Fate of a Fertilized Egg: Why Some Embryos Don't Implant
If the eggs were naturally inseminated, meaning sperm and egg were put together in their culture drop and we allowed nature to take its course. To become pregnant, the following steps must occur: Sperm transport — The sperm must be deposited and transported to the site of fertilization. Egg transport . There are many reasons why couples do not conceive. failure of the egg to fertilize and treat it successfully by injecting sperm microscopically into the egg by a.
Human sperm, for example, will fertilize hamster eggs from which the zona has been removed with specific enzymes; not surprisingly, such hybrid zygotes fail to develop. Zona-free hamster eggs, however, are sometimes used in infertility clinics to assess the fertilizing capacity of human sperm in vitro Figure Figure Scanning electron micrograph of a human sperm contacting a hamster egg.
The zona pellucida of the egg has been removed, exposing the plasma membrane, which contains numerous microvilli. The ability of an individual's sperm to penetrate hamster eggs is more The zona pellucida of mammalian eggs is composed mainly of three glycoproteins, all of which are produced exclusively by the growing oocyte.
Two of them, ZP2 and ZP3, assemble into long filaments, while the other, ZP1, cross-links the filaments into a three-dimensional network. The protein ZP3 is crucial: ZP3 is responsible for the species-specific binding of sperm to the zona, at least in mice. Several proteins on the sperm surface that bind to specific O-linked oligosaccharides on ZP3 have been implicated as ZP3 receptors, but the contribution of each is uncertain.
On binding to the zona, the sperm is induced to undergo the acrosome reactionin which the contents of the acrosome are released by exocytosis Figure Figure The acrosome reaction that occurs when a mammalian sperm fertilizes an egg. In mice, a single glycoprotein in the zona pellucida, ZP3, is thought to be responsible for both binding the sperm and inducing the acrosome reaction.
Note that a mammalian sperm more The acrosome reaction is required for fertilization. It exposes various hydrolytic enzymes that help the sperm tunnel through the zona pellucidaand it exposes other proteins on the sperm surface that bind to the ZP2 protein and thereby help the sperm maintain its tight binding to the zona while burrowing through it.
- Egg meets sperm
- Fate of a Fertilized Egg: Why Some Embryos Don't Implant
- Conception: How It Works
In addition, the acrosome reaction exposes proteins in the sperm plasma membrane that mediate the binding and fusion of this membrane with that of the eggas we discuss below. Although fertilization normally occurs by sperm—egg fusion, it can also be achieved artificially, by injecting the sperm into the egg cytoplasm ; this is sometimes done in infertility clinics when there is a problem with sperm—egg fusion.
The Egg Cortical Reaction Helps to Ensure That Only One Sperm Fertilizes the Egg Although many sperm can bind to an eggnormally only one fuses with the egg plasma membrane and injects its nucleus and other organelles into the egg cytoplasm. If more than one sperm fuses—a condition called polyspermy—multipolar or extra mitotic spindles are formed, resulting in faulty segregation of chromosomes during cell division ; nondiploid cells are produced, and development usually stops. Two mechanisms can operate to ensure that only one sperm fertilizes the egg.
In many cases, a rapid depolarization of the egg plasma membrane, which is caused by the fusion of the first sperm, prevents further sperm from fusing and thereby acts as a fast primary block to polyspermy. But the membrane potential returns to normal soon after fertilizationso that a second mechanism is required to ensure a longer-term, secondary block to polyspermy.
This is provided by the egg cortical reaction. The contents of the cortical granules include various enzymes that are released by the cortical reaction and change the structure of the zona pellucida.
Among the changes that occur in the zona is the proteolytic cleavage of ZP2 and the hydrolysis of sugar groups on ZP3 Figure Figure How the cortical reaction in a mouse egg is thought to prevent additional sperm from entering the egg. The released contents of the cortical granules both remove carbohydrate from ZP3 so it no longer can bind to the sperm plasma membrane and partly cleave more Since it is so much bigger than sperm, the egg is the source of cytosol and organelles,particularly mitochondria, for the future zygote.
This means that the egg is haploid but with sister chromatids still attached to each other. Also unlike sperm, the meiotic division to create eggs, oogenesis, only makes one viable egg. The egg is covered in a thick outer coating known as the zona pellucida, a layer of carbohydrate-covered proteins that surrounds the plasma membrane.
The zona pellucida helps protect the egg and is responsible for mediating the initial meeting of sperm and egg. Cortical granules filled with enzymes line the inside of the cell membrane, and will help make sure that only one sperm can fertilize the egg.
Egg meets sperm (article) | Embryology | Khan Academy
The setting Egg and sperm travel in opposite directions to meet in most often the fallopian tubes. During ovulation, ovaries release an egg into one of the fallopian tubes, and the egg proceeds down the tube toward the uterus, which is being prepared for possible implantation. Part of this preparation involves elevated levels of estrogen and luteinizing hormone LH. LH triggers the ovaries to release the egg, while higher blood estrogen levels stimulate the vaginal membrane to secrete glycogen, which is then metabolized to lactate.
This lowers vaginal pH to as low as 3. However, this environment can also be toxic to sperm, though the semen a basic fluid can buffer the vaginal acidity to preserve sperm cells.When the egg meets sperm
Only about 1 in 1 million sperm that are ejaculated into the vagina will reach the site of fertilization. Estrogen also relaxes the cervix, causes cervical mucus to become watery and more alkaline, and stimulates uterine contractions — all of which help sperm penetrate and navigate the female reproductive system. Relaxing the cervix allows sperm to pass from the vagina into the uterus and reduces a potential physical barrier.
Conditions such as pelvic infections and endometriosis can permanently impair the function of the fallopian tubes, due to scarring or damage to the fimbriae.
The macho sperm myth
Fertilization and Embryo Development Following ovulation, the egg is capable of fertilization for only 12 to 24 hours. Contact between the egg and sperm is random.
Once the egg arrives at a specific portion of the tube, called the ampullar-isthmic junction, it rests for another 30 hours.
Fertilization — sperm union with the egg — occurs in this portion of the tube. The fertilized egg then begins a rapid descent to the uterus. The period of rest in the tube appears to be necessary for full development of the fertilized egg and for the uterus to prepare to receive the egg.
Defects in the fallopian tube may impair transport and increase the risk of a tubal pregnancy, also called ectopic pregnancy. A membrane surrounding the egg, called the zona pellucida, has two major functions in fertilization. First, the zona pellucida contains sperm receptors that are specific for human sperm. Second, once penetrated by the sperm, the membrane becomes impermeable to penetration by other sperm. Following penetration, a series of events set the stage for the first cell division.
The single-cell embryo is called a zygote. Over the course of the next seven days, the human embryo undergoes multiple cell divisions in a process called mitosis. At the end of this transition period, the embryo becomes a mass of very organized cells, called a blastocyst. It's now believed that as women get older, this process of early embryo development is increasingly impaired due to diminishing egg quality.