Fertilization, also known as conception, is the process where a zygote is formed through the fusion of an egg and a sperm. This process begins when sperm enters the female body, traveling through the cervix into the oviduct, which in humans is referred to as the Fallopian tubes or uterine tubes. Upon reaching the oviduct, the sperm encounters an egg (ovum) ready for fertilization. The fusion of the sperm and egg nuclei results in a diploid nucleus, marking the creation of the zygote.
The initial step in fertilization is sperm binding, where the sperm attaches to the jelly coat of the egg, known as the zona pellucida in mammals. This jelly coat, composed of glycoproteins, serves as a barrier that only a few sperm can penetrate. Following sperm binding, the acrosome, a cap on the sperm's head, releases enzymes that dissolve the jelly coat, allowing the sperm to reach the egg's plasma membrane. Once the sperm membrane fuses with the egg membrane, a cortical reaction occurs, releasing cortical granules that change the egg's membrane chemistry to prevent polyspermy, which is the fertilization of an egg by multiple sperm.
After fertilization, the zygote begins to divide through a process called cleavage, which is distinct from typical mitosis as the cells do not grow larger but instead remain the same size while dividing. This results in the formation of a morula, a solid ball of cells. The morula undergoes compaction, where cells bind together, and then differentiation, leading to the development of various cell types in the embryo.
On approximately the fifth or sixth day post-fertilization, the morula transforms into a blastula, specifically termed a blastocyst in mammals. The blastocyst is characterized by a hollow structure containing a fluid-filled cavity called the blastocele. Within the blastocyst, there are two key cell types: the inner cell mass, which will develop into the fetus, and the trophoblast cells, which will form the placenta and supportive tissues. The inner cell mass may also be referred to as the epiblast or embryoblast.
Following the formation of the blastocyst, implantation occurs as the blastocyst embeds itself into the uterine wall, initiating the next stages of development. This process is crucial for establishing a connection between the developing embryo and the mother, allowing for nutrient and waste exchange as the embryo grows into a fetus.