From the so-called blastocyst, the human embryo develops. During gastrulation, the blastocyte produces three different cotyledons: the inner, middle and outer cotyledons. The inner cotyledon is also known as endoderm or endoderm.
The middle is the mesoderm and the outer is the ectoderm. Cell differentiation in three cotyledons is the first differentiation of the embryo into individual cell layers in the developmental biology of tissue animals. Only from these cell layers form the different structures. After further differentiation and the so-called determination, tissue and organs are formed from the cotyledon cells. The cotyledons are formed in the blastula.
As such, the embryonic stage is referred to after the morula stage, which completes the furrowing of the zygote. The early embryo development of mammals is also described by the term triploblastic due to the differentiation in three cotyledons. The cells of the three cotyledons are not yet determined, that is, they are multipotent. What types of tissue they actually become, comes first with the determination that defines the development program of daughter cells of a particular cell.
From about the 17th day, the primitive streak develops during embryogenesis. This strip forms the entry point for profiling and immigration of the 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 inflow time, the epiblast cells move away from the primitive streak and travel in different directions. The first cells, after their migration through the node of the primitive streak, replace the layer of hypoblasts and form the endoblast, from which later the gut and its derivatives develop. The remaining cells, after their migration through the primitive node, go cranial at about the same time, where they form two further structures.
The Prächordalplatte forms cranial to the primitive node. In addition, there develops the extension of the Chorda dorsalis. The remainder of the immigrated cells generates a third cotyledon known as intraembryonic mesoderm. Only at the cloacal membrane and the pharyngeal membrane no middle cotyledon develops. Here, ectoderm and endoderm are in direct contact. Caudally, the cloacal membrane forms the future opening of the rectum and urogenital tract.
Like the mesoderm and the ectoderm, the endoderm is also predominantly relevant to the differentiation of the individual body tissues and organs. The blastula is the starting point of gastrulation. In higher mammals, it is the blastocyst, a hollow sphere made of a single cell layer. This blastocyst is transformed into a bilayered boll germ called gastrula.
The endoderm forms the inner of the two primary germ layers is the endoderm. The exterior of the cotyledons is the ectoderm. The endoderm carries an opening to the outside. This opening is also known as the Urmund or Blastoporus. The endoderm is often called the primary or archenteron. Around the same time as the development of the two primary cotyledons, the mesoderm is formed. The development of the Urmunds makes man a so-called new-moon or Deuterostomier. Unlike the primal mouths, the Urmund develops in the Neumündern for anus. The mouth breaks only after completion of gastrulation on the opposite side of the blastula.
A basic movement of gastrulation is the initial involution of the endoderm into the blastocoelast of the blastula, which appears as an internal and fluid-filled cavity. Due to the deformation of pole cells at the blastula, an evacuated space forms, the internal part of which is the endoderm. The outer part is the ectoderm. The endoderm narrows the primary body cavity during deformation. The prospective endoderm rolls in later. There is an immigration of endodermatic cells. Blastula cells finally fuse the endodermic cells into the blastocoel. In yolk-rich eggs, the prospective ectoderm later also 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, the digestive glands such as the liver or the pancreas and the respiratory tract, the endodermal organs include, above all, the thyroid gland, the urinary bladder and the urethra.
In the context of endoderm, especially genetic diseases play a role. For example, the inner cotyledon may be affected by mutations that cause dysplasia during embryonic development or that may partially or completely miss certain organs.
In endodermal tissue, malformations most commonly affect the urinary tract. However, the liver and pancreas may also be affected. Endodermal dysplasia may be hereditary. However, they can also be triggered by exogenous factors. Known in this context, for example, the so-called cat's eye syndrome. It is a rare and hereditary disease, which is associated with key symptoms such as the vertical-oval gap formation of the iris or a malformation of the rectum.
The cause of the dysplasias is believed to be a developmental defect in the so-called chordablastem. Genetically related cases are associated with a mutation of the RAS homologous gene or the homobox gene. The mutation of these genes should cause a disturbed separation of endoderm and neuroectoderm. In addition to endodermal dysplasia, ectodermal and mesodermal dysplasia and dysgenesis are common causes of congenital diseases and may be associated with endodermal malformations or even overlap causally.