Organogenesis refers to the process of development of organ systems during embryogenesis. In humans, organogenesis begins during the first to second week of embryo and ends around the 61st day of pregnancy with the onset of fetogenesis.
What is organogenesis
Organogenesis refers to the process of developing organ systems during embryogenesis. In humans, organogenesis begins during the first to second week of embryo and ends around the 61st day of gestation
During organogenesis, organs develop from the different cotyledons. Cotyledons are tissue structures that form during embryogenesis. In humans, three germ layers are distinguished. Different organs develop from the endoderm, mesoderm and ectoderm. In addition to the process of natural organogenesis, the development of artificial organs or artificial organ parts in the test tube is also called organogenesis.
Function and task
The most rapid development of the embryo occurs in early embryogenesis. Here, the three germ layers are formed, which then give rise to the organs during organogenesis. The digestive tract, liver, pancreas, thyroid, thymus, respiratory tract, urinary bladder and urethra form from the endoderm, the inner germ layer. Of particular interest is embryonic liver development. The liver, the central metabolic and detoxification organ of the human body, arises from a single bud of the endoderm. Gradual tissue proliferation then gives rise to the mature organ. In this process, the development of the hepatobiliary system can be divided into two steps. First, the development of the functional tissue of the liver, gallbladder and bile ducts occurs. Then the intrahepatic vascular system, i.e. the vascular system within the liver, develops. The skin, nervous system, sensory organs and teeth form from the ectoderm, the upper cotyledon of the embryoblast. The nervous system arises from the neural tube, which in turn forms from the 25th day of development by union of the two neural folds. By the middle of the sixth week, the formation of the neural tube and thus the creation of the nervous system is complete. Bones, skeletal muscles, connective tissue, heart, blood vessels, blood corpuscles, spleen, lymph nodes, lymph vessels, adrenal cortex, kidneys, gonads, the internal sex organs and the smooth muscles of the abdominal organs develop from the mesoderm, the middle cotyledon. The cardiovascular system is the first organ system to start working in the embryo‘s body. As early as the third week of pregnancy, the cardiovascular system is functional. During heart development, the heart temporarily consists of only one atrium and one ventricle. Only through a complicated formation of different walls does a separation into two ventricles and two atria occur. In particular, the cranial development of the embryonic head is an extremely complex process. The investment material for the skull comes from the neural crest, the mesoderm, the two upper pharyngeal arches and the so-called occipital somites. After the completion of organogenesis and the end of embryogenesis, the human form of the unborn child is already clearly recognizable. Gradually, the organs then differentiate during fetogenesis and assume their subsequent final function.
Diseases and disorders
Many clinically relevant diseases can result from disturbances in the various developmental stages of organogenesis. Until the onset of fetogenesis, the unborn is particularly susceptible to external disruptive factors, so there is a higher risk of miscarriage and malformation of the embryo, especially during the first weeks of pregnancy. If incomplete closure of the neural tube occurs during organogenesis, neural tube defects are the result. The malformations can appear in different ways. The most common neural tube defect is anencephaly. In anencephaly, large parts of the brain, meninges and skull bones are not fully developed. Anencephaly develops before the 26th day of pregnancy. Live-born children with this malformation usually die within a few hours after birth. Another neural tube malformation is spina bifida. This malformation develops approximately between the 22nd and 28th day of embryogenesis.Spina bifida is also known as “open back” because the vertebral arch or even the spinal cord membranes are split in two in children with this condition. Neural tube defects are usually caused by folic acid deficiency. Numerous malformations can occur during the complicated process of heart development. Most malformations are caused by disorders during ventricular formation. Ventricular septal defect is one such congenital malformation of the heart. Here, the cardiac septum between the two chambers of the heart has not completely closed. Depending on the size of the defect, a so-called left-right shunt can occur. In this case, oxygen-rich blood flows from the left ventricle into the right ventricle due to the pressure conditions. The additional blood volume puts a strain on the right ventricle. Dilatation of the heart occurs with the risk of subsequent heart failure. Combined malformations may also occur. One such is tetralogy of Fallot. In this case, the ventricular septal defect is joined by an enlargement of the right heart, a narrowing of the pulmonary artery and a so-called “riding aorta”, an anomaly of the aortic arch. Of course, any other organ can be affected by disorders in organogenesis. In particular, the consumption of alcohol and the use of drugs increase the risk that the unborn child will suffer a malformation during organogenesis. A well-known example of drugs that promote malformation is certainly thalidomide. The drug was sold as a sleeping pill under the brand name thalidomide and led to numerous severe damages during embryonic development in the late 1950s. Malformations can also be caused by various pathogens. Infections of the mother with rubella, toxoplasmosis and cytomegaly always pose a risk to the unborn child. X-rays or radioactive radiation can also cause malformations.
