Pregnancy and Lactation II: Lactation and Fertilization of the Ovum PDF

Pregnancy and Lactation II: Lactation and fertilization of the ovum Unit 10. 2 Second Semester Second Medicine  maturation of the ovule In the ovary, the ovum is in the primary oocyte stage, but before it is released from the ovarian follicle, its nucleus divid...

Pregnancy and Lactation II: Lactation and fertilization of the ovum Unit 10. 2 Second Semester Second Medicine  maturation of the ovule In the ovary, the ovum is in the primary oocyte stage, but before it is released from the ovarian follicle, its nucleus divides by meiosis and two identical cells are formed called the secondary oocyte. The secondary oocytes in a second division forms 4 cells each of the 23 pairs of chromosomes loses one of its partners, which becomes incorporated in an oocyte. maturation of the ovule  maturation of the ovule The ovum with 23 unpaired is ovulated into the abdominal cavity and it enters the fimbriated end of one of the fallopian tubes, along with more than a hundred granulosa cells that constitute the corona radiata. The ovum travels through the fallopian tubes to the uterus. It is accepted up to 98 percent of the ova succeed to arrive to the fallopian tube or even enter the opposite fallopian tube.  Fertilization of the ovum After semen in ejaculated or injected into the vagina, some spermatozoids are transported within 5 -10 minutes from the vagina and through the uterus and fallopian tubes to the ampullae of the fallopian tubes. That is one of the purpose of orgams that helps the transport of the sperm with contractions of the uterus and fallopian tubes due to the oxytocin released from the posterior pituitary gland of the female; and also stimulated by prostaglandins in the seminal fluid.  Fertilization of the ovum Fertilization of the ovum normally takes place in the ampulla of one of the fallopian tubes soon after both the sperm and the ovum enter the ampulla, after the spermatozoids penetrate the corona radiata and then bind to and penetrate the zona pellucida surrounding the ovum. For this process to occur capacitation of the spermatozoa is required  Fertilization of the ovum: capacitation process In the seminiferous tubules are constantly produced vesicles with large amounts of cholesterol that are added to the cellular membrane covering the sperm acrosome, toughening this membrane and preventing release of its enzymes. After ejaculation, the sperm escapes from the cholesterol vesicles and the membrane composition in few hours, weakening the acrosome.  Fertilization of the ovum: capacitation process Although spermatozoa are said to be “mature” when they leave the epididymis, their activity is inhibited by multiple factors secreted by the genital duct epithelia, and when they are first expelled in the semen, they are unable to fertilize the ovum. Capacitation of the spermatozoa is required for fertilization The spermatozoa that leave the epididymis have multiple inhibitory factors secreted by the genital duct epithelia, and they are unable to fertilize the ovum. When contact with the fluids of the female genital tract, multiple changes occur : the capacitation of the spermatozoa, that requires several hours. Capacitation of the spermatozoa is required for fertilization When contact with the fluids of the female genital tract, multiple changes occur that activate the sperm for the final processes of fertilization : the capacitation of the spermatozoa, that requires between 1 and 10 hours. Uterus and fallopian tube fluids wash away the inhibitory factors that suppress sperm activity in the male genital ducts in two phases. Capacitation of the spermatozoa is required for fertilization The changes include: 1. The uterine and fallopian tube fluids wash away the various inhibitory factors that suppress sperm activity. 2. In the fluid of the male genital ducts, sperm are exposed to vesicles with large amounts of cholesterol, that is added to the cellular membrane covering the sperm acrosome, preventing release of its enzymes. After ejaculation, the sperm gradually lose their excess cholesterol in few hours. The acrosome becomes much weaker and enzymes can be released.  Fertilization of the ovum: capacitation process With the release of cholesterol, the membrane of the sperm also becomes much more permeable to Ca, that enters the cells and increases the activity of the flagellum. Ca also changes the cellular membrane of the leading edge of the acrosome, making it possible for the acrosome to release its enzymes rapidly and helping the sperm to penetrate the granulosa cells and the zona pellucida. Capacitation of the spermatozoa is required for fertilization Capacitation of the spermatozoa is required for fertilization 3. The membrane of the sperm becomes much more permeable to calcium ions, that enter the sperm and induces the activity of the flagellum. The motion of sperm becomes stronger. Calcium ions also change the cellular membrane of the acrosome, facilitating the release of its enzymes as the sperm penetrates the granulosa cell mass surrounding the ovum, and even more so as it attempts to penetrate the zona pellucida of the ovum itself. Fertilization of the ovum: acrosome reaction The acrosome contains hyaluronidase and proteolytic enzymes. Hyaluronidase depolymerizes the hyaluronic acid polymers in the intercellular cement that holds the ovarian granulosa cells together. The proteolytic enzymes digest proteins in the structural elements of tissue cells that still adhere to the ovum. Fertilization of the ovum: acrosome reaction Sperm must dissolute the granulosa cell layers, and penetrate the zona pellucida to fertilize the ovum with the stored enzymes in the acrosome. Hyaluronidase seems very important in opening pathways between the granulosa cells. When the sperm reaches the zona pellucida of the ovum, the anterior membrane of the sperm binds specifically with receptor proteins in the zona pellucida. Next, the entire acrosome rapidly dissolves and all the acrosomal enzymes are released. Fertilization of the ovum: acrosome reaction Within minutes, the proteolytic enzymes open a way for the sperm head through the zona pellucida to the ovum. In another 30 minutes, the cell membranes of the sperm head and of the oocyte fuse with each other to form a single cell, and the chromosomes of the sperm and the oocyte combine to form a completely new cell genome, containing equal numbers of chromosomes from mother and father. Why does only one spermatozoe enter in the oocyte? Within a few minutes after the first and only spermatozoid penetrates the zona pellucida of the ovum, calcium ions diffuse inward through the oocyte membrane and cause multiple cortical granules to be released by exocytosis from the oocyte into the perivitelline space. The substances released permeate all portions of the zona pellucida and reject binding of additional sperm, including the already bound. Thus, almost never does more than one sperm enter the oocyte during fertilization.  Fertilization of the ovum When the sperm has entered the ovum, the oocyte divides again to form the mature ovum plus a second polar body that is expelled. The mature ovum still carries in its nucleus (now called the female pronucleus) 23 chromosomes. One of these chromosomes is the female chromosome, known as the X chromosome. Later, the 23 unpaired chromosomes of the male pronucleus and the 23 unpaired chromosomes of the female pronucleus align themselves to re-form a complete complement of 46 chromosomes (23 pairs) in the fertilized ovum Fertilization of the ovum