Chapter 59- Sex & Reproduction Asexual Reproduction fission division of the organisms, made possible by mitosis budding part of parent's body becomes separated and differentiates into new individual parthenogenesis females produce offspring from unfertilized eggs honeybees mate, queen stores sperm makes diploid females or haploid male drones (parth.) also found in some lizards hermaphroditism one individual has both testes & ovaries tapeworm makes eggs & sperm earthworms still require another cannot fertilize own eggs some fish are sequential hermaphrodites protogyny first female appears to be under social control protoandry first male Sexual Reproduction union of two gametes egg & sperm formed from ovaries & testes produces zygote mitosis leads to multicellular organism Fertilization external fertilization gametes shed into water usually for brief and well-defined periods seasonal cues in oceans? internal fertilization evolved on land keeps gametes from drying out three strategies: oviparity eggs fertilized internally deposited outside mother ovoviparity fertilized eggs retained, embryos get all nutrients from yolk young hatch fully developed viviparity young develop inside mother, get all nourishment from mother's bloodstream Most mammals have internal & external reproductive organs Male Anatomy external penis & scrotum internal testes (gonads) inside scrotum keeps sperm at lower temperature accessory glands & ducts ducts imply exocrine function Inside testes are seminiferous tubules, highly coiled tubes where sperm are formed. Interstitial cells between the tubules produce testosterone and other hormones. From tubules, sperm pass into tubules in the epididymous, where they remain for 18 hours, gaining motility and fertilizing power. At ejaculation, sperm pass through vas deferens, urethra and out the penis. Male accessory glands include seminal vesicles, the prostate gland and bulbourethral gland. seminal vesicles secrete fluid that contains mucus, amino acids (for coagulation), fructose (energy for sperm) and prostaglandins (causes uterus to contract and carry sperm to egg). the prostate gland surround the urethra, secretes milky fluid (60% of the volume of semen) that contains enzymes & nutrients and is thought to balance acidity of residual urine in urethra and vagina. the bulbourethral glands secrete a viscous fluid that appears to line the urethra (but whose function is largely unknown). sperm are only 1% of the volume of semen one mL of semen contains 50-130 million sperm. Usually 2-5 mL are delivered per ejaculation. Men with only 20 million sperm/mL are considered sterile. Semen is slightly alkaline, to neutralize the acidic environment of the vagina. Enables sperm to move faster. prostaglandins in semen thin mucus at cervix (opening of uterus), stimulate contraction of uterus to move semen along. After ejaculation, semen coagulates, which makes it easier for uterine contractions to move it along, then anticoagulants liquify the semen, at this point sperm begin to swim through female reproductive tract. Female Anatomy- more complicated than men because it has to produce eggs AND maintain the developing embryo. external clitoris & labia internal ovaries gonads ducts & chambers fallopian tubes uterus vagina Egg production takes place in ovaries, there are two, in the abdominal cavity. Each ovary contains many follicles (one egg plus many granulosa cells). Granulosa cells nourish and protect the egg, and produce estrogen. All of a woman's eggs are present at birth. After puberty, granulosa cells begin to secrete estrogen. Usually one egg will mature with each menstrual cycle. Ovulation is when an egg is expelled from the follicle, into the abdominal cavity near an oviduct. Cilia in the oviduct draw the egg into it and the egg is conveyed to the uterus. The uterus is a thick muscular organ whose lining (endometrium) is rich with blood vessels. It narrows to the cervix and then the vagina. Production of sperm & eggs requires meiosis Spermatogenesis Spermatogenesis is a continuous process in males. It starts with the differentiation of primordial germ cells into germinal cells within the embryonic testes. The germinal cells are located near the outer wall of the seminiferous tubules. Numbers of germinal cells increase by mitosis throughout development and early life. After publerty, the germinal cells become primary spermatocytes which are still diploid. They undergo meiosis I to form 2 haploid secondary spermatocytes. Then meiosis II and they become spermatids. Spermatids associate with Sertoli Cells to become mature spermatozoa (sperm). From spermatogonia to motile sperm takes 65-75 days in a human male. Oogenesis Oogenesis begins in the embryonic female. The primordial germ cells go through mitosis in the embryo, producing diploid oogonia. Each oogonium develops into a primary oocyte (within a single follicle). At puberty, a single primary oocyte completes meiosis I each month. The first meiotic division produces a large cell, the secondary oocyte, and a smaller polar body. Ovulation is when the secondary oocyte is expelled from the follicle in the ovary. A second meiotic division only occurs if the secondary oocyte is penetrated by a sperm, and it produces the ovum and another polar body. If the egg is fertilized, the corpus luteum (ruptured follicle) in the ovary secretes additional estrogen and progesterone that maintains the uterine lining during pregnancy. If not fertilized, the egg disintegrates and a new follicle releases a 3 Major differences between spermatogenesis and oogenesis: -only one product of meiosis for eggs; four for sperm -spermatogenesis is continuous, all ova present at birth -spermatogenesis is uninterrupted, long resting periods for oogenesis Hormonal control Testes gonadotropin-releasing hormone (GnRH) from hypothalamus produces: 2 hormones from pituitary FSH tells seminiferous tubules to increase spermatogenesis LH stimulates testosterone production by interstitial cells needed for sperm production these are maintained at constant levels in humans, but fluctuate by season in other mammals Ovaries there are menstrual cycles and estrous cycles estrus: non-primates female about to ovulate sexually receptive endometrium reabsorbed not shed if no egg implanted controlled by seasonal events menstrual: primates endometrium thickens when ovulation about to occur hormones work to coordinate menstrual and ovarian cycles so endometrium ready to receive egg GnRH secreted by hypothalamus tells anterior pituitary to secrete LH & FSH FSH stimulates immature follicles to grow follicles secret estrogen more as it gets bigger estrogen causes GnRH to increase leads to more FSH & LH LH causes ovulation and for the ruptured follicle to become corpus luteum corpus luteum secretes estrogen & progesterone these shut down GnRH so FSH & LH go down as LH goes down, corpus luteum disintegrates est & prog go down if fertilization does occur, fertilized egg releases Human Chorionic Gonadotropin (HCG). Acts like LH to maintain corpus luteum. C.L. secretes est & progest thru 1st trimester.