© 2015. Published by The Company of Biologists Ltd | Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

RESEARCH REPORT

Cellular analysis of cleavage-stage chick embryos reveals hidden
conservation in vertebrate early development

ABSTRACT
Birds and mammals, phylogenetically close amniotes with similar postgastrula development, exhibit little conservation in their post-fertilization
cleavage patterns. Data from the mouse suggest that cellularmorphogenesis and molecular signaling at the cleavage stage play
important roles in lineage specification at later (blastula and gastrula)
stages. Very little is known, however, about cleavage-stage chickembryos, owing to their poor accessibility. This period of chickdevelopment takes place before egg-laying and encompasses
several fundamental processes of avian embryology, including
zygotic gene activation (ZGA) and blastoderm cell-layer increase. We
have carried out morphological and cellular analyses of cleavage-stage
chick embryos covering the first half of pre-ovipositional development,from Eyal-Giladi and Kochav stage (EGK-) I to EGK-V. Scanningelectron microscopy revealed remarkable subcellular details of
blastomere cellularization and subgerminal cavity formation.
Phosphorylated RNA polymerase II immunostaining showed that
ZGA in the chick starts at early EGK-III during the 7th to 8th nuclear
division cycle, comparable with the time reported for other yolk-richvertebrates (e.g. zebrafish and Xenopus). The increase in the number ofcell layers after EGK-III is not a direct consequence of oriented cell
division. Finally, we present evidence that, as in the zebrafish embryo, ayolk syncytial layer is formed in the avian embryo after EGK-V. Our datasuggest that several fundamental features of cleavage-stage
development in birds resemble those in yolk-rich anamniote species,
revealing conservation in vertebrate early development. Whether thisconservation lends morphogenetic support to the anamniote-toamniote transition in evolution or reflects developmental plasticity inconvergent evolution awaits further investigation.
KEY WORDS: Amniote, Cellularization, Chick, Cleavage, Yolk
syncytium, Zygotic gene activation

INTRODUCTION

A mature avian oocyte is fertilized in the infundibulum shortly after
ovulation. From fertilization to egg-laying, the period of preovipositional development covers ∼25 h in the chick, during which
a number of important early embryological events take place. These
1

Laboratory for Early Embryogenesis, RIKEN Center for Developmental Biology,
2
Kobe, Hyogo 650-0047, Japan. Electron Microscopy Laboratory, RIKEN Center for3
Developmental Biology, Kobe, Hyogo 650-0047, Japan. Department ofAgricultural Biotechnology, Seoul National University, Seoul 151-921, Korea.
4
Laboratory for Sensory Development, RIKEN Center for Developmental Biology,
5Kobe, Hyogo 650-0047, Japan. Department of Applied Biological Chemistry,
Faculty of Agriculture, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan.
*Author for correspondence (sheng@cdb.riken.jp)
This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution and reproduction in any medium provided that the original work is properly attributed.

Received 8 October 2014; Accepted 3 February 2015

events include the meroblastic cleavage, cellularization, zygotic
gene activation, blastoderm layer increase and reduction, radialsymmetry breaking, and segregation of epiblast and hypoblast
lineages (Sheng, 2014). Owing to poor accessibility, nocomprehensive molecular or cellular study on pre-ovipositional
embryos has been reported in the literature. General morphologicalcharacterization in the 1970s by Eyal-Giladi and Kochav provided
us with a basic staging system, the EGK stages (Eyal-Giladi, 1991;
Eyal-Giladi and Kochav, 1976; Kochav et al., 1980), and theultrastructure of embryos from the later half of this developmental
period was investigated in the 1990s with electron microscopy by
Etches and colleagues (Watt et al., 1993).
According to the EGK staging system, pre-streak chickdevelopment is divided into 14 sub-stages, from EGK-I to EGKXIV. The pre-ovipositional period includes the stages EGK-I to
EGK-X. These 10 stages are further grouped into two developmental
phases. In the first, from EGK-I to EGK-VI (the cleavage stages),a chick embryo grows mainly by means of cell proliferation(1→10,000) and blastoderm cell-layer increase (1→6). In thesecond, from EGK-VII to EGK-X (the area pellucida formation
stages), the center of the blastoderm above the subgerminal cavity
thins out from six to two cell layers, leading to the formation of two
circular territories: the central area pellucida, which is detached fromthe yolk and contains the epiblast and hypoblast cell layers; and the
peripheral area opaca, which varies in its thickness and maintains
cellular interactions with the yolk. Radial symmetry breaking,one of the most important developmental events in vertebrate
embryogenesis, also occurs during this phase.
With a paucity of molecular and cellular understanding, and withthe exception of primordial germ cells (PGCs) (Tsunekawa et al.,2000), chick embryos during the first phase of pre-ovipositional
development are generally regarded as being undifferentiated,
containing equipotent cells whose morphological variations reflect
their cellularization and proliferation heterogeneity rather than theirfate diversification. These embryos are also considered to beinactive in their preparation for lineage segregation (except for the
PGCs) or radial symmetry breaking because embryonic axis can be
efficiently re-specified with physical perturbation during the second
phase of pre-ovipositional development (Kochav and Eyal-Giladi,1971). Data from other vertebrate models, however, suggest thatmolecular and cellular events taking place during cleavage phase of
the early development are crucial for dynamic signaling, lineage
specification and morphogenetic movements associated with thelate blastula/early gastrula development.
Among those crucial events are the initiation of zygotic geneactivation (ZGA), the separation of outside and inside blastomeres,and the formation of a yolk syncytial layer (YSL). In both mouseand zebrafish embryos, ZGA comes under direct control of key
pluripotency regulators (Foygel et al., 2008; Lee et al., 2013b;
Leichsenring et al., 2013) and is tightly associated with epigenetic
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DEVELOPMENT

Hiroki Nagai1, Maiko Sezaki1, Kisa Kakiguchi2, Yukiko Nakaya1, Hyung Chul Lee3, Raj Ladher4,
Tomohiro Sasanami5, Jae Yong Han3, Shigenobu Yonemura2 and Guojun Sheng1, *reprogramming of parental genomes that, later in development,
controls the specification of both somatic and germ cell lineages(Hackett and Surani, 2013). During mouse early development,
8→16 and 16→32 divisions produce two types of blastomeres:
those located outside, which are biased towards the trophectoderm
fate; and those located inside, which will become the inner cellmass (Rossant and Tam, 2009). This process is regulated bypolarized cell division and post-divisional cell rearrangement(Johnson and Ziomek, 1981; Parfitt and Zernicka-Goetz, 2010;
Sasaki, 2010; Watanabe et al., 2014). The YSL is a special adaptive
feature of telolecithal development. Although eutherian eggs aredevoid of yolk, both birds and reptiles, two other extant amniotegroups, have telolecithal eggs. Studies in zebrafish, a genetic
model for telolecithal embryonic development, suggest that the
YSL is formed at the cleavage stage and that it undergoes highly
coordinated cytokinetic movements with the overlying blastoderm
cells (Carvalho and Heisenberg, 2010). It also regulatesmesendoderm induction by secreting Ndr1 and Ndr2 signals (twonodal related molecules) under the transcriptional control of Mxtx2(Hong et al., 2011), and produces the ventralizing signal BMP2b
under the transcriptional control of MGA, MAX and Smad4
proteins (Sun et al., 2014). To investigate how these events occur inthe chick and whether novel insights into vertebrate early
development can be gained by taking a comparative approach,we collected cleavage-stage chick embryos and performed in-depth
morphological and cell biological analyses.
RESULTS AND DISCUSSION
General description of EGK-I to -V embryos

Pre-ovipositional eggs were retrieved from laying hens using a
non-invasive method described previously (Eyal-Giladi and
Kochav, 1976; Lee et al., 2013a). Because the ultrastructure oflate cleavage-stage embryos and the area pellucida of formationstage embryos had been described previously (Watt et al., 1993), we
focused our attention on the early and mid-cleavage stages, namelyfrom EGK-I to EGK-V. This period covers about 8 hours ofdevelopment. Together with time needed from the fertilization to thefirst cell division, it is considered here to represent the first half of
pre-ovipositional development (∼13 h out of a total of 25 h). Afteregg retrieval, shell and albumen were removed, and embryos werebriefly fixed in 4% PFA with intact yolk for morphologicalpreservation. They were then cut out together with a small amountof underlying yolk to retain embryo-yolk contacts, further fixed in
4% PFA and stored at 4°C for later analysis. Out of a total of 138
embryos collected, 73 were confirmed under a stereo-microscope torepresent stages between EGK-I and EGK-V (supplementarymaterial Fig. S1). Their general morphology is briefly described
here. At EGK-I, all cells are connected to the yolk basally; whenviewed from the apical side, they are not yet enclosed by lateral cell
membrane, except for a few centrally located ones at late EGK-I. At
EGK-II, two types of cells, the central ones with closed lateral
membrane and peripheral ones which are open to the yolk laterally,
can be easily distinguished. From EGK-III to EGK-V, the central,closed cells become progressively smaller as they divide, and thearea they occupy expands as more closed cells are produced from theperipheral open cells after division. In a majority of embryos at all
EGK stages, the central cell cluster is positioned eccentrically in theblastoderm. Compared with those described by Eyal-Giladi and
Kochav (1976), our embryos had their morphology better preserved
and, likely as a result of this, we did not notice any obvious shrinkage
in blastoderm size at EGK-III or large vacuoles at EGK-I and EGKII, as described previously (Eyal-Giladi and Kochav,1976).
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Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

Scanning electron microscopy of EGK-I to -V embryos

To gain a better understanding of cellular morphogenesis during
early cleavages, we performed scanning electron microscopy (SEM)
analysis with EGK-I to EGK-V embryos (Fig. 1A; supplementarymaterial Figs S2-S6; summarized in Fig. 1B). Under SEM, cleavage
furrows and cell boundaries were easily visible. In an EGK-I embryowith eight open cells (Fig. 1A; supplementary material Fig. S2), theorder of three separate cytokinetic events could be estimated based on
the shape and extent of furrow progression. Fracture surface views
indicated that transition from vertical furrow burrowing to horizontal
furrow burrowing took place ∼50-100 µm deep into the yolk
(supplementary material Fig. S2C-E). The transition area had
elaborate membrane protrusions (supplementary material Fig. S2E),
resembling the furrow based body (Bellairs et al., 1978; Gipson,1974). The apical surface was rich in microvilli, with small vesicularbodies at the cortex (supplementary material Fig. S2F). Yolk granules
exhibited fine-graded distribution in their sizes. No obvious transitionzone could be distinguished between yolk granules to be includedin future blastoderm cells (<100 µm below the membrane) and thoseto be allocated to the yolk (supplementary material Fig. S2D,G).
In an EGK-II embryo with 16 laterally closed cells (Fig. 1A;
supplementary material Fig. S3), the center of closed cells was
located clearly off the geometric center of the blastoderm, possibly as
a consequence of eccentric localization of the zygotic nucleus. It is
unclear whether this eccentricity has any predictive value with regard
to where the future posterior side of the epiblast (dorsal side of theembryo) will be. The shape and size of central cells was variable(supplementary material Fig. S3B). Contrary to what had beensuggested based on light microscopy analysis (Eyal-Giladi and
Kochav, 1976), a few central cells at EGK-II had already finished thecellularization process with complete basal and lateral membranes(supplementary material Fig. S3C,D). Numerous small, intracellular
vesicular bodies were present in all cells (supplementary material
Fig. S3D,E), resembling those reported in other yolk-rich vertebrate
embryos (Danilchik et al., 2003; Li et al., 2006). These vesicles arelikely to be involved in the cellularization and cell division processes,
which require a constant supply of new plasma membrane. It is also
possible, however, that they are related to the supply of surface
proteins or to the secretion of extracellular matrix molecules.
At EGK-III (Fig. 1A; supplementary material Fig. S4), the territoryof laterally closed cells from apical surface view expanded significantly
as more peripheral open cells gave rise to laterally closed daughter
cells after division (supplementary material Fig. S4A-C). The apicalmembrane remained rich in microvilli (supplementary material
Fig. S4D), but with reduced abundance at cleavage furrows. A
small subgerminal cavity space appeared under some central cells(supplementary material Fig. S4E) and two-cell thick blastoderm wasobserved occasionally (supplementary material Fig. S4F). This trend
continued at EGK-IV (Fig. 1A; supplementary material Fig. S5). At
this stage, the subgerminal cavity space became obvious under mostcentral cells and started to merge to form a bona fide cavity above theyolk cell surface (supplementary material Fig. S5C-E). A significant
proportion of the blastoderm became two cells thick (supplementarymaterial Fig. S5C,D), with the very central part being three cells thick(supplementary material Fig. S5E). In slightly more peripheral
regions, the subgerminal cavity under newly closed cells had justappeared as an isolated pocket between two closed cells and the yolkcell (supplementary material Fig. S5H). At the basal surface of thesecells, two closely apposed plasma membranes (supplementarymaterial Fig. S5H′, arrows) marked the completion of theircellularization. Interestingly, unlike the fine-graded size distribution
before cellularization (supplementary material Fig. S2G), yolk

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Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

granules at the basal cortex of newly formed cells were much smaller
than those located in the adjacent yolk cells (supplementary material
Fig. S5H′, arrowheads). At EGK-V (Fig. 1A; supplementary material
Fig. S6), all non-edge cells of the blastoderm had completed thecellularization process. Many edge cells had also become fully
cellularized (supplementary material Fig. S6C,E), although some ofthem remained open peripherally. A smooth transition in blastoderm
thickness, from one cell layer at the edge to four cell layers at thecenter, could be observed (supplementary material Fig. S6C,D). Cells
bridging the one-cell and two-cell regions often exhibited wedgeshaped morphology (supplementary material Fig. S6C,F). The
subgerminal cavity became a continuous, expanded space below themultilayered part of the blastoderm, but the one-cell layered part
remained in close association with the underlying yolk cell membrane(supplementary material Fig. S6C-E). Cell-cell contacts among
blastomeres were prominent in both superficial and deep layers(supplementary material Fig. S6G-J).
Zygotic gene activation (ZGA) is initiated at late EGK-II/early
EGK-III

The cellular morphogenetic process from EGK-I to EGK-V issummarized in Fig. 1B. During this period of chick development,
cell (nucleus) number increases from 1 to ∼2000 (Park et al., 2006).
ZGA in mammalian embryos (e.g. in the mouse and human) starts
very early, before the 3rd cell cycle (Vassena et al., 2011; Wang andDey, 2006; Xue et al., 2013; Yan et al., 2013). In the zebrafish, with atelolecithal egg, ZGA was reported to start at the 128-cell stage (8thcell cycle) (Aanes et al., 2011; Harvey et al., 2013; Mathavan et al.,2005). In other yolk-rich embryos, such as Xenopus and Drosophila,
ZGA can be detected at about 128- to 256-cell (nucleus) stage (8th to
9th cell cycle) (Baroux et al., 2008; Tadros and Lipshitz, 2009). It
has been hypothesized that instead of the absolute cell cycle number,the timing of ZGA in large, yolk-rich embryos is determined by the
nucleocytoplasmic ratio and the maternal clock (Tadros andLipshitz, 2009). We stained early EGK stage chick embryos withan anti-phosphorylated-RNA Polymerase II CTD ( p-PolII) antibody
to mark the zygotic gene transcription, DAPI to visualize the nuclei
and E-cadherin to visualize cell boundaries. No specific p-PolII
immunoreactive signals could be detected before late EGK-II stage(Fig. 2A, leftmost column panels). At late EGK-II/early EGK-III(Fig. 2A, 2nd and 3rd column panels), weak nuclear p-PolII signals
were observed in a few centrally located cells. By mid-/late EGK-III
stages (Fig. 2A, 4th and 5th column panels), p-PolII signals became
progressively stronger and could be readily detected in many central
cells. About half of p-PolII-positive cells had strong cytoplasmic
signals at late EGK-III, a phenomenon possibly related to thecytoplasmic-to-nuclear translocation of RNA PolII at the onset of
ZGA, as observed in the mouse embryo (Bellier et al., 1997). At
EGK-IV (Fig. 2A, 6th and rightmost column panels; Fig. 2C,D),
p-PolII signals became more robust and positive cells became moreabundant. During the entire transition period from mid-EGK-II tolate EGK-IV, p-PolII signals appeared in central, smaller cells
sooner and more strongly than in peripheral, larger cells. However, it
is unclear whether this is causally related to changes in the
nucleocytoplasmic ratio because many central cells were weak or
negative for p-PolII signals even when their similarly sized
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Fig. 1. Scanning electron microscopy analysis of EGK-I to -V chick embryos. (A) Apical views of EGK-I to -V embryos under SEM. Scale bars: 500 µm(left); 200 µm (right). Details of the fracture surface are shown in supplementary material Figs S2-S6 and summarized in B. (B) Schematic view ofcellularization and cell proliferation processes during chick development from EGK-I to -V. Completely cellularized blastomeres are shown in blue. Numbers
above the blastoderm indicate the distribution of cell layer numbers.

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Fig. 2. ZGA in EGK-II to -IV chick embryos analyzed by anti-Ser5 phosphorylation of Pol II CTD ( p-PolII) antibody staining and by GFP transgenesis.
(A) Wild-type embryos ranging from mid-EGK-II to late EGK-IV are stained for p-PolII signals. Embryos are co-stained for E-cadherin (E-cad; marking cellboundaries) and DAPI (marking nuclei). Numbers (x/y) under embryo stage labels indicate approximate counts of laterally closed cells (x) and total cells (y).
(Top) Whole-embryo views after DAPI staining. At higher magnification: DAPI (second row), E-cad (third row) and p-PolII (bottom row) staining. Signals for p-PolII
are negative at mid-EGK-II, very weak in late EGK-II, rapidly increase at EGK-III and become very strong at late EGK-IV. From late EGK-III to late EGK-IV,
strong signals are detected both in the nucleus (in some central blastomeres) and in the cytoplasm (in adjacent blastomeres), although the proportion ofcytoplasmic-positive cells decreases gradually (C,D). p-PolII-negative, small blastomeres are also observed in central areas next to similarly sized positive
blastomeres (C). Signals in peripherally positioned blastomeres are much weaker or negative. (B) Embryos from GFP-transgenic male and wild-type female
crosses show strong GFP signals at mid-EGK-IV but no GFP signals at mid-EGK-II. Scale bars: 500 µm.

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Separation of surface and inner cells is not caused byoriented cell divisions

From our scanning EM analysis, late EGK-II/early EGK-III is also
the stage when the cellularization process becomes complete in a
few centrally located cells and the increase in blastoderm cell-layer
number is initiated. This latter process is significant because it
results in the separation of two types of blastomeres: those locatedon the surface and exposed to the external environment; and thoselocated in the inside and shielded from external influences. EyalGiladi and colleagues hypothesized that oriented blastomere
division led to this increase (Kochav et al., 1980). In eutherian
embryos, this separation is directly correlated with the future fateof these cells: trophectoderm for the surface-located cells and inner

DEVELOPMENT

neighbors were strongly positive (Fig. 2C). Supporting our ZGA
observation based on the p-PolII staining data, pre-ovipositional
embryos with GFP transgene insertion under the control of a CMV
promoter (Park and Han, 2012) showed no GFP signals at mid-EGKII (Fig. 2B) and strong GFP signals at mid-EGK-IV (Fig. 2B). Taken
together, our data suggest that ZGA in the chick is initiated at
late EGK-II/early EGK-III (about 64-128 total cell stage; orapproximately during the 7th or 8th cell cycle) and becomesreadily detectable at mid- to late EGK-III (about 128-256 total cellstage; 8th to 9th cell cycle). Chick embryos therefore initiate ZGA
with a developmental timing comparable with that in the zebrafish
and Xenopus embryos, pointing to potential conservation inmolecular mechanisms controlling ZGA in yolk-rich embryos.

cell mass for the inside ones. As mentioned in the introduction, there
are two competing, but not mutually exclusive, theories to explain
this process in the mouse embryo (Johnson and Ziomek, 1981;
Parfitt and Zernicka-Goetz, 2010; Sasaki, 2010; Watanabe et al.,2014). To investigate whether mitotic plane orientation is correlatedwith the increase in cell-layer number in the chick embryo (Fig. 3A),
we performed DAPI staining with EGK-III and EGK-IV embryos inwhich the blastoderm was increasing its thickness from 1- to 2-cells
or from 2- to 3-cells (Fig. 3B). Embryos with many mitotic cells in
anaphase/telophase (e.g. as shown in supplementary material
Fig. S7A,B) were selected. These embryos were sectioned andmitotic plane orientation was measured as indicated in Fig. 3B(embryo, n=5; mitotic nuclei pair, n=240). Data for dividing surface

Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

cells and dividing non-surface (deep) cells were plotted separately(Fig. 3C). The majority (about 3/4) of all surface cells divided with
their mitotic plane oriented at less than 30° angle relative to theblastoderm surface, and about 40% of all surface cell divisions had
an angle of less than 10° angle (Fig. 3C, left). Although dynamic
cytokinetic behavior could not be visualized, those divisions
most likely resulted in generating two surface daughter cells.
Mitotic orientation of dividing deep cells was distributed more
randomly (Fig. 3C, right), with 44% cells dividing at 0-30° angles
and with the rest evenly represented at 30-90° angles. Collectively,
these data indicate that cell layer number increase at cleavage
stages in the chick embryo is not directly caused by oriented cell
divisions.

Fig. 3. Mitotic division orientation and yolk syncytial nuclei. (A-C) Increase in cell layer number is not caused by oriented mitotic division. (A) Schematic
view of three representative mitotic plane angles (0°, 45° and 90°). The last (90°) scenario was hypothesized as the cause for blastoderm cell-layer number
increase by Eyal-Giladi and colleagues. (B) Representative section views of EGK-III to EGK-V embryos stained with DAPI to reveal mitotic cells. Top: one-celllayer region; bottom: 2- to 3-cell-layer region. Only anaphase and telophase nuclei were counted. Mitotic plane orientation was calculated as the angle between
the blastoderm surface and the line passing through two separating nuclei. Red lines: surface-located cell divisions (s). Green line: deep cell division (d). (C) Rose
diagrams showing the distribution of mitotic plane orientation. Left: dividing surface cells (n=143). Right: dividing deep cells (n=97). A vast majority of surface
cells divide with their mitotic planes orientated at a less than 30° angle to the surface, likely producing two surface daughter cells. Mitotic planes of non-surface
cells exhibit a more randomized distribution. (D-H) Yolk syncytial nuclei are detected in three different avian species. (D) DAPI staining of EGK-V (left) and
EGK-VIII (right) chick embryos. DAPI-positive structures (arrows) are detected underneath the yolk cell surface. (E) In post-ovipositional chick (EGK-XI, left
panels) and pre-ovipositional quail (EGK-VIII, right panels) embryos, double-staining with DAPI (nucleus) and Rhodamine-WGA (membrane) reveals that
DAPI-positive signals (arrows) are located underneath the yolk cell membrane. (F) Four examples of zebra finch embryos (two EGK-VI and two EGK-VII)
showing DAPI-positive syncytial nuclei (arrows) located underneath the yolk cell membrane. (G) A composite view (assembled from four images) of a zebra
finch embryo cross-section, showing an intact yolk cell membrane and many syncytial nuclei (arrows) underneath it. (H) A schematic view of the YSL in an
avian embryo.

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A yolk syncytial layer (YSL) is formed during early avian
development

As mentioned in the introduction, the importance of the YSL in early
development has been demonstrated in the zebrafish model. However,whether a similar YSL exists in the avian embryo has not been
investigated. DAPI staining and section analysis suggested that nosyncytial nuclei could be found at stages EGK-I to -IV (not shown).
At EGK-V, occasional syncytial nuclei were detected (Fig. 3D, left),which became brighter, larger and more abundant at later EGK stages(Fig. 3D, right, showing an EGK-VIII embryo) and persisted throughpost-ovipositional stages (Fig. 3E, left, showing an EGK-XI embryo).
These nuclei are to be distinguished from DAPI-positive cells locatedabove the surface of the yolk cell (e.g. as shown in Fig. 3G;
supplementary material Fig. S7F), which are shed from theblastoderm cell mass (as described by Eyal-Giladi and Kochav) andare frequently observed during the blastoderm thinning process from
EGK-VII onwards. To confirm the existence of a YSL in the avian
embryo, we performed similar analysis with quail (Fig. 3E, right) andzebra finch (Fig. 3F,G) embryos. Pre-ovipositional quail eggs wereretrieved and DAPI staining of these embryos revealed the presence ofsyncytial nuclei, especially underneath the area opaca and close to theblastoderm margin (Fig. 3E, right). Zebra finch eggs are laid at an
earlier developmental stage than chicken or quail eggs, and zebra
finch embryos at stages EGK-VI to -IX can be collected easily from
freshly laid eggs (S. S. Mak, C. Alev, H.N., A. Wrabel, Y. Matsuoka,
A. Honda, G. Sheng and R. K. Ladher, unpublished). DAPI staining
and section analysis of EGK-VI to -IX zebra finch embryos supported
our chick and quail data (Fig. 3F,G). Syncytial nuclei were detected
more abundantly in the zebra finch (Fig. 3G, arrows) and co-staining
with a membrane marker (rhodamine-labeled WGA) revealed
that these DAPI-positive signals were located underneath the yolkcell membrane (Fig. 3F). Interestingly, many peripherally locatedsyncytial nuclei in the finch embryo could be found even in wholemount views (supplementary material Fig. S7C-E). There nuclei werelocated away from the blastoderm edge, as revealed by phalloidin
co-staining (supplementary material Fig. S7D,D′,E,E′) and in section(supplementary material Fig. S7F). Taken together, these datastrongly support the hypothesis (Fig. 3H) that a YSL is present in
the avian embryo. Whether this YSL plays a role in patterning theoverlying blastoderm remains to be clarified.
Summary

The first half of pre-ovipositional chick development, from EGK-I
to EGK-V, was investigated in this study (Table 1). During thisdevelopmental period, a chick embryo undergoes ∼11 rounds ofmitotic divisions. The cell number increases from one to ∼2000
and the cell-layer number increases from one to four. Meroblastic
cleavages continue from EGK-I, when all cells are open, to late
EGK-V, when all cells are fully cellularized. Blastomere-yolk
separation starts at late EGK-II and ZGA starts at late EGK-II/
early EGK-III (∼7th to 8th nuclear division cycle). Separation ofoutside and inside blastomeres is not due to oriented cell division,as previously suggested. Syncytial nuclei located underneath theyolk cell membrane can be detected from EGK-V and become
more abundant during the second half of pre-ovipositional
development. The presence of yolk syncytial nuclei is also
observed in quail and zebra finch embryos. Our data on
blastoderm cell-layer increase support the hypothesis that thisprocess is controlled by cellular mechanisms other than oriented
mitotic division. Collectively, our data suggest that many featuresof cleavage-stage development in the chick resemble those in thezebrafish. Whether this conservation is a result of convergent
evolution or is indicative of hardwired molecular and cellularmechanisms regulating vertebrate early development awaits future
investigation.
MATERIALS AND METHODS
Embryo collection

EGK stage chick embryos were collected from laying white leghorn hens
using a non-invasive, manual retrieval (abdominal massage) method as
previously described (Lee et al., 2013a). The retrieval was carried out in theafternoon or early evening and was timed to target eggs in the first half of
pre-ovipositional development. Each collected egg was placed in a Petridish. The eggshell was opened and removed, and the egg yolk was adjusted
so that the embryo was positioned at the top. Excessive albumen covering
the embryo was removed and a few drops of ice-cold 4% paraformaldehyde(PFA) fixative (Alev et al., 2013) were put on the vitelline membrane overthe embryo to preserve the morphology. After a few minutes of fixation atroom temperature, the embryo, together with a small amount of peripheraland underlying yolk, was excised from the bulk of the yolk and placed inPannett-Compton saline solution (Alev et al., 2013). The vitelline
membrane was then carefully removed and the embryo together with
attached yolk was transferred to 4% PFA for further fixation and storage at
4°C. Out of 138 embryos obtained this way, 85 embryos were analyzed forstage distribution under a stereo microscope, and 73 out of 85 were within
the cleavage stages according to the EGK staging system (13 EGK-I, 16
EGK-II, 15 EGK-III, 17 EGK-IV and 12 EGK-V). Pre-ovipositional GFP
transgenic chick embryos and pre-ovipositional quail embryos werecollected using the same abdominal massage method. Post-ovipositionalzebra finch eggs were collected daily from locally maintained colonies andzebra finch embryos were processed for staining using the same protocol as
for the chick embryos.

EGK

I

II

III

IV

V

VI

→

X

Total number of nuclei or cells(median)Nuclear division cycle
Hours post-fertilization
Laterally closed cells (median)
Cell layer number
Basally closed cells
Zygotic gene activation
Yolk syncytium
Cellularization of edge cells

1-16 (∼8)

16-64 (∼32)

64-256 (∼128)

256-1k (∼512)

1k-4k (∼2k)

4k-16k (∼8k)

→

∼55k

1st-4th
5.5
0
1
−
−
−
−

4th-6th
7
∼16
1
−/+
−/+
−
−

6th-8th8.5∼80-902++−−

8th-10th10∼250-3003++−−

10th-12th
13.5
∼All
4
+
+
−/+
−/+

12th-14th
15.5
All
5-6
+
+
+
+

→
→
→
→
→
→
→
→

15th-16th
25
All
1 or 2++++

Avian early development from fertilization at the beginning of EGK-I to complete cellularization by the end of EGK-V is viewed using the following criteria: total
number of cells or nuclei; nuclear division cycle; time post-fertilization; number of laterally closed cells; number of cell layers in central blastoderm; blastomere
basal closure; zygotic gene activation; yolk syncytium and edge cell cellularization. Of practical values among them are the total number of cells/nuclei andnumber of laterally closed cells. However, early development is a continuous process and these criteria should be used only as a general guide.
−, negative; +, positive; −/+, weakly or partially positive.

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DEVELOPMENT

Table 1. Summary of EGK-I to -V embryos

Scanning electron microscopy (SEM)

Samples for SEM analysis were prepared as described previously(Ikenouchi et al., 2013). Briefly, EGK stage chick embryos were fixedwith 2.5% glutaraldehyde and 2% formaldehyde in 0.1 M sodium
cacodylate buffer ( pH 7.4) for 2 h at room temperature, followed by postfixation with 1% OsO4 in the same buffer for 2 h on ice. Post-fixed embryos
were dehydrated in ethanol and then transferred to isoamyl acetate, followedby critical point drying (JCPD-5, JEOL). Specimen surface was coatedusing an osmium coater (Neoc-STB, Meiwafosis). For fracture surface
images after whole-mount SEM, embryo specimen was settled on a small
piece of adhesive tape, and fractured using a fine tungsten needle. All SEM
images were acquired using scanning electron microscope JSM-5600LV(JEOL) with accelerating voltage set at 10 kV.
Immunofluorescence, nuclear staining, imaging and image
analysis

All steps except for the imaging were carried out at 4°C. For phosphorylated
RNA polymerase II detection, fixed embryos were washed in PBTw (PBS with
0.1% Tween-20) for 3×20 min, followed by permeabilization in PBTr (PBS
with 0.1% Triton X-100) for 3×30 min and by blocking in blocking solution(PBS with 1% DMSO, 0.1% Triton X-100 and 5% skimmed milk). Blocked
embryos were incubated overnight with anti-RNA polymerase II CTD pSer5(1:1000, Abcam Cat# ab5131), washed in PBTw, reblocked in blocking
solution and incubated in the secondary antibody (Alexa Fluor 594 goat antirabbit IgG(H+L) (1:300, Molecular Probes Cat# a11012) before imaging.
DAPI (Molecular Probes, D1306) and 36/E-Cadherin antibody (BDTransduction Laboratories, 610182) were used for whole-mount nuclear andE-cadherin staining, respectively. Yolk cell membrane was marked using
rhodamine-labeled wheat germ agglutinin (WGA) (Vector Laboratories, Cat#
RL-1022). Whole-mount fluorescence images were acquired using Olympus
FV1000 microscope. Section fluorescence images were acquired using
Olympus BX51W1 or Olympus FV1000 microscope after processing for10 µm paraffin-embedded section or 12 µm cryosection followed by mounting
in ProLong Gold antifade reagent with DAPI (Molecular Probes Cat#
P36931). Mitotic division angles were measured using Fiji software and data
were plotted as rose diagrams using GeoRose software.
Acknowledgements
We thank Anna Wrabel and Siu-Shan Mak for help with finch colony maintenanceand Kanako Ota and members of the J.Y.H. laboratory for help and coordination
during the collection of pre-ovipositional stage chicken embryos.

Competing interests
The authors declare no competing or financial interests.

Author contributions
G.S., H.N. and J.Y.H. designed the experiments; H.N., M.S., K.K., Y.N., H.C.L., R.L.,
T.S., S.Y. and G.S. performed the experiments; all authors analyzed the data; G.S.
and H.N. wrote the manuscript.

Funding
This work was supported by an internal grant to G.S. from RIKEN Center for
Developmental Biology. Deposited in PMC for immediate release.

Supplementary material
Supplementary material available online at
http://dev.biologists.org/lookup/suppl/doi:10.1242/dev.118604/-/DC1

References
Aanes, H., Winata, C. L., Lin, C. H., Chen, J. P., Srinivasan, K. G., Lee, S. G. P.,
Lim, A. Y. M., Hajan, H. S., Collas, P., Bourque, G. et al. (2011). Zebrafish
mRNA sequencing deciphers novelties in transcriptome dynamics during
maternal to zygotic transition. Genome Res. 21, 1328-1338.
Alev, C., Nakano, M., Wu, Y., Horiuchi, H. and Sheng, G. (2013). Manipulating
the avian epiblast and epiblast-derived stem cells. Methods Mol. Biol. 1074,
151-173.
Baroux, C., Autran, D., Gillmor, C. S., Grimanelli, D. and Grossniklaus, U.
(2008). The maternal to zygotic transition in animals and plants. Cold Spring Harb.
Symp. Quant. Biol. 73, 89-100.
Bellairs, R., Lorenz, F. W. and Dunlap, T. (1978). Cleavage in the chick embryo.
J. Embryol. Exp. Morphol. 43, 55-69.

Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

Bellier, S., Chastant, S., Adenot, P., Vincent, M., Renard, J. P. and Bensaude, O.
(1997). Nuclear translocation and carboxyl-terminal domain phosphorylation of
RNA polymerase II delineate the two phases of zygotic gene activation in
mammalian embryos. EMBO J. 16, 6250-6262.
Carvalho, L. and Heisenberg, C.-P. (2010). The yolk syncytial layer in earlyzebrafish development. Trends Cell Biol. 20, 586-592.
Danilchik, M. V., Bedrick, S. D., Brown, E. E. and Ray, K. (2003). Furrow
microtubules and localized exocytosis in cleaving Xenopus laevis embryos. J. Cell
Sci. 116, 273-283.
Eyal-Giladi, H. (1991). The early embryonic development of the chick, as anepigenetic process. Crit. Rev. Poult. Biol. 3, 143-166.
Eyal-Giladi, H. and Kochav, S. (1976). From cleavage to primitive streak formation:
a complementary normal table and a new look at the first stages of thedevelopment of the chick. I. General morphology. Dev. Biol. 49, 321-337.
Foygel, K., Choi, B., Jun, S., Leong, D. E., Lee, A., Wong, C. C., Zuo, E., Eckart, M.,
Reijo Pera, R. A., Wong, W. H. et al. (2008). A novel and critical role for Oct4 as aregulator of the maternal-embryonic transition. PLoS ONE 3, e4109.
Gipson, I. (1974). Electron microscopy of early cleavage furrows in the chickblastodisc. J. Ultrastruct. Res. 49, 331-347.
Hackett, J. A. and Surani, M. A. (2013). Beyond DNA: programming andinheritance of parental methylomes. Cell 153, 737-739.
Harvey, S. A., Sealy, I., Kettleborough, R., Fenyes, F., White, R., Stemple, D. andSmith, J. C. (2013). Identification of the zebrafish maternal and paternaltranscriptomes. Development 140, 2703-2710.
Hong, S.-K., Jang, M. K., Brown, J. L., McBride, A. A. and Feldman, B. (2011).
Embryonic mesoderm and endoderm induction requires the actions of nonembryonic Nodal-related ligands and Mxtx2. Development 138, 787-795.
Ikenouchi, J., Hirata, M., Yonemura, S. and Umeda, M. (2013). Sphingomyelin
clustering is essential for the formation of microvilli. J. Cell Sci. 126,3585-3592.
Johnson, M. H. and Ziomek, C. A. (1981). The foundation of two distinct celllineages within the mouse morula. Cell 24, 71-80.
Kochav, S. and Eyal-Giladi, H. (1971). Bilateral symmetry in chick embryo
determination by gravity. Science 171, 1027-1029.
Kochav, S., Ginsburg, M. and Eyal-Giladi, H. (1980). From cleavage to primitive
streak formation: a complementary normal table and a new look at the first stagesof the development of the chick. II. Microscopic anatomy and cell population
dynamics. Dev. Biol. 79, 296-308.
Lee, H. C., Choi, H. J., Park, T. S., Lee, S. I., Kim, Y. M., Rengaraj, D., Nagai, H.,
Sheng, G., Lim, J. M. and Han, J. Y. (2013a). Cleavage events and sperm
dynamics in chick intrauterine embryos. PLoS ONE 8, e80631.
Lee, M. T., Bonneau, A. R., Takacs, C. M., Bazzini, A. A., DiVito, K. R., Fleming,
E. S. and Giraldez, A. J. (2013b). Nanog, Pou5f1 and SoxB1 activate zygotic
gene expression during the maternal-to-zygotic transition. Nature 503, 360-364.
Leichsenring, M., Maes, J., Mossner, R., Driever, W. and Onichtchouk, D.
(2013). Pou5f1 transcription factor controls zygotic gene activation in vertebrates.
Science 341, 1005-1009.
Li, W. M., Webb, S. E., Lee, K. W. and Miller, A. L. (2006). Recruitment and
SNARE-mediated fusion of vesicles in furrow membrane remodeling during
cytokinesis in zebrafish embryos. Exp. Cell Res. 312, 3260-3275.
Mathavan, S., Lee, S. G. P., Mak, A., Miller, L. D., Murthy, K. R. K., Govindarajan,
K. R., Tong, Y., Wu, Y. L., Lam, S. H., Yang, H. et al. (2005). Transcriptome
analysis of zebrafish embryogenesis using microarrays. PLoS Genet. 1,
260-276.
Parfitt, D.-E. and Zernicka-Goetz, M. (2010). Epigenetic modification affecting
expression of cell polarity and cell fate genes to regulate lineage specification in
the early mouse embryo. Mol. Biol. Cell 21, 2649-2660.
Park, T. S. and Han, J. Y. (2012). piggyBac transposition into primordial germ cells
is an efficient tool for transgenesis in chickens. Proc. Natl. Acad. Sci. USA 109,9337-9341.
Park, H. J., Park, T. S., Kim, T. M., Kim, J. N., Shin, S. S., Lim, J. M. and Han, J. Y.
(2006). Establishment of an in vitro culture system for chicken preblastodermal
cells. Mol. Reprod. Dev. 73, 452-461.
Rossant, J. and Tam, P. P. L. (2009). Blastocyst lineage formation, early embryonicasymmetries and axis patterning in the mouse. Development 136, 701-713.
Sasaki, H. (2010). Mechanisms of trophectoderm fate specification inpreimplantation mouse development. Dev. Growth Differ. 52, 263-273.
Sheng, G. (2014). Day-1 chick development. Dev. Dyn. 243, 357-367.
Sun, Y., Tseng, W.-C., Fan, X., Ball, R. and Dougan, S. T. (2014). Extraembryonicsignals under the control of MGA, Max, and Smad4 are required for dorsoventral
patterning. Dev. Cell 28, 322-334.
Tadros, W. and Lipshitz, H. D. (2009). The maternal-to-zygotic transition: a play in
two acts. Development 136, 3033-3042.
Tsunekawa, N., Naito, M., Sakai, Y., Nishida, T. and Noce, T. (2000). Isolation ofchicken vasa homolog gene and tracing the origin of primordial germ cells.
Development 127, 2741-2750.
Vassena, R., Boue, S., Gonzalez-Roca, E., Aran, B., Auer, H., Veiga, A. andBelmonte, J. C. I. (2011). Waves of early transcriptional activation andpluripotency program initiation during human preimplantation development.
Development 138, 3699-3709.

1285

DEVELOPMENTRESEARCH REPORTRESEARCH REPORT

Xue, Z., Huang, K., Cai, C., Cai, L., Jiang, C.-y., Feng, Y., Liu, Z., Zeng, Q.,
Cheng, L., Sun, Y. E. et al. (2013). Genetic programs in human andmouse early embryos revealed by single-cell RNA sequencing. Nature 500,
593-597.
Yan, L., Yang, M., Guo, H., Yang, L., Wu, J., Li, R., Liu, P., Lian, Y., Zheng, X.,
Yan, J. et al. (2013). Single-cell RNA-Seq profiling of human preimplantationembryos and embryonic stem cells. Nat. Struct. Mol. Biol. 20, 1131-1139.

DEVELOPMENT

Wang, H. and Dey, S. K. (2006). Roadmap to embryo implantation: clues frommouse models. Nat. Rev. Genet. 7, 185-199.
Watanabe, T., Biggins, J. S., Tannan, N. B. and Srinivas, S. (2014). Limited
predictive value of blastomere angle of division in trophectoderm and inner cellmass specification. Development 141, 2279-2288.
Watt, J. M., Petitte, J. N. and Etches, R. J. (1993). Early development of the chickembryo. J. Morphol. 215, 165-182.

Development (2015) 142, 1279-1286 doi:10.1242/dev.118604

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