Embryonic Phase of Development MCQs

Gastrulation, the process of formation of the three germ layers, occurs during the third week. Epiblast cells form a thickening called the primitive streak, with a primitive knot or node located at its cranial end. Epiblast cells invaginate from this streak to form the mesoderm layer and to replace hypoblast with endoderm. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

All mesoderm is a derivative of the invaginating cells at the primitive streak. The invagination occurs at the beginning of the third week. It is interesting that endoderm cells are also derived from the epiblast during gastrulation, replacing the hypoblast. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

Once the mesodermal layer has formed, the only places at which the ectoderm and endoderm are in direct apposition are the prochordal plate cranially and the cloacal membrane caudally. In these areas, the embryonic ectoderm is fused with the endoderm of the roof of the yolk sac. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 43

Gastrulation is the process that converts epiblast from a bilaminar disc into a trilaminar embryonic disc, consisting of the three main layers: ectoderm, mesoderm, and endoderm. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

The notochord is thought to be an important structure in induction of nervous system development, axial skeleton development and other organogenic events. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

The lining of the gut is endodermal in origin, but the smooth muscle of the gut forms from the splanchnic layer of the lateral plate mesoderm. Visceral peritoneum also forms from splanchnic mesoderm, but the parietal layers of the serous membranes (peritoneum, pleura, pericardium) form from the somatic layer of lateral plate mesoderm. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

Paraxial mesoderm that forms along the notochord and neural tube divides and forms paired cuboidal bodies called somites starting in week three. They develop in a caudal direction from day 20-30 and give rise to most of axial skeleton (skull base, vertebral column, ribs, and sternum), all skeletal musculature, and the dermis of the skin. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51

Tonsils arise from endodermal epithelium lining the 2nd pharyngeal pouch. Kidneys & gonads arise from intermediate mesoderm. Hair is ectodermal. Skeletal muscle is from somites and somitomeres. Cardiac & most smooth muscle is from splanchnic mesoderm. Neural crest develops some vascular smooth muscle. Reference - Textbook of Langman's, 12th Edition, General Embryology Page number - 51