Neurulation
When does the neural plate form a dorsal thickening of the ectoderm?
3rd week of development
What is anencephaly?
fatal deformity that is caused by a complete failure of the neural tube to close
What does the Rhombencephalon divide into?
Metencephalon--> forms the pons and cerebellum
Myelencephalon--> forms the medulla
What develops from the mesoderm? What 3 things come from each somite?
Somites
Muscles (myotome), Bones (sclerotome), Dermis (dermatome)
When do the limb muscles develop?
Around the 7th week, they appear near the base of the limb bud, the limb muscles come from the myotome and they migrate into the limb bud segmentally (myotome pattern)
What happens at the end of the 3rd week of development?
the 2 folds meet and form the NEURAL TUBE and an opening called the NEUROCELE
What are the 3 types of spina bifida? Which one has the worst prognosis? What are the characteristics of each?
Meningomyelocele: MOST SEVERE FORM, failure of caudal neuropore to close, vertebrae don;t form, nerve roots and spinal cord along with meningeal coverings are herniated, can also be accompanied by Arnold-Chiari Malformation
Meningocele: herniation of meningeal membranes, appears as a cystic tumor, NO CNS INVOLVEMENT!
Occulta: Simple defect in the closure of the vertebrae, area of defect is often indicated by a tuft of excessive hair
What is myelination and when does it begin? What myelinates in the PNS and CNS?
Myelination: formation of myelin sheath around a nerve for increased conduction
It begins during the 4th month of fetal life- but the process is not complete until early adulthood
PNS myelinated by Schwann cells developed from neural crest
CNS myelinated by Oligodendrocytes (not from neural crest cells)
When do somites form from the mesoderm?
by the 20th day
What do neural crest cells become?
They will eventually develop into dorsal root and autonomic NS ganglion cells, adrenal medulla, schwann cells and satellite (support) cells.
What are the 3 primary vesicles that form as expansions of the neural tube during the 4th week?
Prosencephalon, Mesencephalon, Rhombencephalon
What is the sulcus limitans? How does it relate to the alar and basal plates?
The sulcus limitans forms as a groove in the inner wall of the neural tube and extends through the spinal cord and the brainstem, it then divides into the alar (dorsal) and basal (ventral) plates.
This happens during the 4th week.
Explain the development of dermatomes.
Each dermatome is associated with a specific spinal nerve
Dermatomes that come into the limb bud will eventually lose their connection with the axial portion as the limb bud gets longer (this is why the mature dermatomal pattern looks like it does)
What structure does the neurocele eventually develop into?
The Ventricular System
What does the Prosencephalon vesicle divide into?
Telencephalon--> forms cortex and basal forebrain
Diencephalon--> forms thalamus and hypothalamus
What do the alar and basal plates eventually become?
Alar plates (dorsal): become the dorsal horn in the spinal cord
Basal plates (ventral): become the ventral horn in the spinal cord
Describe the development of myotomes.
Each myotome is innervated by a single spinal segment associated with a somite, as limb lengthens they divide into flexor/extensor groups
Then they lose segmentation and fuse so individual muscles of a limb come from more than 1 somite and the muscles are innervated by more than 1 spinal cord level
When does the rostral and caudal neuropore close?
The rostral neuropore closes FIRST in the middle of the 4th week
The caudal neuropore closes about 2 days after
What does the mesencephalon vesicle divide into?
The mesencephalon remains a primary vesicle and it forms the midbrain
Where are motor and sensory nuclei located?
Motor nuclei are located most medial
Sensory nuclei are located more laterally
What is the notochord and sclerotome?
Notochord: midline structure, important for proper vertebrate development
Sclerotome: develops into vertebrae (bone)