16.3 Foetal Development
Learning Objectives
By the end of this section, you will be able to:
- Differentiate between the embryonic period and the foetal period
- Briefly describe the process of sexual differentiation
- Describe the foetal circulatory system and explain the role of the shunts
- Trace the development of a foetus from the end of the embryonic period to birth
As you will recall, a developing human is called a foetus from the ninth week of gestation until birth. This 30-week period of development is marked by continued cell growth and differentiation, which fully develop the structures and functions of the immature organ systems formed during the embryonic period. The completion of foetal development results in a newborn who, although still immature in many ways, is capable of survival outside the womb.
Sexual Differentiation
Sexual differentiation does not begin until the foetal period, during weeks 9–12. Embryonic males (with SRY gene) and females, though genetically distinguishable, are morphologically identical (Figure 16.3.1). Bipotential gonads, or gonads that can develop into male or female sexual organs, are connected to a central cavity called the cloaca via Müllerian ducts and Wolffian ducts. (The cloaca is an extension of the primitive gut.) Several events lead to sexual differentiation during this period.
During male foetal development, the bipotential gonads become the testes and associated epididymis. The Müllerian ducts degenerate. The Wolffian ducts become the vas deferens and the cloaca becomes the urethra and rectum.
During female foetal development, the bipotential gonads develop into ovaries. The Wolffian ducts degenerate. The Müllerian ducts become the uterine tubes and uterus and the cloaca divides and develops into a vagina, a urethra and a rectum.
The Foetal Circulatory System
During prenatal development, the foetal circulatory system is integrated with the placenta via the umbilical cord so that the foetus receives both oxygen and nutrients from the placenta. However, after childbirth, the umbilical cord is severed, and the newborn’s circulatory system must be reconfigured. When the heart first forms in the embryo, it exists as two parallel tubes derived from mesoderm and lined with endothelium, which then fuse together. As the embryo develops into a foetus, the tube-shaped heart folds and further differentiates into the four chambers present in a mature heart. Unlike a mature cardiovascular system, however, the foetal cardiovascular system also includes circulatory shortcuts, or shunts. A shunt is an anatomical (or sometimes surgical) diversion that allows blood flow to bypass immature organs such as the lungs and liver until childbirth.
The placenta provides the foetus with necessary oxygen and nutrients via the umbilical vein. (Remember that veins carry blood toward the heart. In this case however, the blood flowing to the foetal heart is oxygenated because it comes from the placenta. The respiratory system