What are the three germ layers and when do they form?

The three germ layers are the ectoderm, mesoderm, and endoderm. They form a trilaminar embryonic disc by the third week of development, giving rise to all tissues and organs in the body.

What is gastrulation and how does it relate to germ layer formation?

Gastrulation is the process during the third week that forms the three germ layers from the embryoblast. It begins with the appearance of the primitive streak in the ectoderm, leading to cell migration and layer differentiation.

How are the germ layers derived according to the current concept?

All three germ layers derive from the epiblast. Epiblast cells migrate to the primitive streak, with some forming endoderm, others forming mesoderm, and the rest becoming ectoderm.

What structures develop from each germ layer?

Ectoderm forms epidermis, nervous system, lens of eye, and enamel of teeth. Mesoderm forms muscles, bones, cartilage, blood vessels, heart, and kidneys. Endoderm forms epithelia of gastrointestinal, respiratory, and urogenital tracts, plus liver and pancreas.

What is the role of the primitive streak in germ layer formation?

The primitive streak appears in the ectoderm and is crucial for gastrulation. Cells from the streak migrate between ectoderm and endoderm to form the intraembryonic mesoderm, helping establish the three germ layers.

What are the extraembryonic structures mentioned, and how do they relate to the germ layers?

Extraembryonic structures include the chorion, amnion, and connecting stalk. They form from extraembryonic mesoderm and support the embryo but are not part of the germ layers that give rise to body tissues.

Germ Layers – Gastrulation & Trilaminar Disc