lec 3 - nervous system development Flashcards
Pre-embryonic period
Begins with fertilization and ends at implantation
weeks 1 & 2
how embryo develops
before nervous system
Embryonic/Critical period
Characterized by cell differentiation and organogenesis
weeks 3-8
1st trimester = most sensitive - susceptible to environment toxins
Fetal period
Characterized by growth of fetus
weeks 9+
Pre-Embryonic Period: Week 1
Fertilization
Cleavage
Blastocyst formation
Implantation
Fertilization:
sperm penetrates a secondary oocyte/mature egg
A zygote is formed when sperm fertilizes an egg
Cleavage:
stage characterized by a series of cell divisions
Blastocyst formation:
a fluid-filled cavity is formed
organizes the cells into an inner (embryoblast) and outer (trophoblast) cell mass
Implantation:
the blastocyst implants into the uterine wall
Pre-Embryonic Period: Week 2
During week 2, a bilaminar embryonic disc forms
The embryoblast differentiates into 2 layers:
> Epiblast
> Hypoblast
Epiblast:
dorsal row of cells that will develop into the entire embryo and all adult structures
A fluid-filled cavity forms in the epiblast cells, which is called the amniotic cavity
Hypoblast:
ventral row of cells that form supporting structures of the embryo
Hypoblast cells migrate down to line the inner surface of the cytotrophoblast cells, and ultimately line a fluid filled space called the yolk sac
The epiblast and hypoblast together form a flat, ovoid shaped disk called the ____
bilaminar embryonic disc
Embryonic Period: Week 3 – Gastrulation
process that transforms the bilaminar embryonic disc into a trilaminar embryonic disc
cells rapidly divide to replace hypoblast cells and create a middle layer of cells, called mesoblasts
results in the 3 primary germ layers, which will form all adult structures:
> ectoderm
> mesoderm
> endoderm
Surface ectoderm will form:
the skin, hair, and nails
neuroectoderm will form the:
nervous system
Mesoderm is a major precursor for:
muscles and connective tissues of the adult body
Embryonic Period: Week 4 – Neurulation
embryonic process that forms the nervous system
occurs during week 4
Ectodermal cells rapidly divide, forming a thickened plate of cells, called the neural plate
The neural plate invaginates down into underlying mesoderm and pinches off, forming the neural tube and neural crest
Neural tube and neural crest (collectively, neuroectoderm) are located in the ___
underlying mesoderm
Ectoderm thickens to form the neural plate due to cell division:
Ectoderm invaginates because it rapidly divides, causing it to buckle in on itself and invaginate down into the underlying mesoderm
Neurulation begins in the middle of the embryo and extends cranially and caudally =
This forms an anterior/cranial neuropore and posterior/caudal neuropore as these are the final regions to complete neurulation and close
By day 28, the neuropores have completed neurulation and are closed
If the neuropores do not close, it may lead to nervous system deficits
Failure of anterior neuropore closure results in ___
anencephaly
cerebrum development affected
Failure of posterior neuropore closure results in ___
spina bifida
tail of spinal cord doesn’t close properly = outpocket into skin
Chiari II malformation
A condition in which brain tissue extends into the spinal canal, present at birth
It occurs when part of the skull is abnormally small or misshapen
Result of neurulation:
Neuroectoderm is in the underlying mesoderm and forms the nervous system
Neural tube forms the CNS
Neural crest forms the PNS, connective tissues of the head, and others
Surface ectoderm overlies the neural tube and neural crest and forms surface structures of the body
> Forms epidermis, hair, nails, enamel, and parts of the ears and eyes
The neural tube forms:
Brain (cerebrum, cerebellum, brainstem)
Spinal cord
Other CNS structures (e.g., pineal gland, part of pituitary gland, retina)
The neural tube forms neurons and glial cells of the CNS:
Neuroepithelial cells differentiate into neurons or glia
Neuroblasts form neurons
Gliablasts form glia
The only glia in the CNS that are not formed from the neural tube are ___
microglia
Microglia are formed from the mesodermal germ layer
Neural tube:
consists of neuroepithelial cells that extend over the entire thickness of the wall and form a thick pseudostratified epithelial layer
Neuroepithelial cells divide rapidly and differentiate into neuroblasts, which form neurons, and gliablasts, which form glial cells
The neuroblasts are formed first, followed by ___
gliablasts
The gliablasts migrate into the mantle and marginal layers of the neural tube to form ___
astrocytes
which are located near blood vessels and neurons and provide support and metabolic functions to neurons
Gliablasts also form __, which migrate to the marginal layer to form ___
oligodendrocytes
the myelin sheath around axons in the CNS
Microglia =
glial cells that form from the mesodermal layer
not derived from ectoderm
phagocytic cells that develop later than other glia cells and migrate into the CNS
Prosencephalon
Forebrain (1°)
> (2°) telencephalon forms 2 large lateral outpocketings that develop into the cerebral hemispheres, basal ganglia, and lateral ventricles
> (2°) diencephalon is characterized by the outgrowth of the optic vesicles and forms deeper brain structures, including the thalamus and hypothalamus and the third ventricle
Mesencephalon
Midbrain (1°)
> mesencephalon (2°) forms the midbrain
> mesencephalon does not change name and forms the midbrain and cerebral aqueduct
Rhombencephalon
Hindbrain (1°)
> forms the secondary vesicles named the
- metencephalon
- myelencephalon
> The metencephalon (2°) forms the pons and cerebellum and the superior part of the fourth ventricle
> The myelencephalon (2°) forms the medulla and lower part of the fourth ventricle
CNS Development: Brain
> cranial end of the neural tube forms the brain and brainstem
> rest of the neural tube forms the spinal cord
> at the cranial end, named vesicles form that create the walls and spaces/cavities in the brain and brainstem
> cranial end of the neural tube first forms 3 primary vesicles, called the prosencephalon, mesencephalon, and rhombencephalon (from cranial to caudal)
> the 3 primary vesicles then develop into 5 secondary vesicles
CNS Development: Spinal Cord
spinal cord develops inferior to the rhombencephalon
marginal layer
mantle layer
neuroepithelial layer
neuroblasts form the mantle and marginal layers of the neural tube
cell bodies of the neuroblasts form the mantle layer and the axons of the neuroblasts form the marginal layer
Myelination of the neuroblast axons by ___ makes the appearance of the marginal layer ___, thus, forming the ___
oligodendrocytes
white
white matter of the CNS
The outer layer of the neural tube is called the ___, which forms the ___
marginal layer
white matter of the spinal cord
The middle layer of the neural tube is called the ___ and forms the ___
mantle layer
gray matter of the spinal cord
The inner layer of the neural tube is called the ___ and forms the ependymal cells that line the ____
neuroepithelial layer
central canal of the spinal cord
The mantle layer differentiates into ____
alar plates = dorsal and basal plates, which are ventral
The alar plates form the ___, while the basal plates form the ____
dorsal gray horn (sensory)
ventral gray horn (somatic motor)
The ____ forms in the region between the alar and basal plates
lateral/intermediate horn (visceral motor)
As a result of continuous addition of neuroblasts to the mantle layer, each side of the neural tube forms thickenings:
a ventral thickening called the ___ and a dorsal thickening called the ____
basal plate
alar plate
The basal plate contains somatic motor neuron cells that form the ____
The alar plate contains cell bodies of interneurons that form the ____
The lateral (intermediate) horn develops between the ventral and dorsal horns at thoracolumbar spinal levels and contains cell bodies for ____
ventral horn of the spinal cord
dorsal horn of the spinal cord
preganglionic sympathetic motor neurons
A longitudinal groove, called the ____, serves as the boundary between the alar and basal plates.
The roof plate and floor plate form at the midline of the neural tube, with the roof plate being the ___ and the floor plate being the ____.
sulcus limitans
dorsal midline
ventral midline
The floor plates are regions that do not contain ___, but instead serve as pathways for nerve fibers crossing from one side of the spinal cord to the other side of the spinal cord.
neuroblasts
The neural crest forms:
Peripheral and cranial nerves and their ganglia
Face and skull bones
Connective tissues in the head
Some other structures in the body as well
PNS Development
Neural crest cells migrate to form PNS structures, including:
> cranial nerve ganglia
> dorsal root ganglia
> spinal nerves
> sympathetic trunk
> collateral ganglia
> adrenal medulla
> enteric nervous system
Neural crest cells also form Schwann cells which create the myelinate sheath around PNS axons
neural crest cells forming the dorsal root ganglia contain the cell bodies of ____ sensory neurons
pseudounipolar
The central process of these sensory neurons grows into the dorsal horn to from the dorsal root, while the peripheral process grows toward the periphery
somatic motor neuron axons grow out of the ventral horn to form the ventral root
