ANIMAL REPRODUCTIONChapter 46

Asexual- mitotic

Sexual- both mitotic and meiotic divisions

-Increases genetic variability due to combination of genes from two parents

-Have variety of phenotypes

Asexual reproduction:

- invertebrates- fission, budding (colonies)

- sea stars, sponges, annelids- fragmentation and regeneration

Advantages:

-enables isolated animals to reproduce

- enables creating lots of offspring in short time

- in favorable environments perpetuates successful genotypes precisely

Reproductive cycle: - periodic nature allows animals to conserve resources

and reproduce when environment is favorable and more energy is available.

- controlled by hormones and environmentPARTHENOGENESIS: - fresh water crustacean- Daphnia- produce eggs of

two types. Haploid adults produce eggs without meiosis

- bees, wasps, ants- males (drones) are produced parthenogenetically

- among vertebrates, several genera of fishes, amphibians and lizards reproduce by a complex parthenogenesis- doubling of chromosomes after meiosis. Ex. whiptail lizards - no males, one female mimics a male.

Hermaphroditism: earth worm

- produce double the number of offspring

Sequential hermaphroditism: individual reserves its sex during its life time

Ex: Caribbean blue head wrasse (reef fishes)- female- first species- largest female becomes male

Oysters- male- first species

Fertilization: - depends on mechanisms that help sperm meet eggs

of the same species. - external- requires moist habitat- prevents gametes

from drying, allow sperm to swim to eggs. Timing is crucial to ensure mature sperm encounter ripe egg.

- temperature or day length cause a whole population to release gametes at once.

- Chemical signals from one individual releasing gametes trigger gamete release in others.

- individuals may exhibit specific mating behaviors leading to fertilization of eggs one female by one male – courtship – which allows mate selection and increases probability of successful fertilization – by triggering release of both sperm and eggs.

Internal- adaptation to terrestrial life

- enables sperm to reach an egg when environment is dry

- requires cooperative behavior which leads to copulation

- requires sophisticated reproductive systems including copulatory organs and receptacles.

Pheromones:

- chemical signals released by one organism that influence the physiology and/ or behavior of other individuals of the same species.

- they are small volatile or water soluble molecules active in small amounts

- function as mate attractants

Ensuring the survival of offspring:

- produce more offspring than can survive to reproduce

- internal fertilization produce fewer zygotes – greater protection of the embryos and parental care of the young.

- protection- tough egg shells, development of embryo within the reproductive tract of the mother, parental care of the eggs and offspring

- embryo develop within the egg

- embryo remain within reproductive tract of the female

- marsupials

- eutherians (placental) - embryos develop entirely within the uterus

Gamete production and delivery:

- Gonads are organs that produce gametes in most organisms

- polychaete worms (annelids) has simplest system, have separate sexes but no distinct gonads- gametes develop from undifferentiated cells lining the coelom

- mature gametes are released from the wall into the coelome

- mature gametes shed through excretory openings or swelling mass of eggs may split the body open, killing the parent

complex reproductive system include - sets of accessory tubes and glands that carry,

nourish, and protect the gametes and developing embryos

- Parasitic flatworms- hermaphrodites- most complex in animal kingdom

- most insects have separate sexes with complex reproductive systems

- in male- sperm develop in a pair of testes and are conveyed along a coiled duct to two seminal vesicles where they are stored.

- During mating sperm are ejaculated into female system

- In female eggs develop in pair of ovaries, conveyed through ducts into vagina, fertilization occurs

- in many species female system includes spermatheca, a sac in which sperm may be stored for a year or more.

- in many nonmammalian vertebrates, digestive, excretory and reproductive systems have a common opening – cloaca

CONCEPT 46.3 - for SDL

Concept 46.4 In humans and other mammals, a complex

interplay of hormones regulates gametogenesis - Spermatogenesis- a continuous prolific

process - each ejaculation of human male contains 100

to 650 million sperm cells - spermatogenesis occurs in the seminiferous

tubules of the testes. - primordial germ cells of embryonic testes

differentiate into spermatogonia(2n) – spermatocytes (n) – spermatids (n) – sperm cells (P. No. 975)

Oogenesis differs from spermatogenesis in three major ways.

- during meiotic division of oogenesis, cytokinesis is unequal- only one cell- secondary oocyte develop into egg; in spermatogenesis all four cells of meiosis develop into sperms

- cells from which sperms develop, continue to divide by mitosis throughout males life, which is not the case in human female

- Oogenesis has long resting periods, sperms are produced from precursor cells in an uninterrupted sequence

The Reproductive Cycles of Females:

- secretion of hormones and reproductive events are cyclic in female

- males produce sperms continuously

Menstrual versus Estrous Cycles:

- menstrual (uterine cycle) - in humans and certain other primates

- ovulation occurs after endometrium thicken and develop a rich blood supply

- in menstrual- endometrium is shed- menstruation

Estrous cycle:

- endometrium is reabsorbed by uterus and no extensive bleeding

- more pronounced behavioral changes

- stronger effect of season and climate

- most mammals copulate only during period surrounding ovulation- this period of sexual activity- estrus- is the only time the condition of vagina permits mating

- estrus= heat

Duration of menstrous cycle

- human 28 days; estrous- rat 5 days; bears and dogs one cycle per year; elephants several

The Human Female Reproductive Cycle:

- it’s an integrated cycle involving two organs, uterus and ovaries- controlled by hormones.

- gonadotropin- releasing hormone (GnRH)- hypothalamus; FSH and LH- anterior pituitory

- concentration of FSH and LH in blood control production of 2 steroid hormones– estrogen and progesterone

- ovarian cycle of hormones control uterine cycle of endometrial growth and loss.

The Ovarian Cycle: - cycle begins with release of GnRH - stimulates pituitory to release small amount of FSH

and LH - they stimulate follicle growth - cells of growing follicles start to make estrogen - several follicle grow, but only one matures others

disintegrate - high level of estrogen stimulates secretion of FSH and

LH particularly LH- positive feedback - LH induces final maturation of follicles - maturing follicle develops an internal fluid- filled cavity

and grows big - follicle and adjacent wall of the ovary rupture

releasing secondary oocyte

- following ovulation, during luteal phase, LH stimulates the transformation of follicular tissue to form corpus luteum, a glandular structure

- corpus luteum secretes progesterone and estrogen

- as these concentration increases, produce negative feed back on hypothalamus and pituitary, inhibiting secretion of FSH and LH.

- near the end of luteal phase, corpus luteum disintegrates, causing decline in estrogen and progesterone level releasing their inhibitory action on hypothalamus and pituitary- stimulates growth of new follicles, initiating next ovarian cycle.

The Uterine( menstrual ) cycle: - hormones secreted by the ovaries estrogen and

progestrone have major effect on uterus. - high amount of estrogen secreted by follicles signals

endometrium to thicken. - follicular phase of ovarian cycle is coordinated with

proliferative phase of uterine cycle. - after ovulation estrogen and progesteron secreted by

corpus luteum stimulate continued development and maintenance of endometrium, arteries and endometrial glands.

- glands secrete a nutrient fluid that sustain an early embryo.

- luteal phase of ovarian cycle is coordinated with secretory phase of uterine cycle.

- rapid drop of ovarian hormones when corpus luteum disintegrates results in menstruation- the menstrual flow phase of uterine cycle and beginning of new cycle.

- estrogen induces deposition of fat in the breasts and hips, increases water retention, affects calcium metabolism, stimulates breast development and influences female sexual behavior.

Menopause: after about 450 cycles, between ages of 46 to 56. ovaries lose their responsiveness to gonadotropins, decline in estrogen production in ovaries

Hormonal Control of the Male Reproductive System:

- principle sex hormones are androgens- testosterone, steroid hormones produced by the Leydig cells of testes, interstitial cells located near the seminiferous tubules.

- primary and secondary characteristics of the male.

- primary sex characters are associated with reproductive system, development of vasa differentia and other ducts, external reproductive structures and sperm production

- androgens determine behavior in mammals

- hormones from anterior pituitary and hypothalamus control androgen secretion and sperm production

Pregnancy or gestation

- correlates with body size

- human-266 days; rodents- 21 days; dogs- 60 days; cows- 270 days; elephants- more than 600 days

Conception, Embryonic development, and Birth:

-fertilization in oviduct, after 24hrs zygote begins cleavage, after 3-4 days zygote reaches uterus as a ball of cells, after a week- blastocyst - a sphere of cells containing a cavity which implants into the endometrium.

- embryo secretes hormones that signal its presence and control mothers reproductive system – human chorionic gonadotropin (HCG)- act like pituitary LH to maintain secretion of progesterone and estrogen by corpus luteum – through first few months of pregnancy

- absence of this decline maternal LH due to inhibitory action of pituitary, results in menstruation and loss of embryo.

First Trimester: - human gestation – 3 trimesters about 3 months

each - First trimester: most radical change both in

mother and embryo - endometrium responds to implantation by

growing over the blastocyst - differentiation of embryo’s body structures now

begins - first 2-4 weeks of development, embryo

obtains nutrients from the endometrium, outer layer of blastocyst- trophoblast, grows out and mingles with endometrium, helping to form placenta; umbilical cord

- first trimester is the main period of organogenesis

- heart begins beating by the 4th week

- by the end of 8th week, all major structures of the adult are present in rudimentary form. Now embryo is called fetus.

- high level of progesterone in mother initiate changes in her reproductive system; increased mucus in the cervix that forms a protective plug, growth of maternal part of placenta, enlargement of uterus, cessation of ovulation and menstrual cycling, breasts enlarge and are tender.

Second Trimester:

- fetus grows about 30cm and is very active

- mother feel the movement

- placenta takes over the production of progesterone

Third Trimester:

-fetus grows to 3-4 kg in weight and 50 cm length

- fetus fills the available space within the embryonic membrane

- mothers abdominal organs compress; a complex of local regulators (prostaglandins) and estrogen and Oxytocin induces and regulates labor

- parturition; lactation (postnatal care)

Parturition is brought about by a series of strong rhythmic uterine contractions.

Process of labor has three stages.

First- opening up and thinning of cervix

Second- expulsion or delivery of baby

Third- delivery of placenta.

Lactation: prolactin stimulates milk production

oxytocin release milk from mammary gland.

Contraception – deliberate prevention of pregnancy.

Prevent release of mature egg and sperms from the gonads, prevent fertilization, prevent implantation.

Modern reproductive Technology – genetic and other congenital disorders.

Amniocentesis and chorionic villus sampling – invasive techniques.

Ultrasound imaging – noninvasive technique.

Solutions for infertility: Assisted Reproductive Technology (ART)

In vitro fertilization (IVF)

Zygote Intrafallopian Transfer (ZIFT)

Gamete Intrafallopian Transfer (GIFT)