Beginning on page 86, Shubin begins discussion on the formation of the zygote from sperm and egg in the section "The Essence in Embryos." For review over sperm production and the menstrual cycle, describe the path of sperm as it travels through the male reproductive parts to be ejaculated out of the urethra and the development of the egg in the female ovum. Describe the complete menstrual cycle in the discussion of egg development, and what happens when sperm does and does not fertilize the egg. Don't forget to include hormones in the discussion.
(Bobby Muttilainen, rmuttilainen@gmail.com)
When a male reaches puberty, the hypothalamus will release the hormone GnRH that targets the anterior pituitary to release the hormones LH and FSH. LH targets the leydig cells to "secrete testosterone and other androgens, which promote spermatogenesis in the [seminiferous] tubules" (Campbell 1010) of the testes. After being developed in the seminiferous tubules, sperm pass through the epididymis in which the sperm are completely matured and become motile. During ejaculation, sperm are moved into the vas deferens that connects from a short ejaculatory duct into the urethra (Campbell 1005). The seminal vesicles, prostate gland and bulbourethral glands produce a fluid called semen that combines with the sperm so that the sperm can move as it travels to the penis. The urethra runs through the penis in which the sperm will be ejaculated out of the male and into the female’s vagina, where the sperm will swim to the fallopian tubes to fertilize an egg.
ReplyDeleteUnlike males, females are born with all of the oocytes that they will have in their life. However, similar to males, the hypothalamus also releases GnRH at puberty to stimulate the anterior pituitary to release FSH and LH. During the follicular phase of the menstrual cycle, “low levels of estradiol inhibit secretion of the pituitary hormones, keeping levels of FSH and LH relatively low” (Campbell 1010). Afterwards, the estradiol secretion by the follicles in the ovaries begin to rise which increase the levels of FSH and LH because the hypothalamus has been stimulated to increase GnRH production. Oogenesis causes to the oocyte to divide by meiosis, but stop at prophase I (Campbell 1009). This is where the oocyte stops development before puberty. Then, FSH stimulates follicles to fully mature the oocyte into the secondary oocyte after meiosis II. One egg and 3 polar bodies are the result. The egg matures in the follicle and at ovulation, and LH causes the follicle to release the egg. At the luteal phase, LH causes the follicular tissue that was left behind to become the corpus luteum, which secretes progesterone and estrogen. At the end of the leuteal phase, LH and FSH levels are low which causes the corpus luteum to disintegrate, which triggers a decline in estrogen and progesterone. Released from the negative feedback relationship with the hypothalamus, GnRH is released again to allow the pituitary secrete LH and FSH to mature more oocytes.
The menstrual cycle consists of the proliferative phase, the secretory phase, and the menstrual flow phase. Estrogen released by the corpus luteum causes the uterus to become thicker with blood vessels in an endometrium lining. Progesterone helps the uterus maintain this lining in case a fertilized egg attaches to the uterus. If the egg is not fertilized, the secretory phase begins in which the levels of estrogen and progesterone drop with the disintegration of the corpus luteum (Campbell 1012). The lining in the uterus is no longer needed for a developing embryo, so the blood vessels in the endometrium constrict to release blood that is shed with the unfertilized eggs. In the menstrual flow phase, new follicles grow in the ovaries and the menstrual cycle starts over again.
Due to the simplicity of the first part of the question, I will not go into the movement of sperm. However, due to the complexity of the menstrual cycle, I will go into greater detail as to the length of the cycle and other key components.
ReplyDeleteThe menstrual cycle consists of 28 days and three phases consisting of the proliferative phase, the secretory phase, and the menstrual flow phase. At the beginning of the cycle, which is reffered to as Day 1, is the day bleeding begins. The flow usually lasts about 3 to 5 days. Around Day 7 some of the eggs in the ovaries start maturing and are almost ready to be released. One egg is released from the ovary on about Day 14. The other maturing eggs stop growing and dry up. The time from menstruation to ovulation, may vary from 13 to 20 days in length. Factors aside from the reproductive system such as sickness, physical exertion, and sudden changes in can upset a regular pattern by shortening/extending it(http://www.fwhc.org/health/moon.htm)
During the next stage ovulation, the egg travels down the fallopian tube toward the uterus. If a single male sperm meets with the egg while it is in the tube pregnancy begins. If fertilization doesn't take place, the egg cell will break apart in a day or two. Around Day 25, hormone levels drop. This causes the lining of the uterus to break down and in a few days it is shed in the menstrual period. This part of the cycle, from ovulation to menstruation, is about the same length in all women. As Claire went into the details on hormones I will not mention them again. The overall purpose of this synopsis was to give us a better understanding of the menstrual cycle with reference to time while making it easier to understand.
Eryk Fundakowski- arthur2446@comcast.net
http://women.webmd.com/tc/normal-menstrual-cycle-topic-overview
http://www.fwhc.org/health/moon.htm