embryology of brain (1) in view with radiology
- 1. Embryology of Brain - DR. ATHIRA PAVITHRAN
- 2. Embryology of Brain •Neurulation •Primary brain vesicles •Flexures of the Brain •Development of Ventricular System •Rhombencephalon •Mesencephalon •Prosencephalon
- 3. Neurulation • At around 16th day of embryonic life ectodermal cells overlying the notochord get differentiated into specialized cells called neuroectodermal cells. These proliferate to form a thick plate called neural plate.
- 4. • By 20th day of embryonic development - Fusion of neural folds starts which begins in the middle and it simultaneously proceeds in the cephalic and caudal directions. Neural crest cells differentiate to form cells of dorsal root ganglia, sensory ganglia of cranial nerves, autonomic ganglia, adrenal medulla, chromaffin tissue, melanocytes, and Schwann cells.
- 6. Primary brain vesicles : •The brain develops from enlarged cranial part of the neural tube. •By end of fourth week of gestation, enlarged cephalic part shows three distinct dilatations called primary brain vesicles : (a) Prosencephalon (forebrain) (b) Mesencephalon (midbrain) (c) Rhombencephalon (hindbrain) •Their cavities form ventricular system of adult brain.
- 7. • During fifth week both prosencephalon and rhombencephalon subdivide into two vesicles, thus producing five secondary brain vesicles.
- 9. Flexures of the Brain 1. Cephalic (mesencephalic) flexure : in the midbrain. Convex dorsally. 2. Pontine flexure : at the middle of rhombencephalon 3. Cervical flexure : at the junction of rhombencephalon and spinal cord making a right angle between them. Convex dorsally.
- 10. Pontine flexure •Hindbrain is folded at its middle so that it forms an acute angle ventrally. The cavity here becomes a diamond-shaped space called fourth ventricle. •It is widest at line of folding (junction of two parts of the hindbrain—pons and medulla oblongata and tapers superiorly to narrow canal of the midbrain, the aqueduct of Sylvius and inferiorly to the central canal in the lower part of medulla oblongata.
- 11. • The thin roof is pulled out to cover the space posteriorly, and the line of folding extends far laterally as roof of the lateral recesses of the fourth ventricle. • At tips of these recesses and inferior angle of the ventricle, the thin roof breaks down forming apertures (lateral foramina of Luschka and median foramen of Magendie) through which the cavity of neural tube communicates with the surrounding subarachnoid space.
- 12. Development of Ventricular System The cavities of brain vesicles form the ventricular system of adult brain : •Lateral ventricles : Twin telencephalic cavities •Third ventricle : Diencephalic cavity •Cerebral aqueduct (aqueduct of Sylvius) : Narrowed mesencephalic cavity •Fourth ventricle: Rhombencephalon cavity
- 13. Rhombencephalon (Hindbrain) The rhombencephalon consists of : 1. Myelencephalon the most caudal of the brain vesicles 2. Metencephalon, which extends from the pontine flexure to the rhombencephalic isthmus Myelencephalon •The caudal part of myelencephalon has a central canal and forms the closed part of medulla oblongata. •Rostrally the central canal expands as the cavity of the fourth ventricle, and thus the rostral part of myelencephalon forms the open part of the medulla oblongata.
- 14. •The floor of fourth ventricle is derived from myelencephalon (medulla) and metencephalon (pons). • The stretched roof plate of rhombencephalic vesicle forms the roof of fourth ventricle. It consists of a single layer of ependymal cells covered by a vascular mesenchyme—(the pia mater.). Pia mater along with covering layer of ependymal cells forms tela choroidea.
- 15. • Owing to active proliferation of vascular mesenchyme, the tuft of capillaries of blood vessels invaginates into the ventricular cavity. These sac-like invaginations consisting of tela choroidea and tuft of capillaries form choroid plexus.
- 16. •Dorsolateral parts of the metencephalon extend medially and dorsally to form rhombic lips. These meet and fuse in the midline over the roof of the fourth ventricle and then grow dorsally to form cerebellum. •The marginal layer of basal plates of metencephalon expands to serve as a bridge for nerve fibers connecting cerebral cortex and cerebellar cortex (cortico- pontocerebellar pathways ) known as pons (pons = bridge). Metencephalon
- 17. Mesencephalon (Midbrain) •It retains a cylindrical form and its narrowed cavity forms the cerebral aqueduct, which is continuous below with the fourth ventricle and above with the third ventricle. •Anterior to the cerebral aqueduct, it gives rise to tegmentum and substantia nigra. The marginal layer of each basal lamina enlarges and forms crus cerebri. These crura serve as pathways for nerve fibers descending from the cerebral cortex to the lower centers in pons, medulla, and spinal cord.
- 18. • The roof plate develops to form bilateral longitudinal elevations separated by a shallow midline groove. Each elevation is subdivided by a transverse groove into upper and lower parts called superior and inferior colliculi, respectively. •Four colliculi (also called corpora quadrigemina) develop into the roof plate dorsal to the aqueduct of Sylvius and form tectum.
- 19. Prosencephalon (Forebrain) Consists of : Telencephalon : forms the cerebral hemispheres Diencephalon : forms the optic cup and stalk, pituitary, thalamus, hypothalamus. Diencephalon •The diencephalon develops from the median portion of the prosencephalon. Its cavity is called the third ventricle. • The primitive diencephalon consists of two thick lateral walls, a thin roof, and floor plates.
- 20. •Each lateral wall presents a sulcus, the hypothalamic sulcus which divides lateral wall into dorsal and ventral regions. •The dorsal region develops into thalamus. The ventral region encroaches on the floor plate and forms hypothalamus.
- 21. •A downgrowth from the floor of anterior hypothalamus, the neurohypophysis, joins an upgrowth from the adenohypophysis, to form hypophysis cerebri (pituitary gland) •The epithalamus comprising pineal gland and habenular nuclei develops posteriorly in the roof plate
- 22. Telencephalon • Telencephalon consists of a median part and two lateral diverticula or cerebral vesicles. • The median part forms a small anterior part of the third ventricle, and the lamina terminalis.The lamina terminalis represents the cephalic end of the primitive neural tube.
- 23. • As the vesicle grows backward it overlaps the diencephalon, mesencephalon, and cerebellar rudiments. • The cerebral hemisphere starts growing in the region of interventricular foramen: 1. forward : frontal lobe 2. dorsally : parietal lobe 3. posteriorly : occipital lobe 4. anteroinferiorly : temporal lobe
- 24. • The lowest parts of medial walls of hemispheres in the region where they are attached to the roof of diencephalon remain very thin and through this the choroid plexus of third ventricle protrudes laterally into lateral ventricle along a line known as choroid fissure. • Above the choroid fissure, the medial wall of the hemisphere thickens to form hippocampus.
- 25. • The hippocampus is displaced posteroinferiorly into the lateral ventricle and the fornix is drawn out as an efferent tract on its medial aspect. • Corpus striatum develops bilaterally in the floor of telencephalon adjacent to thalami. • The fibers that form internal capsule on each side divide the corpus striatum into two parts: (a) dorsomedial portion : Caudate nucleus (b) ventrolateral portion: Lentiform nucleus
- 26. Sources •Langman’s Medical Embryology 12th edition •Textbook of Embyology Vishram Singh
- 27. THANK YOU
Editor's Notes
- #3: Notochord develops at 3rd week
- #4: The neural folds gradually move together toward the midline and finally fuse to form a cylindrical neural tube that loses its connection with the surface ectoderm.
- #7: The prosencephalon gives a rostral telencephalon and caudal diencephalon (interbrain). The telencephalon develops lateral diverticula by evagination, which enlarge, overgrow and cover the caudal diencephalon to form cerebral hemispheres. The diencephalon thus becomes hidden in the lower parts of the cerebral hemispheres and forms thalamus, hypothalamus, epithalamus, etc. mesencephalon gives rise to midbrain. It does not show much changes in early part of development except that its cavity gets progressively narrowed to form cerebral aqueduct. The rhombencephalon divides into rostral metencephalon, which eventually develops into pons and cerebellum, and caudal myelencephalon, which gives rise to medulla oblongata.
- #9: Flexures : - 4 th week of development. The cephalic and cervical flexures are concave ventrally, whereas the pontine flexure exhibits a ventral convexity (Fig. 22.7). The cervical flexure makes a 90° bend between hindbrain and spinal cord, causing the brain to be oriented almost at 90° to the spinal cord. The brain assumes its configuration as a result of differential growth of its vesicles and flexures.
- #12: Cerebrospinal fluid (CSF) is formed in the ventricles, mainly in lateral ventricles by choroid plexuses. The CSF leaves the ventricular system through apertures in the roof of ventricle (viz., foramen of Magendie and foramina of Luschka) into subarachnoid spaces around the brain and spinal cord.
- #14: On either side of midline, the floor consists of the basal and alar laminae, which are separated from each other by a longitudinal sulcus called sulcus limitans. The lamina are a similar to that of spinal cord, contain motor and sensory nuclei, respectively. Each lamina contains two columns, somatic and viscera. But in the brainstem to supply the derivatives of the branchial arches that develop around this region, a special branchial column appears between somatic and visceral columns of each lamina. In addition, a special somatic column appears in the most lateral part of the alar lamina to receive impulses of special sensations of hearing and balance. Thus in brainstem, the basal lamina contains three columns and alar lamina contains four columns.