DEVELOPMENT OF HUMAN BRAIN AND SPINAL CORD

Transcription

1 DEVELOPMENT OF HUMAN BRAIN AND SPINAL CORD LEARNING OBJECTIVES The student should understand the following The origins of nervous system Primary and secondary brain vesicles Development of spinal cord Development of brain stem Development of cerebral hemispheres ORIGIN OF NERVOUS SYSTEM The nervous system develops from the neural plate, a thickened slipper shaped area of ectoderm. The neural plate forms the following neural tube and neural crest. Neural tube.. Differentiates into brain and spinal cord. Neural crest.. Gives rise to ANS most of PNS and ganglia. From neural plate to neural tube. A: The neural plate on the dorsal side of the embryo. B,C,D. The plate folds to form a groove that is transformed into the neural tube. The neural crest cells are pinched off from the tube and develop into sensory ganglion cells, postganglionic autonomic neurons and peripheral glial (Schwann) cells (From Heimer, 1995).

2 The neural tube cranial to the fourth pair of somites develops into the brain. The neural tube caudal to the fourth pair of somites form the spinal cord. 3 primary brain vesicles form in the cranial region Forebrain (prosencephalon) Midbrain (mesencephalon) Hindbrain (rhombencephalon) Development of Spinal cord Spinal cord forms from the neural tube caudal to fourth pair of somites. The lateral walls of neural tube thicken and reduce the size of neural canal, until only a minute central canal of spinal cord is present at 9 to 10 weeks. The neuroepithelial cells give rise to Ventricular zone which forms all neurons and macroglial cells in spinal cord. Then marginal zone of neuroepithelial cells form the white matter of spinal cord. Proliferation and differentiation of neuroepithelial cells in developing spinal cord produces the following

3 Dorsal part forms the ALAR plate, cell bodies in which form the dorsal gray columns. In transverse section, appear as Dorsal horn. Ventral part forms the BASAL plate, cell bodies of which form the ventral and lateral gray columns. In transverse section, appear as ventral and lateral horns respectively. MYELENCEPHALON The caudal part of myelencephalon resembles the spinal cord both developmentally and structurally. The neuroblasts in the alar plates of myelencephalon migrate into marginal zone and form isolated areas of gray matter i.e. gracile nuclei medially and cuneate nuclei laterally. The ventral area of medulla contains the pyramids which consist of cortico-spinal fibers descending from developing cerebral cortex. The rostral part of medulla houses the diamond shaped cavity of fourth ventricle. From medial to lateral on each side following nuclei are formed o General somatic efferent.neurons of hypoglossal nerves. o Special visceral efferent.. Neurons innervating muscles derived from pharyngeal arches. o General visceral efferent.. Some neurons of vagus and glossopharyngeal. Some neurons in the alar plates migrate ventrally and form neurons in olivary nuclei. METENCEPHALON The walls of metencephalon forms the pons and cerebellum, and the cavity of metencephalon forms the superior part of fourth ventricle. The cerebellum develops from thickenings of dorsal parts of alar plates. Some neuroblasts in alar plates migrate to marginal zone and form cerebellar cortex. Other neuroblasts from these plates give rise to central nuclei, the largest of which is dentate nucleus. Cells from alar plates also give rise to pontine nuclei, cochlear and vestibular nuclei and sensory nucleus of trigeminal nerve.

4 MESENCEPHALON Forms the midbrain. The neural canal narrows and becomes cerebral aqueduct. Neuroblasts from alar plates of midbrain into tectum (roof) and aggregate to form four large group of neurons, the paired superior and inferior colliculi. Neuroblasts from basal plates give rise to group of neurons in tegmentum (red nuclei, nuclei of occulomotor and trochlear nerves & reticular nuclei) Substantia nigra also forms from the basal plate. In the end cerebral peduncles become prominent. A-B: Early devlopment of the brain. A midbrain flexure developes in the region of the mesencephalon and a slight bend, the cervical flexure, can be recognized at the junction of the rhombencephalon and the spinal cord. B: A third flexure, the pontine flexure, develops between the metencephalon and myelencephalon and the brain now contains five parts. C: Same as (B) showing the medial surface of the right half of the brain in an 11-mm embryo. D: The medial surface of the brain in a 43-mm human embryo. The diencephalon can be subdivided into the thalamus and hypothalamus.

5 Development of the Cerebral Hemispheres. A and B: The telencewphalic vesicle expands rapidly to cover th erest of the brain, and the posteroventral part of the vesicle curves down and forward to form the temporal lobe. C: The lateral surface of the brain of a 7-month-old foetus. The insular region is bounded by the frontal, frontoiparietal, and temporal opercula. Some of the more pronounced grooves have appeared and the hemisphere can be divided into four lobes: the frontal, parietal, occipital, and temporal lobes and the insula Outlines of the cerebral hemisphere at 25, 38, 53, 68 and 96 mm, drawn to the same scale, showing the considerable enlargement from the end of the embryonic period proper (8 postovulatory weeks). The lowermost drawing presents th entire brain at 25 mm. The uppermost drawing shows the brain in situ at 31 mm in almost natural size.

6 Development of Corpus callosum and fornix. A: Median sagittal section in a 10-week embryo showing the commissural plate and its development into corpus callosum. B: Median section through the corpus callosum and deincephalon in adult human brain. C: 3-D drawing illustrating the realtionship between corpus callosum, fornix and hippocampus. Development of corpus striatum and internal capsule. A: the medial surface of the brain in a 43 mm embryo. The thalamus is developing in the diencephalon, and corpus striatum in the basal part of the telencephalon. Part of the ganglionic eminence can be seen through the interventricular foramen. B, C: transverse sections through the developing brain. ###################################################