Reproduction and Development Flashcards
advantages of asexual reproduction
Asexual reproduction does not provide for genetic diversity in any way. The advantage of asexual reproduction is that a partner is not required, it is not energy-intensive, and the population can be maintained quickly.
Asexual reproduction is a result of mitotic events. Some examples include:
Budding is often seen in hydra and corals. Buds are outgrowths of the parent. This may result in colonial forms of separate but attached organisms, or there may be complete disassociation of the bud from the parent.
Binary Fission is common in invertebrates. Parent organism splits into two individuals of approximately equal size.
Fragmentation and Regeneration are seen in annelids and flatworms. The body is broken into several pieces, and the absent portions are regenerated.
Parthenogenesis is seen in fish, arthropods, amphibians, and lizards. An egg develops without being fertilized.
Hermaphrodism
We think of sexual reproduction in terms of males and females, but the animal kingdom gives us many varieties of this theme. Hermaphrodism, having both male and female capabilities, is common. This strategy means that ANY two individuals can mate, increasing the mating pool. We also see instances where males and females can switch sexes based on the organisms around them to maximize the likelihood of fertilization.
Hormones
Hormones mediate reproductive cycles and often vary with the seasons. Hormones are often released differentially due to environmental cues. The point is to reproduce only when an animal can be maximally successful.
One such environmental cue is temperature. Increasing global temperatures thus impact organisms’ reproductive cycles.
Mechanisms of Uniting Egg and Sperm
One strategy is to make as many eggs and sperm as possible and cast them out. The sheer numbers ensure that they unite.
Another strategy is to be more direct in placing sperm and eggs. This gives rise to copulatory behaviors, like in frogs, where eggs and sperm are deposited in the same location. If we take this idea to the extreme, we see internal fertilization, where the sperm is delivered as close to the egg as possible inside the female body. Thus ensuring maximum protection.
Once gametes unite, there are many strategies to get to birth and additional strategies to ensure the offspring reach adulthood. In this case, we may see many offspring made with hopes that some make it to adulthood, or we may see few offspring with lots of parental nurturing to help them reach adulthood.
Male reproduction system
Sperm are produced via meiosis in seminiferous tubules within the testes. They migrate to the epididymis, where they mature, grow flagella for swimming, and gain acrosomal packs of enzymes needed for fertilization. From there, they migrate to the vas deferens. As sperm move through the vas deferens, the seminal vesicles add nutrients and enzymes to begin to produce semen. This combined fluid enters the prostate and receives additional fluids to form semen fully. Before ejaculate can move from the prostate, fluids are released from the bulbourethral glands ahead of semen to stabilize the pH and clean out the urethra before semen enters the urethra. The urethra also empties the bladder; thus, this cleansing step is necessary to protect the sperm. The urethra is a tube that extends through the penis to the opening of the body.
The penis is the copulatory organ and contains columns of erectile tissue that become engorged with blood to produce an erection and allow female penetration for internal fertilization.
Female reproduction system
Eggs are produced in the ovary inside follicles. Typically, a single follicle matures each month and is released during ovulation. When released, the egg is released and let go into the body cavity; however, fimbriae of the oviducts (fallopian tubes in humans) and cilia sweep the egg into the oviduct. If fertilization is going to happen, it must happen in the oviduct or the resulting embryo will not be adequately developed to implant in the uterus. The egg will continue into the uterus. The uterus is lined with a thick, blood-rich lining called the endometrium. If the egg has not been fertilized, it will be flushed from the body along with the endometrial lining of the uterus during menstruation, passing from the cervix and through the vagina. It will nestle into the endometrium during implantation and continue development if fertilized.
External structures of the female system include the labia majora and labia minora, which cover the vaginal opening and the separate urethral opening. At the frontal edge is the clitoris. This is developmentally analogous to the penis and contains significant nerve endings that react to sexual arousal. Posterior to the vaginal opinion are the vestibular glands. Analogous to the bulbourethral glands, these produce fluids to make internal fertilization easier.
Spermatogenesis
Spermatogenesis takes place in the testes in the seminiferous tubules. Stem cells first go through mitosis to produce a spermatogonium cell. This cell then undergoes meiosis. The completion of meiosis produces four spermatocytes. This process is supported by Sertoli cells. Lydig cells within the testes produce the testosterone necessary for sperm maturation. Early spermatids are pushed toward the epididymis, where they mature into sperm cells. Each mature sperm has a long tail of flagella, a ring of mitochondria, a capsule containing DNA, and an acrosome of digestive enzymes on the head.
Oogenesis
Eggs begin production before birth, and meiosis is not completed until the sperm nucleus penetrates. A single germ cell goes through a mitotic division to establish a primary oocyte arrested in the prophase of Meiosis I. At puberty, a primary oocyte develops each month inside a supporting cell follicle. Meiosis will begin, and a secondary oocyte (and a polar body) will be formed and arrested at metaphase of meiosis II. This secondary oocyte is released at ovulation. If, and only if, sperm enters the oocyte, will it progress through meiosis II.
Through the process, the cytoplasm has been conserved into one cell. The other products of meiosis have been discarded as polar bodies; therefore, one original primary oocyte has produced only one egg.
The follicle, made of supporting cells, ruptures at ovulation to release the secondary oocyte. The “scar” left behind is now called a corpus luteum. This corpus luteum secretes progesterone to build up the endometrial lining for implantation. If no implantation happens, the corpus luteum will degrade but leave behind a tell-tale mark of ovulation.
Hormonal Control of Testes
The production of sperm is controlled via negative feedback. This control is mediated by the hypothalamus in the brain, the anterior pituitary just off the brain, and cells within the testes themselves. The hypothalamus makes gonadotropin-releasing hormone (GnRH) that is fed to the anterior pituitary. The anterior pituitary then secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH tells the Sertoli cells to create an environment suitable for spermatogenesis. At the same time, LH tells the Leydig cells to produce testosterone for sperm maturation. If there is enough sperm, inhibin will be produced, and feedback to the anterior pituitary will shut down FSH production. If there is enough testosterone, it will feedback itself to shut down the anterior pituitary from making LH.
Hormonal control of Female Reproductive Cycles
The hypothalamus makes gonadotropin-releasing hormone (GnRH), which is then passed to the anterior pituitary. The anterior pituitary makes FSH and LH. These hormones influence the ovarian cycle. The ovaries then make estradiol and progesterone, which influence the uterine (menstrual cycle). High levels of estradiol and progesterone inhibit the secretion of GnRH and can shut down ovulation (mechanism of hormonal birth control). High levels of just estradiol stimulate GnRH.
Let’s begin on day 1 of the menstrual cycle and follow what happens. Day one is the first day of menses (bleeding). Estradiol (estrogen) and progesterone are both very low at this point. This triggers menstruation. As menstruation completes, estradiol concentrations increase, stimulating GnRH. This, in turn, begins to increase LH and FSH. This causes the follicle to begin to grow. With a spike of estradiol, we also get a concurrent spike of LH and FSH, which causes ovulation. Immediately after ovulation, LH and FSH drop, as does estradiol. At this point, the corpus luteum takes over and begins the production of progesterone and estradiol. Elevating levels of these hormones stimulate the endometrium to grow. If fertilization does not happen, these hormones will drop off, and menstruation will begin again. If fertilization does happen, the corpus luteum will maintain levels of progesterone and estradiol for implantation.
Menstruation vs. Estrous Cycles
Only humans and some other primates have menstrual cycles. Every female mammal has a thickening of the endometrium, but in most other animals, the uterus reabsorbs the endometrial lining. Humans have lost this trait.
In most other animals, there is an estrous cycle. The estrous cycle influences not only ovulation but also receptivity to sexual activity. Most other animals will only engage in sexual activity at the time of ovulation. Humans will engage in sexual activity often without regard to cycles. A few species, like your house cat, will only ovulate upon mating activity.
Fertilization
In humans, fertilization must occur in the oviduct (fallopian tube). The developing embryo must be in the blastocyst stage for implantation to occur.
Prevention of Polyspermy
Millions of sperm compete to enter the egg at once to fertilize it, but if two try to fertilize at once, the resulting zygote would be triploid and would not develop. The egg has a unique response to ensure that only one sperm can fertilize. First of all, the egg has a jelly coat that protects it. Sperm arriving must dissolve and penetrate this coat with their acrosomal enzymes. Once through the jelly coat, an acrosomal process must bind correctly to the egg’s surface receptors (this ensures the sperm is of the correct species). The egg cell quickly depolarizes if this receptor binds and the membranes fuse. This depolarization is called the fast block to polyspermy.
This also begins the release of cortical granules from the egg cell, creating a space between the egg’s plasma membrane and the vitelline layer surrounding it. The cortical granules do two things: clip off the sperm-binding proteins on the plasma membrane’s surface and create a fertilization envelope around the egg. This process together is a cortical reaction and results in what is called the slow block to polyspermy
Cleavage
A set of rapid cell divisions directly after fertilization results in more cells without subsequent cell growth. Cleavage results in the formation of the blastocyst. The blastocyst will continue development and formation of the inner cell mass.
