Aging&Development Flashcards
define fertilisation age
1 day after last ovulation
measured from time of fertilisation
define gestational age
calculated from the time of beginning of last menstrual period
what is Carnegie staging?
23 stages of embryo development
based on embryo features, not time
covers 0-60 day fertilisation age
when does the embryogenic stage occur?
14-16 days post fertilisation
what happens at the embryogenic stage?
the early embryo is differentiated from the fertilised oocyte
two populations of cells - pluripotent embryonic cells and extraembryonic cells
when does the embryonic stage occur?
16-50 days post fertilization
what defines the embryonic stage?
germ layers
differentiation of tissue types
body plan established
when does the fetal stage occur?
50-270 days post fertilization, i.e second and third trimester
what defines the fetal stage?
presence of major organs
migration of organs to final location
growth
acquisition of metal viability
name the stages from fertilised oocyte to the 200/300 cell structure
what is present at all stages?
1 cell zygote → cleavage stage embryos (2-8cells) → morula 16+ cells → blastocyst
zona pellucida
what is the maternal zygotic transition?
when does it happen?
embryo is dependent on maternal mRNA & proteins (made in oocyte development) until 4-8 cell stage
here embryogenic genes are transcripted (zygotic genome activation) → ↑ protein synthesis and organelle maturation
what is compaction? when does it occur?
outer cells of embryo are pressed against zone and become wedge shaped
outer cells connect via gap junctions and desmosomes → diffusion barrier between inner and outer embryo
outer cells are polarised
occurs after 8-cell stage
what happens to the blastocyst after compaction?
inner cells reorganise to one side to form the blastocoel cavity
what is the blastocoel? how is it formed?
fluid filled cavity
trophoblast pumps sodium ions into it and water osmotically follows
what are 2 functions of the zone pellucida?
prevents polyspermy
protects early embryo
what cells does the inner cell mass consist of?
pluripotent embryonic cells (contribute to final organism)
what is the trophoectoderm? what cells does it consist of?
outer cell layer - extra-embryonic cells i.e. trophoblasts (contribute to structures supporting development)
what is hatching? when and how does it occur?
when the embryo escapes the zone pellucida
at day 5-6
cellular contractions and enzymatic digestions
what happens in peri-implantation events?
@ day 7-9
trophoblasts → syncitiotrophoblasts & some cytotrophoblasts remain
inner cell mass → epiblast and hypoblast
what is the function of syncitiotrophoblasts and cytotrophoblasts?
destroy maternal endometrial cells to create interface between embryo and maternal blood supply
cytotrophoblasts remain to provide source of syncitiotrophoblasts
what do syncitiotrophoblasts secrete?
hcg - human chorionic gonadotrophin
what will the epiblast and hypoblast form?
epiblast → fetal tissues
hypoblast → yolk sac
what is the bilaminar embryonic disc formation? when does it occur?
day 12+
some epiblast cells become separated when the amniotic cavity is formed
cells above → amnion → extra-embryonic membranes
(picture from bottom to top : cytotrophoblast → blastocoel → hypoblast → epiblast → amniotic cavity → amnion → cytotrophoblasts dividing → syncitiotrophoblast → invading maternal endometrium)
what is gastrulation? when does it occur?
15 days post-fertilisation
1. primitive streak forms through epilblast → divides embryo into cranial and caudal ends (and L&R) with primitive pit in the centre and primitive groove towards caudal end
2. epiblast cells invaginate through streak and displace hypoblast cells
hypoblast cells → endoderm
epiblast cells → ectoderm
cells in between → mesoderm
name 5 organs the endoderm forms?
GI tract pancreas liver lung thyroid
name 3 things the ectoderm forms?
tooth enamel
CNS/neural crest
skin epithelia
name 7 things the mesoderm forms?
blood muscle cartialage bone kidenys adrenal cortex gonads
what is the notochord? when and where does it form?
tube structure formed of cartilage like cells
key organising centre for neurulation and ,mesoderm development
forms under ectoderm along embryo midline at ~day13+
explain the process of neurulation
- the notochord signals the neural plate ectoderm to invaginate and form a neural groove
- two neural folds from along the cranio-caudal axis
- neural crest cells form in neural folds
- neural folds move together over neural groove and fuse → hollow neural tube (surrounded by ectodermal epidermis)
- neural crest cells migrate out of neural folds to differentiate
- closure at head end ~day 23 , closure at tail end ~day27
what condition arises when the neural tube fails to close at the head end?
anencephaly
what condition arises when the neural tube fails to close at the tail end?
spina bifida
what germ layer do neural crest cells come from?
ectoderm
name some structure neural crest cells differentiate into
cranial - neurones, ossicles, lower jaw
cardiac - aortic arch
trunk - sympathetic ganglia, adrenal medulla, melanocytes
vagary & sacral - parasympathetic ganglia, enteric NS ganglia
what defects can neural crest differentiation failure lead to?
pigmentation disorders
deafness
cardiac/facial defects
no gut innervation
what is somitogenesis? where does it occur?
formation of somites, commences at head of neural tube down long axis of embryo
how do somites form?
arise from paraxial blocks of mesoderm along the neural tube and notochord
what tissues do somites form?
sclerotome - vertebrae and tip cartilage
dermomyotome → dermatome (dermis, fat & connective tissue) & myotome (muscles)
when and how does the gut tube form?
~day 16 +
embryo folds laterally and ventrally to pinch off part yolk sac → primitive gut (fore, mid & hindgut)
what structures do each the fore, mid and hingut consist of?
fore - oesophagi, stomach, 1st part of duodenum, liver, pancreas, gallbladder
mid - remaining duodenum, jejunum, ileum, ascending colon, proximal 2/3s of transverse colon
hind - distal 1/3 transvers colon, descending colon, rectum, anal canal
what germ layer does the heart form from? when does this occur?
mesoderm ~day 19
beating and pumping blood ~day 22
around when can you detect a fetal heartbeat?
6 weeks gestational age
from what and when do the lungs form?
the lung bud (endoderm) in the 4th week
what germ layer do the gonads form from? what are the initial structures called?
mesoderm
bipotenital gonadal ridges (bipotential = not committed to testis or ovaries)
how do testis and ovary development differ?
XY embryo - presense of SRY gene directs gonadal cells → Sertoli cells → testis formation, leydig cells, testosterone production
XX embryo - absence of SRY → gonadal cells → granulosa cells → ovary development (reinforcement from FOXL2)
what are the 3 broad causes of early pregnancy loss?
errors in embryo foetal development
failure of embryo to implant in uterine lining
inability to sustain development of implanted embryo
define miscarriaage
loss of pregnancy prior to 23 weeks gestation
<12 weeks → early clinical pregnancy loss
12-24 weeks → late clinical pregnancy loss
what percentage of conceptions result in preclinical loss? what does this mean?
60 percent
preclinical means the pregnancy was undetectable, i.e. before a missed menstrual period
what is the likely major cause of early pregnancy loss?
aneuploidy
why does aneuploidy increase with maternal age?
prolonged meiotic arrest in oocytes (prophase I)
homologous chromosomes are held together via cohesin proteins
there is a loss of cohesin with age leading to loss of cohesion between homologous chromosomes
lost cohesion → chromatids can separate and drift during meiotic division → inaccurate spindle segregation → aneuploidy
name 2 cohesin proteins
REC8
SMC2
maintain cohesion between chromatids
how is recurrent miscarriage/pregnancy loss defined in the UK?
three or more pregnancy losses (consecutive or not)
reduced levels of what might contribute to RPL/RM?
LIF in uterine secretions
what aspects do the maternal and paternal genomes contribute to embryo viability?
maternal - restrict embryo fitness to conserve resources for future pregnancies
paternal - promote embryo fitness at expense of mother
what are gestational trophoblastic diseases characterised by?
overgrowth of trophoblastic tissue
what are the benign causes of gestational trophoblastic diseases?
complete and partial hydatidiform moles
what are the malignant causes of gestational trophoblastic diseases?
neoplasias (some arising from hydatidiform moles)
- invasive moles
- choriocarinoma
- placental site trophoblastic tumour
- epithelioid trophoblastic tumour
what is the difference between a complete and partial hydatidiform mole?
complete = no fetal tissue present
how does a complete hydatidiform mole arise?
empty egg fertilised by two sperm cells or by one sperm cell that’s genome duplicates
how does a partial hydatidiform mole arise?
a normal egg is fertilised by two sperm cells or by one sperm cell that’s genome duplicates
what mutations may underly recurrent hydatidiform moles?
NLRP7 mutations
what is an ectopic pregnancy?
implantation of embryo at site other than uterine endometrium (normally Fallopian tube)
what component of cigarette smoke is thought to contribute to ectopic pregnancy?
continine
how is continine thought to contribute to ectopic pregnancy? 2 ways
it up regulates the expression of the PROKR1 receptor in the Fallopian tubes - this is thought to hinder contractility and hence the transfer of the egg to the womb
it induces pro-apoptosis protein expression in the Fallopian tube
how is cannabis thought to cause ectopic pregnancy? 2 ways
tHC components may act on the Fallopian tube and perturb embryo transit
OR
alter the endocannabinoid balance in the fT → disrupted embryo environment
what receptors are reduced in ectopic pregnancy patients?
cannabinoid receptors CB1
10 risk factors for ectopic pregnancy?
previous ectopic pregnancy prior ft surgery STIs endometriosis pelvic inflammatory disease cigarette smoking cannabis use ≥35 yrs history of infertility IVF
what is the non-selective uterus hypothesis?
the uterus allows implantation if poor quality embryos due to changes in uterine mucus expression in women with RPL/RM
pregnancy is detectable but not sustainable
Outline the two types of embryo/foetus nutrition
T1 - histiotrophic - embryo relies on uterine gland secretions and endometrial tissue/capillaries breakdown
T2→term - haemotrophic - maternal blood directly contacts foetal membranes via haemochorial type placenta
what kind of placenta do humans have?
haemochorial-type
what’s the function ofnthe connecting stalk?
links embryo to chorion
what are trophoblastic lacunae?
spaces formed by breakdown of capillaries and uterine glands, filled with maternal blood → intervillous/maternal blood spaces
what are fetal membranes? name them
extra-embryonic tissues that forma tough flexible sac encapsulating the foetus
forms basis of maternal-fatal interface
amnion - inner
chorion - outer
what is the amnion?
inner fetal membrane
arises from epiblast
closed avascular sac with embryo at one end
secrets amniotic fluid from 5th week → fluid sac that surrounds and protects foetus
when does amniotic fluid start being secreted?
from the 5th week