Fate of the Germinal Layers

Once the three germ layers — ectoderm, mesoderm and endoderm — are established at gastrulation, each is committed to giving rise to a characteristic, non-overlapping set of tissues and organs of the adult body, a mapping summarised in Table 2.16 below. In broad terms, the outer ectoderm gives rise to the skin and its appendages (hair, nails and the skin's glands), the sense organs of the eye and ear, and the entire nervous system; the middle mesoderm forms the muscles, connective tissues, the heart and blood vessels, and the urinary and reproductive organs; and the inner endoderm lines the gut and forms the associated digestive and respiratory organs and glands, such as the liver, pancreas and thyroid. This layer-wise commitment of fate is what underlies the histogenesis and organogenesis that follow gastrulation, as the simple three-layered embryonic disc is progressively built up into the distinct organ systems of the developing foetus.