Early Fetal Dev Flashcards
What are the 3 measurements of embryo-fetal age?
Fertilization age (also known as conceptual age):
measured from the time of fertilization (assumed to be +1 day from last ovulation(LO+1))
difficult to know time of fertilization exactly (unless IVF)
Gestational age:
Calculated from the time of the beginning of the last menstrual period (BLMP)
Determined by fertilization date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size charts.
Carnegie stage:
23 stages of embryo development based on embryo features not time
Allows comparison of developmental rates between species
Covers the window of 0-60 days fertilization age in humans
Which phases of development are in the first trimester?
Embryogenic Stage (14-16 days post-fertilization):
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)
Embryonic stage (16-~50 days post fertilization):
Establishment of the germ layers and differentiation of tissue types
Establishment of the body plan
Which phases of development are in the 2nd/3rd trimester and what occurs in them?
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 occurs in the first 3 days(post-f) of embryo development and what happens in the 4-8 cell stage?
Until 4-8 cell stage, the genes of the embryo are not transcribed.
Embryo is dependent on maternal mRNAs and proteins to get through the first divisions
Maternal-to-zygotic transition (4-8 cell stage)
Transcription of embryonic genes (zygotic genome activation)
Increased protein synthesis
Organelle (mitochondria, Golgi) maturation
These mRNA and proteins are synthesized and stored during oocyte development (i.e. pre-ovulation)
Failure to synthesise, store or interpret these mRNAs and proteins during oogenesis can impair embryonic development.
What occurs after the 8 cell stage in embryo development?
Blastocyst formation
Around the 8-cell stage or later:
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
Describe the 4 major structures in a blastocyst and their functions
What does the zona pellucida do in a blastocyst?
Hard protein shell inhibiting polyspermy and protects early embryo
How is the blastocoel formed in a blastocyst?
Fluid-filled cavity formed
osmotically by
trophoblast(trophoectoderm) pumping
Na+ ions into cavity
What does the inner cell mass in a blastocyst do?
Pluripotent embryonic cells that will contribute to the final organism
What does the trophoectoderm in a blastocyst do?
Extra-embryonic cells that contribute to the extraembryonic structures that support development
Why is hatching required, when and how is it done embryonically?
Hatching (day 5-6):
To implant the blastocyst must escape zona pellucida.
Enzymatic digestion
Cellular contraction
What is a morula
An embryo with 16+ cells
What are the 2 divisions of embryonic cell lineages and where in the blastocyst are they derived from in a blastocyst?
Embryonic-Inner cell mass
Extraembryonic-Trophoectoderm
What does the inner cell mass divide into pre-implantation
epiblast: from which the fetal tissues will be derived.
hypoblast: which will form the yolk sac (extraembryonic structure)
What does the trophoectoderm separate into pre-implantation
Trophoblast cells fuse to form syncitiotrophoblast
Syncitiotrophoblast invasion destroys local maternal cells in the endometrium
Creates interface between embryo and maternal blood supply
Cytotrophoblast cells remain individual to provide source of syncitiotrophoblast cells
What occurs day 12+
The bilaminar (two-layer) embryonic disc formation is the final stage before gastrulation
Some cells become separated from the epiblast by the formation of a new cavity – the amniotic cavity.
These amnion cells will contribute to the extra-embryonic membranes.
This leaves a two-layer disc of epiblast and hypoblast, sandwiched between cavities.
Embryo is now ready for gastrulation
At what stage do the syncitiotrophoblasts start to secrete hCG?
Bilaminar embryonic disc formation
What occurs overall in gastrulation and why
The bilayer embryo becomes a trilayer embryo
-Basis for the 3 germ layers
Outline gastrulation(~15 days)
- Thickened structure forms(primitive streak) in the epiblast, near the caudal end which outlines the major bodily axes(cranial->head caudal->tail)
- The primitive streak forms the primitive groove, where cells from the outside migrate in towards it
- (Invagination)Cells detach from the epiblast and slip beneath it, into the hypoblast, replacing the hypoblast cells.
- By day 16 the majority of cells have migrated, forming the mesoderm, the remaining cells in the epiblast from the ectoderm and the hypoblastic cells next to the mesoderm form the endoderm
What organs are related to the endoderm?
GI tract
Liver
Pancreas
Lung
Thyroid
What organs are related to the ectoderm
CNS+neural crest
Skin epithelia
Tooth enamel
What organs are related to the mesoderm?
Blood
Muscle
Gonads, kidneys+adrenal cortex
Bone, cartilage
What is the notochord and what does it do?
Notochord is a rod-like tube structure formed of cartilage-like cells
Forms along the embryo midline, under the ectoderm
Acts as a key organizing centre for neurulation and mesoderm development
Outline 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 which becomes overlaid with epidermis (ectoderm)
- Migration of the neural crest cells from folds
Give 2 conditions caused by the improper formation of the neural tube
Anencephaly (absence of most of the skull and brain) arises from failure to close at the head end (1/10,000 births)
Spina bifida (open neural tube at birth, usually lower spine due to failure to close tail end– varying severity – (0.4-5/1000 births)
At what days does the neural tube head and tail close
Closure at head end: ~ d23
Closure at tail end: ~ d27
Closure at head end precedes formation of brain structures
What germ layer are neural crest cells derived from?
Ectoderm
Where are cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage cells derived from?
cranial NC
Where are aortic arch/pulmonary artery septum, large arteries wall musculoconnective tissue cells derived from?
cardiac NC
Where are dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes cells derived from?
trunk NC
Where are parasympathetic ganglia and enteric nervous system ganglia cells derived from?
Vagral and sacral NC
What do defects of neural crest migration/specification lead to?
Diverse birth defects including pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut.
What is somitogenesis
Formation of somites
Somites: Arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord
How do somites set the bodily axis and what is their rate of budding
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.
Humans 1 pair/90 min, 44 pairs
What 2 types of tissue do somites initially form?
Sclerotome: vertebrae and rib cartilage
and the
Dermomyotome, which in turn sub-divides to form:
Dermatome: gives rise to dermis of the skin, some fat and connective tissues of neck and trunk
Myotome: forms the muscles of the embryo
When and how is the gut tube formed
The primitive gut arises from two types of folding in the embryo:
Ventral folding: where the head and tail ends curl together
Lateral folding: where the two sides of the embryo roll
This pinches off part of the yolk sac to form the primitive gut
Primitive gut is then patterned into foregut, midgut and hindgut
Day16+
What organs are derived from the embryological foregut?
esophagus, stomach, upper duodenum, liver, gallbladder, pancreas
What organs are derived from the embryological midgut?
lower duodenum and remainder of small intestine, ascending colon and first two-thirds of transverse colon
What organs are derived from the embryological hindgut?
last third of the transverse colon, descending colon, rectum and upper anal canal
How is the heart embryologically formed?
Begins as tube of mesoderm around day 19, beating and pumping blood commences around day 22
Fetal heartbeat detectable from ~6 weeks gestational age
How are the lungs embryologically 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 are the gonads embryologically formed?
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.
XX embryos: absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2
