia, young oocytes, and early previtellogenic
oocytes were distributed among the ventral ovarianepithelial cells. These germ cells could be distinguishedfrom the epithelial cells based on the cellular shape and thestaining condition of nucleus (Fig. 7). The nuclei of theovarian epithelial cells were irregular in shape, but thenuclei or early germinal vesicles of germ cells were round(Fig. 8).
Oogonia of about 12 µm in diameter had relatively
large nuclei of about 6 µm in diameter with several
chromatin granules but no nucleoli (Figs. 7, 9). Young
oocytes, those that had not begun oogenetic growth, of
about 12 µm in diameter had early germinal vesicles ofabout 5 µm in diameter with one or a few nucleoli (Fig. 7).
Early previtellogenic growth of the oocytes proceeded
among the ovarian epithelial cells (Fig. 7). These oocytesof more than 20 µm in diameter were surrounded by a thinlayer of egg envelope of about 500 nm thick (Fig. 10).
These enveloped early previtellogenic oocytes had anumerous microvilli, and the surrounding ovarianepithelial cells showed no evidence of secretory activity(Fig. 10). Thus the egg envelope should be autosynthetic
vitelline membrane.
Each of these female germ cells–oogonia, young
oocytes, or early previtellogenic oocytes were whollysurrounded by a few ovarian epithelial cells (Figs. 7, 9), andwere never contacted with the basement membrane of theovarian epithelium (Fig. 9). The epithelial cells wereelongated and curved to surround the female germ cell
(Figs. 7, 9). The basal surfaces of the epithelial cells had alarge number of protuberances that projected deep into thebasement membrane. At the tips of these protuberances,electrondense
granularstructures,
possibly
hemidesmosomes, were very prominent (Figs. 7, 9). The
ovarian epithelium and its basement membrane of theregions surrounding oogonia, young oocytes or earlyprevitellogenic oocytes were thicker than other regions:
They became 15–25 µm and 450–1400 nm in thick,respectively (Figs. 7, 9).
Female germ cells in follicular pouches
Oocytes larger than 60 µm in diameter were notdistributed among ovarian epithelial cells, but were locatedwithin the inner space of each individual follicular pouch(Figs. 3, 11–13). Epithelial cell layer of the follicular pouch
was about 0.3–5 µm in thick (Figs. 3, 11–13), and its innersurface was lined with a basement membrane that was
240–750 nm in thick (Figs. 4, 6, 7, 10). The inner space ofeach pouch was continuous with the hemocoel (Figs. 3, 6,
11–13), and hemocytes were often found within the space
and adjacent to oocytes (Fig. 3).
Most oocytes in follicular pouches larger than 80 µm
in diameter were surrounded by two layers of vitelline
membrane (Figs. 4, 6, 7, 10), but some of these folliculated
oocytes had only one layer of vitelline membrane. The
microvilli were well developed in the oocytes with twolayers of vitelline membrane (Fig. 4). The outer and innervitelline membranes were about 2 µm and 5 µm thick,respectively. In previtellogenic oocytes more than 80 µm
in diameter, the outer layer of vitelline membrane wassomewhat basophilic and more electron dense than thestrongly PAS positive inner layer (Figs. 4, 6, 7).
Oocytes larger than 230 µm in diameter werecategorized into the vitellogenic stage. In the earlyvitellogenic oocytes, small acidophilic yolk granules of 2–4µm in diameter as well as a large number of very prominent
lipid droplets of 2–20 µm in diameter (Fig. 11) first appearedin the periplasm. During vitellogenic growth, the yolk
granules and the lipid droplets increased in size
respectively via successive fusions (Fig. 12). In latevitellogenic oocytes larger than 300 µm in diameter, theyolk granules become large polygonal yolk blocks of 15–85µm at the widest point. The yolk blocks nearly filled theooplasm, and the lipid droplets, 2–50 µm in diameter, were
distributed between them (Fig. 13).
Discussion
Location of germ cells
Our histological observations of two Scolopocryptops
centipedes revealed that the germ cells grew only in theventral region of ovaries along the longitudinal axis. Jangi(1957) also described a similar location of the germ cells inotherscolopendromorph
centipede,
Scolopendra
morsitans. Thus the location of germ cells in the ventral
region of ovary seems to be common among thescolopendromorph centipedes. In contrast, Knoll (1974)
demonstrated that the germ cells developed in the dorsal
region of the ovaries in scutigeromorph species. The
location of germ cells in ovary possible varies among
higher level taxa, and more comprehensive observations
are required covering a wider taxonomic range.
Position of oocyte growth
Several authors have described ovarian structure andoogenesis in several centipede taxa (in scutigeromorph:
Knoll, 1974; in lithobiomorph: Herbaut, 1974; in
scolopendromorph: Jangi, 1957). These authors suggested
that the follicular pouches were the hollows of the ovary,i. e. the epithelial layer of the pouch was continuous withthe epithelial layer of the ovarian wall. The oocytes grewwithin the inner spaces of these pouches, but it was

OVARIAN STRUCTURES IN CENTIPEDES

unclear whether the oocytes were situated in thehemocoelic space or were sandwiched between the follicle
layer and its basement membrane.

Fig. 725

Here, we described the position of oocyte growth in
Scolopocryptops centipedes in detail based on histologicaland ultrastructural analyses (Fig. 14). We clearly

Ovarian epithelium surrounding young germ cells in Scolopocryptops rubiginosus. TEM. Un-stained. Oogonium (og), young oocyte (yo) andearly previtellogenic oocyte (epo) presented among ovarian epithelial cells (oec). Note that many protuberances project deep into the basement
membrane on the basal surfaces of the epithelial cells in this area. Asterisks indicate the hemocoelic space. bm: basement membrane, fec:
follicle epithelium, ivm: inner layer of vitelline membrane, lpo: late previtellogenic oocyte, ovm: outer layer of vitelline membrane. Scale = 10µm.
Fig. 8 Ovarian epithelium surrounding oogonium in Scolopocryptops sexspinosus. Paraffin. Azocarmine G-aniline blue-orange G. Oogonium (og)embedded in ovarian epithelium (oe). Asterisk indicates the hemocoelic space. ol: ovarian lumen. Scale = 10 µm.
Fig. 9 Oogonium and adjacent epithelial tissue in Scolopocryptops rubiginosus. TEM. Double-stained. Oogonium (og) surrounded by a few ovarianepithelial cells (oec), and epithelial cells with many protuberances on the basal surface projecting deep into the basement membrane (bm). Notethe prominant hemidesmosomes (arrowheads) at the tips of the epithelial protuberances. m: muscle, yo: young oocyte. Scale = 5 µm.
Fig. 10 Ovary in Scolopocryptops rubiginosus. TEM. Double-stained. Early previtellogenic oocyte (epo) with microvilli (mv) and vitelline membrane(vm) settled among the ovarian epithelial cells (oec). Note the absence of basement membrane between the oocyte and the ovarian epithelial
cells, and a thick basement membrane (bm) between the follicular epithelial cells (fec) and outer layer of vitelline membrane (ovm) of folliculated
late previtellogenic oocyte. ol: ovarian lumen. Scale = 1 µm.

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Y. MIYACHI AND K. YAHATA

demonstrated that the follicular pouches were hollows of
the ovary, and that the oocytes grew within the innerspaces of these pouches during the late previtellogenic andvitellogenic stages. The space where oocytes grow wasconclusively proved to be hemocoelic rather than thespace between the follicular epithelial layer and its
basement membrane.

Fig. 11 Cross-section of adult ovary in Scolopocryptops rubiginosus. Paraffin.
Alcian blue-PAS-hematoxylin. Dorsal side is oriented towards figure
top. Early vitellogenic oocyte (vo) and late previtellogenic oocyte (lpo)
within individual follicular pouches. Note the strongly stained small
yolk granules (not indicated) and lipid droplets (ld) in the periplasm of
the vitellogenic oocyte. Asterisk indicates the hemocoelic space. fe:follicle epithelium, fp: follicle pore, oe: ovarian epithelium, ol: ovarian
lumen. Scale = 50 µm.
Fig. 12 Cross-section of adult ovary in Scolopocryptops rubiginosus. Paraffin.
Azocarmine G-aniline blue-orange G. Dorsal side is oriented towards
figure top. Vitellogenic oocyte (vo) in the follicular pouch with a numberof late yolk granules (yg) and lipid droplets (ld). Note the presence ofhemocytes (hc) between the oocyte and its follicle epithelium,
indicating the hemocoelic nature of the inner space of the follicular
pouch. Asterisk indicates the hemocoelic space. fb: fat body, fe: follicle
epithelium, fp: follicle pore, oe: ovarian epithelium, ol: ovarian lumen.
Scale = 100 µm.
Fig. 13 Cross-section of adult ovary in Scolopocryptops sexspinosus. Resin.
Azocarmine G-aniline blue-orange G. Dorsal side is oriented towards
figure top. Ripe vitellogenic oocyte (vo) within the follicular pouch withnumber of large block-shaped yolk granules (yg) and lipid droplets (ld).
Asterisk indicates the hemocoelic space. fb: fat body, fe: follicle
epithelium, fp: follicle pore, lpo: late previtellogenic oocyte, oe: ovarianepithelium, ol: ovarian lumen. Scale = 100 µm.

Comparison of ovarian structure among myriapods
In the julid diplopod, Ophyiulus pilosus, Kubrakiewicz(1991a) described the presence of oocytes in hemocoelic
spaces of follicular pouches. This morphological feature isshared with the Scolopocryptops centipedes examined in
the present study.
The observation that growing oocytes wereconnected to the ovarian epithelium via a part of theirfollicle layers was commonly reported for species of all
myriapod groups (e. g., in Chilopoda: Jangi, 1957; Herbaut,
1974; Knoll, 1974; in Symphyla: Tiegs, 1940; in Puropoda:
Tiegs, 1947; in Diplopoda: Sareen and Adiyodi, 1983;
Yahata and Makioka, 1994), and reports of this type wereabsent for other arthropods. Also in the present study, thecontinuity between follicle and ovarian epithelium wasobserved in the ovaries of the Scolopocryptops centipeds
examined. The fact that many previously examined
myriapods have such a follicular continuity could
strengthen the idea that the ovaries of the all four myriapod
groups have a common plan.
Both morphological (e. g., Anderson, 1973; Dohle,
1980; Edgecombe, 2004; Giribet et al., 2005) and molecularanalyses (e. g., Podsiadlowski et al., 2007; Gai et al., 2008)
tend to indicate that the myriapod relationship is as follows(Chilopoda (Symphyla (Pauropoda + Diplopoda))). Oocyte
growth in hemocoelic follicular pouches has been
confirmed in the distantly related myriapod groups,
Diplopoda (Kubrakiewicz, 1991a) and Chilopoda (present
study), but never observed in other arthropods. Occurenceof similar follicular pouches in symphylan (Tiegs, 1940)and pauropodan (Tiegs, 1947) ovaries strongly suggeststhat the hemocoelic manner of oocyte growth is a common
feature among myriapods. If so, the hemocoelic manner
would represent the ground plan of myriapod oogenesis,
and furthermore become a strong candidate of thesynapomorphy of the Myriapoda. To confirm theseassertions, symphylan and pauropodan species should be
subjected to ultrastructural analysis in the future.
Phylogenetic significance of position of oocyte growth
Makioka (1988) suggested that there are several
differences in the structure of the ovaries between two
major arthropod subphyla, the Chelicerata and the
Mandibulata. In the mandibulate-type ovary, the oocytes
grow within the ovarian lumen (Fig. 1B), whereas in thechelicerate type, the growing oocytes are sandwiched
between the ovarian epithelial layer and its basement

OVARIAN STRUCTURES IN CENTIPEDES

27

Fig. 14 Schematic drawing of ovarian structure and oogenesis in Scolopocryptops centipedes. Oogonia (og) and young oocyte (yo) are surrounded bysome cells of ovarian epithelium (oe). Growing oocyte (go) is surrounded by follicle epithelium (fe) and its basement membrane (bm), and thus
growing oocyte is located in hemocoelic space (asterisk). Ripe oocyte (ro) is released from its follicular pouch into ovarian lumen (ol).

membrane (Fig. 1A). The hemocoelic position of oocyte
growth described in a julid diplopod (Kubrakiewicz, 1991a)
and the Scolopocryptops centipedes (present study) iscompletely different from the above mentioned two types.
Hence, we now reached a tentative conclusion that anothertype of ovary can be recognized in arthropods. In this third
ovarian type, the myriapod type, oocytes grow within thehemocoelic space of the follicular pouches, though more
observations are reqired on the ovarian structures ofSymphyla and Pauropoda. If the myriapod type isestablished, we would propose that the type hitherto
known as the mandibulate type should be renamed to thepancrustacean type or the tetraconate type.
Morphological evidence for myriapod monophyly isoften described as elusive (Edgecombe, 2010). If oocyte
growth in the hemocoelic space of follicular pouches is ashared in the all myriapod groups, it could be a robustmorphological support for myriapod monophyly. To
ascertain this, future research should be undertaken inorder to examine symphylan and pauropodan ovaries.
Acknowledgements: We are deeply grateful toprofessors R. Machida and K. Ishida (University ofTsukuba) for their encouragement. We also express our
sincere thanks to Dr. T. Tsutsumi (Fukushima University),Dr. H. Mitsumoto (Keio University), Dr. M. Fukui (Ehime
University), Ms. M. Taira and Mr. T. Shiratori (Universityof Tsukuba) for their technical advice. Thanks are also due
to Dr. M. Sakuma and Mrs. Y. Kurita for providing centipedsamples.
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