This newborn baby is just starting out in life. She has her whole life ahead of her!
Actually, that’s not really true. While most of her life is still ahead of her — including life stages of infancy, childhood, adolescence, and adulthood — this newborn baby is not just starting out in life. She’s already nine months old, and what happened to her during those nine months will help shape the rest of her life. Some of the shortest — but most important — life stages occur before birth. These stages include the germinal, embryonic, and fetal stages. This concept focuses on the earliest of all human life stages: the germinal stage.
What Is the Germinal Stage?
The germinal stage of development is the first and shortest of the stages of the human lifespan. The main events in this stage of development are illustrated in the figure below and described in detail in the rest of this concept. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote (day 0). The germinal stage continues as the zygote undergoes several initial cell divisions to form a solid ball of cells called a morula (days 3-4). It then continues as the morula undergoes additional changes to become a hollow ball of cells called a blastocyst (days 5-7). The germinal stage ends when the blastocyst implants in the endometrium of the uterus (days 8-9). After implantation occurs, the blastocyst is called an embryo, and it will soon obtain nutrients from the mother’s blood via a temporary organ called the placenta. In the germinal stage, however, nutrients must be obtained from cell cytoplasm or secretions in the Fallopian tube or uterus.
Processes in the Germinal Stage
The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation.
Fertilization takes place when a haploid sperm successfully enters a haploid egg and triggers the egg to complete meiosis II. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. Fertilization results in a single diploid cell called a zygote. The nucleus of the zygote contains 46 chromosomes: 23 chromosomes from the nucleus of the egg, and 23 chromosomes from the nucleus of the sperm.
Fertilization usually takes place in the ovary end of a Fallopian tube. Successful fertilization is enabled by three processes: chemotaxis, adhesion, and digestion.
- Chemotaxis is a chemical process in which chemical signals from the egg direct the movement of sperm toward the egg.
- Once a sperm reaches the egg, adhesion occurs. In this process, a sperm “sticks” to the egg, which is enabled by sperm-receptor proteins on the egg. Adhesion causes reactions that block the entry of additional sperm. For example, the egg releases enzymes that digest sperm-receptor proteins on the surface of the egg, preventing other sperm from adhering to the egg.
- After a sperm adheres to the egg, digestive enzymes in the acrosome on the head of the sperm break down the zona pellucida of the egg. The zona pellucida is a protein layer surrounding the cell membrane of the egg (see the drawing below). Digestion of the zona pellucida allows the sperm to enter the egg. The zona pellucida will continue to contain the developing zygote until the end of the germinal stage.
By the second day after fertilization, the single-celled zygote undergoes mitosis to form two daughter cells. Mitosis continues taking place every 12 to 24 hours to produce the first four cells, then eight, and as many as sixteen cells by day 4. These early mitotic divisions are called cleavage. By day 4, the cells form a solid ball called a morula (see figure below). Although cleavage results in more cells, the overall mass of cells making up the morula is still the same size as the initial zygote because the cells are confined within the zona pellucida. A large amount of cytoplasm in the original zygote becomes subdivided among the multiple cells of the morula.
Blastulation is the process of changing the morula into a blastocyst. It occurs from roughly day 5 to day 7 after fertilization. During blastulation, the morula changes from a solid ball of undifferentiated cells into a fluid-filled ball of differentiated cells, as shown in the figure below. The major parts of the fully formed blastocyst are the embryoblast, trophoblast, and blastocoele. The blastocyst remains confined within the zona pellucida until it is ready to implant in the endometrium.
- The embryoblast (inner cell mass) consists of a mass of cells inside the blastocyst. These cells migrate to one end (or pole) of the blastocyst. Embryoblast cells are differentiated cells that will eventually develop into the embryo.
- The trophoblast is the outer cell layer of the blastocyst. Trophoblast cells are differentiated cells that will implant in the uterus and eventually develop into the fetal portion of the placenta and other extraembryonic (outside of the embryo) tissues.
- The blastocoele is a cavity formed by the migration of embryoblast cells to one pole of the blastocyst. The blastocoele fills with fluid secreted by trophoblast cells.
Around day 8 or 9 after fertilization, implantation begins. Implantation is the process in which a blastocyst becomes embedded in the endometrium of the uterus (see the figure below). Implantation is triggered by contact between the blastocyst and endometrium. In response to this contact, trophoblast cells start to proliferate and “hatch” from the zona pellucida. The hatched trophoblast cells start secreting enzymes that digest the mucosa covering the endometrium and break down the extracellular matrix between endometrial cells. These changes allow finger-like projections (called villi) of the trophoblast to penetrate into the endometrium. The projections pull the blastocyst — now called an embryo — into the endometrium until it is fully covered by endometrial epithelium.
- The germinal stage of development is the first and shortest of the stages of the human lifespan. It lasts roughly eight to nine days, beginning with fertilization and ending with implantation in the endometrium of the uterus, after which the developing organism is called an embryo.
- The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation.
- Fertilization takes place when a haploid sperm successfully enters a haploid egg and results in a single diploid cell called a zygote. This usually occurs in a Fallopian tube. Successful fertilization is enabled by the processes of chemotaxis, adhesion, and digestion.
- Cleavage refers to the first several mitotic cell divisions of the zygote. It takes place in the Fallopian tube and results in a solid ball of undifferentiated cells called a morula. The morula forms by approximately the fourth day after fertilization.
- Blastulation is the process in which the morula changes into a fluid-filled ball of differentiated cells called a blastocyst. It generally occurs during days 5 to 7 after fertilization. The major parts of the blastocyst are an internal cell mass called the embryoblast (which will develop into the embryo), an external layer of cells called the trophoblast (which will develop into the fetal part of the placenta and other extraembryonic structures), and a fluid-filled cavity called a blastocoele.
- Implantation is the process in which the blastocyst becomes embedded in the endometrium of the uterus. It occurs around day 8 or 9 after fertilization when trophoblast cells “hatch” from the zona pellucida and penetrate the endometrium.
1. Define the germinal stage of human development.
2. Name four processes that occur during the germinal stage.
3. Describe three processes that enable successful fertilization after ovulation occurs and sperm enter the Fallopian tube.
4. What is cleavage? Where does it take place? What is its end result?
5. What is blastulation? How does the morula change during this process?
6. Identify the major parts of the blastocyst.
7. Define implantation. When and how does implantation occur?
8. List the stages of the developing human organism, in order, from fertilization to the end of implantation.
9. Explain why cells in the zygote are smaller on day 3-4 after fertilization than they were on day 2.
10. True or False: The zona pellucida disintegrates as the morula becomes a blastocyst.
11. True or False: Some cells in the blastocyst do not become part of the embryo.
12. Why do you think it is important that only one sperm fertilizes each egg? What mechanism helps ensure that this happens properly?
13. Put the following events in order of when they occur during early human development, from earliest to latest:
A. differentiation of cells
14. Which has the most cells?
A. the blastocoele
B. the embryoblast
C. the morula
D. the blastocyst
Watch this short animation to help you visualize the processes of early human development, from fertilization through the germinal stage and beyond.
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