firs week of human development

The first week of human development is a time of rapid cell division and differentiation. The fertilized egg, known as a zygote, travels down the fallopian tube to the uterus. Once in the uterus, the zygote implants in the lining of the uterus. The first week of development is divided into three stages: * **Gamete approximation:** This is the stage when the sperm and egg cells come close together. * **Fertilization:** This is the stage when the sperm and egg cells fuse to form a zygote. * **Cleavage:** This is the stage when the zygote divides into many cells. **Gamete approximation:** The sperm and egg cells are attracted to each other by chemical signals. The sperm cell swims up the fallopian tube towards the egg cell. When the sperm cell reaches the egg cell, it releases enzymes that break down the egg cell's outer layer. The sperm cell then enters the egg cell. **Fertilization:** Once the sperm cell enters the egg cell, the two cells fuse together to form a zygote. The zygote contains all the genetic information that will be needed to create a new individual. **Cleavage:** After fertilization, the zygote begins to divide rapidly. The first division occurs within 30 minutes of fertilization. By the end of the first day, the zygote has divided into 16 cells. By the end of the third day, the zygote has divided into 64 cells. The cells are now called blastomeres. The blastomeres are tightly packed together and form a ball-like structure called a morula. The morula continues to divide and grow as it travels down the fallopian tube to the uterus. **Implantation:** The morula reaches the uterus on about the fifth day after fertilization. The zona pellucida, which surrounds the morula, begins to break down. The blastomeres then attach to the lining of the uterus. This process is called implantation. Implantation is a critical event in pregnancy. It is the point at which the embryo begins to receive nutrients from the mother. Implantation can cause some women to experience mild cramping or spotting. At the end of the first week of development, the embryo is a blastocyst. The blastocyst is a hollow ball of cells with an inner cell mass and an outer layer of cells called the trophoblast. The inner cell mass will eventually develop into the fetus. The trophoblast will eventually develop into the placenta, which will provide nutrients to the fetus. The first week of development is a time of rapid change and growth. The embryo is developing at an incredible rate. By the end of the first week, the embryo has already begun to form the basic structures that will eventually become the fetus.