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Fertilization and normal embryonic and early fetal development
Published in Hung N. Winn, Frank A. Chervenak, Roberto Romero, Clinical Maternal-Fetal Medicine Online, 2021
Asim Kurjak, Ritsuko K. Pooh, Aida Salihagic-Kadic, Iva Lausin, Lara Spalldi-Barisic
During the latter half of the 3rd week, drastic changes for morphogenesis (development of the body form) begin. The embryonic disc elongates craniocaudally to become pear-shaped and then slipper like. Cranial to the primitive node, a thickening of the ectoderm appears bilaterally and is called the neural plate because it will eventually develop into the neural tissue. Between the bilateral neural plates, a longitudinal neural groove develops in the midline, flanked by the neural folds bilaterally. Around day 20, the bilateral neural folds begin to fuse with each other to form the neural tube, which is the primordium of the central nervous system (CNS; brain and spinal cord). The rostral (anterior) neuropore normally closes on days 24 to 26, and the caudal (posterior) neuropore closes by 28 days. When the neuropores close completely, three brain vesicles (forebrain or prosencephalon, mesencephalon or midbrain, and rhombencephalon) have already appeared. Rarely the neural tube fails to close completely, resulting in a variety of neural tube closure defects such as anencephaly, encephalocele, myeloschisis, and myelomeningocele.
Developmental Diseases of the Nervous System
Published in Philip B. Gorelick, Fernando D. Testai, Graeme J. Hankey, Joanna M. Wardlaw, Hankey's Clinical Neurology, 2020
James H. Tonsgard, Nikolas Mata-Machado
Encephalocele is a herniation of intracranial contents including the brain and meninges. It is a mass protruding from the skull, most commonly in the occipital area (Figure 9.6). Frontal encephaloceles are much less common, but are more frequent in the Asian population. Basal defects involving the sphenoid or ethmoid sinuses also are seen. The amount of herniated neural tissue in the defect is variable and in part determines severity of the deficits. The cause of encephaloceles is unclear although they are clearly defects in closure of the anterior neural tube. Nasofrontal lesions are thought to be due to defective separation of the neural and surface ectoderm at the site of closure of the anterior neuropore. Occipital encephaloceles may be due to defective segmentation of the bones of the posterior cranium.
The embryonic period
Published in Frank J. Dye, Human Life Before Birth, 2019
Although the process of neural tube formation starts during the third week, it is not completed until sometime during the fourth week. In the meantime, there are openings at the head end and tail end of the tube, referred to as the anterior neuropore and posterior neuropore, respectively (see Figure 8.9B and C). This means that the interior of the neural tube (the neurocoele) is in direct continuity with the fluid-filled amniotic cavity. If these anterior and posterior openings do not close, they will contribute to birth defects known as anencephaly and spina bifida, respectively (see Chapter 19). As you might expect, it is the head end of the neural tube that gives rise to the brain; the balance of the neural tube gives rise to the spinal cord (Figure 8.10).
Acalvaria – report of a case and discussion of the literature
Published in British Journal of Neurosurgery, 2019
Other views suggest that the problem is a failure of closure of the anterior neuropore. No genetic factors have been clearly established, although there are reports of associated abnormalities in acalvaria including holoprosencephaly, hypertelorism and cleft palate.3,4