Human Conception: Fertilization and Early Embryonic Development

The Process of Fertilization

1. The female gamete, or egg, is captured by the fimbriae (finger-like extensions) of the fallopian tube. Thanks to the muscular contractions of the tube's walls and the movement of its cilia, the egg is gently propelled towards the uterus.
2. Sperm that have entered the vagina begin their ascent through the reproductive tract.
3. The surviving sperm continue their journey to the cervix. This region contains glands that produce a specialized mucus fluid. This discharge facilitates the passage of some sperm while trapping others and removing seminal fluid remnants.
4. Once through the cervix, sperm penetrate the uterine cavity, traverse it, and begin their ascent into the fallopian tubes.
5. The meeting of sperm and egg, known as fertilization, typically occurs within the fallopian tube.
6. Several sperm surround the egg and attempt to penetrate its outer layers. To cross the corona radiata (a layer of follicular cells), the sperm secrete an enzyme that helps disperse these cells, allowing the sperm to reach the zona pellucida (a protective glycoprotein layer).
7. At the point where the first sperm makes contact with the egg, it penetrates the egg's membrane. Immediately, the zona pellucida undergoes a change, becoming impervious to the entry of other sperm, ensuring monospermy.
8. Once inside the egg, the sperm's pronucleus fuses with the egg's pronucleus. This fusion completes fertilization, resulting in the formation of a single-celled zygote.

Stages of Embryonic Development

1. Immediately after fertilization, the zygote begins its journey through the fallopian tube towards the uterus. Lacking its own means of mobility, it is transported by the contractions of the tube and the movement of its cilia.
2. Concurrently, the first mitotic division occurs, resulting in two identical cells called blastomeres.
3. These cells continue to divide, forming four, then eight, and so on.
4. This division process, known as cleavage, repeats multiple times, forming a compact ball of cells that resembles a mulberry, called a morula. The morula's size remains approximately equal to that of the original egg.
5. Approximately 5 days after fertilization, the developing embryo, now called a blastocyst, enters the uterus. Within the uterus, the blastocyst continues to divide and differentiate. The larger, inner cells cluster together to form a small mass called the inner cell mass (which will develop into the embryo itself), surrounded by a fluid-filled cavity. The smaller cells are located on the periphery, forming an outer layer called the trophoblast. During this period, the embryo is nourished by secretions from the fallopian tube and uterus.
6. Around 7 days after fertilization, the blastocyst makes contact with the uterine lining (endometrium), which is prepared to receive it. The trophoblast secretes an enzyme that erodes a small area of the endometrium, allowing the embryo to embed itself. This crucial process is called implantation and typically occurs in the upper posterior wall of the uterus, though it can occur elsewhere within the uterine lining.
7. The trophoblast extensions continue to penetrate deeper into the endometrium. These extensions become more numerous, forming root-like structures that invade maternal blood vessels, establishing a connection for the embryo's nutrition.
8. Once the embryo has fully penetrated the uterine lining, it becomes completely enveloped by the endometrium. The trophoblast begins to produce a hormone called human chorionic gonadotropin (hCG). This hormone stimulates the corpus luteum in the ovary, transforming it into the corpus luteum of pregnancy. The corpus luteum of pregnancy secretes large amounts of progesterone, which is vital for maintaining the uterine lining and decreasing the excitability of the uterine muscle (myometrium). This hormonal action prevents uterine contractions that could otherwise expel the embryo. Concurrently, the inner cell mass rapidly develops, marking the beginning of the embryonic stage, which spans approximately the first two months of pregnancy.