How did Theodor Schwann extend cell theory to animals?

In 1838, the botanist Matthias Jakob Schleiden proposed that all plant tissues are composed of cells. Theodor Schwann, a physiologist and friend of Schleiden, extended this concept to the animal kingdom, thereby establishing a unified cell theory for all living organisms.12

Schwann formalized this extension in his 1839 work, Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants. By comparing various animal and plant tissues, he demonstrated that cells are the "elementary particles of organisms" in both kingdoms. He and Schleiden identified common cellular features—specifically the membrane, nucleus, and cell body—to support their conclusion that cells serve as the fundamental unit of biological structure and function. This work was a landmark achievement that bridged the fields of botany and zoology, providing a foundational principle for modern biology.13