1.2: Early fetal development Flashcards
Fertilisation age (conceptual age)
Measured from time of fertilisation
Difficult to know exact time - unless IVF
Gestational age
Calculated from the time of the beginning of the last menstrual period (LMP)
Determined by fertilisation date (+14 days ) if known, or early obstetric ultrasound and comparison to embryo size charts
Carnegie stage
Allows comparison of developmental rates between species - 23 stages based on embryo features not time
Covers the window of 0-60 days fertilisation age in humans
How many stages of embryo development are there and what determines them
23 stages
Dependent on embryo features not time - carnegie stage
Stages of embryo-fetal development
Embryogenic stage 14-16 days post fertilisation
Embryonic stage 16-50 days post fertilisation
Foetal stage 50-270 days post fertilisation
What occurs during the Embryogenic stage
Establishing the early embryo from fertilised oocyte - embryo genesis
Determining two populations of cells : pluripotent embryonic cells, extraembryonic cells
What do pluripotent embryonic cells contribute to
Foetus
What do Extraembryonic cells contribute to
Support structures e.g. placenta
When does the Embryonic stage occur
16-50 days post fertilisation
What occurs in the Embryonic stage
Establishment of germ layers and differentiation of tissue types
Establishment of body plan
Fetal stage occurs
50-270 days post-fertilisation (8-38 weeks)
What happens in the fetal stage
Major organ systems present
Migration of some organ systems to final location
Extensive growth and acquisition of fetal viability - survival outside of womb
First trimester is composed of
Embryogenic stage
Embryonic stage
Second and third trimester is composed of
Fetal stage
4 Stages of the first few days of life following fertilisation
Zygote (1 cell)
Cleavage stage embryos (2-8 cells) : mitotic divisions
Morula (16+ cells) : further mitiotic divisions
Blastocyst (200-300 cells)
What are the genes like in an embryo until the 4-8 cell stage
Genes not transcribed
Embryo dependent on maternal mRNSa and proteins to get through first divisions
MRNA and proteins synthesised and stored during oocyte development
What can failure to synthesise, store or interpret mRNAs and proteins during oogenesis lead to
May impair embryonic development
Maternal-to-zygotic transition (4-8 cell stage)
MZT - embryonic genome activation
Transcription of embryonic genes (zygotic genome activation- not maternal genome)
Increases protein synthesis
Organelle maturation (mitochondria, Golgi)
What starts the formation of the first two cell types
Compaction
What change of shape occurs around the 8-cell stage or later
Change from spherical to wedge-shaped
What happens to outer cells around the 8-cell stage or later
Outer cells become pressed against zona
connect each other through tight gap junctions and desmosomes
Forms barrier to diffusion between inner and outer embryo
Outer cells become polarised
What does blastocyst formation establish
Two cell types
The blastocoel of a blastocyst is
Fluid-filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity
The zona pellucida of the blastocyst is
Hard protein shell inhibiting polyspermy and protecting early embryo
Characteristics of the inner cell mass of a blastocyst
Pluripotent embryonic cells that contribute to final organism
Characteristics of the trophoectoderm
Extra-embryonic cells that contribute to the extraembryonic structures that support development
What happens to the blastocyst during hatching (day 5-6)
To implant, the blastocyst must escape zona pellucida
Enzymatic digestion
Cellular contractions
Separation of embryonic cell lineages I
Morula separates into :
1) inner cell mass (embryonic)
2) trophectoderm (extra-embryonic)
In peri-implantation (day 7-9) Trophoectoderm lineage separates further :
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
In peri-implantation (day 7-9) inner cell mass separates further into:
Epiblast: from which foetal tissues will be derived
Hypoblast: which will form the yolk sac (extraembryonic structure)
Separation of embryonic cell lineages 2 : embryonic branch
Epiblast and Hypoblast
Separation of embryonic cell lineages 2: extra-embryonic branch
Cytotrophoblast, syncitiotrophoblast
What is the bi-laminar embryonic disc
(Two-layer) embryonic disc is the final stage before gastrulation
What happens during the bi-laminar embryonic disc formation (day 12-14)
Some cells separated from the Epiblast by the formation of new cavity - amniotic cavity
These amnion cells contribute to extra-embryonic membranes.
This leaves two-layer disc of Epiblast and hypoblast, sandwiched between cavities.
Embryo is now ready for gastrulation
What does a Syncitiotrophoblast secrete
hCG
Extra-embryonic cells differentiate further into
Syncitiotrophoblast
Cytotrophoblast
Inner cell mass forms
Bilaminar disc
Formation of the primitive streak defines
Head-tail and left-right axes of embryo
Invagination of cells into primitive streak forms 3 germ layers:
Endoderm cells (first through the streak)
Ectoderm remains on the upper (ventral) surface
Mesoderm sandwiched between ends and ectoderm
What 5 organs form the endoderm Germ layer
GI tract
Liver
Pancreas
Lung
Thyroid
What 4 organs form the ectoderm germ layer
CNS
Neutral crest
Skin epithelia
Tooth enamel
What 7 organs form the mesoderm germ layer
Blood
Muscle
Gonads
Kidneys
Adrenal cortex
Bone
Cartilage
What is the notochord
Rod-like tube structure formed of cartilage-like cells
Forms along the embryo midline, under the ectoderm
Key organising centre for neurulation and mesoderm development
What occurs on days 13-17
formation of body plan:
- notochord formation
-neurulation
-somitogenesis
-formation of gut tube
What is neurulation
Formation of the neural tube and CNS
Process of neurulation and neural tube formation
1) notochord signals direct neural plate ectoderm to invaginate forming neural groove
2) creates two ridges running along the cranio-caudal axis
3) neural crest cells specified in neural folds
4) neural folds move together over neural groove and fuse forming a hollow tube
5) neural tube overlaid with with epidermis (ectoderm)
6) migration of the neural crest cells form folds
At what day does the neural tube close at the head end
23
Precedes formation of brain structures
At what day does the neural tube close at the tail end
27
What two developmental defects does failure of neural tube closure cause
Anencephaly - absence of most of the skull and brain ; arises from failure close to head end
Spina bifida- open neural tube at birth, usually lower spine due to failure to close tail end
Pigmentation defects, deafness, cardiac and facial defects, gut innervation defects
Neural crest cells characteristics
Ectoderm-derived, plastic and migrate extensively during development
4 neural crest cells
Cranial NC
Cardiac NC
Trunk NC
Vagral and Sacral NC
Cranial NC give rise to
Cranial neurones, glia, lower jaw, middle ear bones, facial cartilage
Cardiac NC give rise to
Aortic arch, large arteries walls musculoconnective tissue
Trunk NC give rise to
Dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
Vagral and Sacral NC give rise to
Parasympathetic ganglia and enteric nervous system ganglia
What do defects of neural crest migration lead to
Diverse birth defects; including pigmentation disorders, deafness, cardiac and facial defects, failure to inner age the gut
Somitogenesis is
Formation of somites
-divide into two :
Sclerotome - bones and rib cartilage
Deermomyotome - dermatome (dermis of skin), myotome (muscles)
Somites arise from
Paired blocks of paraxial mesoderm flanking the neural tube and notochord
paraxial mesoderm -> condense and bud off into somite pairs
Begins at head end and progresses down 1/90mins in humans (44 total)
How do somites form
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
Rate of budding in humans
1 pair / 80min, 44pairs
What two types of embryonic tissue do somites form
Sclerotome
Dermomyotome
What does Sclerotome tissue form
Vertebrae
Rib cartilage
What does dermomyotome tissue divide into
Dermatology
Myotome
What does dermatome give rise to
Dermis of the skin, some fat and connective tissues of neck and trunk
What does the myotome tissue form
Muscles of embryo
When is the gut tube formed
Day 16+
What two types of folding in the embryo give rise to the primitive gut
Ventral folding - where the head and tail ends curl together
Lateral folding - where two sides of the embryo roll
How is the primitive gut formed
Foldings in the gut pinch off part of the yolk sac
Then patterned into foregut, midgut,and hindgut
Derivatives of the foregut
Esophagus, stomach, upper duodenum, liver, gallbladder, pancreas
Derivatives of the midgut
Lower duodenum, remainder of small intestine, ascending colon, first two-thirds of transverse colon
Derivatives of hindgut
Last third of transverse colon, descending colon, rectum and upper anal canal
How is the heart formed in an embryo
Begins as a tube of mesoderm around day 19,
Beating and pumping blood commences day 22
Foetal heartbeat detectable from 6weeks gestational age
How do lungs arrive in early embryo
Arise from lung bud, and endodermal structure adjacent to foregut - 4th week of development
Lung bud splits into two at end of 4th week, progressively branches through development
How do gonads arise in early embryo
Form from mesoderm as bipotential structures - genital ridges
In XY embryos
Presence of SRY gene on Y detects gonadal cells to become Sertoli cells, triggering development of testes, Leydig cell formation and testosterone production
In XX embryos
Absence of SRY gene leads to gonads adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2
Compaction
outer cells begin to flatten out
inner cells attach firmly to each other
tight junctions form between cells
Gastrulation
forming layers -primitive groove
gives orientation
