Development of CNS Flashcards
What are the two phases of development?
embryogenesis and histogenesis
What are the three phases of embryogenesis?
induction, neurulation, vesicle formation
What is gastrulation?
During the 3rd week after conception epiblasts migrate throug hthe primitive streak to form three germ layers: a dorsal ectoderm, an intermediate mesoderm and a ventral endoderm.
Describe notocord formation
Specialized cells from the mesoderm migrate to the midline to form the notochord. This provides signals that triggers the next step in development.
Describe neural induction
The ectoderm is transformed into the neural plate. This elongated patch of thickened epithelial cells is called the neuroectoderm. It is regulated by chemical signals which come from the notochord. The chemicals coutermand a suppressive signal from BMP4. Without it, the ectoderm forms epidermis.
Describe neurulation
During the 3rd to 4th week, the neural plate thickens and folds up at its edges, rolling into a tube. The ectoderm which was the the dorsal aspect of the notochord becomes the interior layer of the neural tube. Closure of the tube begins at the level of what will become the cervical region of the spinal cord. Theopen ends of the tub (neuropores) close on a predictable schedule. The anterior end closes at day 25 (develops into the brain). The posterior end closes at day 28 (develops into the spinal cord).
What is spinal bifida?
The result of a defect in closure of the posterior neuropore. It is a disorder in which the mininges or even the spinal cord protrude through a defect in the skin and bone in the lower back, compromising nerve supply to the bladder and legs.
What is tethered cord syndrome?
It is a defect in closure of the posterior neuropore. The conus medularis (tip of the spinal cord) and filum terminale (the pia which continues beyond the cord) are abnormally stuck to the vertebral column. With growth this strethces the spinal cord and compromises its function.
What is anencephaly?
A defect in closure of the anterior neuropore. The brain fails to form but the infant often survives because of intact brainstem function.
What is encephalocele?
A defect in closure of the anterior neuropore. The mininges and sometimes the brain herniated through a midline defect in the skull and scalp.
Describe the neural crest?
As the neural tube forms, groups of ectodermal cells that have been at the edge of the developing tube separate to form the neural crest. These cells migrateto various locations to form many specialized structures and most of the peripheral nervous system.
Describe vesicle formation.
After the anterior neuropore closes, the rostral portion of the neural tube subdicides into three vesicles. The prosencephalon, the mesencephalon and the rhombencephalon.
What does the prosencephalon become?
It develops into the “mickey mouse ears” with the telencephalon which become the cerebral hemispheres and the diencephalon turns into the thalamus, hypothalamus, optic nerve and retina.
What does the mesencephalon become?
It doesn’t further divide and becomes the midbrain.
What does the rhombencephalon become?
It elongates to form the metencephalon which becomes the pons and cerebellum and myelencephalon which becomes the medulla.
What are the three bends or flexures of th developing brain?
Midbrain flexure, cervical flexure, pontine flexure
What is the midbrain flexure?
This anterior bend creates a c-shape to the developing brain so that the prosencephalon and rhombencephalon are almost parallel.
What is the cervical flexure?
This anterior bend occurs where the rhombencephalon meets the spinal cord
What is the pontine flexure?
This is a posterior bend at the level of the pons.
What are the two phases of histogenesis?
Cellular differentiation and cellular maturation
What are the progenitor lines that the neuroepithelial cells differentiate into?
neuroblasts and glioblasts
What types of cells do the glioblasts differentiate into?
macroglia: astrocytes, oligodendrocytes, radial glial cells (which after neuronal migration is over) turns into ependymal cells or astrocytes.
Where do microglia differentiate from?
mesenchymal cells
What is the ventricular zone (ependymal layer)?
This region is closest to the central canal which will become the future ventricular system. It is populated by epithelial cells which divide activity.
What is the subventricular zone (called the germinal matrix in newborns)?
Immediately adjacent to the ventricular zone contains neuroblasts capable of dividing
What is the intermediate zone (mantle layer)?
This region contains post mitotic cells
What is the marginal zone (or layer)?
This region is located further from the central canal. It contains few cells at the beginning when it is populated by axons and dendrites of developing neurons
What is the cortical zone (or plate)?
This region is characteristic of developing cerebral hemispheres. It contains post-mitotic cells which have migrated from the subventricular zone.
What are the five concentric zones of the neural tube?
from central canal outwards:
ventricular zone, subventricular zone, intermediate zone, marginal zone, cortical zone
Describe rostrocaudal patterning
There is differentiation and specialization of the central nervous system from the anterior neuropore to the posterir neuropore resulting in the creation of major CNS subdivisions. Inductive signals are secreted for future forebrain, hind brain, Controlled by homeobox genes
Describe dorsoventral patterning
There is differentiation and specialization of the neural tube between dorsal and ventral half. The Alar plate (posterior) and basal plate (ventral) are separated by the sulcus limitans. Ventral-motor, Dorsal-sensory
What part of the spinal cord does the neural canal develop into?
The central canal
What part of the spinal cord does the ventricular zone develop into?
Ependyma
What pat of the spinal cord does the intermediate zone develop into?
Central gray matter
What part of the spinal cord does the marginal zone develop into?
White matter
What do the sensory axons come from?
After the 4th week, the sensory axons from the dorsal root ganglia grow into the dorsal horn of the spinal cord.
