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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.