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Organisation of the body: embryology > Implantation > Flashcards

Implantation Flashcards

1
Q

Stages of fertilisation

A
  1. Capacitation takes place, where the surface of the acrosome is modified by removal of proteins and glycoproteins
  2. Ovum is released from corpus lute and is passed down the uterine tube by the fimbriae at the opening
  3. Spermatozoa binds to the zone pellucida surrounding the ovum
  4. acrosome reaction begins where the acrosome of the sperm breaks down and releases enzymes that dissolve the zone pellucida locally
  5. Cortical reaction takes place and the egg and sperm fuse
  6. meiosis 2 is triggered to continue
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2
Q

What prevents dispermy and polyspermy?

A

Cortical reaction- Enzymes released from the egg contain cortical granules which cause the binding proteins in the zone pelicuda to tighten, preventing more sperm from entering

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3
Q

Zygote definition

A

Fertilised oocyte- DNA of spermatozoon and oocyte contained in the zygote, despite not being in the nucleus

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4
Q

What happens next to the DNA?

A

Spermatozoon’s nucleus becomes the pro nucleus and aligns with the female pronucleus. Nuclear membranes are lost and the DNA is duplicated

DNA condenses into chromosomes and mitosis begins

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5
Q

How long does that process take?

A

18 hours

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6
Q

What is the next process?

A

Cleavage

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7
Q

How long after fertilisation does cleavage take place?

A

24 hours later

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8
Q

Explain cleavage process

A

Unicellular fertilised egg is transformed by consecutive mitotic divisions into a multicellular complex

no growth occurs, the general shape of the embryo does not change- embryo remains in zone pellucida

Subdivides to form many smaller cells called blastomeres

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9
Q

What does the cleavage process form?

A

A morula- formed of 16-32 blastomeres

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10
Q

How many days in does cleavage stop?

A

Day four

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11
Q

The type of cells + why?

A

Totipotent- cells of the morula will give rise to the embryo proper, its associated extra embryonic membranes but also the placenta and related structures

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12
Q

Explain the segregation of blastomeres

A

At the eight-cell stage, the cells begin to flatten and develop an inside-outside polarity that maximises cell contact.

Differential adhesion develops and the outer cell surfaces of the cells become convex and inner become concave.- process known as compaction

Some blastomeres segregate to the centre of the morula and others to the outside.

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13
Q

What are the different layers called?

A

Inner cell mass- centrally placed blastomeres

Trophoblast- blastomeres at the periphery

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14
Q

Function of inner cell mass

A

will differentiate to form the embryo proper, thus is referred to as the embryoblast

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15
Q

Function of trophoblast

A

will form the placenta

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16
Q

Evidence to show how this differentiation takes place

A

Oct4 and nanog are expressed uniformly in the blastomeres

As the inner cell mass and trophoblast form, the Oct4 and nanog expression is switched off in the trophoblast

Mouse embryos that ate Oct4 deficient fail to form the inner cell mass

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17
Q

How is the blastocyst cavity formed?

A
  1. trophoblast differentiates and assembles into an epithelium with adjacent cells adherent to each other, due to the deposition of E-cadherin.
  2. trophoblast cells express a sodium/potassium ATPase
  3. sodium pumped into the interior and water follows due to osmosis
  4. this forms a blastocoelic fluid
  5. as the hydrostatic pressure increases, the blastocyst cavity is formed within the morula
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18
Q

What is the blastocyst?

A

Embryoblast cells accumulate to one side of the blastocyst cavity- forming a compact mass

Trophoblast forms a thin epithelium around it

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19
Q

Two sides within the trophoblast

A

Side of blastocyst containing the inner cell mass is called the embryonic pole

opposite side called the abembryonic pole

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20
Q

What occurs next?

A

Implantation

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21
Q

Stages of implantation- when the blastocyst comes into contact with the uterus

A
  1. Morula reaches the uterus between days 3-4
  2. Day 5- blastocyst hatches from the clear zone pelluicda by enzymically boring a hole in it and squeezing out
  3. Blastocyst becomes tightly adherent to the uterine lining
  4. corpus luteum secretes progesterone which enables the endometrial storm to respond to the blastocyst’s presence
  5. Decidual reaction, where changes in the endometrial stroll cells cause the development of a secretory lining that produces various proteins, cytokines and growth factors- determines the invasiveness of the trophoblast cells
22
Q

What is hCG and function?

A

Human chorionic gonadotropin hCG- a hormone produced by the trophoblast cells on implantation.

Maintains the supply of progesterone

23
Q

What is an ectopic pregnancy?

A

When a blastocyst implants in the peritoneal cavity, surface of the ovary or within the oviduct- abnormal site of the uterus and survives.

Threatens the life of the mother, as the vasculature that forms is liable to rupture

24
Q

Stages of implantation- linked to trophoblast

A

Day 6

  1. Trophoblast at embryonic pole begins to proliferate
  2. Syncytiotrophoblast formed, where the proliferating cells lose their cell membrane and coalesce to form a mass of cytoplasm containing numerous dispersed nuclei
  3. trophoblast cells that line the blastocyst become the cut-trophoblast- retain their cell membranes
  4. syncytiotrophoblast increases in volume throughout the second week as cells detach from the proliferating cytotrophoblast
  5. cut-trophoblast secretes enzymes to break down extracellular matrix between the endometrial cells
  6. syncytiotrophoblast forms finger like processes that penetrate between the endometrial cells and pull the embryo into the uterine wall
25
Q

When is the embryo fully implanted?

A

between days 6-9

26
Q

What happens next to the syncytiotrophoblast?

A

Envelops the entire blastocyst apart from one region at the abembryonic pole

27
Q

What seals the hole?

A

Coagulation plug seals the hole where the blastocyst implanted

28
Q

Before implantation what does the embryoblast reorganise into?

A

Cells differentiate into two epithelial layers by day 8.

Upper layer- epiblast

lower layer- hypoblast- primitive endoderm.

29
Q

What causes them to become the bilaminar disc?

A

Basement membrane laid down between the two layered embryoblast

30
Q

What axis does this form?

A

Ventral (front) hypoblast- dorsal (back) epiblast- closest to endometrium

31
Q

What cavity is next to form and when?

A

Amniotic cavity- day 8

32
Q

how is the amniotic cavity formed?

A
  1. Fluid begins to collect between the cytotrophoblast cells and the epiblast cells
  2. A layer of epiblast cells expand towards the embryonic pole and differentiate into a thin membrane, separating the new cavity from the trophoblast.
33
Q

What is this thin membrane called?

A

amnion

34
Q

What is the next cavity to form?

A

Primary yolk sac

35
Q

Stages of primary yolk sac formation

A

Day 8
1. first wave of hypoblast cell migration. This forms Heuser’s membrane surrounding the yolk sac between the cytotrophoblast

  1. extra embryonic mesoderm forms, filling the remainder of the blastocyst cavity with loosely arranged cells
36
Q

Where is the extra embryonic mesoderm thought to originate from?

A

From the hypoblast/primary yolk sac. potentially from trophoblast too

37
Q

What cavity forms next + when + how?

A

By the 12th day, secondary yolk sac is formed.

  1. primary yolk sac displaced by the second wave of migrating hypoblast cells
  2. primary yolk sac then degraded
38
Q

What is the next cavity to be formed? + when + how?

A

Chorionic cavity- 12/13 days

  1. Extraembryonic mesoderm split into two layers.
  2. Chorionic cavity separates the embryo with its attached yolk sac and amniotic sac from the outer wall
39
Q

How is the connecting stalk formed + function?

A

Growth of extra embryonic mesoderm is faster than hypoblast and epiblast cells.

By day 13, the embryonic disc with its dorsal amniotic sac and ventral yolk sac become suspended in the chorionic cavity by a thick stalk

40
Q

Function of yolk sac

A

Major site of hematopoiesis and presence of primordial germ cells

41
Q

When does the yolk sac disappear?

A

4th week- rapidly overgrown by developing embryonic disc

42
Q

What forms the eventual umbilical cord?

A

extra embryonic mesoderm structure- the connecting stalk and the amniotic membrane

43
Q

In the early weeks of development, how does the embryo receive nutrients?

A

Simple diffusion

44
Q

Why does this become a problem + what it replaces it?

A

Embryo too big, therefore complete exchange does not happen.

Utero-placental circulation formed

45
Q

What is the utero placental circulation?

A

Fetal blood flowing through the placenta, which comes into close proximity to the embryo thus can efficiently exchange cases and metabolites by diffusion

46
Q

Formation of this circulation system stages

A
  1. Day 9 - vacuoles called trophoblastic lacunae form in the syncytiotrophoblast
  2. maternal capillaries then expand to form maternal sinusoids that rapidly anastomose within the lacunae
  3. Cytotrophoblast proliferates locally to form extensions that grow into the syncytiotrophoblasts, induced by the new underlying extra embryonic mesoderm
  4. Extensions grow into the blood filled lacunae, surrounded by a layer of syncytiotrophoblast
47
Q

What are the resulting structures called?

A

Primary chorionic stem villi

48
Q

Explain structures of the three chorionic stem villi

A

primary chorionic stem villi- extension of cytotrophoblast

secondary chorionic stem villi- extra embryonic mesoderm penetrates the centre of the cytotrophoblast

tertiary chorionic stem villi- blood vessels formed within extra embryonic mesoderm

49
Q

What layers must the nutrients diffuse through to reach the embryo?

A

endothelium of the villus capillaries, extra embryonic mesoderm, Syncytiotrophoblast, cytotrophoblast,

50
Q

What is the chorion?

A

outer wall of the blastocyst - formed by extra embryonic mesoderm and the two layers of trophoblast

51
Q

What is the placenta?

A

Temporary organ that connects the developing fetes via the umbilical cord to the uterine wall to allow nutrient uptake, thermoregulation, waste elimination and gas exchange via the mother’s blood

52
Q

What forms the placenta?

A

The cytotrophoblast and syncytiotrophoblast

Organisation of the body: embryology (11 decks)

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