Embryology 4 Flashcards
Give an account of the process of neurulation and formation of the neural tube
The notochord signals the ectoderm to thicken producing the neuroectoderm. The neuroectoderm beings to thicken which gives rise to the neural plate. The neural plate elevates at the lateral edges. This continues until a groove is formed. The lateral edges then fuse together for form the neural tube. Fusion begins at the cervical (neck) region of the embryo and continues cranially and caudally. As the neural tube folds fuses, the tube separates form the surface of the ectoderm. The neural tube will then go onto form the brain and spinal cord.
Day 25: What alteration occurs to the neural tube?
The posterior neuropore and the anterior neuropore need to fuse. The anterior neuropore fuses first on day 25 and the anterior neuropore fuses on day 28. The anterior neuropore will go onto form the brain and the posterior neuropore will go onto form the spinal cord.
Describe the consequences of abnormal neurulation and neural tube defects.
1. Anencephaly
Anencephaly: Failure of the forebrain to form as a result individuals cannot speak, hear, see, comprehend speech and create memories. This occurs due to the failure of the anterior neuropore to fuse. Rarely a baby with anencephaly may survive a few years after birth however most are stillbirth or the newborn dies a few hours after birth. This is as the amniotic fluid will will be compress the brain preventing development. The brain stem is also normally exposed so individuals will not have basic life functions.
Describe the consequences of abnormal neurulation and neural tube defects.
2. Rachischisis
Rachischisis is a condition in which the neural plate fails to fuse to form the neural tube. The therefore flattened neural tissue is exposed to the external environment leading to a very high risk of infection. This anomaly occurs in utero, when the posterior neuropore of the neural tube fails to close by the 27th intrauterine (within the uterus) day. As a consequence the vertebrae overlying the open portion of the spinal cord do not fully form and remain unfused and open, leaving the spinal cord exposed. Patients with rachischisis have motor and sensory deficits, chronic infections, and disturbances in bladder function. This defect often occurs with anencephaly. The condition comes with a very high mortality rate.
What other conditions are classed as neural tube defects though there is not an issue with the neural tube?
- Spina bifida
- Meningocele
- Myelomeningocele
How can neural tube defects be ‘prevented’?
Folic acid. Folic acid is required for adequate neural tube formation. Sufficent levels fo folic acid are needed before conception. Neural Tube Defects are reduced by 50-70% by having sufficient levels of folic acid before conception.
Give an account of the origin of neural crest cells and their derivatives.
There are a subset of neural cells of the lateral regions of the neuroectoderm known as neural crests. As the neural plate folds and fuses, the neural crest cells serrate from the neural plate. These cells migrate laterally and vertically throughout the embryo, differentiating into a variety of structures.
Derivatives:
- Septum in the heart
- Adrenal medulla
- Pre-aortic ganglia
- Glial cells in peripheral ganglia
- Schwann cells
- Pharyngeal arch cartilages
Describe the consequences of abnormal neural crest cell development
If the neural crest cells migrate too far, they can lead to tumours. The NF1 gene is a tumour suppressor gene that is switched off neurofibromatosis leading to increased cell division. Neurofibromatosis is a genetic disorder that leads o tumours throughout the nervous system.
Give an account of folding of the trilaminar embryonic disc in the craniocaudal and lateral axes
The embryo folds due to the rapid growth of the amniotic sac. As a result, the weight of the amniotic sac causes the embryo to fold in both directions:
- Craniocaudal folding
- Lateral folding
The yolk sac stays relatively the same size.
The folding means the the two lateral ends come in close proximity and fuse. This causes the surface ectoderm to fuse in most places to form a tube. They fuse at all levels except for that of the midgut; this is as the yolk sac is in the way. The cranial and caudal end also come in close proximity but do not fuse.
What is the Viteline duct?
The duct that connects the definitive yolk sac with the intraembryonic mesoderm.
Describe ventral wall defects and how they occur..
Ectopia cordis - lateral folds fail to fuse in the thoracic cavity leaving the heart outside the body. Very rare.
Gastroschisis - Lateral folds fold to fail to fuse in abdominal region. This mans the intestines are outside the body wall. For a short period, the intestines must be kept in a septic bag as the baby does not have enough space inside the body for the intestines. As a result, we must wait until the baby has grown and created space for the organ.
