Animal reproduction Flashcards
Fission
Asexual reproduction in some invertebrates
Separation of parent into 2 or more individuals of about the same size
Budding
New individuals arise from outgrowths of existing ones
Hydra and other cnidarians
Fragmentation
Breaking of body into pieces followed by regeneration of lost body parts
Pieces develop into adults
Segmented worms
Parthenogenesis
Development of new individual from unfertilized egg
Rotifers
“Twofold” cost of sexual reproduction
Sexual females have half as many daughters as asexual females
Why sexual reproduction is an evolutionary advantage
Genetic recombination: increased variation of offspring, increased rate of adaptation, shuffling of genes and elimination of harmful genes from population
Ovulation definition
Release of mature eggs at the midpoint of a female cycle
Control of reproductive cycles
Most animals exhibit reproductive cycles related to changing seasons
Reproductive cycles are controlled by hormones and environmental cues
Asexual whiptail lizards
Parthenogenesis
Females switch sexual roles (mounting triggers release of egg)
Hermaphroditic reproduction
2 hermaphrodites can mate
Some can self-fertilize
External fertilization
Eggs shed by female are fertilized by sperm in external environment
Example: female frog secretes eggs into pond, and male frog secretes sperm into pond to fertilize them
Internal fertilization
Sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract
Requires behavioral interactions and compatible sexual organs
Critical timing: environmental cues, pheromones, and/or courtship behavior
Gonads
Organs that produce gametes via meiosis
Female external reproductive structures
Clitorus: arousal
Labia: protect opening into vagina
Ovaries
Female gonads Contain follicles (partially developed egg, oocyte, surrounded by support cells; produce estradiol prior to ovulation)
Oogenesis
Development of oocyte into ovum (egg)
Prolonged process
Ovulation steps
Egg cell is expelled from follicle
Remaining follicular tissue forms into corpus luteum
Cilia in oviduct (fallopian tube) transport egg cell to uterus
Uterus lining (endometrium) contains many blood vessels
Corpus luteum
Follicule tissue minus egg cell
Secretes estradiol and progesterone that helps to maintain pregnancy
If egg isn’t fertilized, this degenerates
Female reproductive opening
Uterus narrows at cervix, then opens into vagina
Vagina opens to outside
Vagina serves as repository for sperm and the birth canal
Male external and internal reproductive organs
External: scrotum and penis
Internal: gonads and accessory glands
Testes
Male gonads
Consist of highly coiled tubes surrounded by connective tissue
Seminiferous tubules
Coiled tubes in testes
Site of sperm formation
Leydig cells
Produce hormones and are scattered between seminiferous tubules
Scrotum
Outer covering of testes
Outside abdominal cavity: temperature is low enough to produce sperm
Sperm flow in ejaculation
Seminiferous tubules -> epididymus (coiled tubules outside of testes) -> vas deferens (muscular tube) -> ejaculatory duct -> urethra
Semen
Sperm + secretions from accessory glands
Seminal vesicles
Contribute about 60% of total volume of semen
Male accessory gland
Prostate gland
Secretes anticoagulant enzymes and sperm nutrients directly into urethra through several small ducts
Male accessory gland
Penis
Composed of 3 cylinders of spongy erectile tissue
During arousal, erectile tissue fills with blood from arteries, causing erection
Spermatogenesis
Development of sperm
Continuous and prolific (millions of sperm are produced each day, and each sperm takes about 7 weeks to develop)
3 ways spermatogenesis differs from oogenesis
- All 4 products of meiosis develop into sperm, while only 1 of 4 develop into egg
- Spermatogenesis occurs throughout adolescence and adulthood
- Sperm are produced continuously without prolonged interruptions as in oogenesis
Spermatogenesis steps
Primordial germ cell in embryo -> Mitosis -> Spermatogonial stem cell -> Mitosis -> Spermatogonium -> Mitosis -> Primary spermatocyte -> Meiosis I -> Secondary spermatocyte -> Meiosis II -> Early spermatid -> Differentiation -> Sperm cells
Sertoli cells
Provide nutrients to developing sperm (spermatids) in spermatogenesis
3 parts of sperm cell
Head: fertilizes egg
Midpiece
Tail
Sperm head components
Acrosome: vacuole that contains enzymes to eat through egg
Nucleus: contains haploid genome
Sperm midpiece
Contains many mitochondria: generate ATP to “swim”
Oogenesis steps
Primordial germ cell in embryo ->
Mitosis ->
Oogonium ->
Mitosis ->
Primary oocyte (present at birth; arrested in prophase of meiosis I) ->
Completion of meiosis I and onset of meiosis II (occurs at puberty) ->
Secondary oocyte (arrested at metaphase of meiosis II) ->
Ovulation and sperm entry ->
Completion of meiosis II ->
Fertilized egg
Gonadotropin-releasing hormone (GnRH)
Secreted by hypothalamus (part of brain that regulates reproduction): stimulated by high levels of estradiol in females and inhibited by high levels of testosterone in males
Directs release of follicle stimulating hormone (FSH) and lutenizing hormone (LH) by anterior pituitary gland
Menstrual cycle
Menstrual flow phase (day 0 to day 5): FSH and LH stimulate follicle to grow, low estradiol and progesterone cause release of endometrium Proliferative phase (day 6 to day 14): follicle matures, increase in estradiol causes spike in LH, LH triggers release of egg cell Secretory phase (day 15 to day 28): corpus luteum forms and secretes estradiol and progesterone, which cause thickening of endometrium After day 28, cycle repeats
Estrous cycles
Characteristic of most mammals (except primates and humans)
Endometrium is absorbed by uterus
Sexual receptivity is limited to a “heat” period
Hormonal control of male reproductive system
Hypothalamus secretes GnRH
GnRH activates anterior pituitary gland, which releases FSH and LH
FSH stimulates sertoli cells, which stimulate inhibin (inhibit anterior pituitary from producing FSH) and spermatogenesis
LH stimulates Leydig cells, which stimulate testosterone (inhibits hypothalamus and anterior pituitary), which stimulates spermatogenesis
Hormone levels are steady, except for morning
Vasectomy
Ties off vas deferens
Birth control
Prevent oocyte development and ovulation
No sloughing of endometrial lining
Condom or abstinence
Prevent entry of sperm into vagina
Spermicides or diaphragm (inserted into vagina)
Prevent sperm movement through female reproductive tract
Morning-after pill
Prevent union of sperm and egg and implantation of blastocyst in endometrium
Tubal ligation
Ties off oviducts
