Myelination in intrauterine period starts at:

Ans: A (20 weeks) Ref: Langman's Medical Embryology, 9th edition, 2004, Page 443 &444Explanation:"Although myelination of nerve fibers in the spinal cord begins in approximately the fourth month of intrauterine life, some of the motor fibers descending from higher brain centers to the spinal cord do not become myelinated until the first year of postnatal life." Ref: Langman's Embryology"Myelination starts in different groups at different times, e.g. the ventral and dorsal nerve roots about the fifth month, the corticospinal fibres after the ninth month. In peripheral nerves the myelin is formed by Schwann ceils (derived from neural crest cells} and in the CNS by oligodendrocytes {which develop from the ventricular zone of the neural tube)." Ref: Grays Anatomy"The process of myelination begins at the fourth month of intrauterine life and is not complete till 2 to 3 years of postnatal life." Ref; Textbook of Human Embryology by RaniMyelinationMyelination of nerve fibers in the spinal cord begins approximately the fourth month of intrauterine life.Myelin is formed by repeated coiling of the Schwann cell membrane around the axon (peripheral nervous system).Oligodendroglial cells forms myelin sheath surrounding nerve fibers in the central nervous system.Also Note:16 weeks would be the most accurate answer. Among given options most approximate answer is 20 weeks.Motor fibers descending from higher brain centers to the spinal cord do not become myelinated until the first year of postnatal life.Tracts in the central nervous system become myelinated at about the time they start to function.Pyramidal tract is last tract to complete myelination, at 2 to 12 years.