human and animal bio exam 2

asexual reproduction advantages vs. disadvantages

advantages:

• large reproductive output

disadvantages:

• no variation/adaptation to changing environment

sexual reproduction advantages vs. disadvantages

advantages:

• variation in offspring

• better adapted to changing environment

disadvantages:

• smaller reproductive output

• males eat half of food resources and don't produce offspring

• STDs

sources of genetic variability in sexual reproduction

• crossing over between homologous chromosomes in prophase 1

• independent assortment of homologous chromosomes in metaphase 1

• random fertilization

types of asexual reproduction

budding, fission, fragmentation/regeneration, parthenogenesis

budding

new individuals arise from outgrowths of existing ones; yeast

fission

separation of parent organism into two or more individuals of approximately equal size; anemone, flatworm

fragmentation/regeneration

parent organism's body is broken into several pieces and each fragment develops into a new individual by regrowing lost body parts; starfish

parthenogenesis

• egg develops without being fertilized

• estrogen level determines ovulation

• haploid or diploid offspring

• XY system offspring are XX

• ZW system offspring are ZZ, WW, or ZW

• komodo dragon, bonnethead shark, water flea, bynoe's gecko, warramaba virgo grasshopper, rotifers

ways to modify meiosis for parthogenesis

• mitotic division of primary oocyte

• combination of first polar body with secondary oocyte

• combination of egg with second polar body

• haploid eggs divides by mitosis instead of fusing with sperm

simultaneous hermaphroditism

• individuals have both male and female reproductive systems

• any two individuals can mate together

• common in stationary animals; barnacles, clams, tapeworms, slugs

sequential hermaphroditism

• individual undergoes a sex-change in its lifetime

• common in animals living in harems (many females one male) when male dies

• protandrous: male first

• protogynous: female first

• only in external fertilization; bony fish, snails, marine worms

aphids

• can alternate sexual and asexual reproduction- asexual in summer

• sexual in fall

sexual reproduction

• two morphologically distinct gametes fuse together

• each gamete contains half the number of chromosomes of somatic cells

• gametes are created by meiosis

sex determination in birds and butterflies

• male: ZZ

• female: ZW

• rare female: WW

choosing a mate

• pheromones

• sexual selection: courtship ritual to determine species similarity

• competition between males

• caring for offspring

steroid reproductive hormones

• mainly produced by gonads, some by adrenal cortex

• invertebrates: ecdysteroids

• vertebrates: androgens (testosterone), estrogens (estradiol), progestogens (progesterone)

gonadotropins

• peptide hormones produced by anterior pituitary

• control steroid hormone synthesis

• follicle-stimulating hormone (FSH), luteinizing hormone (LH)

• release controlled by hypothalamic gonadotropin-releasing hormone (GnRH)

chorionic gonadotropin (hCG)

• produced by the trophoblast chorion (later placenta)

• stimulates corpus luteum to keep producing progesterone during pregnancy

gametogenesis

production of gametes in gonads; spermatogenesis or oogenesis

spermathecae

• sacs in female reproductive system in which sperm can be kept alive for extended periods

• common in insects

male reproductive organs

testes, seminiferous tubules, scrotum, epididymis, vas deferens, ejaculatory ducts, urethra, seminal vesicles, prostate gland, bulbourethral glands,penis, glans, prepuce

seminiferous tubules

produce sperm in testes

scrotum

external sac that maintains testes temp 2C below body

epididymus

sperm complete maturation and gain motility in testes

vas deferens

propels sperm into urethra through ejaculatory duct

ejaculatory ducts

• connect vas deferens and seminal vesicles

• lead to prostate gland and urethra

seminal vesicles

• secrete 60% of seminal fluid

• mucus, nutrients, coagulating enzymes, prostaglandins

prostate gland

• secretes seminal fluid directly into urethra

• buffers, nutrients, coagulating enzymes

bulbourethral glands

• secrete seminal fluid before ejaculation

• mucus, buffers

penis

• three cylinders of spongy erectile tissue

• glans: sensitive head with thin outer layer

• prepuce: foreskin surrounding glans

seminiferous tubules cells

leydig cells:

• produce testosterone- located in surrounding connective tissue (interstitial)

sertoli cells:

• produce androgen-binding protein, inhibin, nutrients, and testicular fluid (sperm transport)

• fill gaps between spermatogenic cells

• dispose of excess cytoplasm from sperm

• form blood-testis barrier through tight junctions

spermatogenic cells

spermatogenesis

mitosis:

• stem cells in contact with epithelial basal lamina

• spermatogonia divide: type A daughter maintains germ cell line, type B moves towards lumen

spermatogonia > primary spermatocytemeiosis:

1. primary spermatocyte > secondary spermatocytes

2. secondary spermatocyte > spermatids (nonmotile)

steps of spermiogenesis

1. golgi apparatus packages acrosomal enzymes in vesicle

2. acrosome positioned at anterior end of nucleus and centrioles at the opposite end

3. microtubules form flagellum of tail

4. mitochondria multiplied and positioned around proximal portion of flagellum

5. removal of excess cytoplasm

6. sperm released from sustentacular (sertoli) cells into lumen

flow of sperm

seminiferous tubules > epididymis > vas deferens > ejaculatory duct > urethra

female reproductive organs

ovaries, oviducts (fallopian tubes), uterus, cervix, vagina, vulva

endometrium

inner lining of uterus

hormonal reproductive regulation in males

1. hypothalamus releases GnRH

2. GnRH stimulates anterior pituitary to secrete FSH and LH

3. FSH stimulates sertoli cells to secrete androgen-binding protein (ABP)

4. LH stimulates leydig cells to secrete testosterone

5. testosterone triggers spermatogenesis

6. testosterone and inhibin (sertoli cells; released when sperm count is high) inhibit hypothalamus and anterior pituitary

capacitation

sperm undergo a change in the female reproductive tract that enables them to penetrate and fertilize an egg

composition of seminal fluid (semen)

• sperm

• mucus: lubricant

• water: liquid medium

• buffers: neutralize acidic environment in male urethra and vagina

• nutrients: nourish sperm

• coagulating enzymes: clot semen in vagina then liquefy it

• zinc: possible association with fertility

• prostaglandins: aid in semen transport; decrease viscosity of cervix mucus

• antimicrobial proteins: protect against pathogens

fibrinolysis

breakdown of a clot

chemicals aiding sperm activity

• chemokinetic: stimulate sperm to swim faster

• chemotaxic: induce sperm to swim towards high concentration of chemical

oogenesis steps

fetal period:

1. oogonia multiply (mitosis)

2. oogonia > primary oocyte (mitosis)

3. primary oocyte arrested in prophase 1 (meiosis)

puberty:

1. primary oocyte > secondary oocyte (meiosis 1)

2. secondary oocyte arrested in metaphase 2 (meiosis)

3. ovulation

pregnancy:

1. secondary oocyte > fertilized egg (meiosis 2)

spermatogenesis vs. oogenesis

spermatogenesis:

• 4 gametes per meiotic division

• spermatogonium (stem cell) present at birth

• puberty to old age

• 1 trillion sperm

• 74 days to produce one gamete

• one sertoli cell sustains many spermatocytes

oogenesis

• 1 gamete per meiotic division

• primary oocyte (prophase 1) present at birth

• fetal development to menopause

• 13-50 years to produce one gamete

• many granulosa cells sustain one oocyte

estrous vs. menstrual cycles

estrous

• reproductive behavior occurs at specific part of ovulatory cycle

• minimal uterine tissue is expelled

menstrual cycle:

• sexual receptivity occurs at several phases of ovulatory cycle

• substantial uterine tissue is expelled

induced ovulation

• release of mature eggs from ovaries is dependent on stimuli generated by copulation

• rabbits, cats, shrews, camelids

spontaneous ovulators

• ovulation results from endogenous processes more or less independent of mating

• humans, rodents, dogs, farms, cows

types of ovulatory cycles

induced ovulators, spontaneous ovulators

hormones childhood vs. puberty

childhood:

• ovaries secrete small amounts of estrogen

• estrogen inhibits GnRH release

puberty:

• leptin from adipose tissue decreases estrogen inhibition

• GnRH, FSH, and LH are released

• adult cyclic pattern achieved in ~4 years

ovarian cycle

follicular phase

1. hypothalamus secretes GnRH

2. GnRH stimulates anterior pituitary to release FSH and LH

3. LH stimulates theca cells to produce androgens

4. FSH stimulates granulosa cells to grow and convert androgens into estrogens (aromatase)

5. inhibin and estrogen (low levels) inhibit hypothalamus and anterior pituitary

6. steep rise in estrogen stimulates hypothalamus and anterior pituitary

7. LH surge stimulates production of secondary oocyte and rupturing of follicle

luteal phase:

1. LH stimulates ruptured follicle to form corpus lutem and secrete progesterone, estrogen, and inhibin

2. progesterone, estrogen, and inhibin inhibit hypothalamus and anterior pituitary

uterine (menstrual cycle)

proliferative phase:

1. estrogen and progesterone secreted by corpus luteum stimulates growth of endometrium

2. growth of endometrial glands

secretory phase:

1. endometrial glands secrete nutrient fluid that can sustain an embryo before implantation

menstrual phase:

1. corpus luteum disintegrates

2. drop in estrogen and progesterone causes constriction of arteries in endometrium

3. uterine lining disintegrates and blood and tissue is released

four essential developmental processes

• cell proliferation and apoptosis

• cell movement or differential expansion

• cell differentiation

• cell-cell interactions

cell movement or differential expansion (development)

• reorganization of cytoskeleton

• convergent extension: sheet of cells becoming longer and narrower

• cell adhesion molecules

• ECM components: fibers direct migrating cells, glycoproteins promote migration by providing anchorage

cell differentiation (development)

• cells are initially determined/committed to a specific pathway and are later differentiated

• differential gene expression

cell-cell interactions (development)

change patterns of gene expression and cell activity

initial cell differences

• unequal distribution of cytoplasmic determinants

• inductive signaling

hox (homeotic genes)

• specify segments of the body along the head-tail axis

• protein product is a transcription factor

• DNA sequence known as homeobox

• order of genes is same order of expression along anterior-posterior axis (collinearity)

homeosis

mutation or misexpression of hox genes that causes body parts to grow in an abnormal place

spatial collinearity

correspondence between ordering of Hox genes along the chromosome and their expression along the head-tail axis

loss of forelimbs in snakes

• Hoxc6 is normally expressed without Hoxc8 and gives rise to forelimbs

• Hoxc6 and Hoxc8 expressed together in snakes prevents forelimb formation

anterior-posterior limb formation

• zone of polarizing activity (ZPA) derived from mesoderm at junction of young limb and body wall

• ZPA secretes sonic hedgehog (Shh)

proximal-distal limb formation

• apical ectodermal ridge (AER) derived from ectoderm overlying mesodermal limb bud

• AER directs limb development in nearby mesoderm called progress zone (PZ)

• AER secretes FGF-2, FGF-4, FGF-8

dorsal-ventral limb formation

* dorsal and ventral ectoderm signaling centers

polyspermy blocks

• fast block - acrosomal reaction (sea urchins)

• slow block - cortical reaction

polyspermy block external fertilization (sea urchins)

1. sperm contacts egg’s jelly coat
2. exocytosis of acrosome content
3. hydrolytic enzymes digest jelly coat
4. acrosomal process protrudes from sperm head and binds to bindin on surface of egg to ensure species sameness
5. membrane depolarization: Na+ channels open and flows into cell
6. bindin receptor blocked to prevent polyspermy
7. sperm nucleus enters egg
8. Ca2+ released from smooth endoplasmic reticulum
9. wave of Ca2+ spreads from site of sperm entry to rest of inner membrane side
10. cortical granules fuse with membrane and release content into perivitelline space
11. cortical enzymes destroy sperm receptors and detach other sperm
12. cortical molecules increase osmotic value in perivitelline space
13. water is drawn in by osmosis and the vitelline layer is pushed further away from membrane
14. vitelline layer hardens and becomes fertilization envelope

polyspermy block internal fertilization (mammals)

1. sperm weaves past corona radiata using surface enzymes

2. sperm binds to molecules in zona pellucida triggering Ca2+ rise within sperm

3. acrosomal contents are released and enzymes digest zona pellucida

4. acrosomal process protrudes from sperm and binds to egg surface receptors

5. sperm and egg membranes fuse and sperm nucleus enters cell

6. cortical reaction triggered by sperm entry

7. zona pellucida hardens and forms fertilization envelope

egg activation

• triggered by Ca2+ surge from cortical reaction

• secondary oocyte completes meiosis II and casts out second polar body

• increase in rates of cellular respiration and protein synthesis

fertilization steps

1. recognition between sperm and egg

2. regulation of single sperm entry

3. activation of egg metabolism

4. fusion of genetic material

corona radiata

layer of granulosa cells surrounding secondary oocyte

cleavage

• rapid series of mitotic divisions that follows fertilization

• involves little or no cell growth or gene expression

• S/M cell cycle

• pattern influenced by amount of yolk and mitotic spindle orientation

order of development

zygote > morula > blastula/blastocyst > gastrula

egg/zygote polarity

• vegetal pole has more yolk; animal pole has less

• differential distribution of proteins and mRNAs

body axis formation in amphibians

• animal pole = anterior; vegetal pole = posterior

• cortical rotation - point of sperm entry determines dorsal side; rotation exposes a gray crescent opposite to this point

• combination of anterior-posterior and dorsal-ventral axes define left-right axis

how is cleavage influenced by yolk

• little or no yolk

  • similar cell sizes (sea urchin)

  • complete cleavage (also frogs)

• larger yolk

  • asymmetrical cell sizes

    • frogs - vegetal hemisphere has fewer but larger cells than animal hemisphere

  • incomplete cleavage - furrows do not penetrate yolk

    • discoidal or superficial

discoidal cleavage

• type of incomplete cleavage

• embryo forms disc of cells called blastodisc on top of yolk

• birds (chicken), reptiles, fish (zebrafish)

superficial cleavage

• type of incomplete cleavage

• yolk is in the center (insects)

• mitosis in absence of cytokinesis results in multinucleated egg

• nuclei migrate to periphery of egg

• membrane grows inward and divides the nuclei into individual cells

how is cleavage influenced by mitotic spindles

• determine cleavage planes and arrangements of daughter cells

• right angle or parallel to animal-vegetal axis - radial cleavage pattern

  • echinoderms (sea urchin), amphibians

• oblique angle to animal-vegetal axis - spiral cleavage pattern

all types of cleavage patterns

radial, spiral, discoidal, superficial, rotational

rotational cleavage

• unique to mammals

• first cell division is parallel to animal-vegetal axis; second occurs at right angles

• slowest within animal kingdom

• not synchronous (no 2,4,8 progression)

• compaction - cells form tight junctions and compact at 8-cell stage

• from 16 to 32-cell stage cells separate into two masses

• inner cell mass develops into embryo

• outer cell mass develops into trophoblast which becomes chorion then placenta

• trophoblast cells secrete fluid which forms the blastocoel and the embryo is now a blastocyst

blastocoel

fluid-filled cavity within a blastula/blastocyst

how long do embryonic cells remain totipotent in mammals

until the 8-cell stage

when does embryo implantation occur

6 days; after blastula/blastocyst has formed

modes of monozygotic twinning

• early embryo is cleaved and the halves develop into separate embryos

• inner cell mass of a blastocyst is split and two separate embryos form enclosed in the same trophoblast

• conjoined twins - inner cell mass does not completely separate or portions rejoin after splitting

where does fertilization occur

upper oviduct

how is early implantation of an embryo prevented

zona pellucida

gastrulation

a blastula transforms into an embryo with three germ layers, body axes, and a primitive gut

gastrulation in sea urchins

1. cells around the vegetal hemisphere flatten and invaginate into the blastocoel

2. some cells migrate and become primary mesenchyme cells

3. invagination becomes archenteron and mesenchyme cells become mesoderm

4. secondary mesenchyme cells attached to top of archenteron send out extensions to overlying ectoderm

5. mesenchyme extensions contract and pull archenteron inward

6. deuterosome - anus forms from blasopore and mouth from other side of contact

blastopore

the origin of invagination during primitive streak formation

gastrulation in frogs

1. cells of dorsal lip originate in gray crescent and invaginate to create archenteron

2. cells move from the embryo surface into the embryo by involution and become endoderm and mesoderm

3. blastopore encircles a yolk plug

4. deuterosome

gastrulation in mammals, birds, and reptiles (not humans)

1. embryo forms a blastoderm and sits on top of a large yolk

2. upper layer of blastoderm (epiblast) moves towards midline of blastoderm then into the embryo toward the yolk

3. midline thickens and is called the primitive streak

4. epiblast cells remaining on top layer become ectoderm

5. epiblast cells detaching and migrating to the middle become mesoderm

6. epiblast cells at bottom layer become endoderm

7. hypoblast cells form sac surrounding yolk and stalk connecting it to embryo

archenteron

primitive gut

gastrulation in humans

1. trophoblast secretes enzymes which break down endometrium

2. trophoblast extends projections which cause capillaries to spill out blood

3. inner cell mass forms flat disk with outer epiblast and inner hypoblast layers

4. epiblast cells move inward through a primitive streak and form mesoderm and endoderm

5. extraembryonic membranes form

amniotes

• organisms that develop within a fluid-filled sac contained in a shell or the uterus known as the amnion

• extraembryonic membranes surround the embryo

protostome vs. deuterostome development

protostome

• mouth forms from blastopore

• spiral and determinate cleavage

• solid masses of mesoderm split and form coelom

deuterostome

• anus forms from blastopore; mouth forms at other side of contact

• radial and indeterminate cleavage

• folds of archenteron form coelom

coelom

principal body cavity between intestinal canal and body wall

derived traits of chordates

• notochord

• dorsal hollow nerve chord

• pharyngeal slits or clefts

• muscular post-anal tail

notochord

• derived from dorsal mesoderm

• core of large cells with fluid-filled vacuoles

• rigid but flexible

• in vertebrates it is replaced by skeletal structures

extraembryonic membranes

• amnion - prevents dehydration and cushions mechanical shock; derived from epiblasts

• chorion - exchanges gases between the embryo and surrounding air, forms the placenta; derived from trophoblast

• allantois - disposal sac for metabolic wastes, forms the umbilical cord; derived from hypoblast

• yolk sac - blood vessels in membrane transport nutrients from yolk into embryo, other nutrients stored in albumen; derived from hypoblast

parturition

labor/birth

initiation of labor

1. placenta secretes HCG which stimulates theca cells in corpus luteum to keep producing progesterone

2. progesterone inhibits uterine contractions

3. fetus secretes cortisol which signals placenta to secrete estrogen

4. increased estrogen causes production of oxytocin receptors by myometrium

5. estrogen causes formation of gap junctions between uterine smooth muscle cells

6. fetus produces surfactant protein A (SP-A) which induces inflammatory response in cervix to soften it

7. uterine stretching by fully grown fetus and pressure on cervix by the head stimulates the release of oxytocin from posterior pituitary

8. oxytocin stimulates uterus to contract and placenta to make prostaglandnd

9. prostaglandins and mechanical stimuli induce more frequent and stronger contractions - positive feedback

myometrium

muscular outer layer of the uterus