Development of the Nervous System Flashcards
Formation of the basic anatomical structures: Background
What is the zero point of embryological development?
Fertilization - the cell membranes of a spermatozoa and an oocyte fuse to form a zygote. All cells are derived from this single cell. The process by which cells assume a specialized structure and function is called differentiation.
**differentiation is clinically relevant for cancer grading
Formation of the basic anatomical structures: Background
Once the zygote is formed, what happens in week 1 of embryological development?
Cleavage - the zygote undergoes cell divisions, resulting in the rapid increase in the number of cells (without an increase in size).
This division occurs while the forming embryo migrates to the fallopian tube.
Once it reaches 16 cells, it is a blastocyte (embryo)
Formation of the basic anatomical structures: Background
What is implantation? When does it occur?
When the embryo reaches 16 cells or greater (between 6 days post fertilization and week 2 of embryological development) the embryo implants into the uterine wall
During implantation, the embryo undergoes morphologic changes, and forms a flat, ovoid, bilaminar plate of cells called the BILAMINAR EMBRYONIC DISC
Formation of the basic anatomical structures: Background
What is gastrulation? When does it occur?
In gastrulation, the bilaminar embryonic disc differentiates into a 3 layered (ectoderm, mesoderm, endoderm) embryonic disc. Differentiation of these three germ layers ultimately forms the tissues and organs of the embryo. the three axes of the body (cranial/caudal, left/right, ventral dorsal) are established by these three germ layers.
Gastrulation occurs at the end of week 2 through week 3 of development.
Formation of the basic anatomical structures: Background
What are the three layers of the embryonic disc that develop during gastrulation, how are they organized, and what arises from each of them?
Ectoderm: outermost; gives rise to nerve tissue
Mesoderm: middle layer; gives rise to muscle tissue and connective tissue
Endoderm: inner layer
Different types of epithelial tissues arise from all 3 layers
Formation of Important Neurological Precursors
What are the important neurological precursors? When are they formed?
Notocord; Somites/Somtomeres
Formed in weeks 3 and 4 of embryological development
Formation of Important Neurological Precursors: Notocord
What is the notocord? What is its function?
A midline mesoderm derived structure.
It underlies the ectoderm and guides the tissue organization of the ectoderm.
Serves as the basis for the axial skeleton
Induces the formation of the neural plate freom the overlying ectoderm
Formation of Important Neurological Precursors: Somites/Somitomeres
What are Somites/Somitomeres? What is their function?
They are rounded, mesoderm derived structures lying parallel to the notocord that serve as the basis for the segmental organization of the body.
As each somite/somitomere develops, it subdivides into three parts:
sclerotome - forms vertebrae
dermatome - forms the dermis of the skin
myotome - forms muscles
Neurulation: When in development does it occur?
Weeks 3 and 4 of embryological development
Neurulation: steps
1) The notocord induces the overlying ectoderm (this part of the ectoderm is also called the neuroectoderm) to form the neural plate.
2) The neural plate comes into existence just dorsal to the notocord. It begins to crease ventrally along its midline, forming a neural groove with neural folds on each side
3) The neural folds come together and fuse, forming the neural tube. the lumen of the neural tube is called the neural canal.
4) While the neural folds are coming together to form the neural tube, a specialization of cells from each neural fold separates from the neural fold to become the neural crest (which is not connected to the neural tube, and forms most of the PNS)
5) The neural tube separates from the surface of the ectoderm and sinks into the posterior body wall. Ultimately, it gives rise to the CNS.
Histogenisis of the Neural Tube:
What kind of cells line the neural canal? What is the role of those cells?
The inner surface of the neural canal is lined with neuroepithelial cells.
These cells are progenitor cells with the capacity to give rise to: more progenitor cells neuroblasts (immature neurons) glioblasts (precursors to glial cells) and ependymal cells
As the neuroepithelial cells divide and migrate, three layers are formed within the neural tube.
As soon as the neuroepithelial cells stop dividing into neuroblasts, the begin to produce glioblasts. After they produce glioblasts, they differentiate in place to produce ependymal cells
Histogenisis of the Neural Tube: Layers of the Neural Tube
Ventricular Zone
This is the inner most layer or the “luminal” surface.
It consists of neuroepithelial cells.
The cells of this layer will give rise to the neurons and some glial cells of the mature nervous system and to the ependymal cells lining the ventricles
Histogenisis of the Neural Tube: Layers of the Neural Tube
Intermediate Zone
Located between the ventricular and the marginal zones.
The cells in this region are post-mitotic neuroblasts (meaning they have undergone their last cell division) that will differentiate into neurons.
Neuroblasts may migrate to their final locations by following the processes of radial glial cells (a type of astrocyte)
The intermediate zone forms the GRAY MATTER (consists of cell bodies) of the CNS
Histogenisis of the Neural Tube: Layers of the Neural Tube
Marginal Zone
This layer is the farthest from the lumen and is therefore called the abluminal surface. It contains the PROCESSES of the cells in the ventricular and intermediate zones and forms the white matter of the CNS.
Molecular Basis of Neural induction: Definition
The stimulation of a specific developmental pathway in one group of cells (the responding tissue) by another closely approximated group of cells (the inducing tissue)
Molecular Basis of Neural induction:
What are the tissues involved in neural induction?
Notocord = inducing tissue
Ectoderm = responding tissue
Molecular Basis of Neural induction:
How does it occur?
Chemical signals released by the inducing tissue activate receptors on the responding tissue, resulting in a change in gene expression.
These changes in gene expression can determine cell identity as well as influence other aspects of neural development
