Neural crest, described as mitotically active cells at the neurosomatic junction can be observed in the Vant embryo and Carnegie Embryo No. 7650. Cited in O'Rahilly R., Muller F. (1987) p.90. There are at least two areas producing neural crest cells at this stage, the mesencephalon and probably the future trigeminal area.
Neural crest migration has been studied extensively in the chick, using grafting techniques to study major migration routes, but also on a more detailed cellular basis using fluorescent antibodies, vital dyes and virally transformed cells. See Gilbert S.F., (1994) for more details.
Neural crest formation is thought to be at its peak in the head region at this stage. It is seen rostrally in the vicinity of the mesencephalon and cranial half of rhombomere A, migrating before closure of the neural groove. Further caudally neural crest is seen migrating from the accousticofacial area with the otic plate probably contributing a significant proportion (rhombomere B). Between these two regions there is a neural crest free area, the future 3rd rhombomere. Neural crest also migrates from the caudal half of rhombomere C after the closure of the neural groove. This contributes to the glossopharyngeal and vagal cranial nerve ganglia
Spinal and occipital neural crest from rhombomere D down to about the level of the eighth somite can be seen in the 10 somite embryo, Carnegie No. 5074. Corner G.W.,(1929)
Neural crest cells contribute to general mesenchyme within the head, the condensations that will from the cranial nerve ganglia, as well as ectomesenchyme to the skull and face. The appearance of the neural crest is thought to be in succession; facial, mesencephalic, trigeminal, vagal, occipital, glossopharyngeal and, finally, spinal. O'Rahilly R., Müller F.,(1994) p.46
According to Yokoh Y., (1968) the neural crest condensation that forms the accousticofacial complex appears at about the 4-5 somite stage followed by the trigeminal condensation at the 7-13 somite stage.
The rhombencephalic and spinal neural crest of a 17 somite embryo has been described by Wen I.C., (1928). The mesencephalic and trigeminal crest are now separate but connected to the neural tube by cellular strands, the latter crest has condensed under the ectoderm at the level of the 2nd rhombomere.
Continuous with the dorsolateral angle of the 4th rhombomere is a condensation of neural crest that is in contact with the overlying thickened ectoderm (probably an epibranchial placode). The crest spreads out into the second arch and under the otic placode and is called the acoustico-facial crest.
Vagal and glossopharyngeal crest are also described in this embryo.
According to Muller F., O'Rahilly R., (1986) mesencephalic neural crest, while still being given off in the least advanced specimens of this stage is not detected in the oldest specimens of stage 11. The mesencephalic crest is thought to contribute to an optic sheath which appears during this stage. At about the 15-16 somite stage the neural crest cells appear from the superior and inferior walls of the optic vesicle and contribute this sheath too. This neural crest production peaks at the 23-25 somite stage. Bartelmez G.W., Blount M.P., (1954) Neural crest cells in this sheath are thought contribute pigment cells to the retina
Neural crest at the level of rhombomere 1 can be observed in Carnegie embryo No.'s. 7611 (16 somites) and 7702 (17 somites)
Neural crest is also still migrating from the 4th, 6th and 7th rhombomeres, and otic epithelium
Neural crest cells can be distinguished to the level of the 9th somite in the least advanced embryos but reach the 13th somite in the 20 somite embryo 2053. Davis C.L., (1923) pp20-21.
In a seventeen somite embryo, neural crest is thought to be distinguishable to the level of the 12th somite. Wen I.C., 1928
The neural crest cells of the trunk take one of two major pathways:
The dorsolateral pathway, between the epidermis and the dermamyotome, to eventually reach the ventral, gut region. These cells will contribute mainly epidermal pigment cells.
The ventral pathway, through the anterior part of the sclerotomes. These cells contribute to dorsal root ganglia, sympathetic neurons, adrenomedullary cells and Schwann cells.
According to Muller F., O'Rahilly R., (1987), formation of neural crest continues from the levels of rhombomere 2,4,6 and 7 forming ganglionic condensations of the trigeminal, acousticofacial, and vagal nerves. Neural crest is only seen migrating from mesencephalon in the least advanced embryos of this stage, e.g. Carnegie Embryo No. 8943 which is transitional from stage 11-12.
Neural crest is also observed leaving the diencephalon in this embryo.
Crest cells are still observed leaving the otic placode and contributing to the otic sheath. Cells are also contributed to the optic sheath from the optic vesicle.
Neural crest cells can be distinguished to the level of the 18th somite in Carnegie Embryo No. 7852 and the 19th in Carnegie Embryo No. 6097 but do not contribute to spinal ganglia yet.
Observations of neural crest production at this stage are scantily recorded;
A new site of neural crest production is found within the telencephalon where terminal -vomeronasal neural crest is being produced. O'Rahilly R., Müller F.,(1994) p 74
Also neural crest is found associated with the accessory nerve. O'Rahilly R., Müller F., (1984).
The otic vesicle is still giving off neural crest cells
Neural crest cells can be distinguished down to the level of the lower lumbar somites. They form longitudinal bands on either side of the tube and extend ventrolaterally at the midline of each segment, particularly in the cervical area where sensory spinal ganglia (dorsal root ganglia) have begun to form. Müller F., O'Rahilly R., (1988b)
Crest cells clinging to the accessory nerve are described by Streeter G.L., (1904) in Embryo Br3 of the His collection (6.9mm long).
Neural crest cells are produced down to the sacral levels, they are spreading ventral to the neural tube in the direction of the dorsal aorta where they will form sympathetic ganglia.
Dorsal root ganglia are also forming
According to Bartelmez G.W.,(1954) neural crest can still be identified migrating from the medial wall of the optic vesicle at this stage e.g. Carnegie No 7394.
The neural crest cells in the occipital, cervical and accessory areas have almost completely formed their relevant ganglia by the end of this stage.
The spinal neural crest extends from the level of the 2nd cervical ganglion to within a short distance of the tip of the spinal cord. The first cervical ganglionic crest has segmented entirely whereas below this level the ganglionic crest is connected by a dorsal bridge.
By this stage the occipital and first cervical crest has more or less completely resolved into separate spinal ganglia, the dorsal connecting bridge of crest cells having disappeared. Streeter G.L., (1904)
Terminal vomeronasal neural crest is still present in the olfactory area.
The terminal vomeronasal crest has condensed and will soon form the terminal and vomeronasal ganglia. O'Rahilly R., Müller F., (1994) Chapter 18 and 19.
Page authored by Amy. Carless@ed.ac.uk