OUTLINE OF NEURAL CREST DEVELOPMENT:

See also lectures from DB (Fall 2000) for week 11; Gilbert text, especially Chs. 10, 12, & 13; and Neural Crest book

Also go to the Histology course (Bio 204), nervous tissue lectures and pictures, week 4

This is a more detailed outline than the material covered in the one class. Remember that there will be no exams in this course; this material is for background and orientation purposes.

Tissue components of nervous systems:

  • Neurons (nerve cells): the cells that actually receive, process, and transmit signals. These cells have long processes that extend out from their cell bodies. These include dendrites that receive signals, and a single long axon that grows out from each nerve cell to synapse with target cells (either other neurons or muscle cells). At the synapse a signal is transmitted to the target cell.
  • All vertebrate nervous systems contain 3 major categories of neurons:
  • Sensory neurons: receive input from the body and environment
  • Interneurons: receive input from the many sensory neurons, process and integrate that input by signaling between interneurons. Interneurons are the category of neurons that make up most of the nerve cells within the central nervous system (brain and spinal cord). Interneurons then send signals to ->
  • Motor neurons: transmit signals to target tissues, which are primarily muscle tissues
  • Support cells (glia): these cells serve a number of supporting functions for nerve cells, including protection (ie, some wrap around long axons to protect them) and metabolic support (transport nutrients from blood vessels to nerve cells). These cells are part of nerve tissue because:
  • they are derived from the same embryonic germ layer components as the neurons: in general, neurons are generated first, and glia later from the same stem cells. This is a relevant point in several of the selected papers.
  • they are found only within the nervous system in association with neurons

    • Organization of vertebrate nervous systems:

  • Central nervous systems contain large brains extending into continuous spinal cords, all containing nerve cells and axons running between them
  • Interneurons send axons throughout CNS: some run in tracts within brain, or to/from brain and spinal cord. Tracts contain specific functional groupings of axons.
  • ALL motor neurons whose cell bodies/nuclei are located in the CNS extend their axons out of the CNS, at which point they are now IN the peripheral nervous system. Their axons travel in bundles in structures called "peripheral nerves" surrounded by glia that are now derived from PNS sources (ie, neural crest).
  • Voluntary motor neurons innervate skeletal muscles directly: axons of neurons located within CNS send axons all the way to their target muscle cells - no intervening nerve cells
  • Autonomic motor neurons innervate the other types of muscle as part of a 2-neuron chain, with the first nerve cell body within the CNS, and the axon reaching out into the PNS -see PNS below
  • NO primary sensory neurons are located within the CNS: they are all in ganglia in the peripheral NS (see below);
  • secondary (inter)neurons in the CNS receive signals from the primary sensory neurons
  • Peripheral nervous system components consist of:
  • Ganglia: contain specific types of nerve cells (ie, the cell bodies/nuclei of the neurons reside here) and glia: two major functional types of neurons reside in ganglia:
  • "Dorsal root"/"spinal" ganglia: ALL primary sensory nerve cells. Ganglia located immediately dorso-lateral to spinal cord/brain. Their axons grow into the CNS to synapse with interneurons.
  • Autonomic/involuntary MOTOR ganglia: two functional types innervate internal organs under "involuntary" control: two functional types counterbalance each other
  • 2nd nerve in 2 neuron motor chain: first nerve cell is located within CNS, sends its axon out into periphery to synapse with 2nd nerve cell in autonomic ganglia.
  • Targets: primary targets are smooth muscle in walls of internal organs, cardiac muscle in heart wall
  • Ganglia physically located in different places:
  • Sympathetic ganglia: located in chains close to the vertebral column
  • Function: triggers "fight or flight" actions (enhance blood flow to skeletal muscles, heart; decrease flow to "vegetative" organs)
  • Parasympathetic ganglia: very small clumps of neurons located mostly within the walls of their target organs
  • Function: triggers vegetative functions (enhance blood flow to GI tract, kidneys, etc; enhance peristaltic contractions of GI tract, etc.)
  • Peripheral nerves: nerve processes (motor axons & sensory dendrites) running together
  • contain more glial cells; surrounded by connective tissues (not of neuroectoderm origin)
  • all peripheral nerves that originate from spinal cord levels (31) contain both motor and sensory nerve processes (motor axons, sensory dendrites)
  • the 12 peripheral nerves that originate from the brain ("cranial nerves") are a mixed bag, with some containing just motor or just sensory processes, and some containing both.
  • Here's a list of the cranial nerves:
  • 1 (olfactory): purely sensory to nose.
  • 2 (optic): purely sensory to eyes. It should be noted that nerve 2 is actually an outgrowth of the brain, and thus does not have a component that comes from neural crest.
  • 3 (oculomotor), 4(trochlear), 6 (abducens): motor nerves to muscles that move the eyeball
  • 5: mixed nerve that innervates tons of things withinand on the surface of the head: muscles, skin, internal mucus membranes. Grows into and innervates derivatives of pharyngeal arch 1, which forms most of the tissues of the upper and lower jaws (thus surrounding the mouth cavity).
  • 7 (facial): mixed nerve to skin and muscles of face; some to tongue,and salivary glands. Grows into and innervates derivatives of pharyngeal arch 2, which later spread all over surface of face and head.
  • 8 (acoustic, or vestibulocochlear): sensory to ears; includes equilibrium sensations from inner ear.
  • 9 (glossopharyngeal): mixed nerve to pharynx, tongue, salivary glands. Grows into and innervates derivatives of pharyngeal arch 3.
  • 10 (vagus): mixed nerve that innervates components in head (pharynx), and is the mother of all nerves to MOST internal organs within the body cavity: carries innervation to parasympathetic ganglia that innervate internal organs down to mid intestinal tract. Grows into and innervates derivates of pharyngeal arches 4 and 6 (no arch 5 develops).
  • 11 (spinal accessory): mixed to muscles and sensation of regions of neck & shoulder
  • 12 (hypoglossal): motor nerve to tongue muscles

    • Events in vertebrate nervous system development: the neural ectoderm forms the neural tube and neural crest cells:

  • The neural ectoderm is induced to form along the anterior-posterior axis of the embryo from the dorsal ectoderm. It remains epithelial in its organization (ie, a sheet of attached cells) and remains initially part of the ectoderm layer.
  • The neural ectoderm forms a thickened plate (called the neural plate) within the ectoderm layer. This plate then begins to rise up to form folds along each side of its long axis, and a depression or groove forms between the folds. The folds grow together and eventually fuse, beginning near the anterior end and proceeding posteriorly. The anterior end will complete fusion long before the posterior end of the neural plate has even fully formed.
  • The fusion of the neural folds produces the neural tube, which then separates from the ectoderm and sinks below it. It is now an epithelial tube, and will remain so as it begins to differentiate.
  • At the same time the neural tube is forming, the cells at the tip or crest of the folds separate from the neural folds on each side to form a population of cells called the neural crest. These cells undergo an epithelial-mesenchymal transformation as they enter the underlying mesoderm. The neural crest cells then must migrate to a number of target destinations before differentiating.
  • THE NEURAL CREST IS A VERTEBRATE "INVENTION".

    • Fates of neural tube and neural crest:

  • Origins of neurons and glia in both neural tube and neural crest:
  • Neurons and glia are derived from the same cells: in general, neurons are generated first and then glia.
  • The neural tube forms the brain and spinal cord. The cells of the neuroepithelial tube proliferate to form multiple layers of neurons and glia. Cells migrate from point of origin across thickness of tube to create layers (more layers in parts of the brain [cerebrum, cerebellum] than the spinal cord or brain stem regions).
  • Neural crest forms a number of neural and non-neural components. Neural crest cells migrate along specific paths through the mesoderm layer to several specific sites where they differentiate into several specific types of cells.
  • Neural tissues:

    Neurons: ganglia are small collections of neurons (outside the brain and spinal cord) formed from neural crest. This includes all sensory neurons and autonomic neurons**

  • The sequence of migration follows specific pathways: first cells to migrate follow the ventral pathway, initially between the neural tube and somite. These cells then migrate through the anterior half of each somite (in the trunk where somites exist).
  • Some cells stop inside the somite: they differentiate into sensory ganglia (neurons and glia). They develop surrounded by projections of the vertebrae that develop from the somite. (Refresh your memory about the 3 parts of the somite and their derivatives: medial sclerotome, lateral myotome and dermatome.)
  • Some cells continue ventrally out of somite through ventral mesoderm:
  • Form sympathetic autonomic ganglia cells (if originate from certain trunk levels)
  • Form parasympathetic autonomic ganglia cells (if originate from certain limited neck regions or most caudal sacral region, and if migrate further into mesoderm of specific target organs)
  • Use handouts and text to examine differences in origins of these two autonomic nerve types along the anterior-posterior axis of the neural tube/neural crest.
  • **Separate ectodermal placodes form bilaterally at specific locations along the cranial part of the neural tube. These placodes take the place of neural crest to form some sensory ganglia and other non-neural tissues associated with some organs of specific special senses that form in the head. These include parts or all of several cranial nerves:
  • 6 cranial nerves have sensory nerve components: 1, 5, 7-10 (see last page for 12 cranial nerves). Nerves 1 and 8 are totally placode derived; sensory components of nerve 5 are totally crest derived; sensory components of nerves 7, 9, and 10 are derived from both. (This information is for reference purposes only, in case you get confused by something you are reading in a paper; it is NOT necessary to try to learn this detail!)
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  • Non-neurons:
  • Later migrating cells follow a path just under the ectoderm (dorso-lateral path). These cells become melanocytes: pigment cells of the skin. They seem to follow this path because it's all that left open (?); cells that take this path have been shown (in some studies) to be able to differentiate into other cell types if transplanted to other neural crest pathways.
  • Remaining derivatives differ greatly in trunk and cranial crest: see Table 13.1 in Gilbert
  • Trunk crest: The other major non-neural trunk derivative is the adrenal medulla endocrine cell that secretes the hormone adrenaline; these cells only receive signals to differentiate on this path if they migrate out from specific trunk levels into the mesoderm forming the rest of the adrenal gland; otherwise their path would take them would become sympathetic ganglion neuronal cells. This alternate selection process has been an extensively studied process for almost a half century!
  • Cranial crest: forms a number of specialized connective tissue components that are formed in the rest of the body from mesoderm. This includes contributions that first migrate to the pharyngeal arches that then each form important structures of the face and neck. Each arch forms specific components. The aortic arch blood vessels that leave the heart and run through the arches also receive contributions from the cranial crest. A number of specific derivatives of crest include cells that form specific bones of the lower face, jaw, and neck, as well as cell components of the teeth, and specific tissue components of organs than form from arch components (thymus, thyroid). These components should be examined only to the point that you can understand the papers you are reading about cranial crest derivatives.

    • Mechanisms underlying neural differentiation:

    A. Original commitment to neuroectoderm fate: (Much of this comes from lecture guides from last year's lectures)

  • The organizer region sends diffusing signals that are required for commitment of the ectoderm to neural fate:
  • The first signals diffuse through the ectoderm from the organizer region in the epiblast (from primitive streak/Hensens' node in amniotes, from dorsal blastopore lip in amphibians) to commit the dorsal ectoderm to become neuroectoderm.
  • Ectoderm commitment to neural ectoderm actually begins with a negative signal: a signal from the organizer inhibits the effect of an epidermis-inducing signal that is released throughout the ectoderm: one form of BMP (bone-morphogenic protein)
  • only the future neural ectoderm cells express inhibitors of this signal, and thus they are the only ectodermal cells free to begin to express the cascade of neuralizing signals. Thus, the neural cell fate could be said to be the true "default" fate. This recent evidence contradicts generations of research that suggested that the "default" fate of ectoderm was to form surface ectoderm (or epidermis) unless it received specific signals to commit to neuroectoderm.
  • The organizer also provides the first signals that specify A-P patterning information: the neuroectoderm is generated in an A-P sequence during gastrulation, with the more anterior portions created first. As each region is created, it receive a different amount of signal from the organizer, which appears to specify increasingly posterior fates to the neuroectoderm as it is generated. (This signal cascade appears to involve differing amounts of retinoic acid.)
  • The notochord (a midline mesoderm structure) then sends signals to the overlying neural ectoderm signalling it to form neural folds and to fuse to form the neural tube and finally to separate from the overlying ectoderm. The notochord continues to send signals that later participate in dorsal-ventral patterning commitment within the neural tube (sending ventral inducing signals while the overlying ectoderm send dorsalizing signals; see below).
  • Cells must be exposed to signals during a critical period for receipt of those signals (ie, during the period in which they express receptors to receive those signals.)

    • B. Patterning commitment within the nervous system: Determines regional specificity in both neural tube and neural crest. This information dictates formation of correct types of neurons and glia.

  • Polarity instructions are issued along both the anterior-posterior and dorsal-ventral axes of the neurectoderm. They affect both the entire neural tube and neural crest
  • effects on crest are important in determining fate in the cranial region
  • The original axis inducing signals are translated into regional specificity by the expression of different complexes of homeotic genes. These genes' products are transcription factors that specify the final step in the identity of each region of the neural tube. They are expressed in overlapping patterns along both axes; each unique combination of these genes activates a unique combination of target "realizator" genes within that individual cell.
  • the range of homeobox genes are activated along the anterior-posterior axis in the midbrain/hindbrain region
  • members of the Pax (paired-box genes found first in insects as part of the zygotic segmentation gene family).
  • The anterior-posterior axis specification creates different regions within the brain and spinal cord
  • The dorsal-ventral axis specification involves inducing the ventral portion of the neural tube to form motor neurons, and the dorsal portion of the neural tube to differentiate into sensory-receiving neurons (neurons that receive sensory input).

    • C. Specific factors directing neural tube differentiation:

  • Factors directing the folding of the neural plate into the neural tube: the answer seems to reside mostly in changes in the cytoskeletal filament arrangements within the cells, causing the cells to change shape while remaining attached to each other.
  • Note that the neural crest is being committed and beginning to migrate while the neural tube is still folding and before separation from the overlying ectoderm. Thus, commitment of neural crest cannot be separated from events in neural tube formation and early differentiation.
  • Factors directing commitment & differentiation to particular types of neurons: the answer seems to reside in:
  • location of neurons within CNS regions because location determines complex of signals received
  • "birth" order of neurons: again, timing of formation exposes cells to different complexes of signals
  • Factors directing outgrowth of axons and dendrites to appropriate parts of the brain or spinal cord, or out into the body: this is the first step in the formation of proper connections between different parts of the brain and spinal cord.
  • A few pioneer axons in a category grow out initially using surface markers on their cells to match up with molecules in the matrix or on the surface of other cells (how they do this is itself an unsolved question). Once these pioneer axons have grown out, others can catch a ride along them in a process that can be compared to a "Hansel and Gretel" breadcrumb trail.
  • Target regions release attractant factors, but those can't operate over long distances, so cells require some kind of instructional directions along the pathway.
  • Factors directing formation of synapses between neurons and targets: this is required for survival of all neurons
  • Similar in both CNS and PNS -see neural crest below

    • D. Specific factors directing neural crest commitment and differentiation: the following is just an outline of the major areas to be covered in reading papers, since this is the focus of those papers:

  • Factors directing original commitment of neural crest cells
  • Original commitment to neural crest fate occurs while still in neuroectoderm as it folds
  • Commitment signals received are based on location: receives specific complex of signals that differ from more ventrally located regions of neuroectoderm
  • Factors directing migration of neural crest cells along particular pathways
  • Time of initial migration correlates with pathways followed (ventral versus dorso-lateral): the extent to which this correlation is a cause and effect relationship is still under investigation. See trunk crest below.
  • The mesoderm through which crest cells migrate lays down pathway instructions within its extracellular matrix that either promote or inhibit migration. Inhibition of migration is key.
  • Crest cells can only migrate through anterior half of each somite because of differences in extracellular materials in two halves of somites.
  • The crest cells have to place matrix-friendly receptors on their cells surfaces at the right time (critical period) or they won't recognize the matrix molecules and will be unable to use them. Thus, crest cells can be signaled to migrate while other mesodermal cells in the area will not because they will not recognize those same migratory signals.
  • Crest migration and commitment have been a great model system to study the issues of matrix directing migration because the crest cells have a different germ layer origin than the mesodermal mesenchyme and matrix through which they migrate, which has made experimental manipulation easy. Furthermore, the use of quail-chick chimeras was pioneered for this area of research: quail cells will be fully incorporated into chick embryos and take the place of chick cells. They can then be identified in several ways - initially researchers used the fact that each quail cell has two nucleoli within each nucleus, which chick cells have one; later antibodies that recognize quail cells were developed.
  • Factors directing final commitment fates: differs in trunk and cranial level crest cells:
  • The trunk level neural crest are generated with the capacity to form all types of trunk crest derivatives, and receive their final commitment instructions from their target destination region via signals that they release.
  • The timing of onset of migration is correlated with pathways followed: research seems to suggest that this is just a co-ordination, NOT an issue of later migrating cells being selectively committed to a more limited fate than earlier migrating cells.
  • Thus, the migratory matrix doesn't seem to play a role in commitment but is essential for getting the cells to the target location.
  • The cranial crest cells are generated with some level of commitment instructions based on their specific location of origin along the anterior-posterior axis. These polarity or axis level instructions come from expression of different combinations of Hox genes within both neural tube and neural crest cells that originate in the hindbrain region.
  • Once they are committed, they still must migrate along appropriate paths to target destinations. The cranial level crest migrates into the pharyngeal arches, and then begins to differentiate within each arch into different neural and non-neural components. Research has shown that cranial crest cells have already received specific instructions before they reach their target arches - if they are transplanted or otherwise migrate to the wrong arch, they still develop into the tissues that they should have become originally, not what is needed at their new home.
  • Types of commitment decisions made by crest cells:
  • Neural versus glial fates
  • Neural commitment: to either:
  • sensory
  • autonomic motor: either parasympathetic or sympathetic
  • these cells use different neurotransmitters: when originally committed, cells manufacture both; final fate determined after reaching final target environment
  • Factors directing formation of synapses between neurons and targets: this is required for survival of all neurons
  • Similar in both CNS and PNS
  • Appropriate targets are determined by matching of cellular surface labels
  • Targets release attractant factors that direct the final selection steps of axons once they reach the vicinity
  • If synapses are not formed because of a lack of appropriate target cells by a specific time, programmed cell death will begin (apoptosis).
  • The number of neurons generated in most areas is in excess of the number required for final innervation of all target cells. This insures that all target cells receive innervation; the excess cells die off.