The endoderm is one of the three embryonic germ layers that form all mature tissues and organs in the body. It forms internal structures, the lining of the digestive system, airways of the lungs, and other internal epithelial tissues.
The endoderm, or entoderm, is one of the three embryonic germ layers that together give rise to all mature tissues and organs in the body. From the inside out, the three cell layers are the endoderm, mesoderm, and ectoderm. Cell layers arise early in embryonic development, when a cavity called the archenteron, a tiny sphere of undifferentiated cells, forms in the center of the blastula, and the cells begin to rearrange themselves into germ layers. The endoderm is the first to form and begins forming in the human embryo about two weeks after fertilization. By the fifth week, the endoderm has already differentiated into organs.
The three cell layers are roughly equivalent to the location of the structures in the fully formed human body, with the endoderm forming internal structures, the ectoderm forming external structures, and the mesoderm forming intermediate structures such as the skeleton, skeletal muscles, and heart . Some animals lack a mesoderm. The archenteron is the precursor to the intestine, or digestive tract, and its appearance marks the beginning of germ layer formation, a process called gastrulation.
Endoderm cells are initially flat, but become columnar, with their height exceeding their width, as they differentiate. They eventually come to form the lining of the digestive system in the human body, except for parts of the mouth, throat and rectum. The endoderm also gives rise to much of the internal epithelial tissue that lines organs and glands.
The endoderm also forms the airways of the lungs: the trachea, bronchi and alveoli. It lines all the glands that open into the digestive tract, including the pancreas and liver, as well as the thymus follicles, which produce T lymphocytes or T cells, and the thyroid gland, which produces a number of important regulatory hormones. It also gives rise to the epithelium of the Eustachian tube and tympanic cavity in the ear, as well as the bladder and urethra.
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