Lecture 10

What is the Embryonic Period?

The time from fertilization to organogenesis, including blastulation and gastrulation; embryos are generally not identifiable as members of a specific species.

What is the Foetal Period?

The time between organogenesis and term, when a fetus can be recognized as a member of a species, ending with expulsion from the uterus.

What is a Conceptus?

The complete product of conception, encompassing everything developed from the zygote, including the embryo proper and extra-embryonic membranes.

What is the ootid stage

A stage where there is both female and male pronuclei and there is abundant cytoplasm present

What defines the zygote?

The zygote is formed when the male and female pronuclei fuse within the ootid.

What is embryonic cleavage?

Subsequent cell divisions within the zona pellucida, partitioning cytoplasm into smaller subunits (blastomeres) without increasing overall embryo size.

How many cells does the first cleavage produce?

The first cleavage produces a 2-cell embryo, with each cell termed a blastomere.

Up to what stage are cells considered totipotent?

Cells up to the 8-cell stage are considered totipotent, having the potential to develop into a complete offspring.

What characterizes the morula stage?

The morula defines the embryo after it reaches 16 cells of blastomeres.

What are trophoblasts?

Compacted outer cells of the morula that develop tight junctions and will eventually develop into the chorion, the foetal component of the placenta.

What is the blastocoel?

A fluid-filled cavity created as outer cells pump sodium into the intercellular space, which then attracts water osmotically.

What are the inner cells in a morula

Inner cells develop gap junctions, facilitating direct cell-to-cell communication.

Will give rise to the embryo proper

What happens when a distinct blastocoel is formed?

The embryo is then referred to as a blastocyst.

What is involved in the hatching of the blastocyst?

Hatching occurs when internal pressure increases due to fluid accumulation, trophoblast cells secrete proteolytic enzymes to degrade the zona pellucida, leading to rupture. The blastocyst itself undergoes rhythmic contractions and relaxations, generating pressure pulses

What is the growth rate of bovine blastocysts post-hatching?

A bovine blastocyst grows from 3 mm to 25 cm within just four days, from day 13 to day 17 post-ovulation.

How do pig embryos grow post-hatching of the blastocyst?

Pig embryos can grow from 2 mm spheres to over 20 cm threads within 24 to 48 hours, with a growth rate of 4 to 8 mm/hour.

What is the mare growth rate post-hatching of the blastocysts

The equine embryo remains spherical and does not exhibit such dramatic elongation because it is encased in a protective glycoprotein capsule.

What is the Yolk Sac?

An early formed structure that develops beneath the ICM derived from primitive endoderm, creating an inner lining to the trophoblast and later regresses in mammals.

How does the amnion develop in the hatched blastocyst

Mesoderm between the ICM and endoderm grows outwards, pushing against the trophectoderm, leading to the formation of "wings" known as amniotic folds. These folds fuse above the embryo, enclosing the embryo within the amniotic cavity. The fusion site creates the chorioamniotic raphe/mesamnion

In which animals does the raphe/mesoamnion persist. What is the significance of this?

Cattle and pigs. Persistence of the mesamnion in cows and pigs necessitates the rupture of the amnion during parturition

What does the amnion provide for the embryo?

The amnion is filled with fluid, providing mechanical protection and acting as an anti-adhesive to prevent embryonic tissues from sticking together.

When can you detect the amniotic slip in a cow

Days 30-45

What is the allantois

AN outgrowth of the embryonic hindgut and is enveloped by mesoderm. It fuses with the chorion

What is the allantochorion/chorioallantois

The fusion of the allantois and chorion, a highly vascular structure which contributes the umbilical vessels to the placenta.

How does the allantois function?

The allantois collects embryonic waste products

How does the chorion grow/form

Formed by the fusion of mesoderm with the trophectoderm cells and grows to surround the conceptus entirely

What is the purpose of the chorion?

The chorion surrounds the entire conceptus and fuses with the allantois to form the allantochorion, the fetal part of the placenta.

What is the primary goal of maternal recognition of pregnancy?

To prevent prostaglandin F2 alpha from inducing luteolysis and maintainining high progesterone levels essential for pregnancy.

When does maternal recognition typically occur?

Between day 11 and day 16 after ovulation, with species-specific variations.

How does luteolysis occur in non-pregnant ruminants?

Oxytocin from the CL stimulates PGF2 alpha production from the endometrium, which travels via the utero-ovairan vein to the ovarian artery leading to the lysis of the CL.

What prevents luteolysis in pregnant ruminants?

The blastocyst secretes interferon-tau, which binds to the endometrium, preventing the synthesis of oxytocin receptors. This prevents the endometrium from secreting PGF2 alpha.

What are the 2 main functions of IFN-tau

• Binds to endometrium to prevent synthesis of oxyctocin receptors

• Stimulates uterine gland secretion for pre-attached embryo

What is a critical requirement for pregnancy in ruminants?

Pregnancy must establish ipsilaterally to the CL to prevent PGF2 alpha from lysing it.

How do pregnant pigs prevent luteolysis?

Blastocysts produce oestradiol which reroutes PGF2 alpha into the endometrial lumen instead of the bloodstream.

How does luteolysis normally occur in a non-pregnant pig

oxytocin (from CLs and pituitary) promotes endometrial PGF2 alpha production. This PGF2 alpha diffuses into capillaries, drains to the uterine vein, and is transported to the ovaries, causing luteolysis

How is luteolysis prevented in a pig

The blastocysts produce oestradiol, which causes PGF2 alpha to be rerouted into the endometrial lumen. It is "dumped" into the uterus rather than reaching the bloodstream and subsequently the CLs

What is essential for sufficient rerouting of PGF2 alpha in pigs?

At least 2 blastocysts must be present in each uterine horn = total of 4 conceptuses

What is unique about the equine blastocyst?

It remains spherical and does not elongate due to a tough glycoprotein capsule until later in pregnancy, preventing it from rapid growth

How do mares prevent luteolysis?

The conceptus must migrate throughout the entire uterus, making contact with the entire endometrial surface between day 12-14, 12-14 times a day.

How does pregnancy recognition differ in dogs? What is the significance of this?

There is no maternal recognition of pregnancy in bitches, as their progesterone level remains similar whether they are pregnant or not. Every bitch either becomes pregnant or pseudopregnant after ovulation.

How do cats differ in terms of maternal recognition of pregnancy?

Queens do not require a signal from the conceptus for CL maintenance if they have bred enough times to induce ovulation as the CLs remain intact for the same duration in both pregnant and non-pregnant queens