early development Flashcards
timeline of human gametogenesis in males
- we make sperms from puberty until death
- from week 20 of embryogenesis until puberty we have immature spermatogonia (diploid sperm progenitor cells) in the testes
- they are waiting for testosterone induced awakening at puberty
timeline of human gametogenesis in females
- weeks 8-birth
- puberty
- ovulation
- menopause
- weeks 8 through 20 of gestation: progenitor germ cells (oogonia) become primary (immature) oocytes arrested in prophase of meiosis 1
- puberty: ovaries still contain essentially 100% of primary oocytes still in meiosis 1. they are now capable of developing into secondary oocytes arrested in meiosis 2 metaphase. Only about 10 oocytes will be in the final race to ovulate during each menstrual cycle. Even at the beginning of menopause, the majority of oocytes are still in the primary oocyte stage
- ovulation: secondary oocyte arrested in meiosis 2 metaphase
- fertilization: membrane fusion stimulates oocyte to complete meiosis 2
- menopause: end of fertile period; no more healthy oocytes available for ovulation (48-55)
cleavage of zygote into blastomere
- happens in the first 2-3 days after fertilization
- no overall increase in size due to presence of the zone pelucida
formation of the morula
- happens by day 3
- consists of outer cells that form the trophoblast and inner cell mass
trophoblast
- component of the morula
- group of cells that will go on to make the placenta and extra-embryonic tissue
inner cell mass
- part of the morula
- cells that will form the embryo
forming the early blastocyst
- further cleavages at day 4-5
- causes shedding of the zone pellucida (hatching) within a day or two
- this permits the embryo to begin the implantation process in the uterus
forming the late blastocyst
- days 12-14
- about a week after implantation which is typically day 6 or 7
implantation is completed by
- the end of the second week post-fertilization
- characterized by the formation of the bilaminar embryonic disc
the three germ layers of the embryo are formed during…
-the third week
zona pellucida prevents..
the fertilized egg from implanting nine the walls of the fallopian tube (ectopic pregnancy)
how many blastomeres will form before the zona pellucida hatches
- 8 blastomeres without any increase in size of the embryo
- this is termed compaction and is cadherin dependent
compaction
- when 8 blastomeres form within the zone pellucida and change shape into the morula without it changing in size
- this is a cadherin dependent process
- this is necessary to establish embryo polarity (inner cell mass and trophoblast)
- occurs in the fallopian tube
polarity of the morula
- ICM forms
- ICM is the source for
- trophoblast forms the
- ICM will form the embryo, amnion, yolk sac, and small portions of the placenta
- ICM is the source of embryonic stem cell
- trophoblast forms most of the placenta
mosaicism can occur via
- mitotic cell divisions via nondisjunction causing translocations or deletions
- the earlier the mutation, the more likely it is to be a complete change and therefore result in a lethal change
- the later it happens, the more likely it is to be mosaicism
- mosaics typically have less severe phenotypes than those with complete non-disjunction, deletion, or translocation events
