Exercises: Gametes, fertilisation and development

Describe the structure of a mammalian sperm cell (head, midpiece, and flagellum) and egg cell (large size and yolk reserves), and relate each feature to its function: motility versus nourishment for the embryo.

Name the gonads and the exit ducts for gametes in the mammalian reproductive system, and indicate at which point in the female tract fertilisation occurs.

Put the stages of fertilisation in order — fusion of gamete nuclei, gamete encounter, egg activation, and sperm penetration — and explain in one sentence what happens during each.

Explain what meiosis is and why it is indispensable for producing gametes: what would happen to the species' chromosome number in each generation if gametes were diploid.

Distinguish between zygote, embryo and foetus in the development of a mammal, indicating the approximate limits of each stage and which processes dominate in them (division, organogenesis, growth).

What is the placenta and which three vital functions does it perform for the foetus (nutrition, respiration, excretion)? Explain why it is important that the blood of the mother and of the foetus do not mix directly.

Explain what the amniotic sac and the amniotic fluid are, and which functions they fulfil during pregnancy (mechanical protection, stable temperature, freedom of movement).

Distinguish between monozygotic (identical) twins and dizygotic (fraternal) twins: how each type originates, what genetic similarity they have, and why the former are always of the same sex and the latter not necessarily.

Look at the image of the mammalian gametes. a) Identify the numbered structures and explain the role of each during fertilisation. b) Reason why the egg cell is thousands of times larger than the sperm cell and what each gamete contributes to the zygote. c) Indicate which barriers the sperm must cross before fusing with the egg nucleus.

The amniotic egg of reptiles and birds incorporates extraembryonic membranes: amnion, chorion, allantois, and yolk sac. Explain the function of each membrane and reason why this adaptation freed vertebrates from reproducing in water.

Following fertilisation, the zygote undergoes cleavage and subsequently gastrulation. Explain what each process involves, what the germ layers (ectoderm, mesoderm, and endoderm) are, and name an adult organ derived from each.

Describe the phases of the menstrual cycle (follicular, ovulation, luteal and menstruation) and the role of the main hormones (FSH, LH, oestrogens and progesterone) in each one, in a simplified way.

Explain what happens in the uterus when there is no fertilisation: how progesterone falls, what menstruation is, and why the cycle stops instead if the embryo implants (which hormone maintains it at first?).

Describe the process of human fertilisation: where it occurs (the Fallopian tube), what the acrosome reaction is and which mechanisms prevent polyspermy (the entry of more than one sperm).

The first weeks of pregnancy are the most sensitive. Explain what organogenesis is, why teratogens (alcohol, some medicines, infections such as rubella) damage mainly in the first trimester, and what foetal alcohol syndrome is.

Describe the phases of childbirth (dilation, expulsion and delivery of the placenta) and the role of oxytocin and of uterine contractions, explaining why it is an example of positive feedback.

A biological expedition discovers in an isolated cave two related amphibian species with very different reproductive strategies:

• Species A: Lays about 1.500 small eggs (diameter < 1,5 mm) that are yolk-poor in temporary ponds with abundant predators.

• Species B: Lays barely 12 large eggs (diameter > 6,0 mm), very rich in yolk, in protected damp crevices, from which fully formed individuals hatch without passing through a free tadpole stage.

1. a) Justify in terms of bioenergetic cost and survival why Species B has been able to dispense with metamorphosis and the aquatic larval stage, relating this to egg type (amount of yolk) and development type.
2. b) A researcher states: "Species A is evolutionarily more efficient because indirect development always implies a lower reproductive cost for the mother." Refute this statement by analysing the adaptive advantages and disadvantages of each strategy depending on environmental stability.
3. c) Resolve the following apparent paradox: eutherian mammals have an egg virtually devoid of yolk (alecithal) and yet exhibit direct development without a larval stage. How have they resolved this evolutionary conflict?

Imagine a hypothetical evolutionary scenario: on an Earth-like planet, a group of aquatic animals first evolves an egg with a waterproof shell and extraembryonic membranes (amniotic) before having developed internal fertilisation mechanisms, retaining external fertilisation in the water.

1. a) Assess the biophysical feasibility of this scenario: why does the emergence of an egg with an impermeable protective shell prior to internal fertilisation represent an insurmountable biological contradiction for the process of gametic penetration?
2. b) Some modern amphibians deposit their clutches on leaves or carry tadpoles on their skin out of open water, but none possess a truly amniotic egg. Based on this fact, justify why internal fertilisation alone was not sufficient for vertebrates to become completely independent of damp habitats.
3. c) Analyse how gravity, desiccation, and gas exchange interact in the aerial environment to impose the observed chronological evolutionary sequence: 1st internal fertilisation, 2nd amniotic egg.

A conservation team seeks to save the northern white rhinoceros from extinction, a species of which only two females survive that are infertile for gestation, but for which frozen semen samples from several males and skin fibroblast cultures from twelve deceased individuals are preserved.

Two biotechnological strategies are evaluated:

• Strategy A: In vitro fertilisation (IVF) using intracytoplasmic sperm injection (ICSI) with oocytes retrieved from the living females and frozen semen, transferring the embryos to southern white rhino females as surrogate gestators.

• Strategy B: Somatic cell nuclear transfer (cloning) using the frozen fibroblasts and enucleated oocytes from the southern white rhino.

1. a) Compare both strategies with respect to future genetic variability: which one allows for greater allelic diversity in the resulting offspring, and why would cloning be detrimental as a sole long-term strategy?
2. b) From the standpoint of cell biology and embryonic development, what specific molecular barriers associated with epigenetic reprogramming of the donor nucleus explain the very high rate of miscarriages and congenital malformations typical of Strategy B compared to Strategy A?
3. c) Evaluate the bioethical limits of applying these technologies: if a company offered to commercialise the cloning of deceased pets, what biological and animal welfare objections would you base your advice against this practice on?

In a gynaecology clinic, the profiles of three women of childbearing age who use different birth control methods are evaluated:

• Case A: Uses a combined oral hormonal contraceptive (ethinylestradiol and levonorgestrel). In the tests on the theoretical day 14 of her cycle, her serum LH and FSH levels are minimal and flat, with no detectable peak.

• Case B: Has had unplanned, unprotected sexual intercourse on day 13 of the cycle (the moment when an ovulation test tests positive due to an LH surge). Twelve hours later, she takes an emergency contraceptive pill containing ulipristal acetate.

• Case C: Has had a copper intrauterine device (IUD) for two years and maintains completely normal ovulatory cycles and hormonal profiles.

1. a) Justify the neuroendocrine mechanism by which the treatment in Case A prevents ovulation: explain what effects synthetic steroids exert on the hypothalamus and pituitary gland, and predict what risk of failure exists if pills are continuously missed for 3 days at the beginning of the pack.
2. b) Provide a reasoned prediction of the expected efficacy of emergency contraception in Case B, taking into account the drug's mechanism of action relative to the biological timing of the ovulatory cycle at which intercourse occurred.
3. c) Compare the contraceptive mechanisms of Cases A and C against two factors: alteration of cervical mucus/endometrium and protection against sexually transmitted infections (STIs). Why does neither of these two methods eliminate the need for condoms in casual sexual encounters?

A couple with long-standing infertility consults an assisted reproduction team. Diagnostic tests reveal an extremely low sperm concentration (0,4 million sperm/ml) with severe hypomotility in the male, while the female presents bilateral tubal obstruction with preserved ovarian reserve.

1. a) Provide a reasoned justification for why intrauterine artificial insemination is contraindicated in this clinical scenario and IVF with intracytoplasmic sperm injection (ICSI) is required. Describe which meiotic stage the oocyte is in at the exact moment of injection.
2. b) Following the ovarian puncture, 8 oocytes are fertilised, successfully developing 4 morphologically viable blastocysts on day 5 of culture. Knowing that the average implantation rate per euploid blastocyst is around 50-60%, argue biologically why single embryo transfer (SET) is preferable to transferring three embryos at once, detailing the perinatal and obstetric risks associated with multiple pregnancy.
3. c) The couple decides to transfer a single embryo, and three high-quality blastocysts are successfully frozen. If they achieve pregnancy and do not wish to have more offspring, analyse the three legal destination options provided by Spanish legislation for these surplus embryos and the ethical dilemmas raised by each.

During the winter of 1944-1945 ("Dutch hunger winter"), the population of the Netherlands suffered from severe malnutrition (< 1.000 kcal/day) for 6 months due to a military blockade. Decades later, epidemiological studies showed that individuals gestated during that period whose mothers experienced famine in the first trimester were born with normal weight, but as adults showed high rates of obesity, hypercholesterolaemia, and type 2 diabetes, associated with stable hypomethylation of the IGF2 gene compared to their unexposed siblings. Conversely, those exposed to famine only at the end of gestation were born with low birth weight, but did not develop obesity in adult life.

1. a) Explain how epigenetic changes (such as DNA methylation at CpG islands and histone modification) modulate gene transcription in the fetus without altering the DNA nucleotide sequence.
2. b) Interpret this case using the "thrifty phenotype" hypothesis: why does a mismatch between the intrauterine environment perceived by the embryo and the postnatal environment of nutritional abundance lead to chronic metabolic diseases in the adult?
3. c) Justify why the long-term impact on health was much more severe and widespread when nutrient deprivation occurred in the first trimester than when it occurred in the last trimester of gestation.

A 29-year-old woman with a history of chlamydial salpingitis presents to the emergency department with sharp, stabbing pelvic pain in the right iliac fossa and 7 weeks of amenorrhea. Laboratory testing quantifies a beta-hCG concentration of 4.200 mIU/ml, but transvaginal ultrasound shows an empty uterine cavity with a decidualized endometrium and a 3,2 cm mass located in the right tubal ampulla.

1. a) Explain the cause-and-effect relationship between the prior infection and the ectopic implantation: what cellular functions of the tubal ciliated epithelium and the tubal smooth muscle have deteriorated to prevent the normal progression of the morula toward the uterus?
2. b) Justify why a proliferated and thickened endometrium is observed in the uterus despite not housing any embryo. What embryonic structure produces beta-hCG and which hormonal pathway maintains endometrial support?
3. c) Predict the immediate pathophysiological consequences if clinical action is not taken before week 9-10 of gestation, comparing the plastic and vascular capacity of the uterine myometrium with the histological limitations of the tubal wall against syncytiotrophoblast invasion.

In a laboratory, the somatic cell nuclear transfer (SCNT) protocol that allowed Dolly the sheep to be obtained is reproduced, using biological material from three different sheep:

• Donor A (Finn Dorset breed, white face): adult female from which cultured in vitro mammary fibroblasts are isolated (differentiated cells, 2n = 54).

• Donor B (Scottish Blackface breed, black face): female from which unfertilized oocytes are retrieved and their meiotic spindle with its chromosome complement is removed.

• Recipient female C (Scottish Blackface breed, black face): female into whose uterus the blastocyst-stage embryo is implanted after electrofusion of the nucleus from A with the enucleated cytoplasm from B.

1. a) Determine, in a reasoned manner, the exact genetic contribution of each donor (A, B, and C) to the newborn lamb, specifying the origin of the nuclear genome, the mitochondrial genome, and its head coloration phenotype.
2. b) After a few years of life, the clone presented premature arthritis and cellular signs of accelerated senescence. Explain why the telomere length in the clone's cells did not correspond to that of a normal newborn and what limitations the nuclear reprogramming of a differentiated somatic cell experiences compared to natural gametogenesis.
3. c) If the goal were not the gestation of the lamb but the extraction of pluripotent stem cells from the blastocyst's inner cell mass to regenerate tissues, what biological and legal difference underpins the approval of this therapeutic cloning versus the universal ban on human reproductive cloning?

Topic Challenge. As a consulting biologist for your region, evaluate how reproductive strategy conditions the survival of threatened Iberian fauna in the face of global change. Prepare a technical report following three phases:

1. Real data search: Access the inventory of the Ministry for the Ecological Transition (MITECO) and select two native species with contrasting biological models: an anamniote vertebrate (amphibian or fish) and an amniote (bird or mammal).

2. Comparative matrix: Build an analytical table contrasting their reproductive structures, gamete morphology, type of fertilization (internal or external), and mode of embryonic (oviparous, ovoviviparous, viviparous) and post-embryonic development (direct or indirect).

3. Reasoned proposal: Write an assessment evaluating the cost-effectiveness balance of each species. Analyze how parental investment and the number of offspring (with juvenile survival rates often below $0,05$ versus high-protection strategies) influence the vulnerability of their populations.

Evaluation criteria:

• Rigor and accuracy in extracting biological data from the official MITECO portal.

• Conceptual precision when classifying structures, gametes, fertilization, and embryonic and post-embryonic developmental stages.

• Quality of the eco-evolutionary reasoning relating reproductive investment to extinction risk.