Final answer:
Sertoli cells are responsible for preventing germ-line cells from developing into eggs and instead supporting their development into sperm within the male gonads.
Step-by-step explanation:
The cells that prevent germ-line cells from developing along the egg pathway and instead help direct them down the sperm pathway of development are called Sertoli cells. These cells are a type of supporting cell, known as a sustentacular cell or sustentocyte, found in epithelial tissue. Sertoli cells secrete signaling molecules that promote sperm production and influence the fate of germ cells, aiding in their development into sperm. They form a physical barrier around the germ cells from the peripheral basement membrane of the seminiferous tubules to the lumen, and their tight junctions contribute to the blood-testis barrier. This barrier is crucial as it protects the germ cells from bloodborne substances and prevents the immune system from reacting to surface antigens on developing sperm cells, which could lead to an autoimmune response.