The mesoderm is the middle cotyledon of the embryoblast. Various tissues of the body differentiate from it. In mesodermal inhibition dysplasias, embryonic development is prematurely interrupted.
What is the mesoderm?
The embryo develops from what is called a blastocyst, also known as an embryoblast. In triploblastic organisms such as humans, the embryoblast has three distinct cotyledons: an inner cotyledon, a middle cotyledon, and an outer cotyledon. The cotyledons give rise to the first differentiation of the embryo. The fetus thus acquires different layers of cells that, over time, give rise to the different structures, organs and tissues. Cotyledons develop from the blastula during gastrulation. The inner cotyledon is also called the entoderm. The outer cotyledon is called the ectoderm. The mesoderm corresponds to the middle cotyledon. Its cells are formed in a human embryo during the third week of embryonic development. During an earlier stage, the epiblast and hypoblast have developed on the embryo. Between these two structures the cells of the mesoderm migrate. The term mesoderm is a term used in ontogenesis, which deals with the development of an individual. The mesenchyme, in particular, develops from the mesoderm. However, the two terms are not synonymous. Mesenchyme is a histological rather than ontological term.
Anatomy and structure
During the third week of development, a primitive streak forms on the embryo. In addition, the intraembryonic mesoderm is formed. During this process, a two-leaf germinal disc is remodeled into a three-leaf germinal disc. This process is called gastrulation. The primitive streak develops on the surface of the ectoderm and is a strip-like condensation of proliferating cells of the outer cotyledon. This stripe determines the longitudinal axis of the later body. At the anterior end, the primitive streak is thickened and develops into the primitive node or Hensen’s node. The median plane of the primitive streak dips to the primitive groove. Cells of the ectoderm dip there. Between the ectoderm and the entoderm, they come to rest and form the median cotyledon. This intraembryonic mesoderm grows to the edges of the germinal disc. At the edges, it becomes the extraembryonic mesoderm. The intraembryonic mesoderm does not form continuously. No mesoderm is formed at the cranial region of the prechordal plate and in the caudal region of the cloacal membrane. In the primitive node, the primitive pit forms, into which some ectoderm cells descend and move on to the prechordal plate. Thus, in the median line, a cell strand called the chordal process forms and serves as an attachment for the chorda dorsalis. The mesoderm tissue adjacent to the chorda dorsalis is subdivided into several sections: axial, paraxial, intermediate, and lateral mesoderm.
Function and tasks
The mesoderm is composed of multipotent embryonic stem cells. These cells have a high mitotic rate. Therefore, they play an important role in morphogenesis. Cell division and differentiation are summarized as morphogenesis. These two processes give an embryo its shape. They give rise to all the necessary tissue types, cell types and organs. The property of multipotency enables embryonic stem cells to differentiate into almost any cell type. It is only through determination that the final developmental program is set for the daughter cells of a cell. Accordingly, determinate cells lose multipotency. Consequently, the cells of the mesoderm are crucial for early development and the first steps of cell differentiation, because they are not yet determined and therefore exhibit multipotency. The mesoderm later differentiates into bone, muscle, vessels and blood. The development of the kidneys and gonads also takes place on the basis of mesodermal tissue. In addition, the connective tissues, the reproductive organs and the lymph nodes including the lymphatic fluid develop from the multipotent tissue via numerous intermediate steps. The extraembryonic mesoderm merely lines the chorionic cavity. The intraembrayonal mesoderm is the developing tissue. The axial mesoderm gives rise to the chorda dorsalis. The paraxial mesoderm becomes somites and the intermediate mesoderm becomes the urogenital system. The lateral plate mesoderm becomes the base of the serosa. A particularly famous development of the mesoderm is to the mesenchyme.From this embryonic connective tissue type, cartilage tissue, bones and tendons, as well as muscle tissue, blood, lymphatic tissue and fatty tissue are formed via differentiation in addition to the actual connective tissue.
Diseases
Cancers are often developmentally differentiated into endodermal, ectodermal, and mesodermal cancers. Ectodermal cancers begin in the tissues of the body surface, that is, the skin and mucous membranes. Cancers of the gastrointestinal tract, liver, pancreas, respiratory organs and genitourinary tract arise from epithelial structures. They are therefore called epithelial tumors and usually correspond to carcinomas. Since the mesoderm becomes bone as well as muscle and nervous tissue, cancers in these tissues are mesodermal cancers. The tumors usually correspond to sarcomas. The leukemias or blood cell cancers are also among the mesodermal tumor diseases. Mutations may also occur in connection with the tissues of the mesoderm. Such mutations usually lead to disgenesias or the so-called inhibition malformations. Inhibition malformations result from an interruption of embryonic development. This results in early arrest of organ development. Hypoplasia, aplasia and agenesis can be the result. In the worst case, organs are missing completely. The causes are of various kinds. Genetically determined inhibitory malformations are just as possible as exogenously determined ones. An example of a mesodermal inhibition malformation is provided by Rieger syndrome, in which iris dysplasia is present and the chamber angle of the eye is also missing.
