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Embryogenesis and development encompass the intricate processes from fertilization to birth, involving critical stages and structures essential for fetal growth and maturation. This knowledge is crucial for the MCAT, as it integrates concepts from biology, physiology, and anatomy.
Embryos initially develop without differentiation in sex, beginning as female by default. Upon fertilization in the ampulla of the fallopian tube, the zygote undergoes rapid mitotic cell division called cleavage as it moves toward the uterus for implantation. During this journey, the embryo is initially supported by the yolk sac until the placenta becomes functional.
Implantation in the uterus marks a crucial phase where the blastocyst, comprising the trophoblast and inner cell mass, embeds into the uterine lining. The placenta, developed from the chorion, facilitates nutrient and gas exchange between maternal and fetal bloodstreams through chorionic villi. This exchange ensures the fetus receives oxygenated blood and nutrients via the umbilical vein. The deoxygenated blood and waste products are carried away by the umbilical arteries.
The fetal circulatory system, unique due to shunts like the ductus arteriosus and ductus venosus, supports circulation in utero. The ductus arteriosus connects the pulmonary artery to the aorta, bypassing non-functioning fetal lungs. At the same time, the ductus venosus connects the umbilical vein directly to the inferior vena cava, allowing nutrient-rich blood to bypass the liver.
Throughout development, cells progress from totipotent to pluripotent states, capable of differentiating into various cell types under the influence of paracrine signals and inducers. The archenteron, formed during gastrulation, develops into the gut, marking the onset of organogenesis. Meanwhile, induction—the ability of cells to influence neighboring cells’ fate—plays a critical role in defining tissue specialization and organ formation.
Protection against pathogens in utero involves the transfer of maternal antibodies across the placental membrane, ensuring temporary immunity for the fetus. This protective mechanism complements the physical barriers provided by the chorion and amniotic sac, which prevent microbial entry.
Developmental stages are susceptible to disruption by teratogens—substances that interfere with normal embryonic development, potentially causing congenital defects or fetal demise. Therefore, understanding these factors is crucial in prenatal care and developmental biology research.
In summary, embryogenesis and development involve a series of complex processes—from fertilization and implantation to organogenesis and placental function—that collectively ensure fetal growth and maturation.