You should be able to: Describe the challenges a sperm faces in making the journ

Question

You should be able to: Describe the challenges a sperm faces in making the journey to the egg *Explain the steps that lead to the fusion of a sperm with anegg Understand the functions of the placenta Understand the ways in which fetal circulation differs from the circulation pattern of an infant Describe the anatomical, metabolic, and physiological changes a woman undergoes during pregnancy .Explain the three stages of labor and what is occurring during each stage Describe the roles of oxytocin and prolactin in breastfeeding . Describe different methods of birth control, including how they prevent pregnancy Describe common bacterial and viral sexually transmitted infections

Explanation / Answer

First one,

The challenges are difficult for a sperm cell right from beginning when it gets released in uterus .Hundreds of millions of sperm vie for a single egg cell. The sperm cells are streamlined in design for this purpose: a long tail to help them move, lots of mitochondria to power that movement, genetic information to pass on, and enzymatic proteins to get into the egg cell. The proteins are stored in a cap at the front of the sperm known as an acrosome - this is the part that first contacts the egg. The tail is called a flagellum, and it uses the energy made by the mitochondria to move the sperm forward. Flagella use a lot of energy, so they’re kept dormant until sperm enter the vagina. Sperm are haploid; they contain one set of 23 chromosomes. They are created by the cellular division process known as meiosis, which creates 4 sperm from a single germ cell. They’re also very small.

The sperms’ target, is the egg. Since it is so much bigger than sperm, the egg is the source of cytosol and organelles,particularly mitochondria, for the future zygote. Unlike sperm, the egg has not completed meiosis - it’s stuck in the Metaphase II stage of division.

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 pH (to as low as 3.8), creating an acidic environment hostile to pathogens (like the ones that cause sexually transmitted infections). However, this environment can also be toxic to sperm, though the semen (a basic fluid) can buffer , new environment is the trigger to activate sperm flagella and increase sperms’ motility.

Only about 1 in 1 million sperm that are ejaculated into the femal reproductive system 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 and reduces a potential physical barrier. Cervical mucus may prevent sperm from passing into the uterus, but during ovulation when the egg is released from the ovaries, the mucus gets thinner and lower in pH. These changes make the mucus a great transport medium for the sperm, and help the sperm continue traveling. The uterine contractions actually help to push sperm toward the correct fallopian tube.

As the sperm approach the egg, they bind to the zona pellucida in a process known as sperm binding. This triggers the acrosome reaction, in which the enzymes of the acrosome are freed. These enzymes then begin to digest the zona pellucida and allow the sperm to tunnel toward the egg’s plasma membrane. When the sperm cell finally reaches the egg cell, the plasma membranes of the two cells fuse together and the sperm releases its genetic material into the egg. At this point, fertilization has occurred, but we’re not done yet!

Fusion also triggers the cortical reaction. When the sperm and egg fuse it triggers a release of calcium ions, which cause the cortical granules inside the egg to fuse with the plasma membrane. As they fuse, these granules release their contents outside of the cell, toward the remains of the zona pellucida. The enzymes of the cortical granules further digest the zona pellucida, making it unable to bind more sperm, while other molecules found in the granules create a new protective layer around the fertilized egg. By creating a new barrier and destroying the initial interface between sperm and egg, the cortical reaction prevents polyspermy, or the fertilization of a single egg by multiple sperm. It’s like entering a hidden temple, but on the way, you set off hidden booby traps that make it impossible to ever enter again. Other sperm reaching the egg now are just shunted away.

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