2

CASE 2


To rule out chromosomal abnormalities and birth defects in the fetus of her 40-year-old gravid patient, the physician performs an amniocentesis and an ultrasound. The chromosomal analysis ruled out any chromosomal abnormalities; however, the amniocentesis showed that the alpha-fetoprotein (AFP) level was elevated. The ultrasound revealed that the fetus had spina bifida cystica.



WHAT FETAL STRUCTURES NORMALLY SYNTHESIZE AFP?


AFP, a glycoprotein, is synthesized by three fetal structures. These structures can be remembered by the acronym “GLY”: gut, liver, and yolk sac. Although small amounts of AFP normally enter the amniotic fluid, elevated concentrations are markers for neural tube defects and defects of the ventral abdominal wall. Because these defects are not covered by skin, AFP freely enters the amniotic fluid from the fetal circulation. AFP is similar to adult albumin and is capable of binding to numerous ligands, such as fatty acids, steroids, bilirubin, and retinoids. Consequently, elevated AFP is a consequence of neural tube defects and not a cause.



HOW DO THE VERTEBRAE AND INTERVERTEBRAL DISCS FORM EMBRYOLOGICALLY?


The vertebrae and intervertebral discs develop from the mesenchyme of sclerotomes around the notochord (Fig. 1-4). Each sclerotome contains two layers of mesenchyme arranged cranially and caudally. The cranial layer of mesenchymal cells is loosely organized, whereas the caudal layer is dense in its arrangement. Between the sclerotomes are mesenchyme and intersegmental arteries. The intersegmental arteries ultimately become the intercostal and lumbar arteries in the thorax and lumbar regions, respectively.


image

FIGURE 1-4 Development of vertebral column. A, Transverse section of a 4-week old embryo. B, Frontal section of embryo showing condensations of cells of the sclerotome. C, Transverse section of a 5-week old embryo showing development of a mesenchymal vertebra. D, Frontal section illustrating the development of the mesenchymal vertebral body and intervertebral disc.


(Moore K and Persaud TVN: The Developing Human, 7e. WB Saunders, 2003. Fig. 15-7.)

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Jun 16, 2016 | Posted by in ANATOMY | Comments Off on 2

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