What is meant by histogenesis? What is its purpose? What ailments or diseases can result from misdirected histogenesis? This question will be answered in the following article.
What is histogenesis?
Histogenesis describes the embryonic development of differentiated tissues with various tasks and functions by a genetically anchored program. This differentiated tissue arises from the undifferentiated fertilized egg.
Function and task
Histogenesis describes the embryonic development of differentiated tissue. This differentiated tissue arises from the undifferentiated fertilized oocyte. The fertilized egg cell is initially pluripotent: the cells contain the complete genetic information and the complete control program to form a human being. For this reason, identical twins can also be formed from an embryonic plant that divides and migrates apart in the first few days after fertilization. Once an egg has been fertilized, the morula forms through many cell divisions. At the time of implantation in the endometrium, the cells in the morula differentiate into “inside” and “outside” – the blastocyst is formed. From the eighth day after fertilization, the embryonic anlage is formed. Another name of the same is embryoblast or germinal disc. It first exists as a two-bladed germinal disc. From the third week after fertilization, further remodeling takes place. The three-leaf germinal disc is formed from the epiblast of the two-leaf germinal disc. It consists of the mesoderm (middle cotyledon), the entoderm (inner cotyledon) and the ectoderm (outer cotyledon). At this time, there is already a right-left asymmetry in the primitive node located in the middle of the germinal disc, which corresponds to the later asymmetry in the arrangement of the thoracic and abdominal organs. Histogenesis plays a particularly important role in the embryonic period, as all organs are laid out in their blueprint during the first weeks of gestation. Cells from cotyledons migrate to different locations to form organs. On the trunk, buds develop from which the extremities emerge. In the face, different structures grow together internally and externally. Thus, the eyes, ears, nose, lips, jaw, palate, and pharynx form.
Diseases and ailments
If something goes wrong in the “building instructions,” the asymmetrical orientation of the primitive node may be incorrectly laid out during the time of development of the trifoliate germinal disc. This causes the cells, which are to form the organs later, to migrate in the wrong directions. The internal organs are then laid out in a different arrangement. This can affect all internal organs (situs inversus totalis) or individual ones. In Kartagener syndrome, the redistribution of organs occurs randomly because the cilia that usually move the cells into places are immobile. People with Kartagener syndrome may therefore have improperly laid out internal organs. Also, sinusitis (sinusitis) and brochnial infections occur more frequently due to the lack of cilia function, as the self-cleaning of the airways fails. Severe disturbances in histogenesis end in intrauterine amniotic death, miscarriage or premature birth. If the children are born alive, severe physical disabilities may result. If the neural tube does not close properly during the embryonic period (day 22 to 28 of embryonic development), spina bifida may occur. Spina bifida can have various precursors and manifestations. Affected individuals may have increased hairiness on the back, which takes on a typical pattern along the spine and extends to the sacroiliac joint. Restricted movement, paraplegia, and problems walking may result. Bladder and bowel control may be impaired. Sometimes spina bifida is associated with other disorders or disabilities, such as hydrocephalus. Cleft lip and palate results from inadequate closure of structures that form the face. In the 5th-7th week of pregnancy, maxillary bulges merge with nasal bulges in the right and left sides of the face. If disturbances occur here, the cleft lip develops. In the 10th-12th week of pregnancy, the palatal processes fuse with the maxillary bulges and, in the front, with the intermaxillary segment. If disorders occur – often associated with the cleft lip – the cleft palate develops.Depending on the severity, this can lead to drinking, swallowing, breathing and speaking disorders. In the case of poor ventilation, there is a subsequent accumulation of throat, nose and ear diseases. Cleft formations, called colobomas, can also occur in the embryonic period on the eyes. In the 7th week of pregnancy, the eye vesicles are transformed into the eye cup. Due to insufficient closure, usually towards the nasal side at the bottom, the coloboma is formed. Colobomas of the eyelids, iris, lens, retina, choroid and optic nerve are possible. In coloboma of the optic nerve (optic coloboma), visual function may be impaired. Strabismus and eye tremor (nystagmus) occur. In the 7th week of pregnancy, the germ cells are created, which will develop into arms and legs, and later into hands and feet. In the 8th week of pregnancy, the beginnings of the later legs and arms are visible. One week later, arms, legs, hands and feet are already formed, whereby the fingers and toes are still connected by webs. If histogenesis is misdirected, malformations of extremities occur: for example, an arm may not be fully developed or supernumerary fingers and toes or clubfeet may be formed. Further damage in histogenesis is also known, for example due to infectious diseases such as rubella, toxoplasmosis, alcohol, nicotine, medication or drug abuse during pregnancy. In areas with radioactive contamination, more and more children were and are born with congenital malformations. In the sixties of the last century, quite a few women took the sleeping pill thalidomide during their pregnancy, which often resulted in severe malformations of their unborn children. Frequently, however, damage in histogenesis is also due to rare chronic diseases of a genetic nature. Information portals have been created for those affected: at the European level these are ‘Orpha Net’ and ‘Eurordis’, and at the German level the ‘Dachverband der Achse’.
