In this blog post, we will explain in an easy and interesting way how life begins in a fertilized egg and grows into a complex organism.
Until the 19th century, the main theory widely accepted to explain the process of life was “preformationism.” According to this theory, all living organisms exist as miniature versions of fully formed adults inside sperm and eggs, and these microscopic structures gradually expand to grow into complete adults. Preformationism was the dominant explanation for the origin of life for a long time, from the thinking of ancient Greek philosophers to the Middle Ages and early modern times. This was closely related to the religious worldview that regarded the manifestation of life as God’s design, and it was far from scientific verification in that it regarded the gradual changes that occur during the process of development as merely external expansion. However, in the mid-18th century, German biologist Caspar Friedrich Wolff opposed this preformationism.
Based on observation and logic, he argued that organisms start as fertilized eggs and gradually form organs during the process of development. This new perspective was called “epigenesis,” and it presented a fundamentally different view from the preformation theory in that development was not the expansion of a fixed blueprint, but a process of gradual formation over time.
In particular, Wolff proposed the hypothesis that a special substance plays a decisive role in the process of growing into an adult, and that this substance functions through a specific mechanism to drive the process of development. At the time, due to a lack of biological understanding at the molecular level, his claim remained a hypothesis, but its validity was gradually proven through scientific experiments.
At the end of the 19th century, German biologists Wilhelm Roux and Hans Driesch attempted an experimental approach to the process of development. By investigating the developmental potential of fertilized eggs and the segments divided from them, they provided empirical support for Wolff’s theory of epigenesis.
Roux hypothesized that the development of an organism is due to the distribution of factors specific to each blastula. To support this hypothesis, he used a red-hot needle to kill one of the blastulae of a two-cell embryo of a frog and then cultured it without separating it. As a result, the surviving blastula developed into an incomplete embryo with half a nerve cord and only one ear. This experiment added weight to the premise that each blastomere plays a specific role independently and contributes to development.
On the other hand, Driesch conducted a more decisive experiment using sea urchin embryos. He separated sea urchin embryos in the early stages of development (two-cell, four-cell, and eight-cell stages) and cultured each blastomere individually. As a result, he confirmed that each blastomere could grow into a complete organism. This experimentally proved that development does not proceed according to a predetermined structure, but is flexibly determined by interactions between cells and the environment. This greatly contributed to redefining the nature of development and had a decisive influence on the direction of developmental biology thereafter.
The scientific understanding of individual development was extended to precise observations of the phenomenon of fertilization. At the moment when a sperm and an egg fuse to create new life, the nuclei of the sperm and egg merge to form the nucleus of the fertilized egg, and it was discovered that the genetic material inherited from both parents combines to form the genetic basis of a new life. In particular, the fact that this genetic material is chromosomes, and that chromosomes are accurately replicated and transmitted without change during the process of cell division, from a fertilized egg to a blastocyst, and then gradually to a complete organism, played a decisive role in scientifically explaining the continuity of life and the phenomenon of heredity.
Through these discoveries, the mechanism of intergenerational transmission of genes and the organic relationship between individual development began to be clarified.
The answer to the question of the continuity of life, which is unbroken and continuous, lies in the stable transmission structure of genetic information. As our understanding of individual development deepened, scientists became interested in how long the totipotency of fertilized eggs and early blastomeres could be maintained.
This research raised the question of whether totipotency could still be preserved in the somatic cells of mature organisms, which soon led to interest in the regenerative abilities of animals. In particular, research on the differences in regenerative abilities between lower and higher animals emerged as another important topic in biology.
Lower and higher animals both start as fertilized eggs and grow into individuals through continuous cell division, and they share the common feature of stopping growth when they reach a certain adult size. However, there are clear differences in their regenerative abilities. For example, sponges can reform into complete adults even after their somatic cells are separated. This shows that lower animals retain their total developmental potential from the fertilized egg stage even after they become adults.
On the other hand, in higher animals such as rabbits and humans, most cells perform specific functions as they differentiate, and their regenerative abilities are extremely limited. It has been found that the limitations of regeneration in higher animals are due to the fact that gene expression regulation mechanisms are much more complex and sophisticated than those in lower animals. Gene expression is not simply the replication or transmission of genetic information, but is precisely controlled according to timing and environment, with numerous regulatory factors involved in the process.
Nevertheless, somatic cells of higher animals also possess species-specific chromosomes that are basically identical to those of fertilized eggs, and the genes contained in these chromosomes can be re-expressed under certain conditions, suggesting that regeneration is not entirely impossible.
If the complex gene expression regulation mechanism of higher animals can be artificially controlled, it may be theoretically possible to induce an entire organism from somatic cells, as in lower animals. The most notable scientific achievement in this regard is the successful replacement of human somatic cell nuclei with egg nuclei and their cultivation to obtain embryonic stem cells.
This supports the fact that somatic cells still retain complete genetic information and can restart the development process under certain conditions. However, this technology is still in the experimental stage, and techniques for directly controlling gene expression regulation factors in somatic cells of higher animals have not yet been developed.
As such, understanding individual development has expanded from the simple observation of developmental processes to various fields of biology, such as genetics, molecular biology, and cell biology, providing profound insights into the origin and continuity of life. In particular, research on how cells regulate genetic information and determine the structure and function of individuals is forming an important theoretical foundation for the development of biotechnology, medicine, and regenerative medicine.
