W7: embryology and pregnancy Flashcards
gametogenesis (FT; EP)
production of male and female games from primordial germ cells
via meiotic cell division in the gonads
fertilisation
- contact sperm and egg
- entry of sperm
- fusion of egg and sperm nuclei
- creates zygote
restores diploid number of chromosomes
cleavage
rapid series of mitotic cell divisions that occur after fertilisation
transforms zygote (single- celled) into morula (multicellular)
zygote develop rapidly and forms smaller cells called blastomeres
advanced morula turns into blastocyst and contains:
blastomeres
inner cell mass (develops into embryo)
surrounding trophoblasts (placenta)
blastocyst
embryoblast (inner cell mass)- develops into embryo
trophoblasts- outer chorionic sac and foetal portion of placenta
hatching
about 5 days after fertilisation, blastocyst grows and eventually the zona pellucida (protective shell) ruptures
blastocyst digests hole in zona pellucida with an enzyme secreted by trophoblasts
the trophoblasts now implant onto the uterus
essential for implantation to occur
implantation
- enzymes are secreted by trophoblasts
- trophoblast pushes its way into the uterine lining by liquifying the endometrial cells
once blastocyst buried in endometrium, starts to secret hcG
= rescues the corpus luteum so progesterone is still secreted (develops uterine lining)
pregnancy is established here
bilaminar disc
bilaminar embryonic disc- between amniotic cavity and yolk sac
amniotic cavity- becomes filled with amniotic fluid
yolk sac- limited function in humans
gastrulation
formation of primitive streak- 3rd week of development
transformation of bilaminar germ disc into trilaminar germ disc
- epiblastic cells invaginate (move inward) to delvelop more layers (2 to 3)
- some cells displace the hypoblast- forming endoderm
- others remain in space between 2 layers- form mesoderm
- remaining epiblast cells- form ectoderm
germ layers
ectoderm (outer tissues)
skin, nails, teeth
nerves
eyes
lining of mouth
mesoderm (middle tissues)
blood, lymph
skeletal/ cardiac muscle
bone, cartilage, CT
reproductive system
endoderm (inner lining)
digestive system
lining of GI tract, bladder, lungs and vagina etc
stem cell
undifferentiated cell hat has ability to form specialise cell types
could either be an embryonic or adult stem cell
totipotent cell
pluripotent cell
multi-potent cell
T: form all differentiated cell types (could form whole organism) e.g. zygote and first few generation of blastomeres
P: form all mature cell ups except placental and extra embryonic cells (cannot form whole organism)
M: form more than one closely related mature cell types in the body e.g bone marrow stem
organogenesis
FT;EP
4th- 8th week embryo starts to develop all the major organs
head and spinal cord are formed
formation of a primitive gut
by 4th wk, upper limbs begin their development
FT; E/FP
by start of 8th wk:
digits on hand beginning to be formed
tail is shorter
eyes open
ears developing
by end of 8th wk:
all limbs formed
digits distinct
embryo clearly human
ATP embryo becomes a human
10-12 weeks
FT;FP
CNS and cardiovascular system have started developing
second trimester , foetal period
12-25 weeks
- rapid growth
- most organs have started maturing
- cardiovascular system started functioning
- head becomes more in proportion to body and limbs lengthen
- brown fat being formed
foetus becomes less vulnerable to damaging affects of drugs, radiation and microbes
by w24, type 2 alveolar cells are starting to produce surfactant in lungs
foetus of 24 weeks could survive with expert neonatal care
third trimester
25-38 weeks
increase in weight from 1.1 to 3.4 kg
nervous system rapidly developing connections
lungs are maturing and baby preparing for life outside the womb
growth of foetus
length of foetus increases almost in proportion to age, 2-3 weeks following implantation
weight of foetus also increases proportionately to the cube of length and age
placentation
formation and growth of the placenta in the uterus
key functions of placenta:
transfer nutrient from mom to fetus
eliminate metabolic wastes from embryo
produce hormones needed to sustain pregnancy
provide immunity and protect against infection
formed from chorionic villi of embryo and decidua basilis of the endometrium of the mom
maternal and foetal circulation don’t mix- o2 and nutrients diffuse across cell membranes from mother circulation into capillaries of the villi
umbilical cord
foetus connected to placenta thru cord
2 arties carry deoxy blood from fetus to mom
one vein carries o2 and nutrients from mum to fetus
amniotic cavity
filled with amniotic fluid
derived from maternal blood
also contains urine from foetus
main functions:
shock absorber for foetus
regulate body temp
prevent drying out
prevent adhesion between skin and surrounding tissues
what are birth defects caused by
disturbance of developmental events involved in formation of particular structure
caused by environmental (teratogens) and or genetic factors
teratogens
any agent that causes a structural abnormality in embryo/ foetus following exposure
infections agents
environmental agents
environmental chemicals
drugs
principles of teratology
the capacity of an agent to produce birth defects depends on:
genotype of the conceptus and maternal genome
developmental stage at time of exposure
dose and duration of the exposure
placental membrane transport
infectious agents
certain viruses can cross placental barrier
rubella: very high risk in first trimester- causes cataracts, cardiac defects, deafness
HIV: growth failure and craniofacial defects
cytomegalovirus: most common virus to cross, result in spontaneous abortion in first trimester, intrauterine growth, retardation
thalidomide
prescribed to treats morning sickness
type of malformation seen was directly related to the time of exposure to the drug
critical period is during organogenesis
35-37= no ears
39-41= no arms
41-43= no uterus
45-47= no tibia
47-49= triphalangeal thumbs
