Development Of The Brain Flashcards
When does the brain develop?
During the 3rd week from the NT, cranial to 4th pair of somites
Fusion of neural folds in the cranial region and closure of rostral neuropore form what?
Three primary vesicles (prosencephalon, mesencephalon and rhombocephalon)
What occurs during the 5th week of development?
Five secondary vesicles form
Forebrain divides into telencephalon and diencephalon
Midbrain -> mesencephalon
Hindbrain divides into metencephalon and myelencephalon
What does the prosencephalon (forebrain) divide into?
Telencephalon and diencephalon
What does the rhombocephalon (hindbrain) divide into?
Metencephalon and myelencephalon
What are brain flexures?
Space into which the NT is developing is limited but there is extensive growth
Causes the NT to bend or flex at two ventral flexures and a dorsal flexure
Includes midbrain, cervical and pontine flexures
When do flexures form?
As the primary vesicles -> into secondary vesicles (5th week)
Which flexures are in the hindbrain?
Cervical and pontine flexures
Describe the cervical flexure
Demarcates hindbrain from the SC
Arbitrarily defined at superior rootlet of C1 roughly at foramen magnum
What does the pontine flexure demarcate?
Metencephalon (rostral) from myelencephalon (caudal)
What does the myelencephalon become?
Medulla oblongata
What does the metencephalon become?
Pons and cerebellum
What does the cavity of the hindbrain become?
4th ventricle (caudal) and central canal (medulla)
Describe the caudal myelencephalon
In this region neuroblast in alar plates migrate into marginal zone and form nucleus gracilis and nucleus cuneatus
Dorsal sensory nuclei associates with afferent tracts of the DCML
Pyramids (ventral) consisting of CST fibers
Describe the rostral myelencephalon
Wide and flat
Pontine flexure causes walls of medulla to move laterally
Roof plate is greatly thinned
Cavity is part of future 4th ventricle (caudal)
In the myelencephalon the alar plates become lateral to the basal plates causing what?
Motor nuclei to be medial to sensory nuclei
Neuroblasts in the basal plate of the medulla develop into what?
Motor neurons and nuclei organize into cell columns
General somatic efferent, special visceral efferent and general visceral efferent
Neuroblasts in the alar plate of the medulla form what?
Neurons that are arranged in columns
General visceral afferent, special visceral afferent, general somatic afferent and special somatic afferent
Some neuroblasts from the alar plates of the medulla migrate more ventrally as compared to afferent columns and form what?
Neurons in the olivary nuclei
Describe the metencephalon
Walls form the pons and cerebellum
Cavity forms 4th ventricle (rostral)
Pontine flexure forces walls of pons laterally and spreads the gray matter in the floor of the 4th ventricle
Neuroblasts of basal plate develop into motor nuclei
What does the cerebellum develop from?
Dorsal parts of alar plates (of metencephalon)
Cerebellar swellings project into 4th ventricle and fuse in median plane, overgrow rostral 4th ventricle, pons and medulla
Alar plates will also form central nuclei, pontine, cochlear nuclei
The ependymal roof of the 4th ventricle is covered with what?
Pia mater derived from hindbrain mesenchyme
What is the tela choroidea?
Formed from pia mater and ependymal roof
Proliferates and invests into 4th ventricle -> forming the choroid plexus which secretes ventricular fluid
The roof of the 4th ventricle evaginates and forms what?
Out pockets which then rupture to form median and lateral apertures
Neuroblasts of the alar plates of the midbrain migrate into what?
The tectum and form the superior and inferior colliculi
Neuroblasts of basal plates of the midbrain form what?
Tegmental nuclei (red nuclei, reticular nuclei, CN III and IV nuclei) Substantia nigra could be basal or alar plate cells that migrate ventrally
The neural canal of the midbrain narrows and forms what?
Cerebral aqueduct which connects the 3rd ventricle to the 4th
Fibers growing from the cerebal cortex form what?
Cerebral peduncles (crus cerebri)
Within the diencephalon there are swellings in the lateral walls of the 3rd ventricle which forms what?
Thalamus, hypothalamus and epithalamus
Describe the thalamus
Develops and bulges into cavity
Fuses at midline in 70% of brains forming interthalamic adhesion
Describe the hypothalamus
Arises from neuroblasts in intermediate zone
Endocrine and homeostatic nuclei develop
Mammillary bodies form on ventral surface
The epithalamus develops from what?
Roof and dorsal portion of the lateral wall of 3rd ventricle
The pineal gland develops as a median outgrowth of the what?
Roof of the diencephalon
What are the two embryonic source of the pituitary gland?
- Hypophyseal diverticulum (Rathke’s pouch): upgrowth of roof of stomodeum (oral ectoderm)
- Neurohypophyseal diverticulum: downgrowth of diencephalon (neuroectoderm)
What is the infundibulum?
Derived from neurohypophyseal diverticulum
Will pass between developing bones of the cranium
Stalk of the pituitary gland connecting it to the brain
Describe the hypophyseal diverticulum
Projects from roof of stomodeum by 3rd week
Constricts its attachment from oral cavity
Passes through the cranium and contacts infundibulum (5th week)
Regresses from oral cavity (6th week)
What does the hypophyseal diverticulum form?
Pars anterior and pars tuberalis
Forms anterior lobe of pituitary gland
What does the neurohypophyseal diverticulum form?
Median eminence, infundibulum and pars nervosa
Forms posterior lobe of pituitary gland
Describe the telencephalon
Consists of a medical part and two cerebral vesicles, primordia of cerebral hemispheres
Cavity forms anterior 3rd ventricle
Telen and diencephalon contribute to 3V
What gives rise to the falx cerebri?
Mesenchyme trapped in longitudinal fissure
When does the corpus striatum of the telencephalon appear?
In the 6th week as a swelling in the floor of each hemisphere
Fibers of IC will pass through
Separates and defines caudate and lentiform (putamen and globus pallidus) nuclei
Medial wall of the cerebral hemisphere becomes very thin and is continuous with the roof of the 3rd ventricle and forms what at this site?
Choroid plexus
What is holoprosencephaly?
Incomplete separation of the cerebral hemispheres and most cases are associated with facial abnormalities (cyclopia, premaxillary agenesis, proboscis, single nostril, hypotelorism, facial clefts)
Impaired forebrain development -> impacts FNP development -> facial anomalies
What is the lamina terminalis?
The largest and most rostral end of commissures
Extends from roof of diencephalon to optic chiasm
Within the lamina, other commissions will form
What does the hippocampal commissure interconnect?
The hippocampi within each hemisphere
What is the corpus callosum?
Largest commissure, links hemispheres along their length
Will growth and extend beyond the lamina terminalis
Anterior portion forms first; posterior portion forms in fetal life
Remainder of lamina terminalis is positioned between what?
Corpus callosum and fornix
Will stretch and form the septum pellucidum which is a thin plate of brain tissue containing nerve cells and fibers
Describe agenesis of the corpus callosum
Complete or partial absence of the corpus callosum
The condition may be asymptomatic but seizures and mental deficiency are common
Associated with more than 50 human congenital syndromes
Describe growth of the cerebral hemispheres
Cortex is initially smooth but growth results development to sulci and gyri
Gyri result from infolding of the cortex
Both increase surface area without increasing volume of neurocranium
Postnatal growth of the brain is from what?
Neurons increasing in size and myelination of axons
Describe the histogenesis of the cerebral cortex
Unique process of migration and differentiation
Cerebral hemispheres initially have zones seen in early NT: ventricular, intermediate and marginal zones
A 4th layer also appears (subventricular zone)
Cortical layers are laid down in a sequence from deep to superficial
Neurons migrate through deeper layer and exit to establish more superficial layers -> “inside out” development
What is lissencephaly?
Incomplete neuronal migration during 3-4 months of gestation
Infant will initially appear normal but later develops seizures, profound mental deficiency and mild spastic quadriplegia
Describe the smoothness of the cerebral surface that can be exhibited with lissencephaly
Pachygyria, broad thick gyri
Agyria, lack of gyri
Neuronal heterotopia: cells in aberrant positions compared to a normal brain
Enlarged ventricles and malformation of the corpus callosum are common
Describe microcephaly
Neurodevelopment disorder where calvaria and brain are small but face is normal sized
Results from a reduction in brain growth
Inadequate pressure from the growing brain leads to a small neurocranium
Accompanied mental deficiencies
What are the causes of microcephaly?
Autosomal recessive primary microcephaly
Ionizing radiation
Infectious agents (cytomegalovirus, Zika virus, rubella virus, toxoplasma Gondii)
Maternal alcohol abuse
What does the telencephalon form?
Cerebral hemispheres, lateral ventricle and part of 3rd ventricle
What does the myelencephalon form?
Medulla and lower part of 4th ventricle
What does the metencephalon form?
Pons and cerebellum
Upper part of 4th ventricle
