EMBRYOLOGY OF THE CENTRAL NERVOUS SYSTEM
Journal: BMJ (1939)

Central nervous system (CNS) embryology is a broad subject. This article serves as a summary of CNS organogenesis as well as a review the framework of embryology, the embryogenesis of the brain and spinal cord, various tests that can be performed in utero to test for CNS anomalies, and problems that may be encountered during embryogenesis, with particular attention to the CNS.The CNS system involves 3 germinal layers: ectoderm, mesoderm, and endoderm.   The ectoderm is the key initiating player in the embryogenesis of the CNS. The ectoderm is further sub-specialized as the (1) surface ectoderm, which differentiates into the epidermis, nails, and hair. The ectoderm is also sub-specialized to form the (2) neural ectoderm, which gives rise to the neural tube and neural crest, which subsequently give rise to the brain, spinal cord, and peripheral nerves.    The endoderm gives rise to the lining of the gastrointestinal and respiratory systems. It also gives rise to abdominal organs such as the liver, pancreas, and bladder.    The mesoderm is differentiated into 3 parts:      Paraxial mesoderm: This part of the mesoderm contains mostly somites which give rise to the axial skeleton, dermis, and muscle.    Intermediate mesoderm: This part of the mesoderm gives rise to the gonads, kidneys, and urogenital structures.     Lateral plate mesoderm is further classified into parietal mesoderm and visceral mesoderm, which give rise to the limb skeleton and muscular wall of the gut tube, respectively.    Embryological Transformations Because these changes do not occur at once, embryology is a complicated subject. The following timing of embryological developments, with particular attention to the CNS, offers a greater understanding of the process.   Weeks 1 to 3: Zygote formation, blastocyst, and gastrulation occur    Mid-fourth week: Embryo is linear and uniform; notochord formation occurs    Late-fourth week: Many forms of differential growth occur; upper limb buds always develop before the lower limb bud    Fifth week: Limb buds more pronounced    Sixth week: Can begin to see eyes and auricular hillocks, which will develop into the external ears    Seventh week: Formation of eyes, ears, and fingers    Late eighth week: Formation of all organ systems    Nine to 12 weeks (11 to 14 gestational age): Embryo has a large head, and small body and this is the time where the body grows in an attempt to catch up with the limbs. The genitalia can be recognized during this period giving a chance for parents to find out the gender of the embryo.    Thirteen to 16 weeks (15 to 18 gestational age): Coordinated limb developments and ossification of skull occur; Ovaries differentiate and contain primordial ovarian follicles that contain oogonia; the eyes face anteriorly, and ears are in place.     Seventeen to 20 weeks (18 to 22 gestational ages): Eyebrows and head hair visible at 20 weeks    Twenty-one to 25 weeks (23 to 27 gestational age): Type II pneumocytes to secrete surfactant. It is after this stage when babies are considered viable.    Twenty-six to 29 weeks (28 to 31 gestational age): Eyelids open; the quantity of white fat increases. The CNS has matured and can control breathing as well as temperature function. Additionally, the bone marrow takes over (from the yolk sac) as the major site of erythropoiesis.    Thirty to 34 weeks (32 to 36 gestational age): Maturation and growth of organs occur   Thirty-five to 38 weeks (37 to 40 gestational age): Baby now has a firm grasp with hands. Testes may have descended in males.