Biology of Human Reproduction Exam 3 Review

Mechanism of fertilization

Sperm binds to the zona pellucida (a glycoprotein coat around the oocyte) → triggers acrosome reaction → sperm penetrates zona → fusion of sperm and oocyte membranes

Why are the zona pellucida & ZP3 receptors important?

Prevents polyspermy and ensures correct sperm-oocyte recognition

ZP3 receptors

ZP3 glycoproteins act as binding sites for sperm receptors, ensuring species-specific fertilization

Events that trigger completion of Meiosis I & II in ovum

• Before ovulation: Oocyte is arrested in prophase I

• Triggered by LH surge → completes meiosis I, forming a secondary oocyte + 1st polar body

• Fertilization triggers completion of meiosis II, forming the ovum + 2nd polar body

Capacitation

Biochemical changes in sperm membrane (in female tract) →

increases motility and membrane fluidity → prepares sperm to penetrate the oocyte

What triggers capacitation?

Occurs upon contact with female reproductive tract fluids

Acrosome reaction

Triggered by binding to ZP3 → release of enzymes (hyaluronidase, acrosin) that digest the zona pellucida for sperm entry

Structure of sperm

Head (nucleus + acrosome), midpiece (mitochondria), tail (flagellum)

How many sperm are typically ejaculated?

~200–300 million sperm

How many sperm get within close range to the egg?

Only a few hundred reach the ampulla (site of fertilization)

Male pronucleus

Sperm nucleus after entering egg

Female pronucleus

Oocyte nucleus after completing meiosis II

What is the fate of male and female pronuclei?

They fuse to form the zygote nucleus (diploid)

Polar bodies

By-products of meiosis; help discard extra chromosomes

Timeline of fertilization after ovulation & transport down the fallopian tube

• Ovulation → fertilization: Within ~12–24 hours post-ovulation (in the ampulla)

• Transport: Zygote travels down the fallopian tube for ~3–4 days before implantation in uterus

Basic characteristics of zygote as it proceeds through the morula & blastocyst

(zygote→morula→blastocyst)

• Zygote: Single cell (diploid)

• Morula: Solid ball of ~16–32 cells (day 3–4)

• Blastocyst: Fluid-filled cavity forms; differentiation into inner cell mass (embryo) and trophoblast (placenta precursor)

Blastocyst

Has a fluid cavity (blastocoel), inner cell mass, and trophoblast layer

Hatching

Blastocyst breaks free from zona pellucida to allow implantation into the uterine wall

Implantation

Trophoblast invades endometrium → forms cyto- and syncytiotrophoblast

hCG

Secreted by syncytiotrophoblast → maintains corpus luteum → continued progesterone production

Placenta

• Provides nutrient/gas exchange, hormone production, and waste removal

• Fetal and maternal tissues with villi for increased surface area

When is pregnancy determined?

Determined by presence of hCG and implantation success

How long does pregnancy last?

In humans, it lasts 38-42 weeks

Stages of labor & delivery

• Stage 1

  • Initial (latent) phase

  • Active phase: Cervical effacement (thinning) and dilation (up to 10 cm)

• Stage 2: Delivery of the baby

• Stage 3: Delivery of placenta

What is the role of oxytocin in labor & delivery?

Stimulates uterine contractions and milk let-down

Cervical effacement

The process by which the cervix thins and shortens in preparation for childbirth

Cervical dilation

The opening of the cervix during labor, measured in centimeters, to allow passage of the baby

Preeclampsia

• a pregnancy-specific, multisystem disorder that is characterized by the development of hypertension and proteinuria after 20 weeks of gestation

• can occur anytime within the antepartum, intrapartum, and even postpartum periods

Consequence of preeclampsia

Potential organ failure, seizures, eclampsia, or premature birth

Gestational diabetes

A form of diabetes that develops during pregnancy, characterized by high blood sugar levels that typically resolves after childbirth

Consequence of gestational diabetes

Increased risk of type 2 diabetes, macrosomia, and complications during delivery

Placenta previa

A condition where the placenta partially or completely covers the cervix, leading to potential complications during delivery

Consequence of placenta previa

Potential bleeding, preterm birth, and increased risk during cesarean delivery

Miscarriage

The spontaneous loss of a pregnancy before the 20th week, often due to genetic abnormalities, health issues, or hormonal imbalances

Consequence of miscarriage

Emotional distress, physical complications, and potential effects on future pregnancies

Preterm labor

Occurs when regular contractions result in the cervix opening before 37 weeks of pregnancy, potentially leading to preterm birth and associated risks for the infant

Consequence of preterm birth

Increased risk of developmental delays, health issues, and long-term disabilities for the infant

Effect of alcohol usage during pregnancy

Fetal Alcohol Syndrome (growth retardation, CNS defects)

Effect of tobacco/nicotine usage during pregnancy

Increased risk of low birth weight, preterm birth, and adverse developmental effects on the infant

Effect of opioid usage during pregnancy

Increased risk of neonatal abstinence syndrome, low birth weight, and developmental problems in the infant

Effect of LSD (lysergic acid diethylamide) usage during pregnancy

Uterine contractions, miscarriage, premature labor, lead to fetal distress, potential congenital abnormalities, and neurobehavioral effects

Effect of marijuana (THC) usage during pregnancy

Affects fetal brain development, linked to low birth weight, preterm birth, neurodevelopmental issues (attention, memory, behavior), increase risk of stillbirth, and growth restriction

Strategies to prevent infections

• Vaccination: Rubella, hepatitis B, influenza

• Screening: STIs (HIV, syphilis, chlamydia)

• Hygiene: Avoid raw meat, unpasteurized dairy (to prevent toxoplasmosis/listeria)

• Safe sex & prenatal care to reduce transmission risks

Maternal complications during pregnancy

• HELLP syndrome

• Eclampsia

• Pulmonary edema

• Acute renal failure

• Placental abruption

• Disseminated intravascular coagulation (DIC)

Fetal complications during pregnancy

• Intrauterine growth restriction

• Intrauterine fetal demise

• Increased risk of cardiovascular disease in growth restricted infants

Eclampsia

• Complication of pre-eclampsia

• Causes seizures

HELLP syndrome

hemolysis, elevated liver enzymes, & low platelet count

Disseminated intravascular coagulation (DIC)

• a life-threatening condition that can occur during pregnancy

• a leading cause of maternal mortality.

• characterized by small or large blood clots throughout the body

What is delivery the only cure for?

Preeclampsia