A topic sentence is the key component in any paper you are writing. Topic sentences are the highway signs of your paper. They tell the reader the purpose of your paper and they signal in which direction each paragraph is going.
Introduction Paragraphs need a topic sentence. This can also be called a thesis statement.
Every body or discussion paragraph also needs a topic sentence.
Source: Kathleen Hanley
This document contains solid explanation of topic sentences in paragraphs.
