The Reproductive System Chapter 50

Sexual vs. Asexual Reproduction Sexual reproduction requires meiosis to produce haploid gametes: sperms and eggs -These are united by fertilization to produce the diploid zygote -Develops by mitosis into a new multicellular organism

Sexual vs. Asexual Reproduction In asexual reproduction genetically identical cells are produced from a single parent cell through mitosis -Single-celled organisms utilize fission -Cnidarians reproduce by budding

Novel Reproductive Strategies Parthenogenesis = Females produce offspring from unfertilized eggs Hermaphroditism = One individual has both testes and ovaries -May be sequential -Protogyny = Female-to-male change -Protandry = Male-to-female change

Novel Reproductive Strategies

Sex Determination In some reptiles, sex determination is environmentally controlled In mammals, it is genetically controlled -Embryonic gonads are indifferent -Y chromosome converts them to testes -A key gene is SRY -Sex-determining region of the Y chromosome

Vertebrate Fertilization Vertebrate sexual reproduction evolved in in the ocean before vertebrates colonized land Marine bony fish use external fertilization -Eggs and sperm are released into the water where union of free gametes occurs Most other vertebrates use internal fertilization -Sperm are introduced into the female reproductive tract

Vertebrate Fertilization Internal fertilization has led to three strategies for development 1. Oviparity = Fertilized eggs are deposited outside mother’s body to complete their development 2. Ovoviviparity = Fertilized eggs are kept within mother to complete development, and young obtain food from egg yolk 3. Viviparity = Young develop within mother and obtain nourishment from her blood

Vertebrate Fertilization

Vertebrate Fertilization Fishes -In most species of bony fish, fertilization is external -Thousands of eggs are fertilized, but only a few individuals grow to maturity -In most species of cartilaginous fish, fertilization is internal -Development of young is viviparous

Vertebrate Fertilization Amphibians -In most species fertilization is external -Eggs of most species develop in water -With some interesting exceptions -Development is divided into embryonic, larval and adult stages

Vertebrate Fertilization

Vertebrate Fertilization (Cont.)

Vertebrate Fertilization (Cont.)

Vertebrate Fertilization Reptiles -Most reptiles are oviparous -Lay amniotic eggs -Most male reptiles use a penis to inject sperm into females -Copulation

Vertebrate Fertilization Birds -All birds are oviparous, laying amniotic eggs -As egg passes along oviducts, glands secrete albumin & the hard calcareous shell -Birds are homeotherms and so incubate eggs to keep them warm

Vertebrate Reproductive Cycles Female reproductive cycles involve periodic release of a mature ovum (ovulation) Most female mammals have estrous cycles -Females sexually receptive to males only around time of ovulation (estrus) Primates have menstrual cycles -Females bleed when shedding inner lining of the uterus (menstruation) -Can copulate at any time in their cycle

Reproduction in Mammals Mammals are of three types: 1. Monotremes are oviparous -Lay eggs 2. Marsupials are viviparous -Give birth to incompletely developed fetuses, which mature in mother’s pouch 3. Placentals are viviparous -Retain young in uterus for long periods of development

Reproduction in Mammals

Reproduction in Mammals (Cont.)

Male Reproductive System When testes form in the human embryo, they develop highly-convoluted seminiferous tubules, the sites of sperm production -Leydig cells produce testosterone -Convert indifferent external genitalia into penis and scrotum Shortly before birth, the testes descend into the scrotum, because sperm need cooler temperature to develop

Sperm Production In the wall of the seminiferous tubule, a spermatogonium divides by mitosis to produce two diploid cells -One later undergoes meiosis, the other remains as a spermatogonium The diploid cell that begins meiosis is called a primary spermatocyte -It undergoes meiosis I, producing two haploid secondary spermatocytes

Sperm Production Each secondary spermatocyte undergoes meiosis II to produce two haploid spermatids The seminiferous tubules also contain supporting cells called Sertoli cells -Nurse developing germ cells -Help convert spermatids into spermatozoa (sperm) by engulfing their extra cytoplasm

Sperm Structure Mature sperm consist of three regions: -Head = Contains a nucleus -Capped by the acrosome, which facilitates penetration of the egg -Body = Consists of many mitochondria -Provide energy -Tail = Consists of a flagellum -Provides locomotion

Male Accessory Sex Organs Sperm are delivered into the epididymis for storage and maturation -From there, sperm enter another long tube, the vas deferens -To the urethra, which empties into the penis

Male Accessory Sex Organs Three glands assist in the production of semen, a complex mixture of fluids & sperm -Seminal vesicles produce a fructose-rich fluid, which makes up 60% of semen volume -The prostate gland contributes about 30% of the bulk of semen -Bulbourethral glands add secretions to make up the last 10% of semen -Also lubricate the tip of the penis

Penis Structure and Function The penis consists of erectile tissue columns -Two corpora cavernosa on dorsal side -One corpus spongiosum on ventral side During erection, these tissues fill with blood -Parasympathetic nerves release nitric oxide (NO), which stimulates dilation of arteries Ejaculation is the ejection from the penis of about 2-5 mL of semen containing an average of 300 million sperm

Male Reproductive Hormones The hypothalamus secretes gonadotropin-releasing hormone (GnRH) -GnRH stimulates the anterior pituitary to produce two gonadotropins -These regulate testicular function

Male Reproductive Hormones Luteinizing hormone (LH) -Stimulates Leydig cells to secrete testosterone Follicle-stimulating hormone (FSH) -Stimulates Sertoli cells to facilitate sperm development, and to secrete inhibin Testosterone and inhibin exert feedback inhibition on LH and FSH, respectively

Female Reproductive System Ovaries develop more slowly than the testes -Female embryo develops a clitoris and labia majora -Ovaries contain microscopic structures called ovarian follicles -Each follicle contains a potential egg cell called a primary oocyte and smaller granulosa cells

The Menstrual Cycle At puberty, granulosa cells secrete estrogen -Triggers menarche, first menstrual cycle -Stimulates secondary sexual characteristics At birth, ovaries contain about 1 million follicles -Each contains a primary oocyte that is arrested in prophase of meiosis I The human menstrual cycle lasts ~ 1 month -Divided into follicular phase, ovulation and luteal phase

The Menstrual Cycle Follicular phase -Several follicles are stimulated to grow under FSH stimulation -But only one achieves full maturity as a tertiary, or Graafian, follicle -Estrogen causes growth of the endometrium -Proliferative phase

The Menstrual Cycle

The Menstrual Cycle Follicular phase -The primary oocyte completes meiosis I -Produces one large secondary oocyte and one tiny polar body -The secondary oocyte begins meiosis II -However, its progress is arrested in metaphase II

The Menstrual Cycle Ovulation -Estrogen stimulates the anterior pituitary to secrete LH about midcycle -The LH surge causes the Graafian follicle to burst, releasing its secondary oocyte into the Fallopian tube -If oocyte is not fertilized, it disintegrates -If it is fertilized, it completes meiosis II, forming a mature ovum and second polar body

The Menstrual Cycle

The Menstrual Cycle (Cont.)

The Menstrual Cycle Luteal phase -LH stimulation transforms the Graafian follicle into the corpus luteum -Secretes estrogen and progesterone -Exert negative feedback on FSH and LH -Cause endometrium to become more vascular and glandular -Secretory phase

The Menstrual Cycle Luteal phase -In the absence of fertilization, the corpus luteum degenerates due to decreasing levels of FSH and LH -The built-up endometrium is sloughed off with accompanying bleeding -Menstrual phase

The Menstrual Cycle Luteal phase -If ovulated oocyte is fertilized, the corpus luteum is maintained by human chorionic gonadotropin (hCG) -Keeps high levels of estrogen and progesterone, and so prevents menstruation until placenta takes over -hCG is produced by the embryo -Tested for in all pregnancy tests

The Estrous Cycle Menstruation is absent in mammals with estrous cycles -The endometrium is cyclically shed, however without bleeding The estrous cycle is divided into four phases: -Proestrus, estrus, metestrus, and diestrus

Female Accessory Sex Organs Fallopian tubes (uterine tubes or oviducts) Uterus = Muscular pear-shaped organ Cervix = Narrow opening Vagina = Initially covered by a membrane called the hymen -Bartholin’s glands secrete lubricant that facilitates penetration by penis During sexual arousal, external genitalia become engorged with blood

Female Accessory Sex Organs Mammals other than primates have more complex female reproductive tracts

Female Accessory Sex Organs (Cont.) Mammals other than primates have more complex female reproductive tracts

Female Accessory Sex Organs Mammals other than primates have more complex female reproductive tracts

Contraception Throughout history, people have attempted to control reproduction without avoiding sexual intercourse The prevention of pregnancy or giving birth is known as birth control -Methods of birth control that act prior to implantation are usually termed contraception

Contraception Abstinence = No sexual intercourse at all -Most reliable way to avoid pregnancy Sperm blockage = Prevention of sperm entry into the uterus -Condoms, cervical caps & diaphragms Sperm destruction = Elimination of sperm after ejaculation -Douches and spermicidal jelly

Contraception Prevention of ovulation -Birth control pills or oral contraception -Contain analogues of progesterone, sometimes with estrogens -Hormone-containing capsules can also be implanted beneath the skin Prevention of embryo implantation -Intrauterine devices (IUD) -“Morning-after pill”, or Plan B

Contraception

Contraception (Cont.)

Contraception Sterilization -Vasectomy in males -Tubal ligation in females

Infertility Infertility is the inability to conceive after one year of contraception-free sexual intercourse Female infertility -Can occur due to a failure at any stage from oocyte production to embryo implantation -Pelvic inflammatory disease (PID) -Endometriosis -Premature ovarian failure (POF) -Hormonal imbalances

Infertility Male infertility -Can occur due to a reduced number, viability, or motility of sperm in the ejaculate -Infection -Hormonal imbalances -Autoimmunity to sperm -Damage to vas deferens or seminiferous tubules

Treatment of Infertility 1. Hormonal treatment -To produce high levels of FSH and LH -Clomiphine (Clomid) 2. Assistive reproductive technology -Artificial insemination -Gametic intrafallopian transfer (GIFT) -In vitro fertilization (IVF) -Intracytoplasmic sperm injection (ICSI)