Final answer:
The zygote divides through mitotic cycles, and the resulting cells differentiate based on tissue-specific gene expression into multiple germ layers—ectoderm, mesoderm, and endoderm—each giving rise to different tissues and organs. This leads to the various specialized cell types in a human.
Step-by-step explanation:
The process by which a single cell, the zygote, can develop into a complex organism like a human with hundreds of different cell types is a fundamental concept of developmental biology. A zygote forms from the fusion of an egg and a sperm and begins life as a totipotent cell, which has the potential to differentiate into any cell type. As the zygote divides through mitotic cycles, it generates many cells that form the embryo. These embryonic cells go through a process of specialization driven by tissue-specific gene expression, where certain genes are activated or inactivated depending on the cell's role. This results in the formation of three primary germ layers: the ectoderm, mesoderm, and endoderm.
Each germ layer is responsible for giving rise to distinct tissues and organs. For example, the ectoderm primarily develops into the nervous system and skin, the mesoderm into muscles, and the endoderm into internal organs like the intestines. As development continues, further specialization and differentiation occur, leading to the formation of the myriad cell types necessary for a fully functional organism. This complex process is orchestrated through carefully regulated gene expression and cell signaling pathways.