Session 1 - Embryology Flashcards
In what week does gastrulation begin?
Week 3
Describe the formation of the neural tube.
Gastrulation produces the notochord.
The notochord then induces neurulation by causing the neural plate to form from overlying ectoderm.
Lateral edges of the neural plate elevate.
Forms a depressed region in the middle called the neural groove.
The neural folds gradually approach each other in the midline and fuse, producing the neural tube.
What is the notochord?
A solid rod of cells running down the mid line of the embryo. Has an important signalling role as it directs the conversion of the overlying ectoderm to neurectoderm.
The neural tube has completely formed by which day of development?
Day 28-32
What causes neural tube defects?
Failure of the neural tube to fuse correctly. Failure can occur caudally or cranially.
Cranial neural tube defects result in what abnormality?
Anencephaly
Caudal neural tube defects result in what abnormality?
Spina bifida
What is spina bifida?
Neural tube defect where the vertebrae of the spine don’t form correctly around the spinal cord.
Spina bifida can occur anywhere along the length of the spine, but they are most common in which region?
Lumbosacral region
What is a significant risk factor for the development of spina bifida?
Lack of folic acid before and in the early stages of pregnancy.
What are the 3 main types of spina bifida?
Myelomeningocele - most severe. Spinal cord and meninges push out and form a sac in the baby’s back.
Meningocele - meninges push out but the spinal cord develops normally.
Spina bifida occulta - most common and mildest. Gap in spine is very small, doesn’t cause any problems, usually unaware of it.
The majority of babies will develop what as a result of spina bifida?
Hydrocephalus
What is hydrocephalus?
Build up of fluid on the brain. This excess fluid puts pressure on the brain which can damage it.
What is anencephaly?
Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. Due to failure of the neural tube to close cranially. It is incompatible with life.
What is rachischisis?
Failure of neural fold elevation meaning there is no formation of the neural tube at all. This is not compatible with life.
How can a neural tube effect be diagnosed?
Raised maternal serum alpha-fetoprotein (indicates an open defect)
Ultrasound scan
Consumption of what substance by mothers before conception and during the first trimester of pregnancy reduces the incidence of neural tube defects?
Folic acid
Before the 3rd month of development, the spinal cord is the same length as the vertebral column. What happens to the vertebral column after the 3rd month, and what does this mean for the spinal roots?
After the 3rd month, the vertebral column grows faster than the spinal cord. The spinal roots still exit at the intervertebral foramen, meaning they must elongate as the vertebral column grows. This forms the cauda equina.
What are the three primary brain vesicles? When do they form?
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
Form after neural tube closure in the 4th week.
What are the five secondary brain vesicles? When do they form?
Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon
Form by the 5th week of development.
Which secondary brain vesicles arise from the prosencephalon (forebrain)?
Telencephalon
Diencephalon
Which secondary brain vesicles arise from the rhombencephalon?
Metencephalon
Myelencephalon
What is the mature derivative of the telencephalon?
Cerebral hemispheres
What is the mature derivative of the diencephalon?
Thalamus
What is the mature derivative of the mesencephalon?
Midbrain
What are the mature derivatives of the metencephalon?
Pons
Cerebellum
What is the mature derivative of the myelencephalon?
Medulla oblongata
What is the role of the ventricular system in the brain?
To cushion the brain and spinal cord within their bony cases.
Blockage of the ventricular system e.g. due to a tumour or infection, can cause what abnormality?
Hydrocephalus
What are the layers of the early neural tube?
From inside to out:
- neuroepithelial layer
- intermediate (mantle) layer (alar (sensory) and basal (motor) plates, formed by neuroblasts)
- marginal layer
Also has roof and floor plates which regulate dorsal and ventral patterning.
What are neural crest cells?
Cells of the lateral border of the neuroectoderm tube which become displaced and enter the mesoderm. In the mesoderm they then undergo epithelial to mesenchymal transition.
What neural crest cell derivatives are found in the nervous system?
Cranial nerve ganglia Spinal (dorsal) root ganglia Sympathetic ganglia (chain and pre-aortic) Parasympathetic ganglia Schwann cells Glial cells Leptomeninges (arachnoid and pia)
What neural crest derivatives are found in the head, neck and midline?
Connective tissue and bones of the face and skull
Odontoblasts
Dermis
C cells of the thyroid gland
What other neural crest cell derivatives are there?
Conotruncal septum (heart)
Melanocytes
Adrenal medulla
Give examples of conditions caused by defects of neural crest cell migration.
Hirschsprung’s disease (aganglionic megacolon). - defect of migration/morphogenesis affecting one structure
DiGeorge syndrome (thyroid deficiency, immunodeficiency secondary to thyroid defect, cardiac defects, abnormal facies). - defect affecting multiple structures.
