Answer:
Human fertilization and development
Fertilization is the process in which haploid gametes fuse to form a diploid cell called a zygote. To ensure that each zygote has the correct number of chromosomes, only one sperm can fuse with one egg.
Stages of human development
Zygotic stage: The zygote is formed when the male gamete (sperm) and female gamete (egg) fuse.
Blastocyst stage: The single-celled zygote begins to divide into a solid ball of cells. Then, it becomes a hollow ball of cells called a blastocyst, attaching to the lining of the mother's uterus.
Embryonic stage: The major internal organs and external features begin to emerge, forming an embryo. In this stage, the heart, brain, and spinal cord become visible. Arms and legs start to develop.
Fetal stage: Once the formed features of the embryo begin to grow and develop, the organism is considered a fetus. Differentiation and specialization of structures happens during this time.
Development overview showing the progression from zygote to blastocyst to embryo to fetus.
Development overview showing the progression from zygote to blastocyst to embryo to fetus.
Differentiation and apoptosis
During development, the number of cells must increase through division so that body axes, tissues, organs, and structures must form. Individual cells become specialized in their structure and function through the process of cell differentiation.
Unnecessary cells also must be removed in order to help form important structures. This occurs is through the process of apoptosis. For example, human hands start out as a paddle-like block of tissue. Eventually, the block was “carved” into fingers by apoptosis of the cells in between the developing fingers.
Microscope images from a scientific paper, showing a developing mouse paw. The cells between the developing digits are stained by a marker that indicates apoptotic cells.
Explanation:
