Key Terms

TermMeaning
GameteA reproductive (sex) cell. In males, sperm; in females, eggs
FertilizationThe process in sexual reproduction in which a male gamete and female gamete fuse to form a new cell
ZygoteCell resulting from fertilization
Diploid (2n)Cell that contains two sets of homologous chromosomes
Haploid (n)Cell that contains only a single set of genes
ApoptosisThe process of programmed cell death
DifferentiationThe process by which cells become specialized in structure and function

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

  1. Zygotic stage: The zygote is formed when the male gamete (sperm) and female gamete (egg) fuse.
  2. 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.
  3. 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.
  4. 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.

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.

Common mistakes and misconceptions

  • Human fertilization occurs in the fallopian tube. Many people believe that human fertilization occurs in the vagina, but this is not the case. Once sperm enter the vagina, they can move through the cervix, into the uterus, and to the end of a fallopian tube. If a sperm is able to penetrate an egg, fertilization occurs.
  • Development is sort of pre-determined. While it IS true that all humans in early development look the same, many of the features that later develop in a fetus are already pre-determined by its genes. For example, the biological sex of the fetus is already decided based on whether it received two X chromosomes (one from each parent) or an X from its mother and a Y from its father, despite the fact that sex-specific characteristics do not appear until later in development.
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