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Human Reproduction, Development and Ageing - Week 4 Lecture Notes

Topic = Fertilisation & Pregnancy

Sperm Structure and Semen

  • Sperm divided into 3 main sections: Head, Middle piece, Tail

    • Head contains the nucleus (haploid set of chromosomes) covered by the acrosomal cap

    • Middle piece contains mitochondria wrapped around axial filaments (microtubules)

    • Tail (flagellum) consists of a principal piece and an end piece; the tail moves via whip-like movements to propel the sperm

  • Ultrastructure details

    • Head includes nucleus with haploid chromosomes and acrosomal cap

    • Acrosomal cap is rich in carbohydrates and glycoproteins and contains enzymes (hylauronidase, neuraminidase, acrosin) essential for penetrating the oocyte coverings

    • Middle piece contains mitochondria around axoneme (9+2 arrangement) surrounded by a fibrous sheath

    • Tail contains axoneme with 9+2 microtubule arrangement for motility

  • Size references

    • Sperm length: 60\text{--}70\,\mu\text{m}

  • Key concepts

    • Freshly ejaculated sperm cannot fertilise an oocyte; capacitation and the acrosomal reaction are required for fertilisation

Semen: Volume, pH, Sperm Count and Normal Ranges

  • Semen (ejaculate) volume: 3\text{--}5\,\text{mL} (variable)

  • Semen pH: 7.3\text{--}7.5

  • Sperm count: 75\times 10^6/\text{mL}

  • Oligospermia definition: sperm count below 15\times 10^6/\text{mL}

  • Azoospermia: zero sperm in ejaculate

  • Semen analysis normality reference: fewer than 15\text{--}20\% should be abnormal

  • Fertilisation potential per ejaculate: fewer than 200 sperm actually reach the ovum; of those, typically only one fertilises the secondary oocyte

  • Site of fertilisation: ampulla of the uterine tube

Semen Composition and Function

  • Seminal vesicles contribute about 60\% of ejaculate volume; secretions include:

    • Fructose, Sorbitol, Glycerol, Prostaglandins, Ascorbic acid, Phosphorylcholine, Flavins, Inorganic ions (K+), Ergothioneine

  • Prostate contributes about 20--30\% of ejaculate volume; secretions include:

    • Spermine, Citric acid, Cholesterol, Phospholipids, Fibrinolysin, Fibrinogenase, Zinc, Acid phosphatase, Phosphate & bicarbonate (buffers)

  • Semen enters vagina and coagulates rapidly; gel-like form may protect sperm from bacteria and cervical mucus; liquefaction occurs within 20--60\,\text{min}; some retrograde coagulation can occur into the penile urethra or bladder

  • Sperm length reference: 60\text{--}70\,\mu\text{m}

Journey to the Oocyte: Transport, Viability, and Guides

  • Sperm are deposited into the vagina and must travel through the cervix and uterus into the uterine tubes (approximately 15 cm total path)

  • Time to reach the oviduct varies: some within 5 minutes, others up to 45 minutes; some may pause on the way

  • Viable sperm can be found in the female reproductive tract up to about 72\,\text{hours} after ejaculation

  • The secondary oocyte can survive for about 1\,\text{day} after ovulation

  • Sperm undergo capacitation and hyperactivation to progress toward the tubal ampulla and oocyte

  • Guidance to the oocyte may involve thermotaxis and chemotaxis; motility hyperactivation helps sperm move through cervical mucus, the corona radiata, and the zona pellucida

Capacitation and Acrosomal Activation

  • Capacitation is required before fertilisation; it occurs primarily in the uterus or fallopian tubes due to secretions

  • Capacitation processes

    • Removal of glycoproteins from the acrosomal cap and associated components

    • Cholesterol and glycoproteins are removed from the acrosomal cap, increasing its fragility

    • Membrane permeability to Ca2+ increases, enhancing motility and metabolic activity

    • Result is a rearrangement/alteration of the cell membrane; must occur before the acrosome reaction

Acrosomal Reaction and Zona Pellucida Penetration

  • Upon contact with the corona radiata and subsequently the zona pellucida (ZP), sperm release hydrolytic enzymes from the acrosome

  • Enzymes include hyaluronidase, neuraminidase, and acrosin; their action digests the zona pellucida to create a pathway for sperm entry

  • Tail movement assists the sperm in penetrating the corona radiata and zona pellucida

  • The first sperm to penetrate initiates the Zona Reaction

  • Zona pellucida becomes impermeable to additional sperm to prevent polyspermy

  • Zona Reaction involves changes in the oocyte plasma membrane and cortical granule exocytosis

Fertilisation: Entry of the Nuclei and Zygote Formation

  • After the acrosomal reaction and zona penetration, sperm nucleus enters oocyte cytoplasm

  • The oocyte completes the second meiotic division, forming the mature ovum and female pronucleus; the sperm tail disintegrates and the sperm head enlarges to form the male pronucleus

  • The male and female pronuclei fuse to form a zygote with diploid chromosome number (zygote = 2n = 46)

  • Fertilisation is considered complete when the zygote reaches the first metaphase of mitosis (first mitotic division)

  • The process restores the diploid chromosome number and creates a genetically unique organism

  • Sex determination is determined at fertilisation by the sex chromosome carried by the sperm: X sperm yields XX (female); Y sperm yields XY (male)

Zygote to Blastocyst: Cleavage and Early Embryogenesis

  • Cleavage begins around 30\,\text{hours} after fertilisation

    • First division yields two identical blastomeres (about Day\ 1: 2-cell stage)

  • By ~Day\ 3-4: morula forms (16 or more cells) — a berry-shaped cell cluster

  • By ~Day\ 4-5: advanced morula (~100 cells) with compaction; inner cell mass and outer trophoblast differentiate

  • The outer layer forms the trophoblast (will become the placenta); inner cell mass forms the embryoblast

  • Blastocoele forms as fluid-filled spaces develop inside, yielding a blastocyst; zona pellucida disintegrates and the blastocyst becomes free to move within the uterine cavity (~24–48 hours of this stage)

  • Implantation-ready: blastocyst implants and progresses to embryonic development

Implantation and Placentation

  • Timing of implantation

    • Blastocyst attaches to endometrium around Day\ 6-7 after fertilisation, typically near the embryonic pole

    • Implantation takes about 5\,\text{days} and is usually complete by 12\,\text{days} after ovulation

  • Early endometrial responses and signaling

    • Endometrium presents a functional zone around the day of blastocyst contact (Day 6–7)

    • Signaling molecules: LIF (Leukaemia Inhibitory Factor), EGF (epidermal growth factor), COX-2, and growth factors/cytokines (estrogens, progesterone)

    • Endometrial microvilli and pinopodes increase surface contact with the implanting embryo

    • Trophoblasts proliferate and differentiate into two layers: cytotrophoblast (inner cellular layer) and syncytiotrophoblast (outer multinucleated layer)

  • Syncytiotrophoblast actions

    • Finger-like projections invade the endometrial epithelium and stroma, secreting substances that break down endometrial tissue to anchor the conceptus

  • Embryonic membranes and placental formation

    • Inner cell mass divides to form the epiblast (future embryo) and hypoblast

    • Yolk sac forms and later contributes to early nutrition before placental circulation is established

    • Placental development involves trophoblast differentiation and placental membrane formation

Placental Structure and Function

  • Early placental components

    • Chorion and amnion membranes; chorionic villi develop to interface with maternal blood in the intervillous spaces

    • Decidua basalis (maternal portion of the placenta), decidua capsularis, and decidua parietalis

  • Placental roles (3 core functions)

    • Gas exchange between mother and fetus (fetal lungs later in development)

    • Fetal nutrition and waste removal via placental diffusion and fetal/maternal circulation

    • Endocrine function: secretion of hormones (e.g., hCG, progesterone, estrogens) to sustain pregnancy

  • Umbilical structures

    • Umbilical vessels: two arteries and one vein; contained within Wharton’s jelly

    • Umbilical cord connects fetus to placenta

  • Placental vasculature and villi

    • Chorionic villi extend into maternal blood spaces (intervillous space) for nutrient/gas exchange

  • Placental exam and rationale

    • Gross and digital placental analyses help establish timing of events, pathophysiology related to pregnancy outcomes (e.g., low birth weight), and mechanisms underlying complications

Prenatal Nutrition and Fetal Development Milestones

  • Early pregnancy nutrition and nourishment

    • In the first week ( fertilisation to implantation ), the ovum receives nutrition from secretions of the fallopian tube and uterus

    • Weeks 2–3: trophoblast digestion and absorption from endometrial glycogen- and lipid-rich cells; a circulatory system is not yet established

    • From Week 4 onward: nutrition through diffusion across placental membranes from the mother

  • Yolk sac and initial nutrition

    • Yolk sac is involved prior to placental circulation establishing

  • Twin pregnancy basics

    • Dizygotic (fraternal) twins: arise from two separate oocytes fertilised by two different sperm; may be the same or different sexes

    • Monozygotic (identical) twins: arise from a single zygote that splits; may result in various chorionic/amnionic configurations

    • Common twin configurations include: dichorionic/diamniotic, monochorionic/diamniotic, and monochorionic/monoamnionic; conjoined twins can occur when splitting is incomplete

Twins: Types and Placental Configurations

  • Dizygotic twins

    • Two oocytes fertilised by two sperm; each twin has its own chorion and amnion

  • Monozygotic twins

    • Splitting timing determines chorionicity/amnionicity (e.g., dichorionic/diamniotic; monochorionic/diamniotic; monochorionic/monoamniotic; conjoined variants)

  • Visual progression of monozygotic twinning

    • Early embryo may split before implantation or after implantation to yield different placental membranes

Placental Pathology and Variants

  • Placenta pathologies of clinical significance

    • Accreta, Increta, Percreta (degrees of placental invasion into the uterine wall)

    • Circummarginate and Circumvallate placentas (abnormal placental edge or ring formation)

    • Bilobed placenta and marginal cord insertion (anomalies affecting placental perfusion and fetal outcomes)

  • Umbilical cord and vessels

    • Wharton’s jelly provides cushioning for the umbilical vessels

  • Placental examination rationale and significance

    • Examines placental size, shape, color, parenchyma; cord analyses; chorionic villi; and placental membranes to assess fetal well-being and potential causal factors of neonatal outcomes

Pregnancy Dating and Key Signs

  • Pregnancy dating conventions

    • Generally counted from the first day of the last menstrual period (LMP); standard duration is 280\text{ days} (40 weeks)

    • Fertilisation occurs approximately 266\text{ days} after fertilisation (assuming a 28-day cycle)

  • Early pregnancy indicators

    • Human chorionic gonadotropin (hCG) appears in maternal blood and urine during early pregnancy

    • Early signs can include missed menses, breast tenderness, nausea, and vomiting

  • Ultrasound dating

    • Ultrasound is commonly used to confirm due dates

Ectopic Pregnancy and Placental Placement Issues

  • Placenta previa

    • A placenta that partially or completely covers the internal os; can cause bleeding and may require cesarean delivery

    • Australian and US data suggest occurrence in roughly 0.3–2.0% of births; maternal mortality around 0.1% due to hemorrhage and shock

  • Ectopic pregnancy

    • 90% occur in the fallopian tubes; most common near the infundibulum, least common near the isthmus

    • Zygote transport delays or stops; tubal pregnancies often rupture and bleed in the first 8 weeks; life-threatening emergency requiring surgical removal of tube and conceptus

  • Rare complications and outcomes

    • Cervical, abdominal, or ovarian pregnancies are rare but potentially life-threatening; implantation in abnormal sites may require surgical intervention; some abdominal pregnancies progress to term in rare cases

Viability, Human Development, and Outcomes

  • Viability and fetal development timeline

    • Early pregnancy proteins (e.g., hCG) mark implantation and early development

    • By Week 1–2, nutrition is provided by uterine/tubal secretions; Weeks 3–4 see placental diffusion beginning

  • Birth and growth data (illustrative cases)

    • Documented cases show variability in term viability depending on placental and systemic factors

Regulatory and Immune Context: Seminal Fluid and Female Reproduction

  • Female response to seminal fluid

    • Seminal fluid delivers sperm and interacts with female reproductive tissues to induce molecular and cellular changes

    • Regulatory T cells induced by seminal fluid help embryo implantation by suppressing inflammation, inhibiting effector immunity toward the embryo, and promoting uterine vascular adaptations that support placental development

  • Consequences and observations

    • Intercourse can influence pregnancy rates after IVF and embryo transfer

    • Greater exposure to partner seminal fluid may relate to lower risk of certain gestational disorders; donor oocytes or donor sperm are linked to higher rates of preeclampsia in some settings

Practical and Ethical Implications in Reproduction

  • Assisted reproduction and embryo transfer considerations

    • Use of donor gametes or surrogacy involves ethical, medical, and social considerations, including risks of complications like preeclampsia in some donor scenarios

  • Placental health as a predictor of outcomes

    • Placental structure and pathology can provide insight into timing and mechanisms underlying pregnancy outcomes and potential long-term health effects for offspring

  • Research and clinical relevance

    • Regulatory T cells and maternal-fetal tolerance; endometrial receptivity and signaling pathways; placental development and fetal programming are active areas of study with implications for reproductive health and chronic disease risk later in life

Notable Definitions and Quick Recap

  • Oligospermia: s\text{ count} < 15\times 10^6/\text{mL}

  • Azoospermia: s\text{ count} = 0

  • Capacitation: removal of inhibitory factors and cholesterol/glycoproteins from the acrosomal cap, increased Ca2+ permeability, and enhanced motility necessary for fertilisation

  • Acrosomal reaction: release of hydrolytic enzymes from the acrosome that digest the zona pellucida and enable sperm penetration

  • Zona reaction: oocyte plasma membrane changes and cortical granule release that prevent polyspermy after the first sperm penetrates the zona pellucida

  • Zygote: initial diploid cell formed by the fusion of male and female pronuclei; 2n = 46

  • Implantation: process by which the blastocyst adheres to and invades the endometrium, typically completed by about 12\ days\ after ovulation

  • Placentation: differentiation of trophoblast into cytotrophoblast and syncytiotrophoblast, formation of placental villi and maternal-fetal circulation

  • Chorionic villi: fetal-derived protrusions that interface with maternal blood for exchange

  • Wharton’s jelly: jelly-like connective tissue within the umbilical cord protecting the vessels

  • Placental pathologies: accreta, increta, percreta; circummarginate/circumvallate placentas; bilobed placenta; marginal cord insertion

  • Common twins: dizygotic (two separate zygotes) vs monozygotic (single zygote split); diverse chorionic/amnionic configurations; conjoined twins possible if splitting is incomplete

  • Pregnancy dating: based on LMP; typical duration 280\text{ days} or 40 weeks; fertilisation-based estimate of 266\text{ days} after conception

  • HCG as pregnancy marker: appears soon after implantation in maternal blood/urine

Important Dates and Ranges (Summary)

  • Semen and fertilisation

    • Ejaculate volume: 3\text{--}5\,\text{mL}

    • pH: 7.3\text{--}7.5

    • Sperm count: 75\times 10^6/\text{mL}

    • Oligospermia: < 15\times 10^6/\text{mL}

    • Azoospermia: 0\,\text{sperm/ mL}

    • Sperm that reach ovum: typically < 200 per ovum

    • Fertilisation location: ampulla of the uterine tube

    • Sperm length: 60\text{--}70\,\mu\text{m}

  • Chromosome/Genetics

    • Zygote chromosome number: 2n = 46

    • Sex determined by sperm chromosome (X → XX; Y → XY)

  • Development timeline

    • First mitotic division: ~30\,\text{hours} after fertilisation

    • 2-cell stage: ~Day 1

    • Morula: ~Day 3–4

    • Advanced morula (~100 cells): ~Day 4–5

    • Blastocyst: ~Day 5–6 post-fertilisation; zona pellucida dissolves

    • Implantation complete by ~Day 12 after ovulation

  • Pregnancy duration and dating

    • 280 days (40 weeks) from LMP

    • 266 days after fertilisation