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.…    



Upper urinary tract

In the cervical portions of the paired blocks of intermediate mesoderm the primitive precursor of the kidney,  the pronephros,  first appears in the fourth week of gestation. 

This structure rapidly regresses in the human embryo.  The midzone mesonephros, however, continues to differentiate, giving rise to tubular structures which,  although ultimately destined to contribute…