Germ Plasm Theory Explained


The germ plasm theory was first proposed by August Weismann in 1893 and it states that multicellular organisms contain germ cells. These cells provide heritable information that can be transmittable information. Somatic cells would then carry out the daily functions of the body. In germ plasm theory, only the places where heritable information needs to be transmitted would contain germ cells. This would mean the reproductive system in humans.

The germ plasm theory was essentially an early attempt to explain genetics. Once the science of modern genetics developed, it was proven through somatic cell nuclear transfer that adult cells retain a complete set of information. This countered the germ plasm theory, which states that genetic information experienced a gradual loss over time.

The Four Hierarchical Levels of Substances in Germ Plasm Theory

In the germ plasm theory, Weismann describes a structure that contains four hierarchical levels of substances within each cell.

The first level, which is also the lowest, are the biphors. These are minute units that will make up the whole cell when they are brought together collectively. In this theory, it is the biphors that are responsible for the growth and metabolism of each cell, involving numerous chemical molecules.

The second level of substances is the determinants. The determinants are the primary constituents of the germ plasm which hold the hereditary data that needs to be transferred to another cell. The biphors work with the determinants in order to determine the variability and phenotype of each cell.

The third level provides us with the id. These are the aggregates of the determinants.

The fourth and final level are the idants. These are the aggregates of the ids and are essentially a recognition of chromosomes and their function.

According to the germ plasm theory, the ids reduce their number through cell division, so each cell contains fewer determinants and ids than previous cells. This led Weismann to claim that only one determinant would be active within body cells and that others would remain inactive, allowing the cell to have a specific phenotype.

Germ Plasm Theory and Regeneration

Since the process of germ plasma had to go across all forms of human and animal life, Weismann recognized the need for his theory to incorporate monogenic reproduction into its process. Some organisms use asexual reproduction, which means a new organism comes from a single parent instead of having the genetics of two parental cells.

Weismann determined that there were two causes that could allow body parts to be regenerated or asexual reproduction to be able to occur.

1. The cells that are able to regenerate lost body parts do so because they have not aged enough to lose their germ plasm.
2. The cells are able to regenerate because they remained inactive and were only activated when there was a specific task that needed to be accomplished.

The issue with these processes, as outlined by the germ plasm theory, is that fission and gemmation are deemed to be distinct processes. Weismann claimed that gemmation was caused by an ability of the germ plasm in a cell to be doubled once an egg was fertilized. By bringing two sources of germ plasm together, one doubled cell could be created.

Fission, Weismann argued, was the bringing together of two determinants instead. This process could then stand out on its own or potentially circulate throughout the bloodstream, activating when it was called upon to transmit information.

The Importance of the Weismann Barrier

For more than 20 years, Weismann defended his theory against critics, often changing components of his theory in response to their criticisms. Yet the very fact that a distinction between soma cells and germ cells was created, leading to the formation that would be come to called the Weismann barrier, would help to lead us toward our modern understanding of genetics and how hereditary information passes from cell to cell.

Modern science has also shown that chemical modification of nucleotide bases can constitute genetic coding and that some information can be transferred when Weismann suggested that it would not be possible to do. These changes, referred to as “epigenetic,” do not alter the nucleotide sequences.

Although the germ plasm theory is not generally considered to be accepted today, Weismann created a decades-long debate that inspired researchers and scientists to understand much more about the basic foundations of the human body. Genetic information may be readily lost in somatic cells in some animals, but as a universal theory, it struggles to stand up underneath the weight of modern scientific findings.