Basics of Embryology Quiz

See The Third Week. Option A is correct. In the third week of development the epiblast (the upper layer of the two-layered embryonic disc) forms three germ layers, which are the three primary sheets of cells from which all the tissues of the body develop. This process, called gastrulation, produces the ectoderm, the outer layer, the mesoderm, the middle layer, and the endoderm, the inner layer. The ectoderm gives rise to the skin's outer layer (the epidermis), hair and nails, and also to the central and peripheral nervous systems and the sensory organs. The mesoderm forms the skeleton, muscles, connective tissue, heart, blood vessels and kidneys. The endoderm forms the epithelial lining of the gastrointestinal tract, together with the liver, pancreas and lungs. So of the tissues listed, the epidermal skin cells are the one of ectodermal origin, whereas heart muscle, gut smooth muscle and renal tubules are mesodermal, and liver cells are endodermal. The word ectoderm itself hints at this, since it comes from the Greek for 'outside skin'.

• Germ layers: gastrulation in the third week forms the ectoderm, mesoderm and endoderm from the epiblast.
• Ectoderm: it gives the epidermis, hair, nails, nervous system and sensory organs.
• Answer: epidermal skin cells are ectodermal, whereas heart, gut smooth muscle and renal tubules are mesodermal and liver cells are endodermal.