An early embryo represents the first stage of development, beginning as a single fertilized egg. This period involves a sequence of rapid, organized events and intense cellular activity. During this time, a simple structure progressively transforms into a more complex form, laying the foundation for a new organism.
The First Week of Development
Development begins with fertilization, the fusion of a sperm and egg, which creates a single cell called a zygote. As the zygote travels through the fallopian tube, it undergoes rapid cell divisions known as cleavage. The cells, or blastomeres, become smaller with each division, so the embryo’s overall size does not increase. Approximately three days after fertilization, the embryo becomes a solid ball of cells called a morula, and its cells compact into a tight sphere.
By the fifth day, the morula develops a fluid-filled cavity, transforming it into a blastocyst. The blastocyst has two distinct cell populations. The outer layer, the trophoblast, will contribute to the placenta. The inner cell mass, or embryoblast, is a cluster of cells inside the blastocyst that will eventually develop into the fetus itself.
The Implantation Process
For development to continue, the blastocyst must attach to the uterine wall in a process called implantation, which occurs between 6 and 12 days after ovulation. The blastocyst first “hatches” from its protective outer layer, the zona pellucida. This allows the trophoblast cells to make direct contact with the uterine lining, or endometrium.
Trophoblast cells produce enzymes that allow the blastocyst to burrow into the nutrient-rich endometrial tissue, establishing a secure connection. As the trophoblast invades the uterine wall, it differentiates and begins to produce a hormone called human chorionic gonadotropin (hCG).
This hormone signals the body to maintain the corpus luteum, a structure in the ovary that produces progesterone. Progesterone sustains the uterine lining, preventing menstruation and supporting the embryo. The presence of hCG in blood and urine is what pregnancy tests detect, providing the first biochemical confirmation of pregnancy.
Establishing the Body Plan
Following implantation, the embryo undergoes gastrulation around the third week. During this stage, the inner cell mass reorganizes into a three-layered structure. These three primary germ layers are the foundational cell populations from which all bodily tissues and organs will arise.
The ectoderm is the outermost layer. It is destined to form structures that interface with the outside world, including the skin’s epidermis, hair, and nails. It also gives rise to the entire nervous system, including the brain and spinal cord, and the cornea and lens of the eye.
The mesoderm is the middle layer, responsible for forming the body’s structural components. This includes the musculoskeletal system, such as bones and muscles. This layer also develops into the circulatory system, including the heart and blood vessels, and the dermis of the skin.
The endoderm is the innermost layer. It gives rise to the linings of the body’s internal tracts, forming the lining of the digestive and respiratory systems. It also forms glands like the liver and pancreas.
Early Developmental Disruptions
The initial weeks of development are a vulnerable period, and the process can be disrupted. A chemical pregnancy is a very early miscarriage that occurs shortly after implantation. The embryo fails to develop, and though hCG may be detectable on a test, the pregnancy ends before ultrasound confirmation is possible.
An ectopic pregnancy occurs when the fertilized egg implants outside the uterus, most often in a fallopian tube. This type of pregnancy is not viable because the fallopian tube cannot support the embryo and may rupture, causing life-threatening internal bleeding. Risk factors include previous pelvic infections or surgeries that have damaged the fallopian tubes.
A molar pregnancy results from a genetic error during fertilization, leading to abnormal growth of placental tissue in the uterus. This condition is characterized by the absence of a viable fetus and unusually high hCG levels. It requires medical intervention to remove the abnormal tissue and monitoring to ensure no remnants remain.