The mesoderm is called the germ layer or middle embryonic sheet of the three that the embryos of triploblastic animals have during the gastrula stage or embryogenesis, the first stages of embryonic development.
Mesoderm also appears in the human embryo, as we are triploblastic animals. The other two layers of the embryo are the endoderm, the inner layer, and the ectoderm, the outer germ layer. the mesoderm is between the two.
Different organs and tissues are formed from each germ layer. From the mesoderm are formed all types of connective tissue, all types of muscle tissue, non-epithelial blood cells (erythrocytes, leukocytes), the mesothelium and other tissues of various organs, for example, in the gonads and kidneys.
In the human species, the mesoderm and other germ layers develop through the process of gastrulation that occurs in the early stages of embryogenesis or embryonic development.
First, the zygote, formed by the union of the egg and sperm, divides to form the morula, a mass of undifferentiated cells. Subsequently, within the morula an orifice is formed, called the blastocoel, which is surrounded by a single layer of cells, called the blastoderm, and thus forms the first embryonic stage blastula.
Around the third week of gestation, gastrulation begins, at the end of which the blastula will have transformed into a gastrula, going from a structure with a single layer of cells to a structure with three different layers: ectoderm, mesoderm and endoderm.
Schematically, gastrulation occurs in these steps:The blastoderm, the single layer of cells in the blastula, will form the ectoderm. Some groups of ectoderm cells begin a process of invagination by folding into the blastula to form the endoderm. By cell division and differentiation of the ectoderm, a third layer is formed, the mesoderm, which remains between the ectoderm and the endoderm.
Evolution of mesoderm in organogenesis
Specific organs and tissues develop from each embryonic germ layer. Nervous tissue (brain, spinal cord, and all peripheral nervous tissue), outer layers of skin, and tooth enamel develop from ectoderm.
From the endoderm, the digestive tract and many of its associated organs are formed. From the mesoderm all other internal tissues arise, mainly of the connective type. Connective tissue is the most abundant fundamental tissue type in the body and includes, among others, muscle, bone, cartilage, adipose tissue, blood and lymph.
By the end of the third week, three parts can be distinguished in the mesoderm: paraxial mesoderm, intermediate mesoderm, and lateral mesoderm. In each of them, the mesenchyme will be formed, a type of connective tissue characteristic of the embryo with a high capacity for differentiation that will form various organs and tissues of the fetus.
Mesoderm also forms the mesothelium, a membrane formed by simple epithelial tissue over a layer of connective tissue that covers certain anatomical cavities: the pleura in the thoracic cavity, the pericardium, the peritoneum in the abdominal cavity (includes extensions of the peritoneum in Organs reproductive organs: the tunica vaginalis of the testes and the perimeter in the uterus).
Paraxial mesoderm appears at the end of the third week of gestation along the neuronal tube. It is formed by a type of cell called somitomeres They appear as a group of approximately 50 pairs that grow from the cephalic end (head) to the caudal end (tail) by the end of the fourth week.
Somitomeres that make contact with the neuronal plate in the head region are known as neuromerases later on will form the head mesenchyme to form prechordal mesoderm. This is where most of the connective tissues of the face and neck will form, including the jaw and other facial bones.
In the dorsal part, somitomeres are differentiated into somites and dorsal somitic mesoderm is formed from which tissues such as cartilage, muscle, bone or the dorsal dermis will be formed.
In the intermediate mesoderm, the urogenital structures, excretory apparatus and gonads. In the upper part of the intermediate mesoderm, the nephrotomy and in the caudal part appears the nephrogenic cord.
The lateral or latero-ventral mesoderm is divided into two layers, the parietal or somatic and the visceral or splenic.
parietal mesoderm and ectoderm form the folds and side walls of the embryonic body. The visceral mesoderm layer together with the endoderm form the walls of the intestinal tube.
From the lateral mesoderm develops the circulatory system, vascular endothelium and serous mesothelial membranes that line the anatomical cavities of the body (peritoneal, pleural, pericardial, etc.).