The first week of an embryo's life is critical to its survival. During this time, the embryo must undergo genetic, metabolic and developmental changes that allow for implantation in the uterine lining. The mother's menses will cease, and development of the placenta will begin to sustain the embryo's future development over the next nine months.
Genetic Changes
Egg and sperm cells each contain only half the genetic information needed to produce a human being. This genetic information is stored in packets of DNA called chromosomes. Fusion of the egg and sperm during fertilization pairs these 23 maternal and paternal chromosomes, restoring the total number of chromosomes in the fertilized egg (zygote) to 46, the normal chromosome number found in every human cell.
Activation of the Zygote's Genes
The zygote initially relies on maternal gene products in the egg, such as hormones, organelles, antibodies, antioxidants and RNAs, that are needed for the first few days of existence. Around day three, the zygote activates its own genes to produce various gene products for continued survival. In their article, "Dirty Tricks of the Egg and Sperm Race," published in the March 2, 2010, issue of "NewScientist," Professors Pat Monaghan and Tim Burkhead make the scientific case for the greater contribution of the egg in determining which inherited genes are turned on in the early embryo.
Compaction and Cell-to-Cell Communication
At the eight-cell stage, the embryo's identical daughter cells are loosely associated with one another. This changes around day four, during a process called compaction, when embryonic cells become tied together by open molecular connections called gap junctions, which permit communication between cells. Because signaling is now possible among cells, the embryo begins to assign specific duties to cells based on their location (or polarity) within the embryo.
Organization and Specialization of Future Cell Types
The outer polar cells form a layer called the trophectoderm, which forms the fetal contribution to the placenta. The inner polar cells, called the inner cell mass, forms the baby's cells. The nonspecialized cells of the inner cell mass can be used at this embryonic stage to create stem cell lines for medical therapy, according to the National Institutes of Health. The blastocyst begins to vigorously pump fluid from its cells, forming a cavity within itself called a blastocoel. The formation of a trophectoderm, inner cell mass and blastocoel complete the reorganization of the early embryo into an implantation-ready blastocyst.
Hatching
The growing blastocyst now has over 100 cells and an expanding blastocoel, which dramatically inflates the size of the blastocyst, causing it to strain at the confines of its protective glycoprotein shell, called the zona pellucida. The expansion of the blastocyst causes the zona pellucida to split open, allowing the blastocyst to "hatch" and attach to the maternal cells lining the uterus.
Implantation and Maternal Communication
The blastocyst attaches and burrows into the uterine lining during embryo implantation, which occurs at the end of its first week of life. The embryo communicates with the mother to preserve the new pregnancy by producing a hormone called human chorionic gonadotropin, which prevents menses and loss of the uterine lining. These embryonic changes in the first week are essential for a successful pregnancy and birth of a baby nine months later.
