reproductive chapter 53 Flashcards
Reproductive Strategies
Most animals reproduce sexually
Requires meiosis to produce haploid gametes (sperm and eggs)
Gametes united by fertilization to produce the diploid zygote
Zygote develops by mitosis into a new multicellular organism
Asexual reproduction
Genetically identical cells are produced from a single parent cell through mitosis
Single-celled organisms utilize fission
Cnidarians reproduce by budding
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Novel Reproductive Strategies
Parthenogenesis
Females produce offspring from unfertilized eggs
Hermaphroditism
One individual has both testes and ovaries
May be sequential – change sex
Protogyny – female-to-male change
Protandry – male-to-female change
Sex Determination
Temperature-sensitive In many fish and reptiles Has evolved many times Genetic XX or XY Humans Embryonic gonads indifferent for first 40 days SRY gene on Y converts gonads to testes Testosterone promotes male development
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
Terrestrial vertebrates use internal fertilization
Sperm are introduced into the female reproductive tract
Internal fertilization has led to three strategies for development
Oviparity
Fertilized eggs are deposited outside mother’s body to complete their development
Ovoviviparity
Fertilized eggs are kept within mother to complete development, young obtain food from egg yolk
Viviparity
Young develop within mother and obtain nourishment from her blood
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
Gives birth to a few, well-developed offspring
Amphibians
In most species fertilization is external
Eggs of most species develop in water
With some interesting exceptions
Reptiles Amniotic egg Extraembryonic membranes Adaptation for life on land Most oviparous reptiles lay eggs and abandon them Leathery shell on egg Other species of reptiles are ovoviviparous Some species are viviparous
Birds Amniotic egg Extraembryonic membranes Adaptation for life on land All birds practice internal fertilization Hard calcareous shell on egg Birds are endotherms Must incubate eggs to keep them warm
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Vertebrate Reproductive Cycles
Female reproductive cycles involve periodic release of a mature ovum (ovulation)
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
Monotremes are oviparous
Lay eggs
Marsupials are viviparous
Give birth to incompletely developed fetuses, which mature in mother’s pouch
Placentals are viviparous
Retain young in uterus for long period of development
Nourished by placenta from mother’s blood
Human Male Reproductive System
Seminiferous tubules
Sites of sperm production
Leydig cells (interstitial cells) produce testosterone
Converts indifferent external genitalia into penis and scrotum
Shortly before birth, the testes descend into the scrotum because sperm need cooler temperature to develop
Gamete production
Spermatogonium (germ cells) divides by mitosis to produce two diploid cells
One later undergoes meiosis, the other remains as a spermatogonium
Primary spermatocyte – diploid cell that begins meiosis
Primary spermatocyte undergoes meiosis I to produce two haploid secondary spermatocytes
Each secondary spermatocyte undergoes a second meiotic division to produce 2 haploid spermatids
Total of 4 haploid spermatids produced
Germinal epithelium
Spermatogonia and meiotic products
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 Head – contains a nucleus Capped by the acrosome, which aids in penetration of the egg Body – consists of many mitochondria Provide energy Centriole acts as a basal body for the flagellum Tail – consists of a flagellum Provides locomotion
Sperm are delivered into the epididymis for storage and maturation
From there sperm enter vas deferens
Then to the urethra
Semen
Complex mixture of fluids and sperm
Seminal vesicles
Produce a fructose-rich fluid, which makes up 60% of semen volume
Prostate gland
Contributes about 30% of the bulk of semen
Alkaline fluid to protect sperm from acidity of vagina
Bulbourethral glands
Add lubricating secretions to make up the last 10% of semen
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
Hormonal control
Hypothalamic GnRH stimulates anterior pituitary to secrete FSH and LH
FSH stimulates the Sertoli cells to facilitate sperm development
LH stimulates the Leydig (interstitial) cells to secrete testosterone
Controlled by negative feedback
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