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The Mammalian Nervous System Development 1 Timing of embryological development Mouse Event Size (mm) Fertilization Implantation of blastocyst Appearance of primitive streak Human day Size (mm) day 1 0.1 - 0.15 1 4.5 0.1 - 0.2 7-12 7 0.2 13 0.4 16 1.0 – 1.5 18 1.5 – 2.5 20 Gastrulation starts; notochord appears 7.5 Neural plate and groove appear First (1-3) somites & neuromeres (in hindbrain) appear 8 2 Timing of embryological development Mouse Event Size (mm) Neural tube starts to close (4-12 somites) Cranial neuropore closes; optic vesicles appear (13-20 somites) 1.2 – 2.5 Human day Spinal nerves start to sprout, cerebral hemispheres become visible 3.3 – 3.9 (mm) day 8 2.0 – 3.5 22 9 2.5 – 4.5 24 3.0 – 5.0 26 4–6 28 Caudal neuropore closes (somites; human 21-29; mouse 30-34) Ventral and dorsal columns start to differentiate in spinal cord and brain stem. Cranial nerve motor nuclei appear (>30 somites) Size 10 3.5 – 4.9 10.5 5–7 32 Sensory and parasympathetic cranial nerve ganglia begin to form 5–6 11 7–9 33 Cerebellum starts to form 6-7 11.5 11 - 14 41 Gestation lasts 19-21 days for mice and about 266 days for humans 3 Fertilization 4 Formation of the blastocyst Week 1: human 5 Week 2 Human Summary Implantation and formation of the embryo 6 Implantation of the blastocyst Germ disk 7 Formation of the yolk sac Definitive yolk sac Aminiotic Amniotic cavity cavity Germ disk Remnants of primary yolk sac 8 Week 3 Human Summary Gastrulation. The primitive streak, the notochord, the neural plate and first somites 9 Appearance of the primitive streak 10 Primitive streak and the embryonic mesoderm and endoderm: Gastrulation The ectoderm, the mesoderm and the endoderm are all derived from the epiblast 11 The notochord Cells entering into the mesodermal layer via the primitive node form a tube (the notochordal process), which is converted into a solid cylinder (the notochord) after fusing transiently with the endoderm. 12 The notochord The notochord contributes to the development of the intervertebral discs The notochord does NOT contribute to the bone structure of the vertebrae The notochord does NOT contribute to the formation of the nervous system. 13 The somites Cells entering the mesodermal layer via the primitive groove and migrating laterally, form the paraxial, intermediate and lateral plate mesoderms. The two paraxial mesoderm columns (one on each side of the midline) develop into somitomeres and then into somites. Note: the first 7 (cranial) pairs of somatomeres do not form somites 14 Scanning electron microscopy somites Notocord & somites Remove ectoderm Days 19 - 20 15 Fates of the somatomeres and somites Somatomere pairs 1 – 7 (cranial region) Appearance on Day 20 Somite pairs 1 – 4 (occipital region) (4 pairs) Occipital bone of the skull, bones around the nose, eyes and inner ears, extrinsic ocular muscles and tongue muscles. Somite pairs 5 – 12 (cervical region) (8 pairs) Occipital bone (somite pair 5) Cervical vertebrae and associated muscles, part of neck dermis and upper limb muscles Somite pairs 13 – 24 (thoracic region) (12 pairs) Thoracic vertebrae and muscles and bones of thoracic wall, part of thoracic dermis, part of abdominal wall and upper limb muscles Somite pairs 25 – 29 (lumbar region) (5 pairs) Lumbar vertebrae, abdominal muscles and dermis, lower limb muscles. Somite pairs 30 – 34 (sacral region) (5 pairs) Sacrum and associated muscles and dermis Somite pairs 35 – 37 (coccygeal region) (3 pairs) ~Day 30 Striated muscles of face, jaws & throat Somite pairs 38 – 44 (embryonic tail region) Coccyx Will degenerate later 16 Development of the nervous system 17 From neural plate to neural groove The prechordal plate and the cranial portion of the notochordal plate induce the overlying epiblast to differentiate into a thick plate of columnar, pseudostratified neuroepithelial cells, or neurectoderm, called Notochordal plate Notochordal tube fuses transiently with the endodermal layer the neural plate First somites appear in mesodermal layer 18 Scanning electron microscopy Neural plate & groove Day 19 - 20 19 Weeks 4 & 5 Human 20 Neurulation Neural crest Neural crest Neural tube notocord 21 Neurulation The neural tube closes in a zipper-like manner, starting in the region of the first 5 somites and proceeding bidirectionally. 22 Cellular structure of the neural tube 23 Cells derived from the neural tube Central canal 24 Fates of cells from the neural crest 25 Fates of cells derived from the neural crest Ganglion = a cluster of a large number of neuronal cell bodies outside of the brain 26 Timeline of Spinal cord and PNS development The neural crest plays a major role in PNS development 27 Formation of peripheral nerves Peripheral nerves are formed from cells derived from both the neural tube and the neural crest Roof plate Floor plate Sulcus limitans 28 Spinal nerve formation 29 Spinal nerve Vertebrae, peripheral nerves & muscles The vertebrae and skeletal muscles are derived from the somites 30 The notocord, sclerotomes & the vertebral column 31 Spinal cord 32 Timeline of brain development pituitary cerebellum 33 Glossary Principal structures Other features Telencephalon Cerebral hemispheres, Hippocampus Lateral ventricle, Choroid plexus, Commissures connecting hemispheres (e.g. corpus callosum) Diencephalon Thalamus Brain region Prosencephalon (forebrain) Relay center between the forebrain and the hindbrain and spinal cord. Cerebral aquaduct (between 3rd & 4th ventricles) Mesencephalon (midbrain) Metencephalon Cerebellum, Pons Myelencephalon Medulla oblongata Rhombencephalon (hindbrain) Third ventricle, Choroid plexus, Pituitary gland Pons relays signals between spinal cord and cerebral and cerebellar cortices Relay center between spinal cord and higher brain centers 34 The Neural Plate & Neurulation 35 The brain: growing & folding 36 The brain: growing & folding 26 days 28 days 35 days mes 50 days Midbrain Future cerebellum Future 4th ventricle cerebrum Future pons Med. Oblongata Spinal cord 37 40 days Development of the cerebellum 60 days 90 days 150 days 120 days 38 The brain: growing & folding mes The telencephalon (future cerebral hemispheres) grow up, over and back over the diencephalon (future [hypo-/epi-] thalamus) The cerebellum grows out of the roof of the future 4th ventricle before it closes 39 The brain and the embryo 40 41 Sources • Human embryology. W. J. Larsen, 2nd edition, 1997 (publ. Churchill Livinstone Inc) • An introduction to embryology. B. I. Balinski, 2nd edition,1970 (publ. W.B. Saunders Company) • The house mouse: atlas of embryonic development. K. Theiler, 1989 (publ. Springer-Verlag) 42