Dr. Rita Levi-Montalcini, a Nobel Prize-winning neurologist who discovered critical chemical tools that the body uses to direct cell growth and build nerve networks, opening the way for the study of how those processes can go wrong in diseases like dementia and cancer, died on Sunday at her home in Rome. She was 103.

Her death was announced by Mayor Gianni Alemanno of Rome.

“I don’t use these words easily, but her work revolutionized the study of neural development, from how we think about it to how we intervene,” said Dr. Gerald D. Fishbach, a neuroscientist and professor emeritus at Columbia.

Scientists had virtually no idea how embryo cells built a latticework of intricate connections to other cells when Dr. Levi-Montalcini began studying chicken embryos in the bedroom of her house in Turin, Italy, during World War II. After years of obsessive study, much of it at Washington University in St. Louis with Dr. Viktor Hamburger, she found a protein that, when released by cells, attracted nerve growth from nearby developing cells.

In the early 1950s, she and Dr. Stanley Cohen, a biochemist also at Washington University, isolated and described the chemical, known as nerve growth factor — and in the process altered the study of cell growth and development. Scientists soon realized that the protein gave them a new way to study and understand disorders of neural growth, like cancer, or of degeneration, like Alzheimer’s disease, and to potentially develop therapies.