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.

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.

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 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?).

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).

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.

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.

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.

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.

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.