Download Head segmentation

The vertebrate
head
Vertebrate characteristics
brain
anterior end
of notochord
ear
eye
Amphioxus: a poorly
differentiated front end
nose
Zebrafish larva: a proper head
Vertebrates have a differentiated head with a brain and paired sense organs
Characteristics
The heads of cyclostomes and gnathostomes are different in
many ways, but the following features are always present:
A mouth and pharynx
Pharyngeal slits (at least in the embryo)
A brain with a series of cranial nerves
Paired nasal sacs (sometimes close together), eyes, and inner ears
A hypophysis on the underside of the brain
A notochord that ends just posterior to the hypophysis
Cartilages or bones around the brain
Head segmentation
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
Goethe, 1790
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
Goethe, 1790
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
cartilages
Goethe, 1790
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
cartilages
somites
Goethe, 1790
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
nerves
somites
Goethe, 1790
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
nerves
somites
Goethe, 1790
Lateral plate mesoderm
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
I think the skull is
constructed from a series
of modified vertebrae.
nerves
somites
Goethe, 1790
Pharyngeal slits
Lateral plate mesoderm
Goodrich 1930: a segmental interpretation of the vertebrate head
Head segmentation
The key question is whether the
head mesoderm is organised into
somite equivalents.
Head segmentation
The key question is whether the
head mesoderm is organised into
somite equivalents.
Head mesoderm
Head segmentation
The key question is whether the
head mesoderm is organised into
somite equivalents.
In amniotes this mesoderm is not
segmented, but it has been
claimed that head segments,
“somitomeres”, are present in
some other gnathostomes (e.g.
Meier 1979, 1981). Other
researchers have not been able to
confirm this.
What about cyclostomes?
Trunk mesoderm
Head mesoderm
Head segmentation
Parasagittal section of lamprey
head from Koltzoff (1901)
Again, it has been claimed that mesodermal head segments are present in lamprey
(Koltzoff 1901, Damas 1944), and in single sections they look convincing…
Head segmentation
Parasagittal section of lamprey
head from Koltzoff (1901)
Again, it has been claimed that mesodermal head segments are present in lamprey
(Koltzoff 1901, Damas 1944), and in single sections they look convincing…
…but this could be deceptive (Kuratani 2008).
Head segmentation
More detailed studies of lamprey head
development (Tahara 1988) show that
the head mesoderm is not segmented,
simply regionalised by the influence of
neighbouring structures such as the
otic vesicle and pharyngeal slits
(Kuratani 2008).
Head segmentation
It seems that the simple
segmentalist view of the
vertebrate head is wrong.
Head segmentation
It seems that the simple
segmentalist view of the
vertebrate head is wrong.
Head segmentation
It seems that the simple
segmentalist view of the
vertebrate head is wrong.
Instead we have a complex
“double segmentation”
reflecting the interaction of
somites, pharyngeal slits
and neural crest streams.
Head segmentation
It seems that the simple
segmentalist view of the
vertebrate head is wrong.
Instead we have a complex
“double segmentation”
reflecting the interaction of
somites, pharyngeal slits
and neural crest streams.
pharyngeal
segmentation
somitic segmentation
Head development
Amniote head development (chicken):
head of animal develops while
gastrulation is still going on at the
rear end.
Head development
Gastrulating region
Amniote head development (chicken):
head of animal develops while
gastrulation is still going on at the
rear end.
The neural tube closes in the head
and differentiates into bulges
(cerebral vesicles) that will form the
main brain regions
Head development
Cerebral vesicles in a human embryo
Head development
Rhombencephalon:
subdivides into rhombomeres
Cerebral vesicles in a human embryo
Head development
The developing brain shows
distinctive and highly
conserved gene expression
patterns (lamprey and
mouse brains in side view,
from Kuratani et al. 2002)
Head development
The adult vertebrate brain carries an
organised series of cranial nerves:
Human cranial nerves,
ventral view, schematic
0: pheromone reception?
I (olfactory): smell
II (optic): sight
III (oculomotor): eye muscle movement
IV (trochlear): eye muscle movement
V (trigeminal): sensory nerve for face,
controls mandibular arch muscles
VI (abducens): eye muscle movement
VII (facial): movement of facial muscles,
sense of taste in anterior part of tongue
VIII (vestibulocochlear): hearing and
movement/balance data from inner ear
IX (glossopharyngeal): sense of taste in
posterior part of tongue, movement of
some throat muscles and salivary glands
X (vagus): sense of taste in epiglottis,
movement of throat muscles, vocal
chords, upper part of gut.
XI (accessory): neck muscle movement
XII (hypoglossal): tongue movement
sensory motor both
Head development
Back to the embryo!
Pharynx of human embryo,
horizontal section
Now two things happen at about the same time: the pharynx starts producing
pouches (the beginnings of gill slits) that divide the side walls of the head into
separate pharyngeal arches…
Head development
Pharyngeal
pouch
Pharyngeal
cleft
Pharynx of human embryo,
horizontal section
Now two things happen at about the same time: the pharynx starts producing
pouches (the beginnings of gill slits) that divide the side walls of the head into
separate pharyngeal arches…
Head development
Pharyngeal
pouch
Pharyngeal
cleft
Pharynx of human embryo,
horizontal section
Now two things happen at about the same time: the pharynx starts producing
pouches (the beginnings of gill slits) that divide the side walls of the head into
separate pharyngeal arches…
…and crest cells start streaming from the mid- and hindbrain into these arches
Head development
Gnathostome embryo
Lamprey embryo
The neural crest streams are quite
similar in lamprey and gnathostomes:
cells from the midbrain and
rhombomeres 1 + 2 go into the
mandibular arch and more anterior
structures, cells from rhombomeres
3-5 go into the hyoid arch, and cells
from more posterior rhombomeres go
into the gill arches (figure from
Kuratani et al. 2001).
Head development
Each arch has the same basic
structure, and is formed from neural
crest and mesoderm sandwiched
between endoderm and ectoderm.
Gnathostome embryo
Lamprey embryo
The neural crest streams are quite
similar in lamprey and gnathostomes:
cells from the midbrain and
rhombomeres 1 + 2 go into the
mandibular arch and more anterior
structures, cells from rhombomeres
3-5 go into the hyoid arch, and cells
from more posterior rhombomeres go
into the gill arches (figure from
Kuratani et al. 2001).
Head development
Rhombomeres in quail-chick chimaeras:
Köntges & Lumsden (1996)
The neural crest cells from different
rhombomeres do not mix freely.
They carry very specific patterning
instructions relating to connectivity.
Head development
The cells from one stream give rise to the connective tissue of a muscle, the regions of bone
where it attaches, and the ganglion of the cranial nerve that innervates it. But the boundaries
between these cell populations do NOT correlate with the outlines of the bones.
Head development
Meanwhile, a series of placodes have developed in the head ectoderm:
these will interact with the developing brain, neural crest and pharyngeal
slits
Head development
Epibranchial
placodes
Meanwhile, a series of placodes have developed in the head ectoderm:
these will interact with the developing brain, neural crest and pharyngeal
slits
Eye development
The eye provides a good example of how the interaction between a placode, the
brain, and migrating neural crest cells creates a major sense organ
Eye development
tunicate
(ocellus)
hagfish
(eye)
lamprey
(pineal gland)
lamprey
(eye)
gnathostome
(eye)
All chordates
have the same
basic type of
light-sensitive cell
Eye development
Eye development starts by
the brain bulging out
towards the lens placode.
The brain will form the
retina and optic nerve, the
lens placode will form the
lens.
Eye development
The lens vesicle detaches from the epithelium and
develops into a lens. The cells transform into long
fibres filled with crystallin - a transparent protein.
Eye development
The lens vesicle detaches from the epithelium and
develops into a lens. The cells transform into long
fibres filled with crystallin - a transparent protein.
Crystallin production is induced by the coexpression
of Sox2 and Pax6.
Eye development
The lens vesicle detaches from the epithelium and
develops into a lens. The cells transform into long
fibres filled with crystallin - a transparent protein.
Crystallin production is induced by the coexpression
of Sox2 and Pax6. Meanwhile, neural crest cells
congregate around the lens and retinal cup to form the
cornea and sclera of the eye.
Eye development
An interaction between Sonic hedgehog (Shh) and Pax6 has a crucial role in controlling
vertebrate eye development. Shh suppresses Pax6. If little or no Shh is produced by the
prechordal plate, the left and right eye fields fuse and cyclopia results.
Eye development
Astyanax mexicanum
If on the other hand too much Shh is produced, Pax6 is suppressed throughout the eye
fields and no eyes are formed at all. This is how blind cave fish have lost their eyes.
Ear development
The ear has a very complex evolutionary history that we will return to later on. However,
in all vertebrates the inner ear originates from the otic placode which sinks in and
forms a vesicle.
Ear development
Process of invagination
Otic cup in mouse embryo
Close-up
The ear has a very complex evolutionary history that we will return to later on. However,
in all vertebrates the inner ear originates from the otic placode which sinks in and
forms a vesicle.
Ear development
Process of invagination
Otic cup in mouse embryo
Close-up
Hair cell
The ear has a very complex evolutionary history that we will return to later on. However,
in all vertebrates the inner ear originates from the otic placode which sinks in and
forms a vesicle. The lining of this vesicle differentiates into hair cells.
Ear development
Process of invagination
Otic cup in mouse embryo
Close-up
Hair cell
The ear has a very complex evolutionary history that we will return to later on. However,
in all vertebrates the inner ear originates from the otic placode which sinks in and
forms a vesicle. The lining of this vesicle differentiates into hair cells.
The tongue
The tongue is an anatomical oddity:
muscles and innervation both loop
round the back of the gill region, but
the skeleton and connective tissue
belong to the pharyngeal arches. The
hypobranchial muscle of lampreys
forms in a similar way.
The head: summary
The vertebrate head is a complex structure formed from the
interaction of ectoderm, neural tube, neural crest, placodes,
mesoderm and endoderm.
There does not seem to be a single unifying segmentation system for
the vertebrate body and head. The head mesoderm anterior to the
otic capsule is not divided into somites.
Although it has minor somitic components, the “segmental”
appearance of the head is principally created by the patterning
effects of rhombomeres and pharyngeal arches.
The rhombomere identity of neural crest cells has a powerful role in
determining musculo-skeletal connectivity.