Early Embryo Development Flashcards
When is fertilisation age measured from?
From the time of fertilisation, assumed to be more than one day from the last ovulation
What other name is fertilisation date known as?
Conceptualisation date
How practical is fertilisation date?
Not very, its also difficult to know the time exactly of ovulation so its not used very commonly
When is gestational age measured from?
From the time of the beginning of the last period
How is gestational age determined?
Fertilisation date + 14 days if known
Early obstetric ultrasound
How practical is gestational date?
More practical, it is used often clinically
What is carnegie age?
23 stages of embryo development based on embryo features not time
What window does carnegie age cover?
0-60 days fertilisation age (in humans)
What are the 3 major stages of time in embryo foetal development?
Embryogenic stage
Embryonic stage
Foetal stage
How many days post fertilisation is the embryogenic stage?
14-16 days
How many days post fertilisation is the embryonic stage?
16- 50 days
How many days post fertilisation is the foetal stage?
50-270 days or 8-38 weeks
What happens in the embryogenic stage?
Establishment of the early embryo from the fertilised oocyte
What are the 2 populations of cells in the embryogenic stage and what do they do?
Pluripotent embryonic cells (contribute to fetus) Extraembryonic cells (contribute to the support structures eg placenta)
What do pluripotent embryonic cells contribute to?
The foetus
What so extraembryonic cells contribute to?
Support structures eg placenta
What happens in the embryonic stage?
Establishment of the germ layers and differentiation of tissue types
Establishment of the body plan
What is present when the foetal stage is reached?
Major organ systems
What stages does the first trimester include?
Embryogenic and embryonic
What stages does the second trimester include?
Foetal stage
When does the transition from embryo to foetus occur?
At the end of the first trimester
How many cells is an ovulate oocyte?
1
How many cells is a zygote?
1
How many cells is a cleavage stage embryo
2-8
How many cells is a morula?
16
How many cells is a blastocyst?
200-300
When is the zona pellucida present?
From ovulated oocyte until blastocyst
Does the embryo transcribe its own genes?
No, it is dependant on materal mRNAs and proteins to get through the first few divisions until it has 4/8 cells. Failure in the mother to synthesise and store these mRNAs during oogenesis can impair development of the embryo
What happens at the maternal to zygotic transition?
Transcription of embryonic genes begins due to zygotic genome activation
Increased protein synthesis
Organelles mature eg mitochondria, Golgi
At what cell stage does compaction occur?
When the zygote is around 8 cells
Describe the process of compaction
Outer cells become pressed against zona pellucida
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 polarized
What are the cell types in the blastocyst? Describe them
Inner cell mass are pluripotent embryonic cells that will contribute to the final organism
Trophoectoderm are extra-embryonic cells that contribute to the extraembryonic structures that support development
What is the zona pellucida?
Hard protein shell inhibiting polyspermy and protects early embryo
What is the blastoceal?
Fluid-filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity and then water following osmotically
What is hatching?
Degredation and weakening of the zona pellucida to allow the blastocyst to escape it
Why is hatching required?
To allow the blastocyst to implant into the uterus
How is hatching achieved?
Enzymatic digestion of the zona pellucida and cellular contractions to allow the blastocyst to escape the seal
What does the morula split into? Describe what each component contributes to
Inner cell mass which contributes to embryonic cells
Trophectoderm which contributes to extra embryonic cells
What is formed when trophoblast cells fuse
Syncitiotrophoblast
What ability does the syncitiotrophoblast have?
It can invade and destroy local maternal cells in the endometrium, the destruction of capillaries allows the embryo to be bathed in blood which is an important interface
What individual cells are a source of syncitiotrophoblast cells?
Cytotrophoblasts
What does the inner cell mass separate into and what are the roles of these cells?
Epiblast from which the foetal tissues will be derived
Hypoblast which will form the yolk sac
What is the yolk sac needed for
Haematopoiesis and gut development
What does the trophoblast give rise to?
Cytotrophoblast
Syncitiotrophoblast
When does bi laminar embryonic disc formation occur?
After day 12
What is the final stage before gastrulation?
Bi laminar embryonic disc formation
Where particularly do changes occur in bi laminar embryonic disc formation?
Epiblast and hypoblast
What are the 2 layers of a bi laminar embryonic disc made of?
Epiblast and hypoblast
What secretes hCG and when?
Syncitiotrophoblast after 12 days
What does the epiblast give rise to?
The epiblast and the amnion
What is the role of the amnion cells?
They contribute to the extra-embryonic membranes
Define gastrulation
The process by which the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc
When does gastrulation occur?
3rd week- day 15
Describe the primitive streak and what it defines
A thickened structure that forms along a midline of the epiblast near the caudal end of the bilaminar embryonic disc
Defines the cranial end (towards the head) and caudal end (towards the feet) as well as the left and right sides of the embryo
When does the primitive streak form?
15 days after fertilisation
What is the primitive pit?
An expansion of the primitive streak at the cranial end creates a primitive node that contains a circular depression called the primitive pit
What is formed by the continuation of the primitive pit?
Primitive groove
What is invagination in relation to gastrulation?
Cells migrate towards primitive streak, detach from the epiblast and migrate into the internal part of the embryo
What cells form the endoderm?
The first cells to carry out invagination and invade the hypoblast
At what day has much of the hypoblast been invaded and replaced?
Day 16
What cells is the mesoderm formed by?
The remaining cells in the space between the ectoderm and definitive endoderm
What is the definitive endoderm?
Layer that the hypoblast cells are completely replaced by
What cells is the ectoderm formed by?
Remaining cells of the epiblast
What 3 layers are formed in gastrulation (from exterior to interior)?
Ectoderm
Mesoderm
Endoderm
When do epiblast cells stop migrating to the primitive streak?
After the mesoderm and ectoderm are formed
What does the epiblast give rise to?
Ectoderm
Mesoderm
Endoderm
What does the endoderm give rise to?
GI tract Liver Pancreas Lung Thyroid
What does the ectoderm give rise to?
CNS and neural crest
Skin epithelia
Tooth enamel
What does the mesoderm give rise to?
Blood (endothelial cells, RBCs, WBCs) Muscle (smooth, skeletal and cardiac) Gonads Kidneys Adrenal cortex Bone Cartilage
What day does notochord formation occur?
Day 13
What is the role of the notochord?
Acts as a key organizing center for neurulation and mesoderm development
What type of cells form notochord?
Cartilage like cells
What is neurulation?
Formation of the neural tube and CNS
Describe the process of neurulation briefly
Notochord signals direct the neural plate ectoderm to invaginate forming a neural groove
This creates two ridges
Neural folds move together over neural groove and ultimately fuse, forming a hollow tube
What is the key signalling structure in neurulation?
The notochord
Which end of the neural tube closes first?
The head end closes first then the tail end
What can happen once the ends of the neural tubes close
The brain structures can be formed
What are the main disorders that occur if the neural tubes don’t close
Anencephaly
Spina bifida
What is anencephaly?
A disorder that occurs when the neural tube doesn’t close at the head end
What is spina bifida?
A disorder that occurs when the neural tube doesn’t close at the tail end, it has varying severity
Where are neural crest derived from?
From the ectoderm
What are some characteristics of neural crest cells?
They are plastic and migrate extensively during development
What are some defects that arise due to neural crest migration?
Pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut
What do somites arise from and how?
From paired blocks of paraxial mesoderm flanking the neural tube and notochord
Blocks of paraxial mesoderm condense and bud off in somite pairs
At what end does somitogenesis occur?
Head end
In humans what rate does budding of somites occur?
1 pair every 90 mins
In humans how many pairs of somites are there?
44 pairs
What 2 types of tissue do somites form?
Sclerotome
Dermomyotome
What does the sclerotome give rise to?
Vertebrae and rib cage
What does the dermomyotome split into?
Dermatome and myotome
What does the dermatome give rise to?
Dermis of the skin, some fat and connective tissues of neck and trunk
What does the myotome give rise to?
Muscles of the embryo
What 2 types of folding allow the formation of the primitive gut?
Ventral folding: where the head and tail ends curl together
Lateral folding: where the two sides of the embryo roll
What are the 3 parts of the gut which develop from the primitive gut?
Foregut
Midgut
Hindgut
When is foetal heartbeat detectable?
Around 6 weeks
What does the heart arise from?
Tube of mesoderm
What gene is specific to the Y chromosome?
SRY gene
What does the SRY gene do?
Directs gonadal cells to become sertoli cells, triggering testis development, leydig cell formation and testosterone production
What does absence of the SRY gene do?
Leads to gonadal cells adopting a granulosa cell fate and ovary development
What reinforcing gene is needed for ovary development etc?
FOXL2
