Embryology Flashcards
Which one of the following correctly describes the order of embryological stages of CNS development?
a. Blastogenesis, gastrulation, dorsal induction, ventral induction, neural proliferation, neuronalmigration, and axonal myelination
b. Dorsal induction, ventral induction, gastrulation, neural proliferation, neuronal
migration, and axonal myelination
c. Gastrulation ventral induction, dorsal
induction, neural proliferation, neuronal
migration, and axonal myelination
d. Neural proliferation, gastrulation dorsal
induction, ventral induction, neuronal
migration, and axonal myelination
e. Ventral induction, gastrulation, dorsal
induction, neural proliferation, axonal
myelination, and neuronal migration
a. Blastogenesis, gastrulation, dorsal induction, ventral induction, neural proliferation, neuronalmigration, and axonal myelination
b. Dorsal induction, ventral induction, gastrulation, neural proliferation, neuronal
migration, and axonal myelination
c. Gastrulation ventral induction, dorsal
induction, neural proliferation, neuronal
migration, and axonal myelination
d. Neural proliferation, gastrulation dorsal
induction, ventral induction, neuronal
migration, and axonal myelination
e. Ventral induction, gastrulation, dorsal
induction, neural proliferation, axonal
myelination, and neuronal migration
Late developing structures, including the cortex, hippocampus and the cerebellum set the stage for differential periods of vulnerability to insults in
a regionally specific manner.
Which one of the following statements regarding gastrulation is most accurate?
a. It is the process by which the bilaminar disc is converted into a trilaminar disc
b. It can result in lipomyelomeningocele if
disturbed
c. It is not dependent on bone morphogenetic protein expression
d. It starts with closure of the cranial
neuropore
e. It occurs from embryonic days 10-12
a. It is the process by which the bilaminar disc is converted into a trilaminar disc
b. It can result in lipomyelomeningocele if
disturbed
c. It is not dependent on bone morphogenetic protein expression
d. It starts with closure of the cranial
neuropore
e. It occurs from embryonic days 10-12
Gastrulation occurs between D14 and D17, and is
the process by which the bilaminar disc (consisting
of epiblast facing the amniotic cavity and the hypoblast facing the yolk sac) becomes a trilaminar disc
with formation of an intervening third layer, the
mesoblast (future mesoderm). On day 14 or 15 a
strip of thickened epiblast/ectoderm (primitive
streak) appears caudally in the midline of the dorsal
surface of the embryo to define the craniocaudal
axis. The cranial end of the primitive streak forms
the primitive (Hendersen’s) node, and shows a central depression called the primitive pit. Ectodermal
cells start migrating towards the primitive streak,
pass inward at the primitive pit to the interface of
ectoderm and endoderm, and then migrate laterally
to form themesoderm. The two paired notochordal
anlagen (primordia) then fuse in the midline to forma single notochordal process (“notochordal integration”; D16). The primitive node defines the craniocaudal axis, the right andleft sides and the dorsal and
ventral surfaces of the embryo. Prospective notochordal cells in the wrong craniocaudal position
undergo apoptosis maintaining segmental notochordal formation. Multiple signaling molecules,
such as bone morphogenetic protein (BMP), fibroblast growth factor (FGF), andWnt are essential for
gastrulation to occur. BMP is very important in
establishing the rostrocaudal polarity. In addition,
multiple factors and genes areimplicatedin patterning the primitive body axis (e.g., brachyury, sonic
hedgehog (SHH), and HNF-beta genes). Defects
in gastrulation (integration or segmental formation)
affect development and differentiation of all three
primary cell layers and cause abnormalities from
the occiput downwards, e.g., split cord malformation (diastematomyelia and diplomyelia), neurenteric, dermoid, and epidermoid cysts, anterior and
posterior spina bifida, intestinal malformation,
duplication and fistula formation, and anterior
meningocele
Which one of the following statements about
primary neurulation is most accurate?
a. Anterior neuropore closure approximately occurs on D19
b. Disjunction results in formation of the spinal canal below the posterior neuropore
c. Fusion of the neural folds starts at the anterior neuropore and proceeds caudally in a zip-like fashion until it reaches the
posterior neuropore
d. Notochord induces the overlying ectoderm to differentiate into a flat area of specialized neuroectoderm called the neural plate
e. SHH/morphogen secretion on D14 causes the neural plate to form median hinge points and start invaginating along its central axis to form a neural groove (with neural folds on either side)
a. Anterior neuropore closure approximately occurs on D19
b. Disjunction results in formation of the spinal canal below the posterior neuropore
c. Fusion of the neural folds starts at the
anterior neuropore and proceeds caudally
in a zip-like fashion until it reaches the
posterior neuropore
d. Notochord induces the overlying ectoderm to differentiate into a flat area of specialized neuroectoderm called the
neural plate
Dorsal induction (3rd-4th weeks; D17-D28)
includes primary neurulation, secondary neurulation and formation of the “true” notochord. Primary neurulation involves separation of
neuroectoderm in the neural plate from cutaneous ectoderm to form the neural tube (brain and spinal cord) as far caudal as S2/3. The steps
are summarized below:
- Neural induction and formation of the neural plate: the notochord induces the overlying ectoderm to differentiate into a fla of specialized neuroectoderm (neural
plate). Relative to Hensen’s node, the neural plate expands cranially and narrows/
elongates the parts on either side of the primary streak—these areas will form the
brain and spinal cord, respectively. This
process is regulated by multiple genes,
including brachyury and Wnt.
* SHH/morphogen secretion on D18 causes
the neural plate to form median hinge
points and start invaginating along its central axis to form a neural groove (with neural folds on either side).
* These folds progressively increase in size
and flex to approach each other, until they
eventually fuse in the midline to form the
neural tube (regulated by PAX3 genes).
Fusion occurs in a zip-like fashion, probably at multiple sites but first at the level of
the 4th somite (future craniocervical
junction).
* The cranial end of the neural tube (anterior
neuropore) closes first at the site of the lamina terminalis on D24-26, followed by the
posterior neuropore on D26-28 to complete primary neurulation. Note that the
posterior neural pore is not located at the
caudal tip of the neural tube. The caudal
part of the spinal cord and the lowest
sacrum portion is formed from the solid
core of neuroepithelium (tail bud) during
secondary neurulation.
* Ectodermal cells progressively disconnecting from the lateral walls of the neural
folds during formation of the neural tube
differentiate into the neural crest cells (form
branchial arch derivatives, dorsal roots/
dorsal root ganglia, autonomic ganglia and
adrenergic cells).
* Disjunction: Immediately after neural tube
closure it becomes separated from the overlying superficial ectoderm (forms the skin)
by dorsally migrating mesenchyme (forms
meninges, neural arches of the vertebrae
and paraspinal muscles).
Which one of the following statements about secondary neurulation and retrogressive differentiation is most accurate?
a. Important for the formation of the conus medullaris but not the filum terminale
b. Involves canalization of a caudal mensenchymal cell mass
c. Is completed by days 24-26 of embryonic development
d. Responsible for the formation of thoracic, lumbar, sacral, and coccygeal neural tube
e. Retrogressive differentiation is a mitotic process
a. Important for the formation of the conus
medullaris but not the filum terminale
b. Involves canalization of a caudal mensenchymal cell mass
c. Is completed by days 24-26 of embryonic
development
d. Responsible for the formation of thoracic,
lumbar, sacral, and coccygeal neural tube
e. Retrogressive differentiation is a mitotic
process
The location of the caudal end of the neural plate
(posterior neuropore) is approximately at the S3
level. The remaining caudal sacral and coccygeal
portions of the neural tube, including the conus
medullaris and filum terminale are formed by secondary neurulation and retrogressive differentiation (days 28-48). During secondary neurulation, a secondary neural tube is formed caudad to the posterior neuropore. A caudal cell mass of undifferentiated, totipotential cells initially appears as a result of fusion of neural ectoderm with the lower portion of the notochord. Multiple small vacuoles then appear in the caudal cell mass and progressively coalesce to form a central canal (canalization), which will merge with the canal formed during primary neurulation. Retrogressive differentiation is an apoptotic process in which a combination of regression, degeneration and further differentiation of the caudal cell mass into the tip of the conus medullaris, ventriculus terminalis, and filum terminale.
Which one of the following statements about
ventral induction is most accurate?
a. It includes development of the primary brain fissure
b. It includes development of the secondary brain vesicles and brain flexures
c. It includes formation of the neural plate
d. It includes formation of the notochord
e. It includes primary neurulation
a. It includes development of the primary
brain fissure
b. It includes development of the secondary brain vesicles and brain flexures
c. It includes formation of the neural plate
d. It includes formation of the notochord
e. It includes primary neurulation
By the end of dorsal induction/primary neurulation
the neural tube is closed and three primary brain
vesicles (prosencephalon, mesencephalon, and
rhombencephalon) are present. During ventral
induction (5th-10th weeks of gestation) the primary brain vesicles differentiate into five secondary
brain vesicles by day 35 (telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon) which then form forebrain, midbrain,
and hindbrain structures. Between the 4th and 8th
weeks, the brain tube folds sharply at three locations. The first of these folds to develop is the
cephalic flexure (between diencephalon and mesencephalon), followed by the cervical flexure
between myelencephalon and spinal cord—both
flexures are ventral and produce an inverted U
shape. The last flexure is dorsally located between
metencephalon and myelencephalon (pontine flexure) and changes the shape to an M. By the 8th
week, deepening of the pontine flexure has folded
the metencephalon (including the developing cerebellum) back onto themyelencephalon. Any insult
during this phase affects the development of brain
vesicles and the formation of the facial skeleton.
Ocular and nasal anomalies are frequently associated with forebrain malformation because the optic
placode and forebrain develop at the same time,
with subsequent formation of the olfactory vesicle
1 week later. The commonly seen forebrain ventral
induction malformations are (1) holoprosencephaly, (2) atelencephaly, (3) olfactory aplasia, (4)
agenesis of the corpus callosum, and (5) agenesis
of the septum pellucidum (septo-optic dysplasia,
cavum vergae and pellucidum). Hindbrain anomalies include vermian dysgenesis (e.g., DandyWalker spectrum).
The disencephalon does not give rise to which one of the following?
a. 3rd ventricle
b. Mamillary bodies
c. Optic vesicle
d. Posterior pituitary
e. Superior colliculus
a. 3rd ventricle
b. Mamillary bodies
c. Optic vesicle
d. Posterior pituitary
e. Superior colliculus
The prosencephalon is the most rostral of the
three brain vesicles and gives rise to a caudal diencephalon and a rostral telencephalon. A pair of diverticula, known as the telencephalic vesicles, appear dorsally and rostrally, which form the
cerebral hemispheres as the central cavities form
the lateral ventricles. The posterior part of the
prosencephalon becomes the diencephalon,
which later develops into the thalami, hypothalamus, epithalamus, optic cups, and neurohypophysis. The central cavity in the region of
diencephalon forms the third ventricle. Simultaneously, two lateral outpouchings (optic vesicles)
grow from the telencephalon on each side. These
optic vesicles form the retina and optic nerve.
Cells of the diencephalon and telencephalon
originate from the germinal matrix lining of the
third and lateral ventricles, respectively. The telencephalon grows rapidly and covers the developing diencephalon, midbrain and hindbrain,
because the outer regions grow more rapidly than
the floor. This growth of the cerebral hemispheres within the developing cranial cavity gives
the characteristic “C” shape to the developing lateral ventricles. The mesenchymal tissue trapped
in the midline between the developing hemispheres develops into the cerebral falx.
Mesencephalon does NOT give rise to which one of the following?
a. Cerebral aqueduct
b. Edinger-Westphal nucleus
c. Pineal body
d. Red nucleus
e. Substantia nigra
a. Cerebral aqueduct
b. Edinger-Westphal nucleus
c. Pineal body
d. Red nucleus
e. Substantia nigra
The mesencephalon undergoes the least amount
of change during the expansion from three primary to five secondary brain vesicles, and forms
the midbrain. The central cavity decreases in size
to form the aqueduct of Sylvius. The neuroblasts
from the dorsal alar plates migrate and appear as
two swellings that form the superior and inferior
colliculi (tectal plate). Some cells of the alar plate
also migrate ventrally to form the red nucleus and
substantia nigra. The basal plate of the mesencephalon forms the midbrain tegmentum (which
include the somatic and general visceral efferent
columns, and crus cerebri).
Which one of the following statements about
the rhombencephalon is most accurate?
a. It contains the cerebral aqueduct at
its center
b. It gives rise to diencephalon and myelencephalon secondary brain vesicles
c. It gives rise to the inferior colliculi and pons
d. It is separated from the mesencephalon by
the isthmus rhombencephalii
e. Pontine flexure indents the rhombencephalon ventrally
a. It contains the cerebral aqueduct at
its center
b. It gives rise to diencephalon and myelencephalon secondary brain vesicles
c. It gives rise to the inferior colliculi and pons
d. It is separated from the mesencephalon by
the isthmus rhombencephalii
e. Pontine flexure indents the rhombencephalon ventrally
With rapid growth of the embryonic brain, the neural tube bends onitselfin a zigzag fashion.Two flexures developed initially are the cephalic and the
cervical flexures, and these are concave ventrally
so the neural tube forms a wide upside-down
U-shaped configuration. The mescencephalon
and rhombencephalon are separated by a constriction (isthmus rhombencephalii).Around 6 weeks of
gestation, the pontine flexure develops dorsally
between the two rhombencephalic vesicles—
metencephalon (future pons and cerebellum) and
myelencephalon (future medulla). This flexure is
concave dorsally, thereby converting the shape of
the developing neural tube into a broad “M” shape.
Hindbrain structures form as follows:
* Pons—develops from a thickening in the
floor and lateral walls of the metencephalon.
* Medulla oblongata—develops from the
thickened floor and lateral walls of the myelencephalon which is continuous inferiorly
with the spinal cord.
* Cerebellum—alar plates of the and rhombic
lips of the metencephalon form the
cerebellum
Which one of the following statements about
cerebellar development is most accurate?
a. Brainstem input to the cerebellum is via parallel and climbing fibers
b. Commences at week 15
c. Golgi cells come to reside in the
molecular layer
d. Granule cells develop axons called Mossy fibers
e. Granule cells migrate inward past Purkinje cells with the help of Bergmann glia
a. Brainstem input to the cerebellum is via parallel and climbing fibers
b. Commences at week 15
c. Golgi cells come to reside in the
molecular layer
d. Granule cells develop axons called Mossy fibers
e. Granule cells migrate inward past Purkinje cells with the help of Bergmann glia
Development of the pontine flexure result in:
* The cranial and the caudal ends of the 4th
ventricle approximate together dorsally.
* The rhombencephalic roof plate is folded
inward towards the cavity of the 4th
ventricle.
* The alar columns are splayed laterally
because of the bending of the pons and
eventually lie dorsolateral to the basal
columns.
Therefore, the roof plate of the developing 4th
ventricle remains thin, is wide at its fold/waist and
tapers superiorly and inferiorly (diamond shaped).
Mesenchyme inserts itself into the roof fold and
forms the plica choroidalis (choroid plexus precursor)which divides the roof of the 4th ventricleinto a superior anterior membranous area (AMA) and inferior posterior membranous area (PMA). The alar laminae along the lateral margins of the AMA become thickened to form two rhombic lips, which enlarge to approach each other and fuse in the midline dorsally (covering the rostral half of the 4th ventricle and overlapping the pons and the medulla). As the rhombic lips grow to form the cerebellar hemispheres and midline vermis, the AMA
regresses by incorporation into the developing
choroid plexus. Growth and backward extension
of the cerebellum pushes the choroid plexus inferiorly, whereas the PMA greatly diminishes in the relative size compared with the overgrowing cerebellum. Subsequently there is development of a marked caudal protrusion of the 4th ventricle, causing the PMA to expand as the finger of a glove. This Blake’s pouch consists ofventricularependyma surrounded by condensation of the mesenchymal tissues and is initially a closed cavity that does not
communicate with the surrounding subarachnoid
space of the cisterna magna. The network between the vermis and the Blake’s pouch progressively becomes condensed, whereas the other portions about the evagination become rarified resulting in permeabilization of the Blake’s pouch to form the foramen of Magendie. The foramina of Luschka also probably appear late into the 4th month of gestation.From superior toinferior, the residual AMA, choroid plexus and residual PMA (i.e., residual
rhombencephalic roof plate) form the definitive
tela choroidea of the 4th ventricle. Folding, transverse fissure formation and foliation result in anterior lobe (cerebellar vermis and hemisphere above primary fissure), posterior lobe (vermis and hemispheres below primary fissure) and a flocculonodular lobe. Development of the cerebellar cortex and deep nuclei (dentate, globose, emboliform, and fastigial) occurs as follows:
* Week 8—Metencephalon consists of typical
ventricular, mantle and marginal layers and
rhombic lips have started to form the cerebellum. The ventricular layer produces four
types of neurons forming the mantle layer
which will subsequently migrate to the cortex: Purkinje cells, Golgi cells, basket cells,
and stellate cells, as well as their associated
glia (astrocytes including Bergmann glia,
and oligodendrocytes).
* Week 12—Two additional layers form: an
external germinal/granular layer derived
from the rhombic lips, from which granular
cells migrate inwards to form a new internal
germinal layer between the ventricular and
marginal layers (cells of the mantle layer
have now dispersed into the marginal layer
where they will form a distinct cortical pattern). External germinal layer also produces
primitive nuclear neurons which also
migrate inwards to form the deep cerebellar
nuclei. Migration of granule cells takes place
along Bergman (radial) glia. Purkinje cells
migrate toward the cortex, it reels out an
axon that maintains synaptic contact with
neurons in the developing deep cerebellar
nuclei. These axons will constitute the only
efferents of the mature cerebellar cortex.
* Week 15—From superficial to deep the cerebellum consists of: external granular layer
(persists until approximately 15 months postnatally), Purkinje cell layer, molecular layer
(stellate, basket cells), and granular layer
(Golgi cells; granule cells and their parallel
fibers),whitematter (Mossy fibers from brainstem nulcei, climbing fibers from inferior
olivary nucleus) and deep cerebellar nuclei.
Which one of the following is important in
dorsoventral patterning of the neural tube?
a. BF-1
b. BMP-4 and BMP-7
c. EMX1 and EMX2
d. FGF-8
e. HOX
f. SHH
a. BF-1
b. BMP-4 and BMP-7
c. EMX1 and EMX2
d. FGF-8
e. HOX
f. SHH
Some of the molecular signals patterning brain and spinal cord development include homeoboxcontaining genes (e.g., HOX, PAX, OTX, EMX). A homeobox is a 180 bp DNA sequence found within genes involved in anatomical development (morphogenesis) and are important in establishing body axes and cellular differentiation:
Homeobox-containing genes coding transcription factors:
* PAX3 and PAX7 are expressed by the entire
neural plate.
* Homeotic (HOX) genes control the body
plan of an embryo along the craniocaudal
axis, e.g., the rhombencephalon is divided
into eight segments called rhombomeres,
which are regulated by an overlapping
HOX gene expression.
* Other homeobox genes are important in
establishment of forebrain and midbrain
boundaries and are expressed even before
the formation of neural fold (e.g., Lim1
and OTX2). Later, once the neural folds
and pharyngeal arches appear, additional
homeobox genes, including OTX1,
EMX1, and EMX2, are expressed in an
overlapping pattern to further specify the
identity of these brain regions.
Other factors:
* Sonic hedgehog (SHH) is a protein
secreted by the notochord and floor plate
which downregulates the expression of
PAX3 and PAX7 in the midline and ventral
half of the neural tube (dorsoventral
patterning).
* Wnt signaling pathway is active in the midline and dorsal half of the neural tube (dorsoventral patterning) and axon guidance.
* Bone morphogenetic protein (BMP-4 and
BMP-7) are growth factors important in
dorsolateral patterning. They are secreted
by the adjacent non-neural ectoderm, maintain and upregulate PAX3 and PAX7 expression in the dorsal half of the neural tube
which stimulates alar plate formation.
* Fibroblast growth factor-8 is secreted by
the anterior neural ridge (an organizing
center in the neural plate) which induces
expression of the brain factor-1 (BF-1) transcription factor that regulates the development of the telencephalon.
Which one of the following best describes
cells forming the mantle layer in the developing neural tube
a. Ependymal cells
b. Glioblasts
c. Neuroblasts
d. Postmitotic young neurons
e. Radial cells
d. Postmitotic young neurons
Except in the telencephalon, neurogenesis establishes the following architecture of the neural tube (from central to peripheral):
1. Central canal.
2. Ventricular layer—neuroepithelial (radial)
cells which give rise to all other layers.
3. Mantle layer—contains cell bodies of postmitotic young neurons which have
migrated laterally from the ventricular layer
and will form eventual gray matter.
4. Marginal layer—outermost layer contains
the axons of neurons in the mantle layer,
and will form eventual white matter (folding of the cerebral hemispheres will alter its position to subcortical).
After production of neurons is waning in the ventricular layer, this layer begins to produce a new cell type, the glioblast which differentiate into glia of the CNS—astrocytes and oligodendrocytes.
Glia provide metabolic and structural support
to the neurons of the central nervous system.
The last cells produced by the ventricular layer
are the ependymal cells; these line the brain ventricles and the central canal of the spinal cord and produce CSF.
Which one of the following statements about
cerebral cortex formation is most accurate?
a. Cortical layers are laid down from most
superficial to deep
b. Germinal matrix zone is superficial to the
ventricular zone
c. Intermediate zone contains axons of cortical pyramidal neurons
d. Migration of cortical pyramidal neurons
occurs tangentially
e. The neocortex usually has four layers in
the adult
c. Intermediate zone contains axons of cortical pyramidal neurons
The cerebral cortex is made up of several cell
layers (or laminae) that vary in number from three in the phylogenetically oldest parts to six in the
dominant neocortex. Compared to the rest of
the CNS, cerebral cortex has an “inside-out”
arrangement of gray and white matter.
* Proliferating cells of the ventricular layer
undergo a series of regulated divisions to
produce waves of neurons that migrate
peripherally (on radial cell processes spanning the full thickness of the cortex) and
establish the neuronal layers of the cortex.
The first wave of neurons form a cortical
layer is termed the preplate.
* Axons extend from preplate cells back
towards the ventricular zone producing an
intermediate zone (white matter).
* As neurogenesis proceeds, new neurons
are increasingly formed in an accessory germinative zone lying deep to the ventricular
zone, called the subventricular (germinal
matrix) zone.
* Multiple cortical layers are laid down in a
sequence from deep to superficial, that is, the
neurons of each wave migrate through the
preceding layers to establish a more superficial layer. This is thought to be mediated by
reelin (glycoprotein) secreted by transient
Cajal-Retzius cells which migrate to the
marginal layer (lamina I) tangentially after
being born in a dorsal midline telencephalic
structure.As such, after normal cortical histogenesis has been achieved in principle, only
lamina II-VI persist in the adult.
* As the production of neurons tapers off, the
ventricular layer gives rise to various kinds
of glia and then to the ependyma.
More numerous but smaller than the pyramidal
neurons are theinhibitoryinterneurons—the granule cells, which originate in the ganglionic eminences of the ventral telencephalon and migrate dorsally into the cortex via a tangential route.
Which one of the following is the first to form in the developing brain?
a. Anterior commissure
b. Genu of corpus callosum
c. Hippocampal commissure
d. Posterior commissure
e. Splenium of corpus callosum
a. Anterior commissure
The commissures that connect the right and left
cerebral hemispheres form from a thickening at
the cranial end of the telencephalon, which represents the zone of final neuropore closure. This area can be divided into a dorsal commissural plate and a ventral lamina terminalis:
* 7th week—anterior commissure forms in
the commissural plate and interconnects
the olfactory bulbs and olfactory centers
of the two hemispheres.
* 9th week—hippocampal (forniceal) commissure forms between the right and left
hippocampi.
* 9th week (late)—corpus callosum linking
together the right and left neocortices along
their entire length. The most anterior part
(the genu) of the corpus callosum appears
first, and its posterior extension (the splenium) forms later in fetal life.
Which one of the following statements about
the developing spinal cord are most accurate?
a. Alar columns form the intermediolateral
horn
b. Alar columns form the ventral horns
c. Dorsally the floor plate marks where the
paired basal columns meet
d. Laterally, the alar and basal plates abut at a groove called the sulcus limitans
e. Ventrally the roof plate marks where the
paired alar columns meet
d. Laterally, the alar and basal plates abut at a groove called the sulcus limitans
Cell bodies in subependymal zone: in the spinal
cord cells remain near the subependymal zone
to form the central gray matter of the spinal cord
(mantle layer) and extend axonal processes toward the periphery of the spinal cord.
Axons (white matter) surrounds gray: the surrounding spinal cord white matter is comprised of local and ascending white matter tracts generated in the spinal cord gray matter and descending tracts from supranuclear sources.
Starting at the end of the 4th week, the neurons
in the mantle layer of the spinal cord become
organized into four plates that run the length of
the cord: a pair of dorsal (alar) columns and a pair
of ventral (basal) columns. Laterally, the two
plates abut at a groove called the sulcus limitans,
dorsally the roof plate and ventrally the floor plate
(both non-neurogenic). The cells of the ventral
columns become the somatic motoneurons of
the spinal cord and innervate somatic motor structures such as the voluntary (striated) muscles of
the body wall and extremities. The cells of the
dorsal columns develop into association neurons
receiving synapses from afferent (incoming) fibers
from the sensory neurons of the dorsal root ganglia, and either synapsing with ipsilateral/contralateral motoneurons to form a reflex arc or it may
ascend to the brain. The outgoing (efferent) motor
neuron fibers exit via the ventral roots. In most
regions of the cord—at all 12 thoracic levels, at
lumbar levels L1 and L2, and at sacral levels S2-
S4—the neurons in more dorsal regions of the
ventral columns segregate to form intermediolateral cell columns. The thoracic and lumba intermediolateral cell columns contain the visceral
motoneurons that constitute the central autonomic motoneurons of the sympathetic division,
whereas the intermediolateral cell columns in
the sacral region contain the visceral motoneurons
that constitute the central autonomic motoneurons of the parasympathetic division.
Embryological terms:
a. Ectoderm
b. Endoderm
c. Induction
d. Mesenchyme
e. Mesoderm
f. Neural crest
g. Notochord
h. Paraxial mesoderm
i. Primitive streak
j. Sclerotome
k. Somite
For each of the following descriptions, select the
most appropriate answers from the list above.
Each answer may be used once, more than once
or not at all.
1. Population of cells arising from the lateral
lips of the neural plate that detach during
formation of the neural tube and migrate
to form a variety of cell types/structures.
2. The first morphological sign of gastrulation.
2 EMBRYOLOGY 23
3. The process in which one embryonic region interacts with a second embryonic region, thereby influencing the behavior or differentiation of the second region.
1—f, Neural crest; 2—i, Primitive streak;
3—c, Induction
