BSC 116, Notes on Reproduction
BSC 116, Notes on Reproduction BSC 116
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This 6 page Class Notes was uploaded by Ashley Bartolomeo on Sunday April 3, 2016. The Class Notes belongs to BSC 116 at University of Alabama - Tuscaloosa taught by Professor Harris in Spring 2016. Since its upload, it has received 17 views. For similar materials see Principles Biology II in Biological Sciences at University of Alabama - Tuscaloosa.
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Date Created: 04/03/16
Lectures 28 & 29 Reproduction Overview Variation in reproductive modes among animals o Sexual vs. asexual reproduction o Gonochoristic vs. hermaphroditic o Internal vs. external fertilization Human gametogenesis & hormonal regulation o Spermatogenesis in males: constant o Oogenesis in females: cyclic Pregnancy & early development Vertebrates Are Not Really Representative of Animal Reproductive Modes Sexual reproduction: fusion of haploid (n) gametes (sperm & egg) to form diploid (2n) zygote o What about asexual reproduction? Gonochoristic: separate male and female sexes o What about hermaphroditic species? Internal fertilization: gametes mingle in female’s reproductive tract o What about external fertilization? Many Animals Rely On Some Form of Cloning to Reproduce Asexual reproduction: new individuals without sex (i.e., meiosis); = cloning Fission: one individual splits into two of similar size o E.g., sea anemone Budding: one individual grows from another o E.g., Hydra o E.g., colonial corals Fragmentation: breakage, followed by regeneration o E.g., starfish Linckia Parthenogenesis: offspring develop from unfertilized eggs o E.g., Daphnia o E.g., Aspidoscelis Why Isn’t Asexual Reproduction More Popular? Asexual reproduction presents a paradox o If each female produces two offspring, then a population without sex doubles every generation o Population with males constant o Asexual populations should quickly replace sexual populations by natural selection But, clones are not as adaptable to changing environments o Sex produces new genotypes: populations flexible to new climates, pathogens, etc. o E.g., if everyone is identical, then a successful pathogen infects everyone Would expect asexual reproduction to be favored in stable environments, sex in more variable o E.g., Daphnia switches seasonally o E.g., Campeloma asexual up north, sexual down south (more parasites) In Many Taxa, It Is Advantage to Not Have Separate Sexes The sex of an individual is determined by its gonads o Males have testes that produce sperm o Females have ovaries that produce ova (eggs) Vertebrates tend to be gonochoristic: separate sexes o i.e., an individual has either testes or ovaries Many animal species do not have separate sexes: hermaphrodites o E.g., tapeworms, flukes: simultaneously testes & ovaries o E.g., some oysters: sequentially, testes then ovaries o Esp. among invertebrates, hermaphrodites among generally gonochoristic species not uncommon In Many Taxa, Gametes Mix Outside Both Parent’s Bodies Many aquatic species rely on external fertilization o Spawning: both sperm & eggs released to water o Some species synchronize based upon environmental cues E.g., palolo worm: whole population reproduced on same night o Some species individuals interact more E.g., frog: have specific courtship behaviors Internal fertilization is necessary in terrestrial habitats; various mechanisms, typically cooperative o Many species lack specialized copulatory organs E.g., spiders pass bundle of sperm to female using a leg E.g., reptiles (+ birds) have cloaca: common opening of digestive, excretory and reproductive systems E.g., hermaphroditic earthworks and flukes: reciprocal fertilization Many species (including vertebrates) have specialized copulatory organs In Contrast to Males, For Females Reproduction is Costly & Highly Regulated For complex sexually reproducing vertebrates with separate sexes and internal fertilization, many different events need to be synchronized o Especially for females o E.g., production of gametes (gametogenesis) = oogenesis must be coordinated with systems to support fertilized embryo o Eggs relative large: DNA + nutrients for embryo For males, spermatogenesis (production) of sperm is simple: lots all the time Spermatogenesis different from oogenesis in 3 key ways: o Number of gametes formed by meiosis Spermatogenesis: all four products become gametes Oogenesis: only a single meiotic egg is produced o Timing of meiotic divisions Spermatogenesis: occurs continuously throughout adulthood Oogenesis: much of the process is completed before birth; mature gametes produced until about age 50 o Pace of meiotic divisions Spermatogenesis: sperm produced in continuous sequence from precursors to mature gametes Oogenesis: long pauses in the process Human Male Reproductive Anatomy Is Typical of Other Primates External male sex organs o Penis: urethra & erectile tissue Relies on hydrostatic pressure (blood) during copulation Other mammals have a bone (baculum) to maintain rigidity o Scrotum: contains testes outside the body Spermatogenesis happens at lower temperature than body Testicle: testis within a scrotum Internal sex organs o Testes: composed of highly coiled seminiferous tubules, produce sperm Leydig cells (endocrine): make testosterone o Epididymis: highly coiled, 6 m tube Sperm takes three weeks to mature Ejaculation: the process of getting sperm from the inside to the outside o From epididymis to muscular vas deferens, and around the bladder o Ejaculatory duct opens to urethra Sperm mixed with products of three glands to make 2-5 mL of semen o 70-130 million sperm per mL o Seminal vesicles: most of the volume; contain mucus, sugar, ascorbic acid, and prostaglandins o Prostate gland: adds more fluid, including anticoagulant enzymes o Bulbourethral glands: secretes mucus to neutralize any urine (acidic) in urethra Gametogenesis (Spermatogenesis) In Males is Designed To Make Lots of Sperm All The Time Spermatogonium: diploid (2n) stem cells, within testes Primary spermatocyte: 2n Secondary spermatocytes: n x 2 o Products of meiosis I Spermatids: n x 4 o Products of meiosis II Sperm cells: n x 4 o Mature in seminiferous tubules o Become motile in epididymis Gametogenesis in both males & females is under hormonal control Hormones come from hypothalamus, anterior pituitary and the gonads themselves o GnRH (gonadotropin-releasing hormone) from hypothalamus o FSH (follicle-stimulating hormone) & LH (luteinizing hormone) from pituitary Topic hormones that target gonads o Gonads produce sex hormones Males: androgens, like testosterone Females: estrogens, like estradiol and progesterone Sex hormones don’t only regulate gametogenesis: involved in secondary sexual characteristics o Androgens influence male traits: male fetal development, bird songs, deep voice, facial hair, sexual behavior, aggressiveness o Estrogens influence female traits: sexual behavior, development of breasts, water retention, calcium metabolism Hormonal control of spermatogenesis is based on negative feedback Hormonal control of spermatogenesis is relatively simple GnRH in hypothalamus, leads to release of FSH and LH o FSH: acts on sertoli cells in seminiferous tubules Nourish developing sperm Promote spermatogenesis o LH: acts on Leydig cells between seminiferous tubules Secretes testosterone, which promotes spermatogenesis Beginning at puberty, constant androgen production controlled by negative feedback o Testosterone inhibits both hypothalamus and anterior pituitary o Sertoli cells produce inhibin: regulates pituitary Female Reproductive Anatomy Has Gonads & Endocrine Glands as Well as a Womb Eutherian females must have organs to produce gametes and carry a developing fetus Paired ovaries: are the female gonads o Outer layer of follicles: oocytes (partially developed eggs) surrounded by support cells 1-2 million follicles at birth Only ca. 500 mature between puberty and menopause o After ovulation (release of ovum), follicle becomes corpus luteum that makes estrogen Degrades without fertilization Oviducts (= fallopian tubes) lead to uterus (=womb) o Connects to vagina via cervix o Uterus lined by endometrium: many blood vessels to support developing fetus Mammalian females as have mammary glands that produce milk Oogenesis Produces a Few Big Cells Rather Than Many Small Cells Oogenium: diploid (2n) stem cell, within follicle of ovary Primary oocyte: 2n o Present at birth Secondary oocyte: n (+ polar body) o 1 per month; stops mid-meiosis II Fertilized egg: 2n (+ polar body) o Ovulation + sperm initiates completion of meiosis II o Develops into 2n zygote Follicle becomes corpus luteum The Ovarian Cycle is One of Two Reproductive Cycles Ovarian cycle: produces ovum o Lasts about 28 days o Under control of same hormones as spermatogenesis 1. GnRh from hypothalamus 2. FSH & LH from anterior pituitary 3. FSH stimulates follicle growth 4. Growing follicle secretes estradiol (slowly increase during follicular phase); low estradiol inhibits anterior pituitary 5. But increasing estradiol stimulates pituitary 6. Leads to spike in FSH & LH, leads to more estradiol, which leads to more gonadotropins (positive feedback) 7. Maturing follicle ruptures and releases secondary oocyte (ovulation) 8. Luteal phase follows ovulation; LH stimulates development of corpus luteum, which secretes estradiol & progesterone a. Inhibits hypothalamus by negative feedback b. Decrease in gonadotropins causes corpus luteum to degrade In the absence of pregnancy, the whole ovarian cycle starts again The Uterine Cycle is Synchronized with The Ovarian Cycle Hormonally Uterine cycle: prepares the uterus to support a fetus o Happens in coordination with the ovarian cycle Estradiol from follicles causes endometrium to thicken; proliferative phase 9. After ovulation, estrogens stimulate development of uterine lining, including arteries and glands; secretory phase 10. When corpus luteum disintegrates, hormone levels drop, and endometrium degrades, releasing blood Menstrual flow phase Coordination of ovarian and uterine cycles until menopause o Rare; why give up reproductive potential? o Other mammals have seasonal or annual estrous cycle (“in heat”) when females are receptive Fertilization Leads to Pregnancy and Temporary Disruption of the Cycles Fertilization (“conception”): sperm fuses with mature oocyte in oviduct o 24 hours: first cleavage (deuterostomes, radial) o 2-3 days zygote reaches uterus o 1 week: blastocyst (hollow ball of cells), implants in endometrium o Develops into fetus Embryo produces hGC (human chorionic gonadotropin) o Works like LH to keep corpus luteum from degrading o Keeps progesterone levels up o hGC can be detected in mother’s urine; pregnancy test Pregnancy: one or more embryos in the uterus The human gestational period is typically 38 weeks First 2-3 weeks: embryo gets nutrients directly from endometrium Placenta forms from embryonic and material tissue; blood vessels from both exchange nutrients, gases, wastes, etc. After 8 weeks of organogenesis, embryo termed a fetus Birth/ Labor is Induced by Both Estradiol & Oxytocin At the end of 38 weeks, a complex interaction between hormones (estradiol & oxytocin) and local regulators (prostaglandins) leads to labor = child birth o Dilation of the cervix o Contraction of the uterus to push fetus out thru vagina o More contraction to deliver placenta
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