The conception of a new human being occurs when the sperm from the father unites with the egg from the mother, resulting in a fusion of both parents' genetic material. Fertilization occurs soon after ovulation, usually in the mother's fallopian tube. The first cell divisions will occur here on the way down to the uterus. A newly fertilized egg is called a zygote. The zygote will divide to become a morula. From the morula stage, it becomes a blastocyst, then an embryo. The embryonic stage ends 10 weeks after conception, marking the beginning of the fetal stage.
Zygote
The zygote is the product of newly fused sperm and egg cells. The single cell will begin to divide as it descends the fallopian tube toward the uterus, doubling the number of cells with each division. The zygote stage will end around three days after fertilization at the eight-cell stage.
Morula
The eight-celled zygote undergoes a division to become a 16-celled cluster called a morula. The word morula is Latin for "mulberry" and refers to the cell cluster's appearance. The morula consists of two different types of cells -- those that make up the inner cell mass and those that make up the outer cell mass. The inner cell mass will become the embryo, while the outer cell mass will become the placenta.
Blastocyst
About five days past fertilization, the morula reaches the uterus and changes shape to become a hollow ball of cells called a blastocyst. The cavity within the blastocyst is called a blastocoel. By day six, the blastocyst lands somewhere in the uterine lining; the cells of the outer cell mass, or trophoblast, will begin to penetrate it. This event is called implantation.
Placental Formation
By about 14 days past fertilization, cells of the trophoblast have established a primitive connection with the mother's circulatory system. This connection between mother and baby will continue to develop, becoming the placenta that will help nourish and support the developing child throughout the pregnancy.
Gastrulation
The embryonic stage begins with a process called gastrulation. This event begins about 15 days past fertilization and results in the formation of three different tissue types called ectoderm, mesoderm and endoderm, which will give rise to all of the embryo's organs and tissues. Gastrulation is an especially sensitive period of development because cell types are beginning to differentiate. For example, exposure to high doses of alcohol during this time can result in defective formation of the brain and facial features.
Embryo
The embryonic period takes place between the third and ninth weeks of development. This time is marked by the formation of the organs, referred to as organogenesis. The nervous system will form from the embryonic ectoderm by a process called neurulation. The ectoderm will also form the skin, hair, nails, pituitary gland, mammary glands, sweat glands and tooth enamel. Embryonic mesoderm will give rise to muscle, cartilage, bone and subcutaneous connective tissue, as well as the vascular and urogenital systems. Embryonic endoderm will become the lining of the gastrointestinal tract, respiratory tract and bladder. The embryonic period ends 10 weeks past fertilization, marking the beginning of the fetal period.
References
- "A Child is Born (4th Edition)"; Lenart Nilsson and Lars Hamberger; October 2004
- "Developmental Biology (7th Edition)"; Scott Gilbert; March 2003
- "Langman's Medical Embryology (10th Edition)"; T.W. Sadler; 2006
