L4 (prenatal) Flashcards
Preformationism
ultimate nativist theory of development in which prenatal life starts with a fully-formed individual (either in the sperm or egg) that gets bigger
Canalization
developmental reduction in plasticity of cells (i.e. reduction in ability to change as we age)
Beginning of life according to Beng of West Africa
life begins when newborns choose not to return to afterlife (wrugbe) rather they are born as a reincarnation of an ancestor
you know they’ve chosen to stay when their umbilical stump drops off a couple weeks post-birth
Gametes
haploid sex cells (eggs and sperm)
Haploid cells
contain only half the genetic material found in other diploid cells (23 chromosomes instead of 23 pairs of chromosomes)
Eggs vs sperm
- much bigger (the largest cell) and formed prenatally
- smaller and formed throughout the lifespan
How many sperm fertilize eggs?
millions of sperm are ejaculated but only ~200 reach the egg (survival of the fittest)
Zona reaction
chemical reaction that blocks other sperm from entering an egg
takes around 6 hours for sperm to penetrate by 6-7 inches
What happens to sperm when it fertilizes an egg?
sperm tail falls off and the rest goes into the egg
Zygote
fertilized egg with 23 chromosomes from mom and 23 chromosomes from dad
3 principles of prenatal development
- cephalocaudal (head first)
- basic first then more specialized (e.g. arm bud, hand paddle, then fingers)
- important first (e.g. brain, heart)
4 processes that transform a zygote into an embryo then into a fetus
- cell division (mitosis) resulting in the proliferation of cells
- cell migration
- cell differentiation
- apoptosis
Cell migration
movement of cells from their point of origin to somewhere else in the embryo
Cell differentiation
embryo’s unspecialized stem cells are transformed into ~350 different types of cells
Apoptosis
selective death of certain cells that enables prenatal development
3 periods in prenatal development
germinal (conception to 2 weeks), embryonic (3rd to 8th week), fetal (9th week to birth)
Germinal period
lasts until the blastocyst is implanted in the uterine wall where rapid cell division takes place
after conception, zygote > blastomere > morula > blastocyst
When are twins determined?
germinal period
Monozygotic twins
identical, share the same genetic material, and result from the division of inner cell mass (usually share the same placenta)
Dizygotic twins
fraternal, share 50% of genetic material, and result from two different eggs getting fertilized by two different sperms
What happens to MZ twins when inner cell mass split happens in the first 2 days of the germinal period?
they get their own amniotic sacs and sometimes even their own placentas
What happens to MZ twins when the inner cell mass split happens late?
they may share the same amniotic sac and risk having umbilical cord entanglement; become conjoined if very late
Is twinning hereditary?
identical twinning is uncommon and random while fraternal twinning is hereditary (tendency for hyper-ovulation is passed down)