Embryology Flashcards
1
Q
What are the different periods of development
A
- Zygote (Day 1 - Week 2)
- Embryo (Week 3 - 8)
- Foetus (Week 9 - Month 9)
2
Q
What is the period of the zygote
A
- From conception through to implantation, days counted after fertilisation, days
- 2: 4 cell stage
- 3: Morula
- 4-5: Early blastocyst enters uterus
- 6-7: Blastocyst implantation and proliferation of trophoblast cells
- 12: Implantation complete
- 14: Bilaminar embryo
3
Q
What occurs in week 1 and day 12 of development (Z)
A
- Blastocyst implantation begins day 6-7, nourished by uterine secretions
- Trophoblast cells of blastocyst adhere to site with proper receptors and chemical signals
- Day 12: Embryo completely embedded, formation of embryonic disc, inner cell mass divides into two layers (epiblast and hypoblast), hollow cavities visible (amnion / yolk sac)
4
Q
What occurs in week 2 of development (Z)
A
- Blastocyst composed of cytotrophoblast (inner) and syncytiotrophoblast (outer anchored to endometrium, long protrusions, enzymes, multi-nucleated, large SA for gas exchange)
- Amnion: Sac filled with amniotic fluid, epiblast cells, form transparent sac of amniotic fluid, envelopes embryo (protection), temperature regulation, weightless environment for movement
- Yolk Sac: Sac at ventral surface of embryo, forms part of digestive tube, source of earliest blood cells and blood vessels
- Allantois: Small out-pocketing at caudal end of yolk sac, structural base for umbilical cord, becomes part of urinary bladder
- Chorion: Helps form placenta, encloses embryonic body and all other membranes, syncytiotrophoblast plus cytotrophoblast
5
Q
Briefly list what happens in week 3 of development (E)
A
- Formation of the tri-laminar embryo
- Changes in the chorion (formation of chorionic villi)
- Gastrulation begins / formation of 3 germ layers (ectoderm, mesoderm, endoderm)
- Neurulation
6
Q
What is the chorionic villi (E)
A
- Anchoring villi penetrate intodecidua basalisand anchor chorion vesicle to wall of uterus
- Absorbing villi branch in inter-villus spaces (maternal blood)
- Primary villi (small, nonvascular, cytotrophoblast penetrate synctiotrophoblast)
- Secondary villi (extra embryonic mesoblast in centre, surrounded by cytotrophoblast and synctiotrophoblast)
- Tertiary villi (same structure as 2nd but vascularised by umbilical artery / vein)
- Diffusion between chorionic blood vessels and maternal blood in lacunae
- Oxygen and nutrients diffuse into embryo
- Carbon dioxide and waste products diffuse from embryo
7
Q
What is gastrulation (E)
A
- Conversion of bilaminar embryo into tri-laminar embryo
- Epiblast cells migrate up to primitive streak and cells push down through primitive groove and form mesoderm
- Mesoderm spreads between epiblastic and hypoblastic layers of bilaminar embryonic disc
- Epiblast (forms ectoderm), hypoblast (forms endoderm) and middle layer (mesoderm)
- Areas where embryonic mesoderm cells are absent (cloacal / oral membrane)
8
Q
What is neurulation (E)
A
- Ectoderm cells differentiate to form neural plate
- Notochord forms from mesodermal cells soon after gastrulation is complete
- Notochord causes inward folding of ectoderm at neural plate and thickening on sides (neural crest - become PNS)
- Ends of neural plate come together and fuse to form neural tube (becomes CNS)
- Embryonic mesoderm adjacent to notochord and neural tube (paraxial) form somites
- Notochord degenerates except in region of intervertebral disc (nucleus pulposus)
9
Q
What occurs in week 4 of development (E)
A
- Embryonic folding, heart beat, formation of limb buds and branchial / pharyngeal arches, formation of optic pits / vesicles and nasal placodes
- Folding of cranial and caudal ends and lateral folding of embryo
- Head fold brings primitive heart region and oral membrane onto ventral surface
- Lateral folding forms abdominal walls
10
Q
What occurs in week 4.5 of development (E)
A
- Placentation, regional differences in placental organisation
- Chorionic villi invaded by new blood vessels, extend to embryo as umbilical arteries (2) and vein (1)
- Placenta consists of foetal / maternal tissues, blood separated by chorionic villi
- Placenta forms from chorion and endometrium to allow embryo / foetus to exchange nutrients and waste, semipermeable (diffusion of nutrients and gasses), metabolic transactions sustain embryo
- Chorionic villi provide SA for exchange, nutrient and gas exchange happens without actual blood exchange
11
Q
What occurs in week 5-8 of development (E)
A
- 5: Alimentary canal is continuous, oral membrane (mouth), cloacal membrane (anus), primitive gut
- 7: Feet and hands emerge, rudimentary skeleton, eyes, nose and mouth shape
- 8: Sexual development, if male, SRY on Y chromosome triggers formation of testes or ovaries
12
Q
What occurs in weeks 9-12 of development (F)
A
- External genitalia form
- Organ systems laid out
- Teratogens no longer lethal but can produce major defects
13
Q
What occurs in month 3 of development (F)
A
- Movement but cannot yet be felt by mother
- Digestive system and excretory systems functioning
- Reproductive system contains immature ova or sperm cells
14
Q
What occurs in months 4-6 of development (F)
A
- Sucking, swallowing, breathing, movements
- Heart beat can be heard with stethoscope
- Sweat glands functioning
- Vernix (protects skin from chapping)
- Lanugo (fine hair helps vernix stick to skin)
- Visual and auditory senses are functional
15
Q
What occurs in months 7-9 of development (F)
A
- Age of viability: 22-28 weeks, survival outside the womb is possible, weight is ~2kg (at end of 7th month), mostly for size increase and maturity
- Lungs finish developing last, receive oxygen from placenta
- Surfactant develops late (keep alveoli inflated)
- Fully developed lungs by 37 weeks
- Growth, activity slows, sleep increases
- Birth occurs
- Stop breathing via placenta’s oxygen supply (first breath)
