The mesoderm develops into muscle, the skeletal system, some organs, and connective tissue. The ectoderm differentiates into the nervous system and skin. As the embryo continues to develop, individual cells continue to differentiate. These differentiated cell types are made from what were initially the same types of pluripotent embryonic stem cells.
Embryonic stem cell differentiation: emergence of a new era in biology and medicine
An assortment of physiological mechanisms guides certain cells towards particular developmental pathways, creating varying cell types. Every cell contains DNA within the nucleus , containing the blueprint to build many different proteins in the cell. Different signals can cause embryonic cells to select specific parts of the DNA which can then be used to synthesize proteins, eventually building different cell types.
Differentiation of cells in the embryo is brought about by both internal cellular factors as well as extracellular factors that act on the cell from the outside. Much remains to be understood about the exact molecular interactions that govern cellular differentiation. It is understood, however, diversifying the ratio of and types of internal and external influences on certain cells, allows many divergent cell types to arise.
There are two main types of cellular development that pertain to embryos: mosaic development or regulative development. In mosaic development which is not characteristic of mammals, but of organisms such as annelids differentiation occurs in steps that are set in order and progression, without input occurring between neighboring cells. On the other hand, regulative development involves the interaction of adjacent cells, within what is known as embryonic fields.
Mesoderm - Wikipedia
The advantage of regulative development is the flexibility that it confers to differentiation. The process of embryonic differentiation is crucial to proper animal development. The processes involved in embryonic differentiation continue to be explored and have relevance to studies involving embryonic stem cells and in vitro cell differentiation.
As scientists continue to study the physiological mechanisms of embryonic development the process of embryonic differentiation should continue to be understood in greater and greater detail. Other animals such as chordates produce a third layer the mesoderm between these two layers, making them triploblastic.
Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. In , Heinz Christian Pander discovered three primordial germ layers while studying chick embryos. Between and , Robert Remak had further refined the germ cell layer Keimblatt concept, stating that the external, internal and middle layers form respectively the epidermis, the gut, and the intervening musculature and vasculature.
Among animals , sponges show the simplest organization, having a single germ layer. Although they have differentiated cells e. Diploblastic animals, Cnidaria and Ctenophora , show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues.
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All higher animals from flatworms to humans are triploblastic , possessing a mesoderm in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs. Fertilization leads to the formation of a zygote.
During the next stage, cleavage , mitotic cell divisions transform the zygote into a hollow ball of cells, a blastula. This early embryonic form undergoes gastrulation , forming a gastrula with either two or three layers the germ layers.
In all vertebrates , these progenitor cells differentiate into all adult tissues and organs. In the human embryo , after about three days, the zygote forms a solid mass of cells by mitotic division, called a morula.
This then changes to a blastocyst , consisting of an outer layer called a trophoblast , and an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida and undergoes implantation. The inner cell mass initially has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears. The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called the endoderm , pushing the hypoblast out of the way this goes on to form the amnion.
The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm.
The endoderm is one of the germ layers formed during animal embryonic development. Cells migrating inward along the archenteron form the inner layer of the gastrula , which develops into the endoderm.
The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tract except part of the mouth and pharynx and the terminal part of the rectum which are lined by involutions of the ectoderm. It also forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.
The endoderm forms: the pharynx , the esophagus , the stomach , the small intestine , the colon , the liver , the pancreas , the bladder , the epithelial parts of the trachea and bronchi , the lungs , the thyroid , and the parathyroid. The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation , some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.
The mesoderm has several components which develop into tissues: intermediate mesoderm , paraxial mesoderm , lateral plate mesoderm , and chorda-mesoderm.