Description
Fertilization occurs when a sperm cell merges with an egg cell, creating a zygote, which is the first stage of a new life. This process takes place in the fallopian tube. The zygote then undergoes rapid cell division, known as cleavage, forming a multi-cellular structure called a blastocyst. As the blastocyst moves toward the uterus, it continues to divide and develop. Approximately 5-7 days after fertilization, the blastocyst implants itself into the uterine lining, initiating pregnancy. Early development involves the formation of the foundational structures and systems that will shape the embryo.
Résumé
- Fertilization can be either in vivo or in vitro in humans, or external or internal in the broader ecosystem. In vitro fertilization (IVF) involves creating embryos from oocytes and sperm in a petri dish. Oocytes are collected from ovaries through ultrasound-guided follicular aspiration, matured, and fertilized. Semen, either conventional or sex-frozen, is used for fertilization.
- Oocyte collection can be done surgically or, more commonly, through the trans-vagile ovum pickup (TVS) technique, especially in cattle. This involves restraining the animal, administering an epidural, and using an ultrasound probe to visualize and aspirate follicles using a needle. The recovered oocytes are then transported to the lab in a physiological saline solution with antibiotics, maintaining a temperature of 22-24°C.
- The in vitro fertilization process involves three main steps: COC (Cumulus-Oocyte Complex) washing, sperm purification, and fertilization. The COC washing removes hormones and nutrients from the maturation media. Sperm purification involves washing sperm cells from the extender and seminal plasma, using methods like percol purification, sperm swim-up, or glass wool filtration.
- Intra cytoplasmic sperm injection (ICSI) is used when the male has poor sperm quality. It involves injecting a selected sperm cell directly into a mature oocyte using micro manipulation devices. Robotic ICSI has made the process easier, with robotic arms holding the egg while the embryologist fertilizes it.
- In vivo fertilization involves the natural journey of sperm towards the ovum, with ejaculation releasing sperm contained within seminal fluid into the vagina. Sperm navigate through the cervix, uterus, and fallopian tubes guided by chemical signals (chemotaxis). Capacitation, a process occurring in the fallopian tubes, prepares the sperm for fertilization by removing a glycoprotein layer covering the acrosomal cap.
- The acrosome reaction is a process where perforations develop in the sperm's acrosome, releasing enzymes that facilitate fertilization. Fertilization itself is the fusion of a single sperm and ovum to form a zygote, typically completing within 12-24 hours after ovulation. It involves the sperm penetrating the corona radiata and zona pellucida, eventually fusing its plasma membrane with the ovum.
- After fertilization, the zygote undergoes cleavage, repeated mitotic cell divisions within the zona pellucida, increasing the number of cells (blastomeres). The blastomeres become smaller with each division. The zygote divides into two cells, then four, then eight, and by the third day, a morula (16 blastomeres) is formed.
- The morula reaches the uterine cavity by the fourth day and transforms into a blastocyst. The blastocyst, typically formed 5-6 days after fertilization, is a hollow sphere composed of the blastocoel (fluid-filled cavity), inner cell mass (embryoblast), and outer cell mass (trophoblast). The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast, aiding in implantation.
- By the end of the sixth day, the blastocyst adheres to the uterine lining (opposition), followed by the syncytiotrophoblast invading the endometrium (implantation). The pre-embryonic or germinal period lasts 1-3 weeks, the embryonic period is 4-8 weeks, and the fetal period spans from 9 weeks to birth. Gametogenesis, encompassing oogenesis and spermatogenesis, is the process of mature gamete development.
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