EMBRYOLOGY

1. CNS System
Themba Hospital FCOG(SA) Part 1 Tutorials
By Dr N.E Manana

2. CNS
• The central nervous system (CNS) appears at the beginning of the
third week as a slipper shaped plate of thickened ectoderm, the
neural plate, in front of the primitive node
• Its lateral edges soon elevate to form the neural folds
• The neural folds continue to elevate, approach each other in the
midline, and fuse, forming the neural tube
• Fusion begins in the cervical region and proceeds in cephalic and
caudal directions (neuropores)

4. CNS
• The cephalic end of the neural tube shows three dilations, the
primary brain vesicles:
(1) The prosencephalon, or forebrain
(2) The mesencephalon, or midbrain
(3) The rhombencephalon, or hindbrain
• Simultaneously, it forms two flexures: (1) the cervical flexure at the
junction of the hindbrain and the spinal cord and (2) the cephalic
flexure in the midbrain region

5. CNS
• When the embryo is 5 weeks old, the prosencephalon consists of two
parts:
(1) The telencephalon, the primitive cerebral hemispheres;
(2) The diencephalon, characterized by outgrowth of the optic vesicles
• Rhombencephalic isthmus, separates the mesencephalon from the
rhombencephalon
• The rhombencephalon also consists of two parts:
• (1) The metencephalon (the pons and cerebellum)
• (2) The myelencephalon
• The boundary between these two portions is marked by the pontine
flexure
• The lumen of the spinal cord, is continuous with that of the brain vesicles,
forming ventricles

6. • 18.5

7. Spinal cord
• The wall of a recently closed neural tube consists of neuroepithelial cells.
• Once the neural tube closes, neuroepithelial cells begin to give rise to
another cell , primitive nerve cells, or neuroblasts
• They form the mantle layer, a zone around the neuroepithelial layer
• The mantle layer later forms the gray matter of the spinal cord.
• Myelination of nerve fibers in the outermost layer takes on a white
appearance and therefore is called the white matter of the spinal cord

9. Spinal cord
• As a result of continuous addition of neuroblasts to the mantle layer, each
side of the neural tube shows a ventral and a dorsal thickening
• The ventral thickenings, the basal plates, which contain ventral motor horn
cells, the dorsal thickenings, the alar plates, form the sensory areas
• A longitudinal groove, the sulcus limitans, marks the boundary between
the two
• Roof and floor plates, do not contain neuroblasts; they serve primarily as
pathways for nerve fibers crossing from one side to the other
• In addition to ventral motor horn and the dorsal sensory horn, a group of
neurons accumulates between the two areas and forms a small
intermediate horn (sympathetic portion of the autonomic nervous system)

10. Neuroblasts

13. Neural Crest Cells
• During elevation of the neural plate, a group of cells appears along each
edge (the crest) of the neural folds
• Crest cells migrate laterally and give rise to sensory ganglia (dorsal root
ganglia) of the spinal nerves and other cell types
• During further development, neuroblasts of the sensory ganglia form two
processes
• In the spinal cord, they either end in the dorsal horn or ascend through the
marginal layer to one of the higher brain centers
• The peripherally growing processes join fibers of the ventral motor roots
and thus participate in formation of the trunk of the spinal nerve.

15. Spinal Nerves
• Motor nerve fibers begin to appear in the fourth week, arising from
nerve cells in the basal plates (ventral horns) of the spinal cord
(ventral nerve root)
• Dorsal nerve roots form as collections of fibers originating from cells
in dorsal root ganglia (spinal ganglia)
• Central processes from these ganglia form bundles that grow into the
spinal cord opposite the dorsal horns.
• Distal processes join the ventral nerve roots to form a spinal nerve
• Almost immediately, spinal nerves divide into dorsal and ventral
primary rami.

16. Myelination
• Schwann cells myelinate the peripheral nerves with each cell
myelinating only a single axon
• These cells originate from neural crest, migrate peripherally, and wrap
themselves around axons, forming the neurilemma sheath
• Beginning at the fourth month of fetal life, many nerve fibers take on
a whitish appearance as a result of deposition of myelin, which is
formed by repeated coiling of the Schwann cell membrane around
the axon
• The myelin sheath surrounding nerve fibers in the spinal cord has a
completely different origin, the oligodendroglial cells

17. Positional Changes of the Cord

18. Rhombencephalon: Hindbrain
• Consists of the myelencephalon, the most caudal of the brain vesicles, and
the metencephalon which extends from the pontine flexure to the
rhombencephalic isthmus
• The myelencephalon is a brain vesicle that gives rise to the medulla
oblongata, It differs from the spinal cord in that its lateral walls are everted
• The basal plate, similar to that of the spinal cord, contains motor nuclei.
These nuclei are divided into three groups: (1) medial somatic efferent (2)
intermediate special visceral efferent (3) lateral general visceral efferent
• In the metencephalon and the mesencephalon, the column contains
neurons of the abducens (VI) , trochlear (IV), and oculomotor(III) nerves,
respectively.

19. Rhombencephalon: Hindbrain
• In the myelencephalon, the column is represented by neurons of the
accessory (XI), vagus (X), and glossopharyngeal (IX) nerves
• The general visceral efferent group contains motor neurons that
supply involuntary musculature of the respiratory tract, intestinal
tract, and heart
• The alar plate contains three groups of sensory relay nuclei, the
somatic afferent (general sensory) , the intermediate, or special
afferent, group receives impulses from taste buds and The medial, or
general visceral afferent, group receives interoceptive information
from the gastrointestinal tract and heart

22. Metencephalon
• The metencephalon, similar to the myelencephalon, is characterized
by basal and alar plates
• Two new components form (1) the cerebellum, and (2) the pons
• Each basal plate of the metencephalon contains three groups of
motor neurons: (1) the medial somatic efferent (2) the special visceral
efferent (3) the general visceral efferent group
• The alar plates of the metencephalon contain three groups of sensory
nuclei: (1) a lateral somatic afferent , which contains neurons of the
trigeminal nerve; (2) the special afferent (3) the general visceral
afferent group

23. Cerebellum
• The dorsolateral parts of the alar plates bend medially and form the
rhombic lips
• As a result of a further deepening of the pontine flexure, the rhombic lips
compress cephalocaudally and form the cerebellar plate
• In a 12-week embryo, this plate shows a small midline portion, the vermis,
and two lateral portions, the hemispheres
• A transverse fissure soon separates the nodule from the vermis and the
lateral flocculus from the hemispheres
• During further development, a number of cells formed by the
neuroepithelium migrate to the surface of the cerebellum to form the
external granular layer
• In the sixth month of development, the external granular layer gives rise to
various cell types.

26. • 18.21

27. Mesencephalon: Midbrain
• Each basal plate contains two groups of motor nuclei: (1) a medial
somatic efferent , represented by the oculomotor and trochlear
nerves (2) a small general visceral efferent group,
• The marginal layer of each basal plate enlarges and forms the crus
cerebri.
• These crura serve as pathways for nerve fibers descending from the
cerebral cortex to lower centers in the pons and spinal cord.
• With further development, a transverse groove divides each elevation
into an anterior (superior) and a posterior (inferior) colliculus

28. • 18.23

29. Prosencephalon: Forebrain
• The prosencephalon consists of the telencephalon, which forms the
cerebral hemispheres and the diencephalon, which forms the optic
cup and stalk, pituitary, thalamus, hypothalamus, and epiphysis
• The diencephalon, which develops from the median portion of the
prosencephalon
• The roof plate of the diencephalon consists of a single layer of
ependymal cells covered by vascular mesenchyme. Together, these
layers give rise to the choroid plexus of the third ventricle
• The most caudal part of the roof plate develops into the pineal body,
or epiphysis.

30. Prosencephalon: Forebrain
• The alar plates form the lateral walls of the diencephalon.
• A groove, the hypothalamic sulcus, divides the plate into a dorsal and
a ventral region, the thalamus and hypothalamus, respectively
• The hypothalamus, forming the lower portion of the alar plate
• Differentiates into a number of nuclear areas that regulate the
visceral functions, including sleep, digestion, body temperature, and
emotional behaviour.

31. • 18.24

32. Hypophysis or Pituitary Gland
• The hypophysis, or pituitary gland, develops from two completely
different parts:
(1) an ectodermal out pocketing of the stomodeum ,Rathke’s pouch
and
(2) Infundibulum
• By the end of the 2nd month the Rathke’s pouch loses its connection
with the oral cavity and is then in close contact with the infundibulum
• Cells of the anterior wall of the Rathke’s pouch proliferate and form
the Adenohypophysis
• The infundibulum gives rise to the stalk and neurohypophysis

33. • 18.26

34. Telencephalon
• Consist of two lateral outpocketings, the cerebral hemispheres and
the lamina terminals
• The cavities of the hemispheres the lateral ventricles, communicate
with the diencephalon via the foramina of Monro
• The hemispheres arise at the beginning of the fifth week
• The corpus striatum starts to appear
• The Hemisphere wall consist of a single layer of ependymal cells
covered by vascular mesenchyme, and the form the choroid plexus
• In the wall of the choroid fissure the hemisphere forms the
hippocampus

35. Telencephalon
• With further expansion, the hemispheres cover the lateral aspect of the
diencephalon, mesencephalon, and cephalic portion of the metencephalon
• The corpus striatum likewise expands posteriorly and is divided into two
parts: (1) a dorsomedial portion, the caudate nucleus, and (2) a
ventrolateral portion, the lentiform nucleus
• At the same time, the medial wall of the hemisphere and the lateral wall of
the diencephalon fuse, and the caudate nucleus and thalamus come into
close contact
• Continuous growth of the cerebral hemispheres in anterior, dorsal, and
inferior directions results in the formation of frontal, temporal, and
occipital lobes, respectively

36. • 18.27

37. Cortex Development
• The cerebral cortex develops from the pallium , which has two
regions: (1) the paleopallium, (2) the neopallium
• Neuroblasts migrate to a subpial position and then differentiate into
fully mature neurons.
• At birth, the cortex has a stratified appearance due to differentiation
of the cells in layers
• The motor cortex contains a large number of pyramidal cells, and the
sensory areas are characterized by granular cells.
• Neural crest cells and ectoderm of the frontonasal prominence to
form the olfactory placodes

38. • 18.29

39. Commissures
• In the adult, a number of fiber bundles, connect the right and left
halves of the hemispheres.
• The most important fiber bundles make use of the lamina terminalis
• The first of the crossing bundles to appear is the anterior commissure
• The second commissure to appear is the hippocampal commissure, or
fornix commissure.
• The most important commissure is the corpus callosum. It appears by
the 10th week of development

40. • 18.30

41. Cerebrospinal Fluid
• Secreted by the choroid plexuses in the brain ventricles
• These plexuses are modifications of the ependymal layer and produce
approximately 400 to 500 mL of CSF per day
• The fluid circulates through the brain ventricles leaving the lateral
ventricles
• Some CSF enters the spinal canal and some exits the fourth ventricle
through its median and lateral apertures to enter the subarachnoid
space that surrounds the CNS
• CSF is absorbed into the venous system from the subarachnoid space
through arachnoid granulations

42. Cranial nerves
• By the 4th week the nuclei for all the 12 cranial nerves are present
• All except the olfactory and the optic nerves arise from the brain stem, and
of these ,only the oculomotor arises outside the region of the hindbrain
• Neuroepithelium proliferation centers in the hindbrain establish 8
segments the rhombomeres, which give rise to IV, V, VI, VII, IX, XI and XII
• Motor neurons for cranial nuclei are within the brainstem, while sensory
ganglia are outsideof the brain
• Not all cranial nerves contain both motor and sensory fibers
• Sensory ganglia originate from the ectodermal placodes and neural crest
cells

43. • 18.40

44. • Table 18.2

47. Autonomic nervous system
• ANS can be divided into two parts: a sympathetic portion in the
thoracolumbar region and a parasympathetic portion in the cranial and
sacral regions
• In the fifth week, cells originating in the neural crest of the thoracic region
migrate on each side of the spinal cord toward the region immediately
behind the dorsal aorta
• They form a bilateral chain of segmentally arranged sympathetic ganglia
• Some sympathetic neuroblasts migrate in front of the aorta to form
preaortic ganglia, such as the celiac and mesenteric ganglia
• Sympathetic organ plexuses

49. • 18.42

50. Suprarenal Gland
• Develops from two components: (1) a mesodermal portion, which forms
the cortex, and (2) an ectodermal portion, which forms the medulla
• During the fifth week of development, mesothelial cells between the root
of the mesentery and the developing gonad begin to proliferate and
penetrate the underlying mesenchyme
• Here, they differentiate into large acidophilic organs, which form the
primitive cortex
• Cells, smaller than those of the first wave, later form the definitive cortex
• Neural crest cells invade its medial aspect, where they are arranged in
cords and clusters

51. • 18.43

52. Parasympathetic Nervous System
• Neurons in the brainstem and the sacral region (S2–S4) of the spinal
cord give rise to preganglionic parasympathetic fibers
• Fibers from nuclei in the brainstem travel via the: the oculomotor (III),
Facial (VII), Glossopharyngeal (IX),and vagus (X) nerves.
• Postganglionic fibers arise from neurons (ganglia) derived from neural
crest cells and pass to the structures they innervate

53. Thank you