[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Nervous system and senses](https://durgical.com/en/embryology/#a-nervous) › Spinal cord: alar and basal plates and the ascent of the cord [Embryology](https://durgical.com/en/embryology/) · [Nervous system and senses](https://durgical.com/en/embryology/#a-nervous) # Spinal cord: alar and basal plates and the ascent of the cord - Embryonic structure The tail part of the neural tube forms the spinal cord: sensory neurons sit at the back, motor neurons at the front, and its end 'rises' as the spine grows. Updated 2026-11-01 **See it in 3D:** [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) · [Conus medullaris](https://durgical.com/en/anatomy/conus-medullaris/) · [Filum terminale](https://durgical.com/en/anatomy/filum-terminale-internum/) · [Cauda equina](https://durgical.com/en/anatomy/cauda-equina/) · [Dura (dural sac)](https://durgical.com/en/anatomy/spinal-dura/) ## Key points - Alar = sensory (dorsal); basal = motor (ventral); the sulcus limitans divides them. - SHH (notochord, floor plate) ventralizes; BMP (roof plate) dorsalizes. - Conus medullaris: L3 at birth, L1–L2 in adults. - Sympathetic lateral horn from T1 to L2. - Dorsal root ganglia come from neural crest. ## What it is The spinal cord forms from the neural tube caudal to the fourth pair of somites. Its walls organize into a dorsal alar plate (sensory) and a ventral basal plate (motor), separated by the sulcus limitans. Because the vertebral column outgrows the cord, the conus medullaris ends progressively higher: this explains the cauda equina and where a lumbar puncture is done. ## Key facts - Wall layers: ventricular (ependymal), mantle (gray matter) and marginal (white matter). - Basal plate → anterior motor horns; alar plate → posterior sensory horns. - Intermediate (lateral) horn: preganglionic sympathetic neurons from T1 to L2. - Roof and floor plates: no neurons; they carry fibers crossing the midline. - Third month: the cord fills the whole vertebral canal; at birth it ends near L3; in adults near L1–L2. - Pia mater continuing past the conus forms the filum terminale; roots below the conus form the cauda equina. ## Structure - SHH, secreted by the notochord and then by the floor plate, ventralizes the cord (motor neurons). - BMPs from the ectoderm and roof plate dorsalize the cord (sensory neurons, PAX3/PAX7). - Dorsal root ganglion neurons come from the neural crest, not from the tube. ## What it becomes - Basal plate: somatic and visceral motor neurons (ventral roots). - Alar plate: association neurons of the dorsal horn that receive the dorsal roots. - Neuroepithelium: neurons, astrocytes, oligodendrocytes and ependyma; microglia come from mesoderm. - Tube lumen: the central (ependymal) canal. ## Step by step - Week 4: the posterior neuropore closes and the caudal tube is complete. - The neuroepithelium proliferates and forms the mantle and marginal layers. - Ventral SHH and dorsal BMP divide the wall into basal and alar plates. - Motor neurons send axons out ventrally; the crest forms the dorsal root ganglia. - From the third month the vertebral column grows faster than the cord. - At birth the conus lies near L3; during childhood it rises to its adult level. ## Markers - SHH - BMP4 - PAX3 - PAX7 - NKX2.2 - Olig2 ## Clinical notes - Lumbar puncture is done below the conus (L3–L4 or L4–L5) to avoid the cord. - In tethered cord the filum or a lesion fixes the cord, which is stretched as the child grows: pain, neurogenic bladder, leg weakness, scoliosis. - A hair tuft, dimple or lipoma over the low back can mark occult dysraphism. ## Memory aid 'Alar feels, basal moves': with the patient lying face down, the dorsal side is up, and that is the side that feels. ## Test yourself Think of the answer, then tap the question to see it. ## Which plate of the cord gives rise to anterior horn motor neurons? The basal plate, ventral to the sulcus limitans. ## Which signal secreted by the notochord ventralizes the neural tube? Sonic hedgehog (SHH), later also made by the floor plate. ## At what vertebral level does the cord end at birth, and why does it rise? Near L3; the vertebral column outgrows the cord, so the conus ends ever higher (L1–L2 in adults). [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/embryology/spinal-cord-alar-and-basal-plates-and-the-ascent-of-the-cord/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) - [Conus medullaris](https://durgical.com/en/anatomy/conus-medullaris/) - [Filum terminale](https://durgical.com/en/anatomy/filum-terminale-internum/) - [Cauda equina](https://durgical.com/en/anatomy/cauda-equina/) - [Dura (dural sac)](https://durgical.com/en/anatomy/spinal-dura/) ## Related - [Neurulation and closure of the neural tube](https://durgical.com/en/embryology/neurulation-and-closure-of-the-neural-tube/) - [Neural crest cells and their derivatives](https://durgical.com/en/embryology/neural-crest-cells-and-their-derivatives/) - [Brain vesicles: from three to five and what each becomes](https://durgical.com/en/embryology/brain-vesicles-from-three-to-five-and-what-each-becomes/) - [Neural tube defects: anencephaly, spina bifida and encephalocele](https://durgical.com/en/embryology/neural-tube-defects-anencephaly-spina-bifida-and-encephalocele/) - [Chiari malformations](https://durgical.com/en/embryology/chiari-malformations/) - [Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acid](https://durgical.com/en/embryology/molecular-regulation-shh-wnt-fgf-bmp-hox-genes-and-retinoic-acid/) - [Vertebrae: resegmentation of the sclerotomes](https://durgical.com/en/embryology/vertebrae-resegmentation-of-the-sclerotomes/) ## Related conditions - [Spina bifida (myelomeningocele)](https://durgical.com/en/conditions/spina-bifida/) - [Neurogenic bladder](https://durgical.com/en/conditions/neurogenic-bladder/) - [Scoliosis](https://durgical.com/en/conditions/scoliosis/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: [13.1 The Embryologic Perspective - Anatomy and Physiology 2e](https://openstax.org/books/anatomy-and-physiology-2e/pages/13-1-the-embryologic-perspective) (OpenStax, 2022); [Tethered Spinal Cord Syndrome](https://www.ninds.nih.gov/health-information/disorders/tethered-spinal-cord-syndrome) (National Institute of Neurological Disorders and Stroke, 2024); Langman's Medical Embryology, 13th ed., ch. 18: Central Nervous System (Wolters Kluwer, 2015). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/spinal-cord-alar-and-basal-plates-and-the-ascent-of-the-cord/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/spinal-cord-alar-and-basal-plates-and-the-ascent-of-the-cord/