Week 15: Prenatal Development
Germinal Period: The first two weeks after the sperm and egg unite.
During this time, the newly formed zygote grows and divides in a process called cleavage
(which starts about 24 hours after conception). After cleavage occurs a few times, the resulting
mass of cells is called the morula: A solid mass of blastomeres resulting from a number of
cleavages of a zygote.
Epigenetic Modification: Every cell has the same DNA in it (except gametes) but our body
needs to produce different types of cells which is done by epigenetic modification
(environmental factors) which basically turn genes on a off based on the need for different types
Stem Cells: An undifferentiated cell that can divide and produce any one of a variety of
• Stem cells have the same DNA as every other cell, but has not yet undergone epigenetic
• Each stem cell has the capacity to become any specific type of cell with the proper
• Cells are very flexible, and any cell has the capacity to develop into a healthy infant (how
identical twins are formed)
Blastocyst: Up until now, the developing organism has been in the fallopian tube, where
conception occurs and is travelling down into the uterus. As the morula enters it, the uterus
begins to fill with fluid
• Cells differentiate forming 2 layers called the inner cell mass: The mass of cells inside
the morula that eventually will form the embryo, and the trophoblast: the cells that form
the outer layer of a blastocyst which protects and transmits nutrients to it (develops into
extra-embryonic tissues including placenta)
• After this, the morula is now called a blastocyst: the stage a fertilized egg reaches five
to six days after fertilization.
Layers of Cells: after implantation in the uterine wall the embryonic period begins and lasts
about 8 weeks. The embryo consists of hundreds of cells. The trophoblast layer transforms into
two parts, one being the amniotic sac containing the embro and fluid, and the second part is the
placenta which attaches to the inside of amniotic sac and the umbilical chrost of the embryo
• Placenta: acts as a filter and protective barrier, prevents blood of mother and embro from
mixing, transfers nutrients and oxygen from the mother’s blood to the developing
organisms , eliminate waste and protects organism from chemicals/infections from the
mother • Embryo: splits into 3 layers: the endoderm: The innermost layer of the three primary
germ cell layers of the embryo, which develops into digestive system, urinary tract and
lungs, the mesoderm: The middle layer of the embryo, lying between the ectoderm and
the endoderm., transforms into muscles, bone and the circulatory system, and the
ectoderm: The outermost of the three primary layers of the embryo, which develops into
skin, hair, teeth and central nervous system.
Neural Tube: The embryo's precursor to the central nervous system.
Embryo turns in on itself and forms a small tube of ectoderm inside the embro. This tube begins
to develop into the brain and spinal cord, in a process called neurulation. All cells of the central
nervous system are grown inside this tube. The growth of new neurons is called neurogenesis
beginning 6-7 weeks after conception.
Neural Migration: The process through which neurons move, grow, and connect as the basic
neural tube develops into a more mature brain.
• Genetic instruction for outline for neural growth
• Timing and location of neurogenesis, interaction with glial cells and chemical and
environmental signals all influence migration
• During em