Early Fetal Development

Question 1

What are the 3 ways to measure embryo-fetal development time?

Answer 1

Embryo-fetal developmental progression can be measured in different ways:​

1. Fertilization age (conceptual age) = measured from the time of fertilisation (assumed to be +1 day from last ovulation)
2. Gestational age: calculated from start of last period (day 0), and ovulation happens 14 days after this
3. Carnegie stage: based on presence or absence of certain embryo features ​

Question 2

Why is fertilisation age not very practical?

Answer 2

Usually a period of time has elapsed after fertilisation so days/ weeks post fertilisation, this is v difficult to know in practice ​

Variaiblity in time between intercourse and fertilisation in natural conception

Can be inferred if we know time of ovulation

Only can know exact time if using IVF

Question 3

What is gestational age?

Answer 3

Calculated from the time of the beginning of the last menstrual period (LMP)

Determined by fertilization date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size charts

Question 4

What is carnegie stage?

Answer 4

Based on embryo features rather than time

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

Question 5

What are the 3 stages of pregnancy?

Answer 5

Embryogenic stage
Embryonic stage
Fetal stage

Question 6

What is the Embryogenic Stage?

Answer 6

14-16 days post-fertilization - part of first trimester

Form early embryo from fertilised oocyte and formation of 2 cell types: ​

Pluripotent embryonic cells (contribute to organs of fetus)​

Extraembryonic cells (contribute to the support structures e.g. placenta)​

Question 7

What is the Embryonic Stage?

Answer 7

16-50 days post fertilisation - part of first trimester

Establishment of germ layers, differentiation of tissue types, and body plan​ - major organ systems are specified

Question 8

What is the Fetal Stage?

Answer 8

50-270 days post-fertilisation (8-38 weeks) - so transition from embryo to fetus occurs roughly at end of first trimester​, and fetal stage makes up the 2nd and 3rd trimesters

Major organ systems now present but these are now migrating to their final location

Extensive growth and acquisition of fetal viability​ - ability of foetus to survive outside the womb
​

Question 9

What is the developmental trajectory for the first few days of life?

Answer 9

Ovulated oocyte after fertilisation = zygote​

Zygote undergoes mitotic division (cleavage) = 2 cell embryo to 4 cell embryo to 8 cell embryo (these are cleavage stage embryos)​

8 cell embryo proceeds with further mitotic divisions to form a morula (16 cell)​

Morula forms an blastocyte (around day 5)​

This developmental trajectory is happening as the oocyte/ early embryo is travelling along the fallopian tube and into the uterus to be implanted​

The zona pellucida (hard, protein shell) that surrounds the oocyte at ovulation is present at all these stages so all these mitotic divisions are happening within the constriction of the ZP​

Question 10

What is the first major developmental event?

Answer 10

Maternal-to-Zygotic transition

Question 11

What is the Maternal-to-Zygotic transition?

Answer 11

Occurs at the 4-8 cell stage

We start to get transcription of embryonic genes and lose the reliance on maternal mRNA and proteins​ - i.e. embryotic genes take over and begin transcription

Embryo increases protein synthesis and organelle (mitochondria, Golgi) maturation​

​

Question 12

How does the cell divide prior to Maternal-to-Zygotic transition?

Answer 12

No genes are transcribes

Divisions dependent on maternal mRNA and proteins

Question 13

What is the second major developmental event?

Answer 13

Compaction

Question 14

What happens during Compaction?

Answer 14

Early events (compaction and blastocyst formation) separate the embryo into embryonic (inner cell mass) and extra-embryonic cells (trophectoderm)​

Compaction occurs at the 8 cell stage

Gives us our first 2 cell lineages (2 distinct cell types):​

1. Outer cells: tightly bound wedge-shaped outer cells (green)​

As outer cells become pressed against zona they change from spherical to wedge-shaped.

2. Inner cells: shielded from outside (pink)​

Outer cells become polarised and connect to each other through tight gap junctions and desmosomes = to form barrier to diffusion between cells inside the embryo and the outside of the embryo

Question 15

What is the third major developmental stage?

Answer 15

Blastocyte formation stage - establishes the 2 cell types

Question 16

How do the 2 types of cells formed in the Compaction stage develop further in the Blastocyte formation stage?

Answer 16

These 2 cells types develop further:

Outer cells form outer shell of blastocysts

Inner cells clumping together at one end and there is formation of the blastocoel cavity

Once this happens the morula is a blastocyst

Question 17

What is the zona pellucida and why is it important?

Answer 17

Hard protein shell inhibiting polyspermy (multiple sperm fertilisation) and protecting the early embryo

Question 18

What is the purpose of the inner cell mass at the blastocyte formation stage?

Answer 18

Gives rise to the pluripotent embryonic cells that will contribute to the final organism

Question 19

What is the purpose of the outer cell mass at the blastocyte formation stage?

Answer 19

AKA the Trophoectoderm/ trophoblast:
Give rise to extra-embryonic cells that contribute to the extraembryonic structures that support development​ e.g. placenta

Question 20

What is the formation of the Blastocoel at the blastocyte formation stage?

Answer 20

Fluid-filled cavity formed osmotically

Trophoblasts pump Na+ ions into centre of the embryo, and water follows this osmotically to form large fluid-filled space in middle​

Question 21

What is hatching and why is it important?

Answer 21

Escape of the blastocyst from the zona pellucida shell - around day 5-6​
Essential for further development and implantation (into endometrium)

Question 22

How does hatching occur?

Answer 22

Blastocyst secretes enzymes = enzymatic digestion

And cellular contractions of the embryo

Both contribute to weaken zona pellucida

Question 23

Once the embryo has implanted into the endometrium, it undergoes peri-implantation events.

What are the peri-implantation events?

Answer 23

Further differentiation of the embryo - 7-9 days post fertilisation

1. Trophectoderm lineage separates
2. Inner cell mass separates

Question 24

What does the trophectoderm lineage separate to?

Answer 24

Trophectoderm lineage separates into the syncitiotrophoblast and the cytotrophoblast

The trophoblast cells fuse to form syncitiotrophoblast, which is invasive - invades uterine endometrium and starts to degrade endometrium cells to breakdown capillaries = allows syncitiotrophoblasts to bathe in maternal blood

This creates interface between embryo and maternal blood supply ​

Cytotrophoblast cells remain individual cells and contrinue to divide (proliferate) to add cells to the syncitiotrophoblast cells

​

Question 25

What does the inner cell mass separate to?

Answer 25

Epiblast and hypoblast Epiblast = from which the fetal tissues and organs will be derived Hypoblast = which will form the yolk sac (extraembryonic structure important in gut development + early haematopoesis), this lines the under-side of the epiblast and faces the cavity​ ​ ​

Question 26

What is the final developmental stage?

Answer 26

Occurs around day 12 Formation of the bilaminar (two-layer) embryonic disc is the final stage before gastrulation At this stage:​ Syncitiotrophoblast continues to expand into the endometrium to firmly implant embryo in endometrium and begin secreting hCG​ ​ Some of the epiblast cells become separated from the main block of epiblasts by the formation of a new cavity – the amniotic cavity. The epiblast cells at the top of the amniotic cavity give rise to the amnion and these amnion cells will contribute to the extra-embryonic membranes The epiblast cells below the amniotic cavity will be the ones that give rise to the fetal tissue structures and organs There is a layer of hypoblast below the epiblasts which faces the blastocoel, together they form a bilaminar embryonic disc At this stage it is ready for gastrulation​ ​

Question 27

What forms the basis of pregnancy testing?

Answer 27

Detection of beta hCG subunit in blood/urine is basis of pregnancy testing

Question 28

What is gastrulation?

Answer 28

The process whereby the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc: Ectoderm (upper) Mesoderm Definitive endoderm

Question 29

What happens during the third week of development?

Answer 29

The bilaminar embryonic disc differentiaties to establish three primary germ layers = this is called gastrulation

Question 30

What happens during the primitive streak formation in gastrulation? What is the purpose of the primitive streak?

Answer 30

Occurs around 15 days after fertilisation A thickened structure forms along the midline of the epiblasts on the upper surface called the primitive streak from the caudal end of the bilaminar embryonic disc Formation of the primitive streak defines head / cranial and tail / caudal ends, and the left-right axes of embryo​

Question 31

What occurs to the primitive streak at the cranial end of the embryonic disc?

Answer 31

Primitive streak expands to form a primitive node | This contains a circular depression - primitive pit

Question 32

What happens to the primitive pit?

Answer 32

This depression continues along the midline of the epiblasts​, towards the caudal end Forms the primitive groove

Question 33

So what makes up the primitive streak?

Answer 33

The primitive groove + primitive pit + primitive node = primitive streak​

Question 34

What is invagination?

Answer 34

Cells of the epiblast migrate inwards towards the primitive streak and dive into the primitive groove They detach from the epiblast And lip beneath it into the interior of the embryo

Question 35

What do the first invaginated cells do?

Answer 35

The first cells to invaginate through the primitive groove invade the hypoblasts and displace its cells and eventually the hypoblast cells are completely displaced by a proximal cell layer (the definitive endoderm)​

Question 36

What happens by day 16?

Answer 36

Majority of hypoblast has been replaced Remaining cells of the epiblast are referred to as the ectoderm

Question 37

What does the ectoderm form?

Answer 37

Most exterior, distal layer

Question 38

What do the other invaginated cells do?

Answer 38

Remain in the space between the ectoderm and definitive endoderm - form a germ layer - the mesoderm

Question 39

Recap what happens during gastrulation for the formation of the 3 germ layers: Ectoderm (upper) Mesoderm Definitive endoderm

Answer 39

Thickened structure forms along the midline of the epiblasts on the upper surface from the caudal end of the bilaminar embryonic disc = primitive streak Primitive streak expands, forms primitive node Primitive node has a depression = primitive pit Depression continues along the midline of the epiblasts​, towards the caudal end = primitive groove The epiblast cells start to migrate towards primitive groove and dive into it​ The epiblast cells that slip through the groove differentiate to form different layers:​ 1. First vaginated cells = definitive embryonic endoderm cells which replace the ventral hypoblast layers​ 2. Form embryonic mesoderm layer​ The epiblast cells that don’t dive into the groove are called the embryonic ectoderm layer​

Question 40

What are the properties of the 3 germ layers?

Answer 40

These germ layers are multipotent - they can differentiate into any tissue or organ​

Question 41

What organs are formed by the ectoderm?

Answer 41

CNS and neural tract Skin epithelia Tooth enamel

Question 42

What organs are formed by the mesoderm?

Answer 42

Blood (endothelial cells, red and white blood cells) Muscle (smooth, skeletal and cardiac) Gonads, kidneys and adrenal cortex Bone, cartilage

Question 43

What organs are formed by the endoderm?

Answer 43

GI tract Liver, pancreas Lung Thyroid

Question 44

What is the first major event after gastrulation?

Answer 44

Notochord formation at day 13 A rod- like tube structure made of cartilage-like mesoderm cells forms from end of primitive streak and grows forward towards the cranial end of the embryo, along the midline It grows underneath the ectoderm along the embryo midline.

Question 45

What is the function of the notochord?

Answer 45

Acts as a key organizing centre for neurulation and mesoderm development Releases growth factor signals

Question 46

What is neurulation?

Answer 46

Process of CNS formation Neural plate is an area of thickened ectoderm that sits on top of the embryo. Signals from the notochord beneath the ectoderm will move up through the embryo to direct the neural plate to form the neural tube​ Neurulation refers to the folding process in which the neural plate bends up and later fuses to form the hollow tube that will eventually differentiate into the brain and the spinal cord of the central nervous

Question 47

What does the neural plate do?

Answer 47

Around day 15​ In order to form CNS, the notochord signals direct the neural plate ectoderm to invaginate forming neural groove​ This creates two ridges (neural folds) running along the cranio-caudal axis​ Neural crest cells (Ectoderm) specified in neural folds​ ​ ​

Question 48

What happens a few days later, after neural fold formation?

Answer 48

Neural folds move towards each other (up and across) over neural groove​ Ultimately the neural folds fuse, forming a hollow tube (neural tube) running across the whole embryo​ Neural tube overlaid with epidermis (ectoderm)​ Neural crest cells from folds will migrate away​ - to populate different tissues

Question 49

What happens to the neural tube?

Answer 49

Neural tube overlaid with epidermis (ectoderm derived) Initially open at each end as it runs along the length of the embryo, closes at later days Must close for formation of brain structures

Question 50

When does the neural tube close at the head end? When does the neural tube close at the tail end?

Answer 50

Day 23 Day 27

Question 51

What conditions are caused by failure of neural tube closure?

Answer 51

Failure of neural tube closure = common developmental defect 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)​

Question 52

What are the features of neural crest cells?

Answer 52

Endoderm derived Highly plastic and migrate extensively during development Give rise to a variety of cell types

Question 53

How are neural crest cells classified?

Answer 53

By where they end up in the embryo

Question 54

What are the different neural crest cells?

Answer 54

Cranial NC = cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage Cardiac NC = aortic arch/pulmonary artery septum, large arteries wall musculoconnective tissue TrunkNC = dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes Vagral and Sacral NC = dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes

Question 55

What do defects of neural crest migration/specification lead to?

Answer 55

``` Diverse birth defects including: Pigmentation disorders Deafness Cardiac defects Facial defects Failure to innervate the gut ```

Question 56

What is the next key stage after neuralation?

Answer 56

Somitogenesis

Question 57

What happens during somitogenesis?

Answer 57

Created formation of blocks of medoderm along the axis of the embryo - called somites On either side of neural tube we have blocks of specialised mesoderm cells called paraxial mesoderm. Blocks of paraxial mesoderm condense and bud off in somite pairs starting from cranial end and progressing to the caudal end. Rate of ‘budding’ or appearance of somite pairs is species-specific, as is the number of pairs so humans form new pair per 90 minutes and this leads to around 44 pairs in the embryo​

Question 58

What do somite pairs look like on embryo imaging?

Answer 58

Chevron structures | Blocks lining up along the back of the embryo

Question 59

What 2 types of embryonic tissue do the somites initially give rise to?

Answer 59

1. Sclerotome = vertebrae and rib cartilage​ 2. Dermomyotome = sub-divides to form:​ Dermatome = dermis of the skin, some fat and connective tissues of neck and trunk​ Myotome= muscles of the embryo​

Question 60

From what does the primitive gut arise?

Answer 60

Primitive gut arises around Day 16 from 2 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 These 2 types of embryonic folding pinch off part of the yolk sac to form the primitive gut

Question 61

What subsequently happens to the primitive gut?

Answer 61

Patterned into foregut, midgut and hindgut Foregut = esophagus, stomach, upper duodenum, liver, gallbladder, pancreas Midgut = lower duodenum and remainder of small intestine, ascending colon and first two-thirds of transverse colon Hindgut = last third of the transverse colon, descending colon, rectum and upper anal canal

Question 62

What are the features of heart development?

Answer 62

Begins as tube of mesoderm around day 19 By day 22, it is beating and pumping blood Fetal heartbeat detectable from ~6 weeks gestational age

Question 63

What are the features of lung development?

Answer 63

Arise from the lung bud (a structure that sits adjacent to the foregut), an endodermal structure, beginning in the 4th week of development Lung bud splits into two at the end of the 4th week, and progressively branches through development.

Question 64

What are the features of lung development?

Answer 64

Arise from the lung bud, an endodermal structure that sits adjacent to the foregut, beginning in the 4th week of development Lung bud splits into two at the end of the 4th week, and progressively branches through development (gives rise to the bronchi and bronchioles)

Question 65

What do the gonads form from?

Answer 65

Gonads are mesodermal derived structures Initially they are bipotential (i.e. not committed to the testes or ovaries) = 2 thickenings of mesoderm towards the caudal end known as the gonadal ridges In 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 therefore masculinisation of embryos​ In XX embryos, absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2 (transcription factors) essential for ovary development​ ​