Embryology

Question 1

Embryo

Answer 1

typically refers to product of fertilisation, until early gestation (e.g. some organogenesis)

Question 2

Fetus

Answer 2

typically from early gestation onwards, recognisable as a species

Question 3

Conceptus

Answer 3

used at all developmental stages to indicate both the offspring and its placenta

Question 4

Pre- implantation embryo development

Answer 4

After fertilisation, a zygote develops into a blastocyst prior to implantation (attachment) into the uterus

Question 5

The first cleavage – 2 cell stage
Time, level of possible differentiation

Answer 5

~24 hours post fertilization, the first cleavage (cell division) occurs, forming a 2 cell embryo (2 daughter cells)
Each daughter cell is a blastomere, and at this stage is totipotent – it can give rise to a individual
Identical twins are formed by spontaneous splitting of embryos between 2-8 cell stages

Question 6

4 cell, 8 cell and morula stages
Timing for each of the 3 stages.
When does embryonic genome activation occur

Answer 6

Ongoing cleavage produce 4 and 8 cell stage embryos at 36-48h and 48-72 h post fertilisation (species dependent)
After 8 cell stage, blastomere # becomes hard to count – now a morula (~4-7 days post fertilisation)

Division are all occurring within the ZP – overall size is not changing, original cytoplasm being divided into smaller cells
At 8-16 cell stage = embryonic genome activation

Question 7

The blastocyst

Answer 7

Formation of tight and gap junctions between blastomeres in the morula creates 2 differentiated cell populations: (form blastocyst)
- inner cell mass (ICM) ~ joined by gap junctions = future embryo
- trophoblast (aka trophectoderm) ~ joined by tight junctions = future placenta

Question 8

Post-hatching embryo development, different species + which part is growing

Answer 8

Highly species dependent
- rodents, primates = implant soon after hatching, minimal growth
- i.e. blast hatches out of zona and implants in uterus straight away
- cow, ewe, sow = significant growth/elongation (25-80cm) prior to implantation
(noodle)
- mare, bitch, queen = significant growth, but remains spherical prior to
implantation
This extreme growth is driven by expansion of extra-embryonic tissues (i.e. future placenta)

Question 9

Uterine support for early embryo (list 5 of ingredients) + what is it driven by?

Answer 9

Prior to implantation (and even for a little while afterwards), embryo development is supported by uterine histotroph (aka uterine milk)
Secretions from endometrial glands,
rich in:
- growth factors
- nutrients (aa, lipid, carb, vit)
- cytokines & hormones
- ions
- enzymes
Histotroph driven by Progesterone (pregnancy hormone)

Question 10

The quiescent uterus

Answer 10

Prior to implantation, myometrial contraction moves embryos around to ensure adequate spacing (most relevant in polytocous species)
- Important in animals with more than 1 foetus
As pregnancy progresses, rising P4 blunts response to oxytocin – dampens down uterine motility (myometrial contraction)

Question 11

You want to collect blastocysts from superovulated, inseminated ewes for an embryo transfer program.
How long after ovulation (fertilisation) would you flush, and where would you find embryos

Answer 11

Not B because looking for blastocyst, not elongated conceptus (day 15 is elongated conceptus)
Not D because would find 2 cell embryo
Is E.

Question 12

Gastrulation

Answer 12

Prior to implantation, embryos start the next stage of embryonic development –
gastrulation
1. Inner cell mass becomes a bilaminar embryonic disc, with pluripotent epiblast
and hypoblast layers
2. The primitive streak forms in epiblast (establish cranial/caudal axis)
3. Three germ layers from (tripoblastic):
Ectoderm, mesoderm, endoderm

Question 13

Germ layers

Answer 13

Cells in the 3 germ layers (aka primary cell layers) give rise to all body tissues during organogenesis

Question 14

Cell potency

Answer 14

Totipotent cells (e.g. blastomeres in a 4 cell embryo) = can create all tissues, including the placenta
Pluripotent cells (e.g. inner cell mass in a blastocyst) = can create all tissues, except the placenta
Multipotent cells (e.g. neural stem cells) = can create multiple related cell types, but not other tissues

Question 15

Embryo Implantation (attachment)

Answer 15

Eventually, the embryo will implant into the uterine endometrium (attachment via trophoblast cells)
Implantation is:
- non invasive in cow, ewe, sow, mare
- invasive in the bitch, queen, rodents and primates
Once attached, the placenta will develop from the trophoblast and support embryo growth

Question 16

The window of receptivity

Answer 16

Implantation is a complex molecular event – requires the developmental embryo and endometrium to be in synch
- the endometrium is only receptive to implantation for a short period – the
window of receptivity (implantation)
If doing an embryo transfer program, need to synchronise estrous in donors and synchronise estrous in our recipients.
Note: FSH stimulates superovulation

Question 17

Species timing of implantation

Answer 17

Remember: this is time since fertilisation.

Question 18