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Alobar holoprosencephaly, cyclopia and enigmatic
associations in newborn rabbits
Serge Bonin, Michel Gruaz, and Esther van Praag
Holoprosencephaly is a serious and complex congenital malformation of the brain
caused by a defect of the medial cleavage of the forebrain into two cerebral
hemispheres. It is characterized by typical craniofacial abnormalities.
The word cyclopia comes from the ancient
Greek Κύκλωψ (Kyklops), meaning “round
eye”. It refers to strange beings in the
Greek mythology, Giants with only one eye
in the middle of their faces, whose aspect is
otherwise normal. Their parents from
Ouranos (Heaven) and Gaia (Earth) gave
birth to three children with particularly
dangerous strength, power and weapons.
There were the Uranian Cyclops,
blacksmiths, builders and pastors. Some see
it as an analogy with the central eye of
volcanoes rather than living things. Other
civilizations also have beliefs or legends in
which beings appear with only one central
eye. This is the case with the Irish and the
Ossetians. The reality of a man or animal
born with cyclopia is, unfortunately, tragic,
Figure 1: Extremely rare discovery: Flemish Giant newborn presenting a combination of major cerebral
and facial morphological abnormalities.
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as most cases are not viable. But they can
be the source of these terrifying tales of
giant monsters.
There is also a
small freshwater
crustacean − the
Cyclops, which has
only a simple median
eye. Hence its name.
Cyclopia in vertebrates
Embryonal development of the nervous
system and the brain is complex. It is
formed from the notochord, a cartilaginous
structure that will become the neural plate
and the vertebral bodies. The neural
induction mechanism will result in the
formation of a neural tube, which will
gradually close along the entire length from
the cervical region to the caudal region.
Closure defects cause different pathologies
in the newborn:
- A failure to close the anterior opening of
the neural tube (neuropore) is
characterized by the absence of a brain in
the embryo (anencephaly), i.e. the
appearance of a brain outside the skull
(hernia of the brain or encephalocele)
(Figure 2).
- A failure to close the posterior opening of
the neural tube causes a malformation of
the spine called spina bifida.
Coordination of gene expression
The processes involved in the formation
of the brain and the development of motor
neurons are under the control of the Sonic
Hedgehog glycoprotein (SHH). The
Hedgehog protein signaling pathway
(formation of organs during embryonic
development) plays, indeed, a vital role in
the regulation of digits formation (number of
fingers and toes) and the brain organization.
A concentration gradient appears when the
molecule spreads throughout the embryo
creates. This contributes to the appearance
of the dorso-ventral axis. The effect on cells
depends on its concentration.
An abnormal concentration of the Sonic
Hedgehog protein will affect the brain
development. It can have a genetic origin
like, e.g., a mutation of genes involved in
the regulation of SHH. Fibroblast growth
factor (FGF) genes also appear to play a role
Figure 2: Newborn rabbits presenting a failure to close the neural tube during the embryonic development,
causing an encephalocele (left) or a spina bifida (right,
https://www.devtox.org/nomenclature/ml_imag_im.php?img=1.1117.5226-3-01).
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in controlling the concentration of SHH in
the developing brain. Finally, a plant alkaloid
steroid is able to block the SHH signaling
pathway.
Brain formation
The development of the brain is gradual
and takes place above the spinal cord (Table
1, Figure 3). Different structures are formed
by budding.
Prosencephalon: this structure will divide
into two secondary vesicles, the
diencephalon and the telencephalon. The
latter will divide into approximately
symmetric left and right cerebral
hemispheres.
Mesencephalon: (midbrain) derives from the
middle cerebral vesicle of the primary
encephalon. It provides the motor control of
muscles and the sensory reflexes of vision
and hearing.
Rhombencephalon: represents the posterior
part of the brain. It is subdivided into two
portions (metencephalon and
Embryologic divisions
Anatomic Divisions
Encephalon
Prosencephalon
Telencephalon
Brain
Diencephalon
Mesencephalon
Mesencephalon
Brain stem
Rhombencephalon
Metencephalon
Annular
protuberance
(pons)
Cerebellum
Cerebellum
Myelencephalon
Medullary
brain
Brain stem
Spinal cord
Spinal cord
Table 1: Division of the brain during the development of the vertebrate embryo.
https://fr.wikipedia.org/wiki/Rhombencéphale
Figure 3: Development and division of the neural tube and formation of different parts of the brain.
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myelencephalon) which will give rise to the
cerebellum and the medullary brain
(medulla oblongata).
Median longitudinal fissure
During embryogenesis, the longitudinal
division of the primitive brain into two
cerebral hemispheres and two ventricles is
closely coordinated by different genes.
These regulate the synthesis of signaling
proteins that will tell cells of the primitive
brain to form the right and left hemispheres.
The inactivation of several genes or the
mutation of a gene will have serious
structural consequences on the brain. These
are associated with specific and more or less
serious congenital malformations of the skull
and face. Depending on the severity of the
anomaly, the alteration of the brain
development leads to a partial, longitudinal
cleavage or a complete absence of cleavage
of the forebrain into two distinct cerebral
hemispheres
This set of birth defects is called
holoprosencephaly. Individuals born with
such a defect are usually not viable.
Holoprosencephaly exhibits a great
variability of phenotypes. The earlier the
alteration of the forebrain cleavage appears
in the development of the embryo, the more
the pathology will be severe.
Alobar holoprosencephaly
Alobar holoprosencephaly is the most
severe condition of birth defects of the brain
and face (Figure 4). Individuals affected by
this malformation are generally not viable.
Causes
The causes have various origins.
Alobar holoprosencephaly is often
associated with chromosomal aberration. It
can also have a multigenic origin in humans.
Forty percent of cases are caused by
mutations in genes involved in the
production and spread of a specific molecule
in the neural tube. The latter induces the
correct differentiation of the ventral
structures of the brain. Sixteen mutated
target genes have been identified to date in
man. They act together to regulate the
activity of this molecule. Any defect at this
embryonic stage will result in the
appearance of a cyclopia-like
holoprosencephalic phenotype. The disease
is, thus, characterized by great genetic
complexity
It has also been suggested that cyclopia
may be caused by the recessive cy gene in
rabbits. Other genetic causes include high
inbreeding, e.g. between members
belonging to the same nest (2nd degree).
A plant can, moreover, cause
holoprosencephaly when ingested by
pregnant females whose embryos are at an
early stage of development. it is the
Californian corn lily, also called California
Figure 4: Craniofacial features suggestive of
alobar holoprosencephaly in
humans.
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false hellebore (Veratrum californicum). This
plant contains steroidal alkaloids:
cyclopamine and jervine (Figure 5). These
molecules block the Hedgehog signaling
pathway and, thus, induce a developmental
delay in embryos. Teratogenic molecules like
thalidomide are also known to induce
holosencephaly in rabbits
Finally, environmental causes cannot be
ruled out, such as a pre-existing disease
linked to the cholesterol metabolism,
hyperglycemia or a viral infection in
pregnant rabbits
Pathogenesis
The absence of cleavage and
diverticulation of the primitive brain into two
cerebral hemispheres results in alobar
holoprosencephaly. The brain is, thus,
organized as a single ventricular cavity
shaped like a horseshoe. The longitudinal
fissure between the cerebral hemispheres is
absent. As a consequence, nervous fibers
interconnecting the two cerebral
hemispheres (corpus callosum) are also
absent. The basal ganglia and rudimentary
thalami (diencephalon) are fused in the
midline. The brainstem and cerebellum are
usually present, but they are malformed.
There is a single anterior cerebral artery.
The brain is lined with cerebral
parenchyma at the front and demarcated by
a thin wall at its posterior. The ventricular
system, which represents a collection of
chambers located inside the brain, merges
and forms a single chamber
(monoventricle). At the back of the brain,
this monoventricle may become a fluid
pocket (dorsal sac) that puts pressure on
the peripheral brain parenchyma. The size of
the monoventricle varies. Indeed, the
amount of cerebrospinal fluid (CSF) in the
cerebral ventricular system is regulated by
the Sylvius aqueduct. In individuals with
alobar holoprosencephaly, the opening of
this aqueduct is absent (atresia),
leading to an accumulation of
fluids and, consequently, a
dilation of the single cerebral
ventricle (hydrocephalus).
This malformation also causes
facial abnormalities of the midline
(Figures 6, 7, 8). The most
evocative are:
- The presence of a single eye
socket located on the midline.
In the case of cyclopia, it
contains a single eye. In
synophthalmia, there is a
doubling of the internal
structures of the eye to
varying degrees, or the
presence of two joined eyes.
In the case of cebocephaly, the
two hypoteloric eyes are
distinct and very close
together. The optic nerves are
Figure 5: Californian corn lily (Veratrum californicum)
causes holoprosencephaly when ingested by
gestating females whose embryos are at an
early stage of development.
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usually fused into a single nerve, but may
also be absent;
- Absence of the olfactory system and the
nose (arrhinia). The olfactory tracts are
merged into a single tract, but may also
be absent;
- Presence of a proboscis, also called
tubular nose. It results from the absence
of development of the lining of the frontal
bud. This appendix is located on the
supraorbital midline of the head
(ethmocephaly), above the eye. Indeed,
due to the non-separation of the ocular
buds, the nose cannot migrate down the
face during embryonic development.
Enigmatic association
Alobar holoprosencephaly with cyclopia is
Figure 6: Newborn rabbit with ethmocephaly-appearing alobar holoprosencephaly, with a proboscis
above a single orbit, a duplex eye formation and an atrophied nasal apparatus.
https://www.devtox.org/nomenclature/ml_manus.php?mno=110325066&spc=3).
Figure 7: Cyclopia in a rabbit newborn, presenting the Astomia agnathia type of malformation,
with a single eye on the face, no nose, atresia of the jaws and rotation of the ears .
Video: JoAnna Bova https://www.youtube.com/watch?v=vQwMhyuUUwA
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exceptionally associated with other
malformations ranging from absence of the
jaw to a failure of ears migratation from a
prone position to a cranial position during
embryogenesis (otocephaly). The ears
remain at the level of the cheeks, on the
side of the head. Studies in humans have
shown that this very serious form of
Figure 8: Severe craniofacial malformations in a newborn rabbit, with caudal positioning of the ears,
no nose (arrhinia), no mouth (astomy) and no jaws (agnatia).
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holoprosencephaly affects mostly females.
The animal world is not spared. Such
individuals have been observed in guinea
pigs, sheep, mice and rabbits (Figures 7, 8).
They frequently suffer from other birth
defects. The most common one is an
abnormal spatial arrangement of the
internal paired or single organs in the thorax
and/or abdominal cavity (heterotaxis),
relative to the right/left symmetry axis in a
normal individual. Causes and pathogenesis
of this association remain enigmatic. It may
be linked to the lack of inducing activity of
the prechordial plaque (which will later
develop into the mouth opening) on tissues
of the prosencephalic portion of the neural
tube and the cells of the neural crest. In the
skull, part of these cells normally
differentiates into cartilage, bone and
ganglia. Others contribute to the formation
of the jaw and the middle ear, among
others. Not in these individuals. This does,
however, not explain all the clinical
manifestations observed in those affected
individuals
Conclusion
The development of cranial and facial
structures and those of the brain is the
result of complex interactions between gene
coordination and different signaling
mechanisms. An anomaly or defect can
occur at any time during this crucial stage of
embryonic development, explaining the
broad clinical spectrum with a great
variability of phenotypes in affected
newborn rabbits.
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