Theodor Schwann's many contributions to biology include…

Theodor Schwann's many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin in extracts from the stomach lining and thus the first isolation of an animal enzyme, the discovery of the organic nature of yeast, and the invention of the term metabolism.

Among the first to break with the traditional vitalism, Schwann is working towards a physico-chemical explanation of life, having in 1837 showed digestion to depend essentially on the presence of a fermenting agent he called pepsin.

Schwann had also examined the question of spontaneous generation, which led to its eventual disproof.

In the course of his experiments, he has discovered the organic nature of yeast.

In fact, the whole germ theory of Pasteur, as well as its antiseptic applications by Lister, can be traced to Schwann's influence.

In 1839, when Schwann is dining with Matthias Jakob Schleiden (who in 1837 had viewed and stated that new plant cells formed from the nuclei of old plant cells), the conversation turns on the nuclei of plants and animal cells.

Schwann remembers seeing similar structures in the cells of the notochord and instantly realizes the importance of connecting the two phenomena.

The resemblance is confirmed without delay by both observers, and the results soon appear in his famous Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals, in which he declares that "All living things are composed of cells and cell products."

Thus, cell theory is definitely constituted.

In the course of his verification of cell theory, in which Schwann traverses the whole field of histology, he proves the cellular origin and development of the most highly differentiated tissues including nails, feathers, and tooth enamel.

His generalization becomes the foundation of modern histology, the study of the microscopic anatomy of cells and tissues of plants and animals, and in the hands of Rudolf Virchow (whose cellular pathology is an inevitable deduction from Schwann) is to afford the means of placing modern pathology on a truly scientific basis.