Endoderm

The endoderm is one of the three main layers that form very early in an embryo. It is the innermost layer, with the other two layers being the ectoderm on the outside and the mesoderm in the middle. Cells that move inward along a structure called the archenteron create the inner layer of an early stage of development called the gastrula, which becomes the endoderm.

Visualization of the three germ layers of a gastrula.

In animals, the endoderm starts as flat cells but later changes shape. It becomes the lining for important body systems, including the respiratory tract, which we use to breathe, and the gastrointestinal tract, which helps us digest food. This happens through special signals that tell the cells where to go and what to become.

In plants, the endoderm is found in the innermost part of the cortex, or bark, of young shoots and roots. It is often just one layer of cells. As the plant grows older, this endoderm layer can change and become stronger through a process called lignify.

Formation of the Endoderm Layer

The endoderm is the innermost layer of cells in very early embryos. It forms through a process called the nodal signaling pathway. In all animals with a backbone, this layer develops where there is a lot of nodal signaling.

In humans, the endoderm starts to form organs after about 5 weeks of embryonic development.

Production of Tissues

The endoderm forms the inner linings of important body tubes, including those for digestion and breathing. These linings help our bodies take in nutrients from food and exchange gases when we breathe. Special patterns guide the endoderm cells to create organs such as the liver and pancreas.

Layer	Category	System
General	Gastrointestinal tract	the entire alimentary canal except part of the mouth, pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm), the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas
General	Respiratory tract	the trachea, bronchi, and alveoli of the lungs
General	Endocrine glands and organs	the lining of the follicles of the thyroid gland and the epithelial component of the thymus (i.e. thymic epithelial cells).
	Auditory system	the epithelium of the auditory tube and tympanic cavity
	Urinary system	the urinary bladder and part of the urethra