The germ layer, also known as the reproductive epithelium, is less used and is a group of cells when the animal embryo is formed. All animals have germ layers in which the vertebrate germ layer structure is particularly pronounced, while the sponge animal's germ layer is the simplest and usually produces two to three major tissue layers (sometimes referred to as primary germ layers). Radiation-symmetric animals (eg, coelenterates) have two germ layer structures, including endoderm and ectoderm; bilaterally symmetrical animals have three germ layer structures, more than radiation-symmetric animals, endodermal and external mesoderm between germ layers. All cells within the germ layer eventually develop into various tissues and organs of the animal.
Germ layer differentiation
Germ layer differentiation is the process of differentiation and development from the original outer and middle mesoderm to different tissue structures. Structural differentiation is accompanied by functional differentiation, often irreversible, and is the result of the interdependence and interaction of the three germ layers. Different tissues are organically linked together to form the various primitive organs of the embryo.
Initially, the posterior ectoderm of the gastrula is thickened to form a nerve plate under the induction of the notochord and the parameal mesoderm. As the notochord lengthens, the nerve plate is widened and elongated, while the central depression is a nerve groove, and the ridges on both sides are nerve pleats. On the 22nd day, the nerve groove began to close. The first step is to close the body plane from the 4th to 5th, and then continue to the head and tail. While the nerve folds heal, some of the cells outside the nerve plate become neurotropic, about between the neural tube and the ectoderm. The neural tube and nerve stalk further differentiate into central and peripheral nervous systems, and the remaining ectoderm differentiates into epidermis and its derivatives, as well as mucosal epithelium and glandular epithelium of each natural hole.
The most primitive endoderm is a layer of cells that surround the inner surface of the primitive intestine. As the gut becomes differentiated into the foregut, midgut, and hindgut, the endoderm gradually differentiates into the epithelium from the pharynx to the rectum, the glandular epithelium in the gut, and the epithelium of the liver, pancreas, gallbladder, and the like. The epithelium of the larynx of the anterior intestine and the ventral side of the esophageal primordia differentiate into the larynx, trachea, bronchi and lungs of the respiratory system. The urethra and bladder and other epithelium in the urinary system are differentiated from the hindgut. The thyroid, parathyroid gland and thymus are differentiated from the original pharynx.
As the gut is formed, the notochord and its mesoderm are separated. The notochord is a supporting organ of the vertebrate embryonic stage, and finally the degeneration disappears or forms the nucleus pulposus in the intervertebral disc. The adult spine forms a mesoderm around the notochord. The layer is a layer of cells. With the formation of the cylindrical embryo body, the embryonic mesoderm Hyperplasia, forming the upper mesoderm (ie, the somite), the middle mesoderm, and the lower mesoderm on both sides. The embryo body continues to grow, the sports gradually increase, and further differentiate into the dermatomes, the tendon and the osteogenesis. The lower mesoderm divides into two layers, the outer layer called the mesoderm and the body wall ectoderm, and the inner layer is called dirty. The wall mesoderm is associated with the visceral endoderm. The mesoderm is divided into the genital cord and the reproductive ridge between the two mesoderm. They are the primordium of the genitourinary organs. Most mesoderm cells are dispersed to form a network of mesenchyme, which differentiates into connective tissue, cardiovascular system, most muscle tissue, cartilage and bone tissue of various organ tissues.
In human embryos, fertilization leads to the formation of zygotes. In the next stage, cleavage, split cell division transforms the zygote into a hollow balloon, the blastocyst. This early embryonic form undergoes gastric germ formation, forming a gastric germ with two or three layers (embryo layer). About three days later, the zygote formed a solid cell mass by mitotic division. It then becomes a blastocyst, which consists of an outer layer called the trophoblast and an inner cell mass called the blastocyst. The blastocyst is filled with uterine fluid, which is removed from the zona pellucida and implanted. The inner cell mass initially has two layers: secondary cells and ectoderm. At the end of the second week, the original streaks appear. The osteoblasts in this region move toward the original streak, sneak down into it, and form a new layer called endoderm, pushing the secondary blasts away (continuing to form the amnion). The second layer, the mesoderm. The top layer is now called the ectoderm.