The egg and sperm join creating the fertilized cell, which divides and multiplies forming a sphere of cells that will be an embryo. These cells differentiate into three tissues: endoderm, mesoderm and ectoderm.

Endoderm

• Innermost layer.
• Will form the viscera, digestive system (stomach, intestines...), lungs, liver, pancreas, bladder, urethra, glands such as the thyroid and thymus.
• This tissue is reflected in the concha of the auricle. It is innervated by the vagus nerve.
• It is related to metabolic activities of the internal organs and the autonomic nervous system.

Mesoderm

• Middle layer.
• Forms the muscles, both of the locomotor system as well as smooth and cardiac muscle. Joints, connective tissue, bones, circulatory and lymphatic systems. Urogenital organs. Suprarenal glands.
• It is represented in the antihelix, scaphoid fossa, triangular fossa and in parts of the helix. Innervated by the trigeminal nerve.
• It allows muscle actions and mobilizing defense mechanisms.

Ectoderm

• Outermost layer.
• Forms the skin, hair, eyes, teeth, nose. Peripheral nerves, spinal cord, brainstem, thalamus, hypothalamus. Sweat glands, pituitary, pineal.
• It is represented in the lobule and in the tail of the helix. Innervated by the cervical plexus.
• It is the layer that communicates us with the environment.