Fertilization and Development

Fertilization   The union of sperm and ovum – –Forms a zygote  The ovum completes meiosis II following fertilization   After fertilization, chemical reactions occur preventing additional sperm from entering the ovum

Early Development  A fertilized egg is a zygote  The zygote divides (mitosis) to become a morula (solid ball of cells)  The morula hollows out to become a blastocyst  The inner cell mass undergoes gastrulation to form a tube with three layers –The developing embryo is now called a gastrula c Morula Inner cell mass

Early Development Implantation of the blastocyst into the uterine wall

Primary Germ Layers  Layers are formed by differentiation of cells  Ectoderm (outside) –Skin –Brain and spinal cord  Mesoderm (middle) –Muscle –Bones –Blood  Endoderm (inside) –Lungs –Digestive tract –Liver

Multiples

Twins  Dizygotic = Two zygotes –Two sperm fertilized two separate eggs –Occurs when 2 oocytes are ovulated –These twins are as alike as any other siblings  Monozygotic = One zygote –Genetically identical –3 different types depending on where the split occurs

3 Types of Monozygotic Twins

From Embryo to Fetus  At 3-4wks the neural tube forms –The neural tube will form the brain and spinal cord –All other organs organize around the neural tube  At 8 weeks, the embryo becomes a fetus –All of the rudimentary features are formed

Life’s Greatest Miracle