Entoderm: Structure, Function & Diseases

The endoderm is the inner cotyledon of the blastocyte. Various organs develop from it by differentiation and determination, such as the liver. If this embryogenetic development is disturbed, organ malformations may occur.

What is the endoderm?

The human embryo develops from the so-called blastocyte. During gastrulation, the blastocyte gives rise to three different cotyledons: the inner cotyledon, the middle cotyledon, and the outer cotyledon. The inner cotyledon is also known as the entoderm or endoderm. The middle one is the mesoderm and the outer one is the ectoderm. In tissue animal developmental biology, cell differentiation into three cotyledons is the first differentiation of the embryo into individual cell layers. Only from these cell layers the different structures are formed. After further differentiation and the so-called determination, tissues and organs form from the cotyledon cells. The cotyledons develop in the blastula. As such, the embryonic stage is called after the morula stage, which completes the furrowing of the zygote. Early embryonic development in mammals is also described by the term triploblastic because of the differentiation into three cotyledons. The cells of the three germ layers are not yet determined, i.e. they are multipotent. What types of tissues they actually become only emerges with determination, which establishes the developmental program of daughter cells of a particular cell.

Anatomy and structure

Beginning around day 17, the primitive streak forms during embryogenesis. This stripe forms the entry site for profiling and immigration of epiblast cells. During their migration, these cells form pseudopodia and lose contact with each other. This phenomenon is known as gastrulation. Depending on their origin and the time of influx, the epiblast cells move away from the primitive streak and migrate in different directions. The first cells, after migrating through the node of the primitive streak, replace the layer of hypoblasts and form the endoblast, which later develops into the intestine and its derivatives. The remaining cells, after their migration through the primitive node, move cranially at about the same time, where they form two more structures. The prechordal plate forms cranially of the primitive node. In addition, the process of the chorda dorsalis develops there. The remainder of the migrated cells generate a third cotyledon known as the intraembryonic mesoderm. Only at the cloacal membrane and pharyngeal membrane does no middle cotyledon develop. Here, the ectoderm and endoderm lie directly on top of each other. Caudally, the cloacal membrane forms the future opening of the rectum and urogenital tract.

Function and tasks

Like the mesoderm and ectoderm, the endoderm is predominantly relevant to the differentiation of individual body tissues and organs. The blastula is the initial site of gastrulation. In higher mammals, it is the blastocyte, which is a hollow sphere made of a single layer of cells. This blastocyte is remodeled into a two-layered cup germ called gastrula. In this process, the inner of the two primary cotyledons is the endoderm. The outer of the cotyledons is the ectoderm. The endoderm carries an opening to the outside. This opening is also known as the primordial mouth or blastoporus. The endoderm is often called the primordial gut or archenteron. At about the same time as the two primary cotyledons develop, the mesoderm develops. The further development of the primordial mouth makes the human being a so-called new mouth or deuterostomes. Unlike the primordial mouths, the primordial mouth develops into the anus in the new mouths. The mouth does not break through until gastrulation is complete on the opposite side of the blastula. A basic movement of gastrulation is the initial invagination of the entoderm into the blastocoel of the blastula, which appears as an internal and fluid-filled cavity. Deformation of pole cells on the blastula results in the formation of an airless space, the inner portion of which is the entoderm. The outer portion is the ectoderm. The entoderm constricts the primary body cavity during deformation. The prospective endoderm later curls up. Immigration of endodermal cells occurs. Blastula cells eventually cord off the endodermal cells into the blastocoel. In addition, in yolk-rich eggs, the prospective ectoderm later overgrows the endoderm.Gastrulation overlaps with the onset of subsequent processes, such as neurulation. The endodermal tissue forms various organs in later stages of embryonic development. In addition to the gastrointestinal tract, digestive glands such as the liver or pancreas, and the respiratory tract, endodermal organs include the thyroid gland, urinary bladder, and urethra.

Diseases

In the context of the endoderm, genetic diseases play the most important role. For example, the inner cotyledon may be affected by mutations that cause dysplasia during embryonic development or cause certain organs to be partially and even completely absent. In endodermal tissue, malformations most commonly affect the urinary tract. However, the liver and pancreas may also be affected. Endodermal dysplasias may be hereditary. However, they can also be triggered by exogenous factors. A well-known example in this context is the so-called cat’s eye syndrome. This is a rare and hereditary disease associated with leading symptoms such as vertical-oval clefting of the iris or malformation of the rectum. The cause of the dysplasias is thought to be a developmental defect at the so-called chordal ablasm. Genetically caused cases are associated with a mutation of the RAS homologous gene or the homobox gene. Mutation of these genes is thought to cause impaired separation of the endoderm and neuroectoderm. In addition to endodermal dysplasias, ectodermal and mesodermal dysplasias and dysgenesias are also a common cause of congenital disease and may coincide or even causally overlap with endodermal malformations.