Human gestation spans a period of 38 weeks, from fertilisation to birth. Conventionally, pregnancy is divided into three trimesters, each of 3 months’ duration. The formation of organs and systems (embryogenesis) takes place principally between the third and 10th weeks of gestation. Throughout the remainder, the fetal organs undergo differentiation, branching, maturation and growth.
In each ovulatory cycle a small number of germ cells (primary oocytes) within the ovary are stimulated to resume the long-arrested meiotic division. Of these, usually only one progresses to be extruded from the ovary into the fallopian tube at the midpoint of the menstrual cycle. At the time of fertilisation the protective zona pellucida of the oocyte is penetrated by the fertilising spermatazoon, thereby triggering the final meiotic division to create the definitive oocyte and second polar body (consisting of nonfunctional DNA).
Fertilisation is defined as the fusion of the nuclear DNA of the male and female gametes (spermatazoon and definitive oocyte). In the ensuing 5 days, the fertilised zygote undergoes a series of mitotic doubling cell divisions termed cleavage (Figure 1.1).
Implantation of the spherical mass of cells (blastocyst) into the primed uterine endometrium occurs approximately 6 days after fertilisation.
Proliferation of the embryonic cell mass over the ensuing 10 days is accompanied by the appearance of two cavities – the amniotic cavity and the yolk sac. The embryo is destined to develop from the cells interposed between these two cavities.
The ectodermal tissues of the embryo derives from the layer of cells on the amniotic surface of the embryonic disc, whereas the endodermal derivatives have their origins in the layer of cells adjacent to the yolk sac (Figure 1.2). Inpouring of cells from the amniotic surface via the primitive streak creates a third layer of embryonic tissue, the intraembryonic mesoderm, which subdivides into paraxial, intermediate and lateral plate mesoderm.
It is from the intermediate block of intraembryonic mesoderm that much of the genitourinary tract is derived. Figure 1.1 Key stages in the 5-6 days from fertilisation to implantation.
Figure 1.2 Embryonic disc at 16 days. Formation of intraembryonic mesoderm by inpouring of cells at the primitive streak.
Segmentation and folding of the embryo begins during the third and fourth weeks of gestation, and towards the end of this period the precursor of the embryonic kidney begins to take shape.
In vitro fertilisation (IVF)
Many of the more severe congenital abnormalities of the genitourinary tract frequently carry a significant risk of infertility or subfertility.
Examples in males include oligospermia or azoospermia due to cryptorchidism, impaired ejaculation in men with a history of posterior urethral valves or bladder exstrophy and erectile dysfunction in men with neurological impairment associated with spina bifida. Causes of infertility or reduced fertility in females include Turner syndrome, spina bifida and cloacal anomalies.
With the development of in vitro techniques, the outlook for fertility is not always as bleak as was once the case. In the technique of intracytoplasmic sperm injection (ICSI), spermatozoa are harvested directly from the testis or epididymis and a single spermatozoon is selected and injected into an oocyte. The fertilised zygote is then inserted into the uterus. This technique is particularly applicable to men with obstructive azoospermia or oligospermia. Other techniques such as gamete intrafallopian transfer (GIFT) or zygote intrafallopian transfer (ZIFT) may also be applicable, depending upon the cause of infertility.
David FM Thomas FRCP FRCPCH FRCS
St James’s University Hospital
Patrick G Duffy MB BCH BAO FRCS(I) FEBU
Great Ormond Street
Hospital for Sick Children
Anthony MK Rickwood MA FRCS
Formerly of Royal Liverpool
Hospital for Sick Children