The Analysis of Development

The zygote undergoes a transformation process to develop into an adult animal capable of responding to stimuli and performing various physiological functions. This raises the question: how does the zygote undergo this transformation, and what enables the adult to carry out its functions?

Development through Pre-Formation or Epigenesis

Aristotle proposed two hypotheses to explain development: Pre-Formation and Epigenesis.

Pre-Formation suggests that the new individual is already fully formed within the egg or sperm.

Epigenesis, on the other hand, proposes that the egg or sperm does not contain pre-formed structures. Instead, these structures develop later in their proper positions, using material present in the egg. The development process is influenced by genes inherited from the parents, implying that the embryo is shaped by the action of these genes. The organization of the embryo is achieved through epigenesis, a process controlled by genes.

The Challenge of Differentiation

If the organization of the embryo occurs through epigenesis, how do the dividing cells of the zygote differentiate into three embryonic layers? Furthermore, how do these layers give rise to different structures? In 1924, Hans Spemann and Hilde Mangol conducted a series of experiments to address these questions.

Experiment No: 1

They excised the ectoderm, which normally becomes the nerve tube, from an amphibian embryo and placed it in a separate dish. Although the embryo from which the ectoderm was removed healed and survived, it either lacked a functional nervous system or had a defective one. Moreover, the isolated piece of ectoderm did not form a nervous system, even though it remained alive and healthy.

Conclusion: They concluded that the piece of ectoderm needs to remain connected to the embryo for the proper development of the nervous system. Subsequently, they conducted another experiment.

Experiment No: 2

SPEMANN'S EXPERIMENT

They cut a flap of ectoderm from the top of an embryo but left it attached. Then, they removed the underlying mesoderm and discarded it. Finally, they folded the flap of ectoderm back into its original position. Although the ectoderm healed and appeared healthy, it failed to develop into a nervous system.

Conclusion: The removal of mesoderm prevented the ectoderm from differentiating into nerve tissue. This indicated that the mesoderm somehow influenced the ectoderm's differentiation into nerve tissue. They proceeded to conduct another experiment.

Experiment No: 3

Using two embryos in the early gastrula stage, they removed a piece of mesoderm immediately in front of the dorsal lip of the blastopore from one embryo. From the second embryo, they removed an equivalent-sized piece of mesoderm from the area 180 degrees opposite to the dorsal lip, and in its place, they transplanted the piece of mesoderm from the first embryo. The transplanted mesoderm formed a blastopore and migrated inside the host embryo. As a result, two nervous systems were formed: one in front of the embryo's original dorsal lip and another in the area where the transplantation occurred. This created a Siamese-twin-like embryo with two brains and spinal cords.

Conclusion: This experiment demonstrated that the mesoderm appears to control the differentiation of nervous tissue.

Spemann referred to the dorsal lip area as the primary organizer because it was the only tissue capable of inducing the development of a secondary embryo in the host. He termed this inductive event "primary induction" as he believed it to be the first inductive event in development. Spemann's experiments earned him the Nobel Prize in 1935.

Embryonic Induction

Embryonic induction refers to the phenomenon where cells of one type direct the development of other cells. For example, in Spemann's experiment, the mesodermal cells from the dorsal lip region induced the ectoderm to form a nerve tube. This process of one group of cells influencing the development of other cells is known as embryonic induction.

The Challenge of Cell Differentiation

Cell differentiation involves cells becoming structurally and functionally specialized for the diverse tasks in complex multicellular organisms. While all cells share some "housekeeping" activities such as cell respiration and protein synthesis, specialized cells possess unique sets of structural proteins and enzyme systems. The question then arises: how do cells achieve this structural and functional specialization?

Differentiation of specialized cells relies on the production of specific proteins by distinct groups of cells. Cells must actively express specific portions of their genetic information (genome) while repressing other parts. For example, certain regions of the genome are utilized to produce brain cells, while different regions are responsible for producing kidney cells.

In summary, the zygote's transformation into an adult organism involves the processes of epigenesis and embryonic induction. Epigenesis describes the development of structures from material present in the egg, guided by the action of genes received from the parents. Embryonic induction occurs when one group of cells directs the development of other cells. The differentiation of cells into specialized types is achieved through the selective expression of specific genes, leading to the production of unique proteins and the establishment of distinct structures and functions.

Hans Spemann's pioneering experiments on amphibian embryos provided valuable insights into these processes and earned him the Nobel Prize. The field of developmental biology continues to investigate the intricate mechanisms underlying the transformation of a single zygote into a complex, functioning organism.