Age + Dev - Early Fetal Development Flashcards
What are the 3 ways in which embryo-fetal development can be measured?
→ fertilization age
→ gestational age
→ carnegie age
What is fertilization age?
measured from the time of fertilization (assumed to be +1 day from last ovulation)
Why is fertilization age not very useful?
difficult to know time of fertilization exactly unless it’s IVF
What is gestational age?
→ calculated by time from the beginning of the last menstrual period (lmp)
→ determined by fertilzation date (+14 days) if known, or early obstetric ultrasound + comparison to embryo size charts
What is the carnegie age?
→ based off of 23 stages of embryo development which are based on embryo features rather than time
→ allows comparison of development rates between species
→ covers window of 0-60 days of fertilzation age in humans
How is pregnancy split up into stages to measure embryo-fetal development?
→ embryogenic stage
→ embryonic stage
→ fetal stage
What is the embryogenic stage?
→ 14-16 days post-fertilisation establishing the early embryo from the fertilized oocyte
→ Determining two populations of cells:
- Pluripotent embryonic cells (contribute to fetus)
- Extraembryonic cells (contribute to the support structures eg placenta)
What is the embryonic stage?
→ 16-~50 days post fertilisation
→ Establishment of the germ layers and differentiation of tissue types
→ Establishment of the body plan
What is the fetal stage?
→ ~50 to 270 days post-fertilization or ~8 to ~38 weeks
→ Major organ systems now present
→ Migration of some organ systems to final location
→ Extensive growth and acquisition of fetal viability (survival outside the womb)
What stages of embryo-fetal development occur in the first trimester of pregnancy?
→ embryogenic stage
→ embryonic stage
What stages of embryo-fetal development occur in the 2nd + 3rd trimester of pregnancy?
fetal stage
What does the ovulated oocyte become after fertilisation?
zygote
How many cells in a zygote?
1 cell
What is the next stage of development for a zygote?
cleavage division into “cleavage stage embryos”
How many cells are there in cleavage stage embryos?
2-8 cells
What is the next stage of development for cleavage stage embryos?
morula
How many cells are there in morula?
16+ cells
What is the next stage of development for morula?
blastocyst
How many cells are there in blastocyst?
200 - 300 cells
Where do the first few days of life take place in the female gonads?
happens while egg migrates from ovaries through fallopian tubes to the uterus
What is the first major developmental transition in the first few stages of life?
maternal-to-zygotic transition
What is the maternal-to-zygotic transition?
→ embryo stops being dependent on maternal mRNAs + proteins to get through first divisions
→ transcription of embryonic genes occurs + zygotic genome activation
→ increased protein synthesis
→ organelle (mitochondria + golgi) maturation
How does zygote survive before zygotic genome activation?
Dependent on maternal mRNAs + proteins (that were synthesised + stored during oocyte development) to get through the first divisions
What happens when mRNA + proteins are not properly synthesised or stored or interpreted properly?
Can impair embryonic development
When does the maternal-zygotic transition occur in terms of cell stage and human development time?
→ 4-8 cell stage
→ 2 days post-fertilization
What is the process that forms the morula + blastocyst?
Compaction
How does compaction produce a compacted morula?
→ Outer cells become pressed against zona
→ Change from spherical to wedge-shaped.
→ Outer cells connect to each other through tight gap junctions and desmosomes
→ Forms barrier to diffusion between inner and outer embryo
→ Outer cells become polarised
What is the arrangement of the compacted morula?
2 distinct cell populations:
→ inner
→ outer
How does a morula formation change to form a blastocyst?
Inner cells + outer cells reorganise with the formation of the blastocoel cavity
What are the different parts of the blastocyst?
→ zona pellucida
→ trophectoderm
→ inner cell mass
→ blastocoel
What is the purpose of the zona pellucida for the blastocyst?
Hard protein shell inhibiting polyspermy and protects early embryo
What is the purpose of the trophectoderm for the blastocyst?
Extra-embryonic cells that contribute to the extraembryonic structures that support development
What is the purpose of the inner cell mass for the blastocyst?
Pluripotent embryonic cells that will contribute to the final organism
What is the purpose of the blastocoel for the blastocyst?
Fluid-filled cavity formed
osmotically by
trophoblast pumping
Na+ ions into cavity
How does that blastocyst implant in the uterus?
blastocyst must escape zona pellucida
How does that blastocyst escape the zona pellucida?
→ enzymatic digestion
→ cellular contractions
What are the separate embryonic cell lineages of the morula?
→ inner cell mass = embryonic
→ trophectoderm = extra-embryonic
How many days does it approximately take for the blastocyst to develop from a fertilised ovum?
6 days
How does the trophectoderm lineage separate during peri-implantation events?
→ syncitiotrophoblast
→ cytotrophoblast
How does the inner cell mass lineage separate during peri-implantation events?
→ epiblast
→ hypoblast
What are the syncitiotrophoblasts?
→ Trophoblast cells fuse to form the syncitiotrophoblast
→ Syncitiotrophoblast invasion into the uterine endometrium destroys local maternal cells in the endometrium
→ Creates interface between embryo and maternal blood supply
→ secretes beta hCG subunit
What are the cytotrophoblasts?
→ remains around the blastocoel cavity, remaining individual to provide a source of syncitio-trophoblasts
What are the epiblasts?
→ layer closest to the cytotrophoblasts
→ from which the fetal tissue will be derived
What are the hypoblasts?
→ layer furthest from the cytotrophoblasts
→ will form the yolk sac (extra-embryonic structure)
When do the peri-implantation events occur?
day 7-9 approximately
What is the next step after the peri-implantation events? When does it occur?
→ bi-laminar embryonic disc formation
→ day 12+
What is the process of the formation of the bi-laminar embryonic disc?
→ some of the epiblasts separate from the border of the cytotrophoblasts to create 2 cavities
→ new cavity closer to the uterus = amnion cavity
→ 2-layer embryonic membrane disc made up of epiblasts + hypoblasts separates the 2 cavities
→ allows the embryo to be ready for gastrulation
What are the amnion cells? What are their purpose?
→ epiblast cells that line the amniotic cavity + cytotrophoblasts
→ amnion cells contribute to extra-embryonic membranes
Why can a pregnancy be detected using a pregnancy test usually after the formation of the bi-laminar embryonic disc formation?
→ syncitiotrophoblasts secrete beta hCG subunit
→ can now be detected in blood + urine by pregnancy test
What happens after the formation of the bi-laminar embryonic disc formation?
gastrulation
What is the process of gastrulation?
→ thickened structure forms in the caudal end of the epiblasts of the amniotic cavity called the primitive streak
→ primitive streak contains the primitive node + pit = groove
→ epiblasts pass through the groove down into the hypoblast layer = INVAGINATION
→ first epiblasts to invade hypoblasts = new layer called definitive endoderm
layer
→ remaining epiblasts above this = ectoderm
→ cells remaining in space between endoderm and ectoderm = mesoderm
At what point does gastrulation occur?
15 days +
What are the 3 germ layers that form from the epiblast cells?
→ ectoderm
→ mesoderm
→ endoderm
Why are the germ layers so important?
they’re precursor tissues to organs of fetus
What tissues and organs develop from the endoderm?
→ GI tract → liver → pancreas → lungs → thyroid
What tissues and organs develop from the ectoderm?
→ CNS
→ neural crest
→ skin epithelia
→ tooth enamel
What tissues and organs develop from the mesoderm?
→ blood (endothelial cells, red + white blood cell) → muscle (smooth, skeletal, cardiac) → gonads → kidneys → adrenal cortex → bone → cartilage
What is the purpose of the notochord?
acts as a key organising centre for neurulation + mesoderm development
What is neurulation?
forming of the neural tube + CNS
What is the process of neurulation?
→ Notochord signals direct the neural plate ectoderm to invaginate forming neural groove
→ Creates two ridges (neural folds) running along the cranio-caudal axis
→ Neural crest cells specified in neural folds
→ Neural folds move together over neural groove
→ Ultimately neural folds fuse, forming a hollow tube
→ Neural tube overlaid with epidermis (ectoderm)
→ Migration of the neural crest cells from folds
When does the head of the neural tube close?
the head end closes at day 23 approximately
When does the tail end of the neural tube close?
day 27
What happens after the closure of the neural tube?
formation of the brain structures
What can the failure of the neural tube closure cause?
developmental defects :
→ anencephaly
→ spina bifida
What is anencephaly?
→ absence of most of the skull and brain
→ arises from failure to close at the head end
→ 1/10,000 births
What is spina bifida?
→ open neural tube at birth, usually lower spine
→ due to failure to close tail end– varying severity
→ 0.4-5/1000 births
What are the different neural crest cells?
→ cranial NC
→ cardiac NC
→ trunk NC
→ vagral + sacral
What do the cranial neural crest cells develop into?
→ cranial neurones → glia → lower jaw → middle ear bones (ossicles) → facial cartilage
What do the cardiac neural crest cells develop into?
→ aortic arch
→ pulmonary artery septum
→ large artery walls
→ musculo-connective tissues
What do the trunk neural crest cells develop into?
→ dorsal root ganglia → sympathetic ganglia → adrenal medulla → aortic nerve clusters → melanocytes
What do the vagral + sacral neural crest cells develop into?
→ parasympathetic ganglia
→ enteric nervous system ganglia
What can defects of the neural crest migration or specification lead to?
→ pigmentation disorders
→ deafness
→ cardiac + facial defects
→ failure to innervate gut
What is somitogenesis?
→ formation of somites
→ segmentation of body axis
What are somites?
arise from paired blocks of paraxial mesoderm flanking the neural tube + notochord
What is the process of somitogenesis?
→ Blocks of paraxial mesoderm condense and bud off in somite pairs
→ One of each pair either side of the neural tube.
→ Somitogenesis commences at the head end and progresses down the long axis of the embryo
→ Rate of ‘budding’ or appearance of somite pairs is species-specific, as is the number of pairs.
What are the 2 types of embryonic tissue formed from somites?
→ dermomyotome
→ sclerotome
What is sclerotome?
vertebrae + rib cartilage
What does dermomyotome split into?
→ dermatome
→ myotome
What is a dermatome?
gives rise to dermis of skin, some fat + connective tissue of neck + trunk
What is a myotome?
forms muscles of the embryo
When does the primitive gut form?
day 16 +
What 2 processes form the primitive gut?
→ ventral folding : where the head + tail ends curl together
→ lateral folding : where the 2 sides of embryo roll
How does the the primitive gut form?
ventral folding + lateral folding pinch off part of the yolk sac to form the primitive gut
How can the primitive gut be split up?
→ foregut
→ midgut
→ hindgut
What is a part of the foregut?
→ oesophagus → stomach → upper duodenum → liver → gall bladder → pancreas
What is a part of the midgut?
→ lower duodenum → jejunum → ileum → ascending colon → 2/3 of transverse colon
What is a part of the hindgut?
→ last third of transverse colon
→ descending colon
→ rectum
→ upper anal canal
How is the heart derived?
→ begins as tube of mesoderm at day 19
→ beating + pumping blood commences around day 22
From when is a fetal heart beat detectable?
approx. 6 weeks gestational age
How are the lungs formed?
→ Arise from the lung bud, and endodermal structure adjacent to the foregut, in the 4th week of development
→ Lung bud splits into two at the end of the 4th week, and progressively branches through development
How do the female gonads develop?
Forms from mesoderm as bipotential (i.e. not committed to testis or ovary) structures known as gonadal/genital ridges
absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2
How do the male gonads begin to develop?
Forms from mesoderm as bipotential (i.e. not committed to testis or ovary) structures known as gonadal/genital ridges
XY embryos: presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells, triggering testis development, Leydig cell formation and testosterone production
