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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.
The embryonic period
Published in Frank J. Dye, Human Life Before Birth, 2019
When the neural folds fuse to form the neural tube, a region of ectoderm (neuroectoderm) disappears from the surface and the balance of the ectoderm (epidermal ectoderm) closes over it. Actually, a small population of ectodermal cells disappears from the surface but is not incorporated into the neural tube; this population of cells is called the neural crest (see Figure 8.9A). Neural crest cells migrate from their initial position and give rise to a variety of structures in the developing embryo; for example, ganglia (collections of nerve cells outside the CNS), pigment cells (some of which provide us with a tan when we are exposed to the sun), and some cells (medulla) of the adrenal glands (which produce the hormones that prepare us for “fight or flight”).
Skeletal Embryology and Limb Growth
Published in Manoj Ramachandran, Tom Nunn, Basic Orthopaedic Sciences, 2018
Rick Brown, Anish Sanghrajka, Deborah Eastwood
The ectoderm forms the neural tube by a process called neurulation. In the midline, ectodermal cells become elongated, causing a relative thickening in that region (the neural plate). The two edges of the neural plate thicken to form the neural groove and neural folds. The neural tube forms by the fusion of the edges of these folds, which begins near the anterior end of the embryo and proceeds in anterior and posterior directions. The neural groove closes by the third week, except at its ends (the anterior and posterior neuropores), which normally close by the end of the fourth week. Failure of the anterior neuropore to close results in anencephaly, whilst neural tube defects (NTDs) are most commonly thought to arise from a failure of the posterior neuropore to close.
Congenital Spinal Lipomatous Malformations. Part 1. Spinal Lipomas, Lipomyeloceles, and Lipomyelomeningoceles
Published in Fetal and Pediatric Pathology, 2020
Very early, the embryonic primitive streak forms the mesoderm and produces notochordal cells in Hensen’s node that probably induce the overlying ectoderm to thicken into the neural plate [17, 31]. The neural plate, covering much of the posterior early embryo, folds medially at its lateral borders with the embryonic ectoderm, possibly by compression from the ectoderm assisted by force generated by the mesoderm [41]. When the lips of the neural folds form at the ectoderm-neural plate borders and move medially to close dorsally and mesially, the primary neural tube is formed. The posterior neuropore at the caudal end of the primary neural tube closes at the level of the future S3–S5 vertebrae (Figure 2). The primary neural tube detaches (disjunction) from the ectoderm to complete primary neurulation. Failure of disjunction or a failure of posterior neuropore closure may cause open neural tube defects, the most common malformations of spinal dysraphism. The most common open neural tube defects over the spine include myelomeningoceles and craniorachischisis totalis [17, 40, 42–44].
Congenital Spinal Lipomatous Malformations. Part 2. Differentiation from Selected Closed Spinal Malformations
Published in Fetal and Pediatric Pathology, 2021
Embryonic formation of a nonterminal myelocystocele begins as incomplete (limited, segmental) dorsal closure of the lips of the neural folds of the primary neural tube. The unapposed neural folds also suffer focal failure of disjunction from embryonic ectoderm [1]. The result is limited dorsal myeloschisis (focally open neural folds). The original continuity of the neural folds with embryonic ectoderm is maintained as a permanent focal attachment, or tether, of the spinal cord by the skin. The roof ostium formed in the primary neural tube is thought to be so minor that the fetal dermis simply covers it over. The closed skin at the adhesion site may be dystrophic (interrupted areas of epidermal atrophy and dermal fibrosis) [49, 54].
Spina bifida and pediatric cancers
Published in Pediatric Hematology and Oncology, 2020
Julia E. Heck, Pei-Chen Lee, Chia-Kai Wu, Chung-Yi Li, Di He, Noah Federman, Fei Yu, Jorn Olsen, Beate Ritz, Onyebuchi A. Arah, Johnni Hansen
Spina bifida is a birth defect resulting from failed closure of the neural folds in the first month of gestation, resulting in vertebrae that do not completely form. Global incidence varies by time period and region, with a meta-analysis estimating global risk as 3.5-24.3 per 10,000 pregnancies.1 It is detected at ultrasound or via alpha-fetoprotein testing in the second trimester. Spina bifida occulta (or closed spinal dysraphism) is the mildest form of the condition and can go unnoticed because the meninges and nerve tissue are covered by skin. Spina bifida aperta (or open spinal dysraphism) is more severe and can result in sensory, motor, or orthopedic impairments, and bowel and bladder dysfunction.