How is cell development defined? What are its functions, what are its tasks in embryogenesis? What diseases can occur that affect cell development? All of this is discussed below.
What is cell development?
After fertilization occurs, the two half sets of chromosomes from the sperm and egg attach to each other and cell division begins. Figure shows morula stage. The mother’s egg and the father’s sperm each have half the set of chromosomes. After fertilization occurs, both half sets of chromosomes attach to each other and cell division begins. From the combination of these two hereditary factors, a unique human being is created. From now on, every single cell in the body has the same genetic information, the DNA. From the 2,4 and 8 cell stage, the morula develops on the third to fourth day after fertilization. Two days later, the morula has further developed as a germinal vesicle, with an inner cell mass, a cavity and an outer cell layer. During this time, the germinal vesicle must implant into the endometrium and establish deeper contact and exchange with the maternal organism. A great deal of energy is required for the developmental steps that are now about to take place. The germinal vesicle burrows so deeply that it is enveloped by the endometrium. Still all cells are pluripotent, they have the ability to differentiate into all kinds of cell types like clones or stem cells. There has first been a spatial distribution at the beginning of implantation. The cell mass of the germinal vesicle always faces the endometrium, the cavity faces the outside. During implantation, various differentiation processes take place: a cotyledon is formed at the site of the cell mass as a disc consisting of two layers: the ectoderm and the endoderm. Below the ectoderm, the anmion cavity is formed, which becomes the later amniotic sac with the amniotic fluid. During gastrulation, the germ has completely burrowed into the endometrium. At the same time, during the third week of development, further cell migration and cell divisions have taken place inside. The endoderm is simultaneously forming the yolk sac, and the ectoderm has increased somewhat in girth. The internal amniotic cavity has increased in size. Most importantly, the mesoderm has formed between the endoderm and ectoderm – the trifoliate germinal disc has emerged. At the outermost points of the germ, the mesoderm is absent. Here a cloacal membrane and a pharyngeal membrane will form. Axes of “above” and “below” have now also formed – the primitive streak has emerged. The ectoderm gives rise to the central and peripheral nervous systems and the skin. The mesoderm forms the skeleton, muscles and vessels; the endoderm the intestine, lungs and liver. With the formation of the primitive streak, the early phase in embryogenesis has been initiated, in which the formation of the organ systems is now decisive. This embryonic period lasts from about the third to the eighth week of development.
Function and task
As mentioned earlier, all body cells possess the identical genetic information. Over time, only certain genes are activated and others are deactivated in individual cells. If a pluripotent cell is to develop into a nerve cell, inducers will activate only those genes within that cell that are responsible for forming a nerve cell from that cell. The same scheme is followed in the development of specific cells, such as skin cells, blood cells, and all other cell and tissue types. This specialization work of embryonic cell development is now particularly active between the third and the eighth week of development: in addition to further development, remodeling, “demolition” and reverse development also occur. At the head end of the primitive streak lies the primitive node, whose cells are responsible for the growth of the cephalic process. As early as day 19, the neural plate and vascular system are formed. Embryonic hematopoiesis is begun. Four days later, the neural tube is formed. After the fourth week of development, the primitive streak is virtually absent. The neural tube is already the higher stage of development towards the spinal cord and brain and has replaced the chorda dorsalis (dorsal cord) coming from the mesoderm, which is practically completely regressing. On the 22nd day, the heart already begins to beat.On the 29th day the eye vesicles develop, one day later already the buds of the upper limbs, on the 32nd day those of the lower limbs. The embryo has now assumed a curved shape. One day later, the eyes and the cerebellum are laid out. On day 36, the ear bud and hand plate emerge. Two days later, the eyes pigment, and the lenses have already been put on in the preliminary stage. The foot plates are also put on. From day 41, the embryonic tail is regressed. Its remnants form the coccyx. External auditory canal and finger buds appear. On the 44th day, the eyelids are formed, and the nose and toe buds are laid down. 48 hours later, the embryo somewhat abandons its strongly curved posture. The outer ear is formed. The membranes of the bladder, genitals and anus break through. From the 49th day, the fingers are separated. On day 51, the vascular system under the scalp develops strongly. The nasal septum is formed and the palate is formed. On the 56th day, embryogenesis is complete. The chin and nasal cavities have been created. The external sexual organs develop. From the 9th week of pregnancy, the embryo has become a fetus, and the head makes up half of its length. All organs, tissues and the human form have been laid out in their basic features and must now slowly be further differentiated, grow and mature in function. The organs gradually take up their work. Until the formation of the liver, the yolk sac had the task of taking over metabolic functions. After that, the yolk sac is formed back.
Diseases and ailments
After innumerable genetically controlled processes occur during embryogenesis, a variety of aberrant processes are possible. During the first 14 days of germ development, malformations due to errors in genetic control lead to unnoticed spontaneous abortion. After implantation, the embryo is very sensitive to harmful substances, for example in the form of nicotine, alcohol, drugs, medications and X-rays. If mutations and malfunctions are very severe, miscarriage or premature birth will occur. In anencephaly, the skull has not closed during the embryonic period. As a result, the brain mass has leaked out and been decomposed by the amniotic fluid. If a child is born with anencephaly, it can survive only a few hours or days because it lacks all control functions, depending on the extent of the damage. If the parts of the face do not fuse together properly in the seventh week of pregnancy, a cleft lip and palate may result. The manifestations and extent vary. The children usually have difficulties in sucking, drinking, swallowing and speaking. In addition, the ventilation of the ear-nose-throat area through the cleft is not optimal, so that infections are more likely to occur there. Starting from the buds of the arms and legs, the extremities grow in length within a few days. If growth stops prematurely, the lower leg and feet or forearm and hands, for example, are missing. There are fused toes and fingers or supernumerary fingers and toes. Some deformities of extremities are part of a syndrome. In the case of Bardet-Biedl syndrome, there is a metabolic disorder of the cilia with involvement of the eyes as retinitis pigmentosa, hearing loss, and supernumerary toes. In addition, there is obesity, diabetes, and short stature. Malformations are found in the liver and gall bladder; the kidneys are prone to disease. In the field of ophthalmology, there are malformations such as incompletely formed eyes, congenital cataracts, clefts of the iris, choroid or optic nerve, and eyeballs that are too small or too large. Optic nerves may be equipped with too few nerve tracts, leaving the affected person functionally blind, depending on the severity. In Leber’s optic atrophy, the optic nerves of both eyes are affected. The mitochondria in the nerve cells of the optic nerve, which provide the required energy, do not have full functionality due to the genetic disease. This leads first to problems in the perception of the colors green and red, and later to central visual field defects and a massive loss of central visual acuity. Another genetic disease affects the cilia, which are located in every body cell and play a major role in cell migration during embryogenesis. Their job is to transport substances. In Usher’s case, they do not have full functionality. The auditory and visual sensory cells degenerate. The loss of hearing precedes the loss of visual function.Those affected are increasingly unable to compensate for their hearing loss (although this can be compensated for by hearing aids) through vision, as visual function is destroyed over time by degeneration, as in retinitis pigmentosa. Some genetic metabolic diseases result in low life expectancy, as in Hunter’s disease. Genetic diseases are inherited dominantly or recessively to a large percentage. Among relatives or in spatially remote areas, recessive diseases are more likely to occur. However, they are rare. Those affected often spend years searching for a diagnosis or therapy. Clinical competence centers have been established. Various umbrella organizations and portals have emerged to pool knowledge, such as ‘Achse’, ‘Orpha net’ and ‘Eurordis’.
