Which of the following is a derivative of the ectoderm of the trilaminar embryo?
The neural plate, a thickened area of ectoderm, gives rise to the nervous system. The first step is the development of a groove in the neural plate. On either side of this groove are the raised neural folds. These neural folds then begin to fuse in the midline at approximately the midpoint of the neural plate. Fusion proceeds in a rostral and caudal direction; once completed, there still remains an open area of the "tube" at both the rostral and caudal ends. These openings are referred to as the rostral and the caudal neuropore. The rostral neuropore closes around day 26 and the caudal neuropore closes on approximately day 28. With the closure of the caudal neuropore, closure of the neural tube is completed. It has been convincingly shown that folic acid plays an impoant role in the closure of the neural tube. By giving women of childbearing age folic acid supplements, the rate of neural tube defects can be reduced dramatically. Yet, based on the embryology of the neural tube (with closure of the tube roughly by day 28), it is obvious that in order for folic acid supplementation to work, it should be staed pregestationally. Unfounately, most women do not seek prenatal care until after the neural tube has undergone its period of closure. Bone, muscle, and spleen are all derivatives of the mesoderm. This mesoderm is formed at the end of the second week post-feilization, when some cells of the primitive streak migrate laterally between the endodermal and the ectodermal layer. Germ cells are derived from the endodermal layer of the trilaminar embryo. They migrate as primordial sex cells from the endoderm of the yolk sac to the urogenital ridge.
Bone
Germ cells
Muscle
Spinal cord