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Gametes & Embryos > Embryo Cleavage, Blastulation, Implantation, & Hatching > Flashcards

Embryo Cleavage, Blastulation, Implantation, & Hatching Flashcards

1
Q

Where in the oviduct does fertilization take place?

A

Ampulla

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2
Q

How long do human embryos take before “hatching” in the uterus?

A

5 days

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3
Q

What happens to the zygote during its journey to the uterus?

A
  1. It undergoes mitotic divisions called cleavage division.

2. Note: daughter cells (blastomeres) don’t grow yet.

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4
Q

What is “compaction” and when does it occur?

A
  1. Embryo blastomeres split so that only few face external environment.
  2. Occurs at 8-16 cell stage.
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5
Q

What are the two cellular “lineages” that are initiated after compaction?

A
  1. “Trophectoderm” or “trophoblast” which forms a part of the placenta
  2. “Inner cell mass” which forms embryo and some extra embryonic membranes.
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6
Q

What must happen to the endometrium in order for implantation to successfully occur and for how long must it happen?

A
  1. It must be “primed” by ovarian steroids.
  2. Usually requires several days of this.
  3. Note: knowledge of the timing and synchronizing of this is important for successful in vitro fertilization.
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7
Q

Why is it possible for an embryo to have an odd number of blastomeres despite mitotic division producing two cells each time?

A

Because the human embryo doesn’t cleave synchronously so different cells cleave at different times which can lead to an odd number of blastomeres at certain times.

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8
Q

Before the blastocyst stage of the embryo, what kind of cleavage occurs and how long does it last?

A
  1. “Slow cleavage” without G1 or G2 phase occurs.

2. Takes 12-24 hours between each cell division.

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9
Q

When does a cleaving embryo’s cell division cycle begin to represent the “normal” cycle seen in adults?

A

At blastocyst stage: includes G1, S, G2, and M.

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10
Q

In the absence of blastomere growth during embryo cleavage, what happens?

A

Blastomeres cleave and are thus continually reduced in size as the embryo remains ~100microns in diameter, until the blastomeres finally reach the size of normal somatic cells.

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11
Q

What are four important organelles found in the blastomere cytoplasm?

A
  1. Maternal mRNA
  2. Ribosomes
  3. Mitochondria
  4. Golgi apparatus
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12
Q

The early cytoplasm of an embryo is derived from and dependent on what?

A

Maternal inheritance - products of oogenesis (new mRNA are not synthesized yet); NOTE: any deficiency or flaw in oocyte maturation causes embryo development and pregnancy loss.

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13
Q

Describe the events and times during which the first few embryo cleavages occur.

A
  1. First cleavage begins (one day) after fertilization and ~30 hours after ovulation.
  2. Cell cycles of early mammalian embryos are 12-24 hours and are the slowest in the animal kingdom (fun fact?).
  3. Cleavage continues and by day 3 zygote reaches uterus..
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14
Q

Contrast between the position of early mammalian cleavage and that of invertebrates.

A
  1. Mammalian = ROTATIONAL cleavage: 1st cleavage is normal meridional (straight up and down) and second is where one blastomere divides equatorially and the other meridionally.
  2. Invertebrates and early vertebrates = RADIAL cleavage: cleavage planes oriented obliquely (at a slant/angle) to the polar axis of the oocyte.
  3. Note: Also, asynchronous divisions are characteristic only of early mammalian embryos.
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15
Q

When does the zygotic genome get “turned on” and the embryo cease to rely on maternal factors?

A
  1. Mice/Goats: 2-cell stage.

2. Humans: 4-8 cell stage when maternal material is degraded and zygotic genome kicks in.

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16
Q

After what cell stage do embryos move into the uterus?

A

8-cell stage.

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17
Q

In the early luteal phase, progesterone helps transfer the embryo into the uterus how?

A
  1. By relaxing the isthmus sphincter.
  2. By changing the oviductal and uterine musculature.
    Note: occurs around day 3-4.
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18
Q

What is the first sign of embryo differentiation?

A

Compaction.

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19
Q

How does compaction occur?

A
  1. Formerly loosely-associated blastomere cells at the 8-16 cell stage now develop extra cellular matrix that increases their adhesion to one another and form a compact ball of cells:
  2. Arrangement now stabilized by tight junctions at outside edges of blastomeres, sealing off the inside of the sphere.
  3. Gap junctions (connexins?) form between small blastomeres to let molecules pass in between.
  4. E cadherin (Ca2+ dependent) and other cell-adhesion proteins are expressed (E-cadherin antibodies will prevent compaction from occurring or cause decompaction if it has already occurred).
  5. Water moves osmotically (K+/Na+/ATPase activity) into intercellular space, forming segmentation cavity (soon to be called the blastocoelic cavity).
20
Q

After compaction, what is an embryo referred to as?

A

A morula.

21
Q

What do the external blastomeres following compaction give rise to?

A

Trophectoderm.

22
Q

At what point are the inner cell mass and trophectoderm differentiated?

A

64-cell stage or 3.5 days.

23
Q

How many cells make up the original inner cell mass?

A

~15 cells

24
Q

What four things do the inner cell mass go on to form?

A
  1. Embryo
  2. Yolk sac
  3. Amnion
  4. Portion of the placenta
25
Q

How many cells make up the original trophectoderm?

A

~45-cells

26
Q

After compaction, what two growth factors further blastocoele expansion?

A

EGF and TGF-alpha

27
Q

How does the inner cell mass support the trophectoderm?

A

By secreting proteins like FGF4.

28
Q

At what point is the embryo called a blastocyst?

A

After 4.5 days (and after the inner cell mass and trophectoderm have formed).

29
Q

Contract differential gene expression in trophectoderm vs. inner cell mass after the blastocyst stage has formed.

A
  1. Trophectoderm = cells synthesize protein “eomesodermin,” which activates more proteins characteristic of trophoblast layer.
  2. Inner cell mass = cells retain expression of Oct4 & Foxd3 (two transcription factors associated with pluripotency).
  3. NOTE: It’s actually not that simple - species are different as well (OCT4 expressed in both human lineages but only inner cell mass in mouse).
30
Q

What is “hatching” and when does it occur?

A
  1. When blastocyst frees itself from zona pellucida (which makes it more sticky so it can implant in the embryo wall).
  2. ~ day 5
31
Q

What are two functions of the zona pellucida prior to hatching?

A
  1. Prevents blastomeres from falling apart before compaction.
  2. Prevent embryo from sticking to oviduct wall.
32
Q

Based on mouse studies, how does hatching occur?

A
  1. Small hole in fibrillar matrix of zona pellucid is lysed by trypsin (which is a protease found on the trophoblast cell membrane).
  2. The expanded blastocyst is squeezed out.
33
Q

After hatching, how big is the blastocyst that remains free in the uterine lumen and until when does it remain free-floating?

A
  1. 200-300 cells

2. ~ 6 days

34
Q

When does implantation begin?

A

~ day 6-7.

35
Q

What are four specific types of genes involved in embryo hatching?

A
  1. C-adhesion and migration molecules
  2. Epigenetic regulators
  3. Stress-response regulators
  4. Immunoresponse regulators
36
Q

What are five C-adhesion and migration molecules involved in hatching?

A
  1. E-cadherin
  2. Neuronal cell adhesion molecule
  3. Lectin
  4. Galactose binding
  5. Soluble 7h
37
Q

What are two epigenetic and stress-response regulators involved in hatching?

A
  1. DNMT1

2. SIN3 yeast homolog A

38
Q

What are four immunoresponse regulators involved in hatching?

A
  1. Interleukin-2-inducible T-cell kinase
  2. Interleukin-4-R
  3. Interferon-receptor 2
  4. Neurotrophin
39
Q

What five proteins and molecules are involved in the adhesion of the trophoblast to the extra cellular matrix of the uterine endometrium for implantation to occur?

A
  1. Collagen
  2. Laminin
  3. Fibronectin
  4. Hyaluronic acid
  5. Heparin sulfate receptors
40
Q

What on the trophoblast cells bind to what on the extra cellular matrix of the uterus for implantation to occur?

A
  1. Integrins (trophoblast)

2. Fibronectin (endometrium)

41
Q

What two hormones control the uterine endometrial cycle and subsequent receptivity of the uterine to the hatched embryo?

A

Estrogen and progesterone.

42
Q

What seven genes regulate estrogen and progesterone (for implantation to occurr)?

A
  1. HOXA-10
  2. Stromal cytokines
  3. Epidermal growth factor
  4. Beta-tumor growth factor
  5. Leukemia inhibitory factor
  6. Interleukin-1
  7. Insulin like growth factor
43
Q

Summarize the first 6-7 days following fertilization of an oocyte.

A

Day 0 = pronuclear stage (nearest infundibulum)
Day 1 = 2-cell stage (near ampulla?)
Day 2 = 4-cell stage (ampulla?)
Day 3 = 8-cell stage (isthmus?)
Day 4 = Morula after compaction ( ~16-cell stage in isthmus?)
Day 5 = Blastocyst (into uterus lumen?)
Day 6 = Hatching (into uterine endometrium)

44
Q

Ectopic pregnancies happen in which species?

A

Only humans.

45
Q

What (3) autocrine and paracrine factors are involved in the early growth of embryos?

A
  1. Insulin growth factors 1 & 2
  2. Epidermal growth factor
  3. Platelet-derived growth factor A
    Note: these were either derived from maternal factors or the embryo itself.

Gametes & Embryos (15 decks)

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