BIOL 2302 Ch 26: Reproduction, Fetal Development and Heredity

erection reflex

• a spinal reflex influenced by the brain

• triggered by

  • psychogenic stimulation

    • thoughts, sights, etc.

  • reflexogenic stimulation

    • physical touch

• both types activate autonomic pathway from brain→ sacral spinal cord (? reflex centers)

• parasympathetic stimulation

  • vasodilation of penile arterioles→ erection

• sympathetic inhibition

  • vasoconstriction of penile arterioles→ flaccid state

sexual arousal

• reflexogenic or psychogenic trigger

  • can incorporate all the senses

• 4 stages

  • excitement

  • plateau

  • orgasm

  • resolution

• male specific

  • urethra opens and widens (for ejaculation)

  • scrotum skin thickens

  • cremaster muscle elevates testes

• common to M and F

  • increased heart rate

  • increased blood pressure

  • increased breathing rate and depth

  • erect nipples

  • sex flush (reddening of skin, often on chest/face)

male sexual reponse

1. excitement

   1. originates in erection reflex center of sacral spinal cord

   2. erection can be reflexogenic (physical) or psychogenic (mental)

   3. penis goes from unstimulated→ partial→ full erection

   4. testes elevate

2. plateau

   1. requires continual erotic stimulation

   2. pre-orgasmic emission begins

   3. prostate enlarges

   4. scrotum thickens

   5. testes elevate further and increase in volume

3. orgasm

   1. loss of voluntary muscle control

      1. ejaculation (emission + expulsion of semen)

      2. penile and urethral contractions

      3. internal urethral sphincter contraction

      4. seminal vesicle and prostate contractions

      5. anal sphincter contraction

   2. followed by refractory period (no new erection/orgasm possible for a while)

4. resolution

   1. reproductive tissue return to their resting state

   2. erection disappears

   3. scrotum thins

   4. testes descend

female sexual response

1. excitement

   1. vaginal lubrication

   2. uterine fibrillations (minor contractions)

   3. clitoris becomes erect

   4. labia swell

   5. uterus elevates above bladder

2. plateau

   1. external indicators of sexual arousal continue to increase

   2. labia deepen in color

   3. uterus continues to elevate

   4. vaginal walls widen (tenting effect)

   5. sexual arousal signs intensify

3. orgasm

   1. strong vaginal muscle contractions apply greater pressure on the penis

   2. vaginal dilation to receive ejaculate

   3. no refractory period (can have multiple orgasms)

   4. rhythmic uterine contractions

4. resolution

   1. reproductive tissues return to their resting state

   2. external cervical os dilates to aid sperm migration

male ejaculate

• seminogelin

  • from seminal vesicles

  • coagulates semen, sticks sperm to vaginal wall after ejaculation

• motility inhibitor

  • prevents sperm from struggling in the coagulant, wasting energy

• PSA (prostate specific antigen)

  • breaks down seminogelin within 20-40 mins, freeing sperm

  • liquefant

• prostaglandins

  • induce reverse peristalsis in uterus to draw sperm inwards

  • reduce cervical mucus viscosity (ease sperm entry)

• hCAP-18

  • anti-microbial protein that prevents bacterial growth in female reproductive tract

• factor III

  • coagulation/clotting and healing of vaginal microabrasians

• PSAP (prostate specific acid phosphatase)

  • potent anti-nociceptive

  • reduces pain perception during/after sex

seminogelin

• From: Seminal vesicles

• Function: Coagulates semen, sticks sperm to vaginal walls

motility inhibitor

Prevents sperm from wasting energy while trapped in the coagulate

PSA prostate specific antigen

• From: Prostate

• Function: Breaks down seminogelin (20–40 mins later), frees sperm

prostaglandins

• Induce reverse peristalsis in uterus (pull sperm inward)

• Reduce cervical mucus viscosity to ease sperm entry

hCAP 18

• Anti-microbial protein

• Protects against bacterial growth in female tract

factor III

Promotes clotting and healing of vaginal microabrasions

PSAP prostate specific acid phosphatase

reduces pain perception (anti-nociceptive) during/after sex

fertilization

• sperm deposited in vagina

• sperm travel thru cervix→ uterus→ uterine tube

• reaches egg in 30 mins- 2 hrs

• 1 sperm binds egg membrane

• egg blocks other sperm (polyspermy prevention)

• sperm and egg nuclei fuse→ zygote (1 cell)

capacitation

final maturation step; reqd for fertilization

sperm shed protective layers and gain ability to swim forcefully

prostasomes fuse to sperm head, adding survival and guidance factors

Ca2+ influx increases motility

1. insemination

   1. sperm enters vagina

2. initial capacitation

   1. last step of sperm maturation

   2. sperm activate and start swimming

3. sperm reservoir

   1. sperm rest in uterine tube isthmus until ovulation

4. hyperactivation

   1. activity of sperm

   2. triggered by progesterone from the oocyte

      1. progesterone binds catsper channel→ Ca2+ rushes in

      2. sperm swims vigorously, follows chemical signal

5. cumulus penetration

   1. sperm pushes thru cells around eggs

6. zona pellucida penetration

   1. sperm breaks thru egg’s outer shell

7. fertilization! (1 sperm fuses w/ egg)

fertilization

1. capacitated sperm contacts zona pellucida ZP

   1. ZP= species-specific barrier

   2. only sperm of same species can bind

2. acrosome rxn

   1. acrosome (enzyme sac on sperm head) ruptures

   2. released proteases digest zona pellucida so sperm can pass thru

   3. also rearranges sperm plasma membrane for fusion

3. digestion of zona pellucida

   1. sperm uses enzymes + hyperactive movement to burrow through

4. penetration into perivitelline space PVS

   1. sperm enters space btw ZP and oocyte

   2. must find area w/ microvilli to bind and fuse

5. sperm-oocyte membrane fusion

   1. triggers Ca2+ release in oocyte

   2. activates oocyte→ completes meiosis II

   3. blocks polyspermy, starts development

fusion of egg and sperm

• oocyte has microvilli all over its surface, except for overlying the oocyte’s pronucleus

• microvilli serve as docking site for sperm, ensuring they dock away from pronucleus

• oocyte is activated after sperm-egg fusion

• PLCz (a component of the sperm’s plasma membrane) triggers release of Ca2+ from oocyte’s ER

  • Ca2+ oscillations trigger

    • block polyspermy

    • complete meiosis II

    • recruitment of maternal mRNA

    • activation of zygotic genome

blocking polyspermy

• cortical rxn happens immediately after oocyte activation

• cortical granules migrate towards the oocyte plasma membrane and release their contents into the perivitelline space PVS

• these materials form a new barrier against further sperm fusion

• prevents extra DNA (euploidy issue)

syngamy- 2nd meiotic division

• Ca2+ release triggers meiosis II

• second polar body is expelled

• oocyte has haploid female pronucleus

• sperm pronucleus is unpacked

  • packaging proteins in sperm pronucleus replaced with maternal histones

• paternal mitochondria destroyed

• maternal mitochondria activated

  • only maternal mitochondria passed to embryo

• maternal RNA destroyed

• male+ female pronuclei fuse= diploid zygote (1 cell)

early embryogenesis

• initial cleavage (day 1)

  • zygote (1 cell) divides → 2 cell stage

    • mitosis begins within 24 hrs

  • maternal RNA degraded

    • zygotic genome takes over control

• morula stage (day 2-3)

  • continued cleavage→ 4 cell→ 8 cell→ 16 cell morula

  • still inside zona pellucida

  • moves slowly thru uterine tube toward uterus

  • uterine tube contractions pause morula to sync with uterus readiness

• compaction and polarization (day 4-5)

  • blastomeres compact→ form tight ball

  • asymmetric division begin→ polarity established

  • morula enters uterus

• blastulation (day 6-7)

  • fluid enters morula→ forms hollow blastocyst

  • trophoblast (outer layer)→ forms placenta

  • inner cell mass→ forms embryo

  • blastocoel→ fluid filled cavity

  • zona pellucida is shed→ embryo ready to implant

initial cleavage

• day 1 embryogenesis

• zygote (1 cell) divides → 2 cell stage

  • mitosis begins within 24 hrs

• maternal RNA degraded

  • zygotic genome takes over control

morula stage

• day 2-3 embryogenesis

• continued cleavage→ 4 cell→ 8 cell→ 16 cell morula

• still inside zona pellucida

• moves slowly thru uterine tube toward uterus

• uterine tube contractions pause morula to sync with uterus readiness

compaction and polarization

• day 4-5 embryogenesis

• blastomeres compact→ form tight ball

• asymmetric division begin→ polarity established

• morula enters uterus

blastulation

• day 6-7 embryogenesis

• fluid enters morula→ forms hollow blastocyst

• trophoblast (outer layer)→ forms placenta

• inner cell mass→ forms embryo

• blastocoel→ fluid filled cavity

• zona pellucida is shed→ embryo ready to implant

prep for uterine implantation

• apposition

  • uterine walls swell (stromal edema) to bring blastocyst and endometrium closer

  • helps incoming blastocyst make initial, weak contact with uterine lining

• attachment

  • blastocyst attaches with inner cell mass facing uterus (polarity matters)

  • trophoblast cells contact uterine epithelium→ differentiate into

    • cytotrophoblast (retains structure)

    • syncytiotrophoblast (invades uterus)

• penetration

  • blastocyst burrows into endometrial stroma for access to uterine nutrients

  • syncytiotrophoblast forms a barrier btw blastocyst and maternal cells

apposition

• stage of implantation

• Uterine walls swell (stromal edema) to bring blastocyst and endometrium closer

• Helps blastocyst make initial, weak contact with uterine lining

attachment

• stage of implantation

• Blastocyst attaches with inner cell mass facing uterus (polarity matters!)

• Trophoblast cells contact uterine epithelium → differentiate into:

  • Cytotrophoblast (retains structure)

  • Syncytiotrophoblast (invades uterus)

penetration

• stage of implantation

• Blastocyst burrows into endometrial stroma to access uterine nutrients

• Syncytiotrophoblast forms a barrier between blastocyst and maternal cells

hypoblast

• layer of inner cell mass closest to blastocoel

• will eventually form the extraembryonic endoderm

epiblast

• layer of inner cell mass farther from blastocoel

• gives rise to embryo proper

• extraembryonic tissue origin

blastodisc

• hypoblast + epiblast layer

• gives rise to yolk sac, amnion, chorion

  • yolk sac- early hematopoesis, derived from hypoblast

  • amnion- surrounds fetus as fluid-filled sac, derived from epiblast

  • chorion- placenta, derived from cytotrophoblast and syncytiotrophoblast

• extra embryonic tissue

gastrulation

• process where epiblast forms the 3 primary germ layers (blastocyst becomes an embryo)

  • ectoderm

    • forms from the remaining epiblast cells (outer layer)

  • mesoderm

    • forms btw the ectoderm and endoderm, from migrating epiblast cells

  • endoderm

    • forms from epiblast cells that replace the hypoblast (inner layer)

• primitive streak

  • the region where migration of cells occurs to form these layers

  • epiblast cells migrate inward

  • endoderm forms from cells that replace the hypoblast

  • mesoderm orms btw endoderm and epiblast

  • ectoderm forms from remaining epiblast cells

  • result: 2 layers (epiblast and hypoblast)→ 3 layers: ectoderm, mesoderm, endoderm

• these 3 layers give rise to all tissues and organs in body

ectoderm

• Forms from the remaining epiblast cells (outer layer).

• a primary germ layer

mesoderm

• Forms between the ectoderm and endoderm, from migrating epiblast cells.

• a primary germ layer

endoderm

• Forms from epiblast cells that replace the hypoblast (inner layer).

• a primary germ layer

primitive streak

• the region where migration of cells occurs to form these layers

• epiblast cells migrate inward

• endoderm forms from cells that replace the hypoblast

• mesoderm orms btw endoderm and epiblast

• ectoderm forms from remaining epiblast cells

• result: 2 layers (epiblast and hypoblast)→ 3 layers: ectoderm, mesoderm, endoderm

embryo folding

• embryo folds along 2 axes

  • cephalic-caudal (longitudinal) axis

    • forms head (cephalic) and tail (caudal) ends

    • embryo bends toward the endoderm and folds back on itself

  • transverse (lateral) axis

    • sides of embryo fold inward and fuse at midline

    • pinches off most of yolk sac, leaving vitelline duct

• caused by rapid growth of embryo and amnion, slower growth of yolk sac

• result

  • cylindrical embryo with defined body shape

    • embryo divided into cephalic and caudal regions

  • embryo divided into 3 distinct layers

    • external ectoderm

    • intermediate mesoderm

    • internal endoderm

mesoderm fate

• depends on location after migration during gastrulation

• 4 types

  • chordamesoderm (midline)

    • forms notochord→ induces neural tube formation

    • establishes anterior-posterior axis

    • forms prechordal plate→ gives rise to mouth

  • paraxial mesoderm (next to midline)

    • forms somites and head mesenchyme

    • somites→

      • sclerotome: cartilage, tendons

      • myotome: skeletal muscle

      • dermomyotome: dermis, connective tissue

  • intermediate mesoderm

    • forms urogenital system- kidneys, ureters, gonads

  • lateral plate mesoderm

    • forms circulatory system, body cavity linings, spleen, adrenal glands, appendicular skeleton cartilage

intermediate mesoderm

• forms urogenital system- kidneys, ureters, gonads

midline mesoderm

• forms notochord→ induces neural tube formation

• establishes anterior-posterior axis

• forms prechordal plate→ gives rise to mouth

paraxial mesoderm

• forms somites and head mesenchyme

• somites→

  • sclerotome: cartilage, tendons

  • myotome: skeletal muscle

  • dermomyotome: dermis, connective tissue

lateral plate mesoderm

• Splanchnic mesoderm (inner layer) → forms:

  • Circulatory system

  • Visceral lining of body cavities

• Somatic mesoderm (outer layer) → forms:

  • Body wall lining

  • Appendicular skeleton cartilage

  • Spleen and adrenal glands

ectoderm

• nervous tissue

• epidermis and derivatives

• sense organs

• lens of eye

• teeth enamel

• mouth and anus

• pituitary and adrenal glands

endoderm

• internal lining of respiratory, GI, urinary and reproductive tracts

• portions of liver, gallbladder, and pancreas

• palantine tonsils

• thyroid and parathyroid glands

• thymus

placenta

• structure

  • chimeric organ (2 genetically diff tissue sources)

    • consisting of maternal (uterine) and embryonic (chorion) tissue

  • several lobes (cotyledons), fed by umbilical vessels

  • covered in villi sitting in maternal blood-filled spaces

• fx

  • material exchange btw mother and developing embryo/fetus

    • nutrients, gases, wastes

  • endocrine organ

    • produces hormones required for pregnancy (hCG, progesterone, etc)

  • hemochorial type

    • embryonic tissue directly contacts maternal blood

    • minimizes barrier for efficient exchange

ectopic pregnancy

• development of the embryo or fetus outside of the uterus

• occurs when something blocks the passage of the fertilized ovum

placenta previa

• the placenta implants in the inferior uterus, near to/covering the internal os of the cervix, leading to spontaneous abortion or premature birth

preeclampsia

• sudden pregnancy induced hypertension

dystocia

• difficult labor due to an abnormal fetal position or inadequate vaginal canal

• may lead to cesarean section

deliver of physiologically immature baby

• classified as a baby that weighs less than 2500g at birth

• carries substantial risk to the baby

uterus growth during pregnancy

• mostly from hypertrophic and hyperplastic growth of the uterine myometrium

  • starts growth around week 4

  • doubles in size by end of first trimester

  • by week 20, fundus (top of uterus) reaches umbillicus

  • in 3rd trimester, grows beyond umbillicus

  • by term, reaches xiphoid process of sternum

  • displaces and compresses nearby organs- often causes discomfort

• grows all the way xiphoid process of the sternum by term

mammary glands during pregnancy

• grow in response to placental hormones

• areola and nipple become darker in response to melanocyte-stimulating hormone MSH

pregnancy hormones

• 1st trimester

  • hCG (human chorionic gonadotropin) peaks early

  • progesterone and estrogen produced by ovary

• 2nd trimester

  • ovary and placenta produce hormones, with placenta taking over production of progesterone and estrogen

• 3rd trimester

  • placenta produces hCG, progesterone, and estrogen

  • progesterone and estrogen rise throughout pregnancy and reaches peak towards end of 3rd trimester

    • estrogen peaks right before labor

• other hormones:

  • placental lactogen PL

  • prolactin PRL

  • relaxin

  • corticotropin releasing hormone CRH

human chorionic gonadotropin hCG

• produced by chorion (syncytiotrophoblast cells)

• fx

  • maintains corpus luteum

  • stimulates progesterone production

  • enhances implantation

  • regulates trophoblast differentiation

  • promotes maternal-fetal immunological balance

• detectable around 10 days after ovulation (home pregnancy test detects)

• peaks around 2k 10 of pregnancy, then declines and plateaus

progesterone

• produced by corpus luteum (1st trimester), then placenta (2nd and 3rd trimesters)

• fx

  • prevents uterine contractions

  • supports uterine and mammary enlargement

  • maintains pregnancy by inhibiting maternal gonadotropins and suppresses parturition (giving birth)

estrogen

• produced by corpus luteum (1st trimester), then placenta (2nd and 3rd trimesters)

• fx

  • promotes uterine and mammary enlargement

  • relaxes pelvic ligaments

  • facilitates uterine contractions to support parturition

placental lactogen PL

• produced by syncytiotrophoblast cells

• shifts fuel away from mother→towards fetus

  • increase maternal blood glucose and lipolysis

• anti-insulin effect: decreases maternal insulin sensitivity

prolactin PRL

• produced by placenta and mother’s anterior pituitary gland

• shifts fuel towards mother in periods of insult/stress/illness

• helps protect maternal survival in adverse conditions

relaxin

• produced by syncytiotrophoblast cells

• plays important roles in osmoregulation and cardiovascular adaptation

• opposes parturition (birth) and supports fetal growth

  • reduces uterine contractions

corticotropin releasing hormone CRH

• produced by syncytiotrophoblast cells

• increases DHEA production→ synthesize estrogen

• serves as an initial signal for parturition around wk 34-35

cervical effacement, dilation

• stage 1 of parturition

• prep birth canal for fetal passage

• phase 1: effacement

  • effacement= thinning of cervix

  • 0→100% effacement of cervix

  • preps cervix to start dilating

• phase 2: dilation

  • cervix dilates from 1→ 7cm

    • 1 cm= cheerio

    • 4 cm= cracker

    • 7 cm= soda can

  • contractions become longer, stronger, more frequent

  • lasts 5-9 hrs (1st baby birth) or 2-5 hrs (subsequent births)

• phase 3: transition

  • cervix dilates from 7→ 10 cm

    • 10 cm= bagel

  • most intense and painful phase

  • contractions last 60-90 secs, very close together

  • fetus moves into pelvic basin, creating urge to push

  • ends when full dilation (10 cm) is reached→ ready for delivery

• effacement contractions→ dilation of cervix→ transition

effacement

• phase 1, step 1 of parturition

• effacement= thinning of cervix

• 0→100% effacement of cervix

• preps cervix to start dilating

dilation

• phase 2, step 1 of parturition

• cervix dilates from 1→ 7cm

  • 1 cm= cheerio

  • 4 cm= cracker

  • 7 cm= soda can

• contractions become longer, stronger, more frequent

• lasts 5-9 hrs (1st baby birth) or 2-5 hrs (subsequent births)

transition

• phase 3, step 1 of parturition

• cervix dilates from 7→ 10 cm

  • 10 cm= bagel

• most intense and painful phase

• contractions last 60-90 secs, very close together

• fetus moves into pelvic basin, creating urge to push

• ends when full dilation (10 cm) is reached→ ready for delivery

fetal expulsion

• to expel baby from uterus

• contractions less strong than during transitional stage; controllable

• contracts last around 1 min, every 1-3 mins (variable)

• generally 30 min-2 hrs w/ first baby, usually shorter in subsequent births

• active pushing helps move fetus thru birth canal

• crowning (baby’s head visible at vaginal opening) → birth is near

• baby usually emerges face down, then rotates to align shoulders

• final pushes completely deliver baby

• stage 2 of parturition

placental expulsion

• 3rd and final stage of labor

• occurs 15-30 mins after birth

• uterine contractions help detach and expel placenta

• around 200 mL blood loss is normal

• marks delivery completion

• afterward, uterus begins involution (shrinking back to pre-pregnancy size), which takes up to 6 weeks postpartum

• stage 3 parturition

parturition

1. cervical effacement

2. cervical dilation

3. transition

4. expulsion of fetus

5. expulsion of placenta

• fetal stress or “placental clock” triggers labor

• this increases corticotropin-releasing hormone CRH in fetal blood

• CRH causes fetus to release adrenocorticotropic hormone ACTH, increasing fetal cortisol

  • fetal cortisol

    • increases estrogen levels in mother

    • decreases progesterone

  • higher estrogen

    • boosts oxytocin production in placenta

    • increases oxytocin receptors in uterus

    • stimulates prostaglandin PG production in uterus

• oxytocin + prostaglandins cause uterine contractions

• contractions press fetus against cervix→ triggers fetal ejection reflex

• fetal ejection reflex makes mom’s pituitary release more oxytocin

• more oxytocin=stronger contractions→ positive feedback loop

• strong contractions continue until birth occurs

sexually transmitted diseases

• infections that spread thru sexual contact

• ex.,

  • nongonococcal urethritis

  • chlamydia

  • syphillis

  • gonorrhea

  • vaginitis

  • herpes simplex

  • human papilloma virus

  • AIDS

contraceptive devices

Most Effective (<1 pregnancy per 100 women/year)

• Implant: 0.05%

• IUD (LNG): 0.2%

• IUD (Copper T): 0.8%

• Male sterilization (vasectomy): 0.15%

• Female sterilization: 0.5%

• Tip: After procedure, little or nothing to remember

  • Vasectomy & hysteroscopic sterilization: use backup method for first 3 months

Moderately Effective (6–12 pregnancies per 100 women/year)

• Injectable: 6% → Get repeat shots on time

• Pill: 9% → Take daily

• Patch: 9% → Change weekly, use as directed

• Ring: 9% → Replace monthly

• Diaphragm: 12% → Use every time you have sex, correctly

Less Effective (18–24+ pregnancies per 100 women/year)

• Male condom: 18%

• Female condom: 21%

• Withdrawal: 22%

• Sponge: 24% (parous women), 14% (nulliparous women)

• Tip: Use correctly every time you have sex

Least Effective (>24 pregnancies per 100 women/year)

• Fertility-awareness methods: 24%

• Spermicide alone: 28%

• Tip:

  • Fertility-awareness: Abstain or use condoms on fertile days

  • Standard Days & Two-Day Methods = easier, possibly more effective