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BIOC19 Animal Developmental Biology > Lecture 8 > Flashcards

Lecture 8 Flashcards

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1
Q

Where does fertilization occur in mammals?

A

Fertilization occurs in the AMPULLA

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2
Q

What is completed after sperm entry?

When does the first cleavage happen?

A

MEIOSIS is completed after sperm entry.

The first cleavage happens a day later.

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3
Q

What is cell cleavage?

A

Cell cleavage is the period of rapid cell division after fertilization.

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4
Q

In cell cleavage, the egg divides into many smaller cells, but does cell volume or size change?

A

No, in cell cleavage the egg divides into many smaller cells but cell volume and size DOES NOT change.

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5
Q

What makes cell cleavage in mammals unique?

A

Mammalia cell cleavage has a unique nature:

  • relatively slow (12-24h apart)
  • blastomeres organized in unique orientation
  • pattern of cleavage within a species doesn’t change (genetic control?)
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6
Q

In most species, what controls cell cleavage?

A

In most species, MATERNAL FACTORS control cell cleavage

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7
Q

Maternal factors control the following:

6 things

A

Ribosomes (protein translation)

DNA polymerases (DNA replication)

Lipids (formation of new cell membranes)

RNA (gene expression)

Structural proteins, enzymes, etc

ATP, nucleotides, amino acids, and other building blocks/metabolites

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8
Q

Once maternal factors are exhausted, what takes over?

What is this process called?

A

As maternal factors are exhausted, ZYGOTIC GENOME starts to take over

This is called MID-BLASTULA TRANSITION (MBT)

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9
Q

In mammals, when is the zygotic genome activated?

A

during the first cell cycle

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10
Q

True or False: In mammals, cleavage divisions are synchronous.

A

False

Not synchronous

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11
Q

Following MBT, what occurs?

A

Maternal to zygotic transition (MZT)

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12
Q

What happens during MZT?

A

MZT takes place as

the cell cycles lengthen

gap phases are introduced

and the time spent in S phase is increased

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13
Q

What is the subsequent transition that occurs alongside MBT and MZT?

A

Early gastrula transition (EGT)

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14
Q

What is EGT?

A

Early gastrula transition (EGT)

When the embryo acquires the ability to perform apoptosis

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15
Q

Describe the somatic cell cycle in general.

A

Somatic cell cycle:
G1, S, G2 and M phase

G1 - growth and prep for DNA synthesis

S - DNA replication

G2 - growth and prep for mitosis

M - mitosis and cytokinesis

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16
Q

Describe the cell cycle during early cell cleavage. How is it modified?

What is this coordinated by?

A

In early cell cleavage, the cell cycle is modified so it only have S and M phases

This is coordinated by maternal factors in the egg

MPF = mitosis promoting factor (two subunits: cyclin B and cdc2/cdk1)

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17
Q

What are cell division patterns also called?

A

cleavage patterns

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18
Q

Cleavage patterns variations are dependent on:

A

amount and distribution of yolk protein within cytoplasm

distribution of cytoplasmic determinants

type of cell-cell interactions

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19
Q

Many eggs have a polarity based on the presence of _____

A

yolk

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20
Q

What is the vegetal pole?

A

yolk-rich part of the zygote that contains cells that divide slowly

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21
Q

What is the animal pole?

A

the yolk-poor part of the zygote that contains cells that divide rapidly

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22
Q

Where is the zygote nucleus generally displaced? Which pole?

A

The animale pole

the yolk-poor part of the zygote that has rapidly dividing cells

23
Q

What are the types of cell cleavage patterns?

A

Holoblastic (complete) cleavage

Meroblastic (imcomplete) cleavage

24
Q

What is holoblastic cleavage?

Give an example.

A

complete cleavage

the entire egg is divided into smaller cells

i.e. amphibians and mammals
animals with low amount of yolk

cleavage furrow extends thru an entire egg

25
Q

What is meroblastic cleavage?

Give an example.

A

incomplete cleavage

when only a portion of the cytoplasm is cleaved

i.e. insects, fish, reptiles, birds
animals with large amount of yolk (sufficient to nourish the embryo throughout development)

cell furrow does not go through the yolky part of the egg (membrane formation is inhibited by the yolk)

26
Q

What are the types of holoblastic cleavage?

A

isolecithal cleavage

mesolecithal cleavage

telolecithal cleavage

centrolecithal cleavage

27
Q

What is isolecithal cleavage?

A

It is a type of holoblastic cleavage

Occurs in embryos with sparse, evenly distributed yolk (Isolecithal = equal yolk)

e.g. mammals, sea urchins

28
Q

What is mesolecithal cleavage?

A

It is a type of holoblastic cleavage

Occurs in embryos with an intermediate sized yolk that is concentrated in one hemisphere of the egg (vegetal pole)

e.g. amphibians

Complete cleavage but occurs relatively slowly in the vegetal pole –> asymmetric cell division

29
Q

What is telolecithal cleavage?

A

It is a type of holoblastic cleavage

Most of the cell has dense yolk and only one small area at the animal pole that is free of yolk undergoes division

e.g. birds, fish

Discoidal cleavage: Cell division occurs only in a small disc of cytoplasm that is free of yolk

30
Q

What is centrolecithal cleavage?

A

It is a type of holoblastic cleavage

Yolk in the center of the egg

division occurs only in the rim of cytoplasm around the cell periphery (superficial cleavage)

e.g. insects

31
Q

At the end of the cleavage, what is the embryo referred to as?

A

blastula

32
Q

At the end of the cleavage, the embryo is referred to as the blastula. What are the individual cells of the blastula called?

A

blastomeres

33
Q

Blastomeres enclose a fluid-filled cavity called _____

A

blastocoel

insulates cells from signalling molecules and provides space for gastrulation

34
Q

Blastomeres enclose a fluid-filled cavity called the blastocoel.

What is the role of blastocoel?

A

insulates cells from signalling molecules and provides space for gastrulation

35
Q

Many mammalian embryos undergo __________ cleavage.

A

rotational cleavage

36
Q

What is rotational cleavage?

A

first cleavage is a normal meridional cleavage

but in second cleavage, one blastomere divides meridionally and the other equatorially

37
Q

Mammalian blastomeres can contain an odd number of cells. Why?

A

Because mammalian blastomeres do not divide synchronously (at the same time)

Cell number does not increase exponentially (i.e. 2, 4, 8, 16)

38
Q

What is the unique feature in mammalian cleavage?

A

E-cadherin becomes expressed (8 cell embryo)

changes adhesion properties

blastomeres become tightly attached and form a compact ball of cells

39
Q

Mammalian cleavage - compaction

Compacted 8-cell embryo divides to form _________

A

16-cell morula

40
Q

Compacted 8-cell embryo

What are the properties of the 16-cell morula ?

A

16-cell morula

contains a group of outer cells (trophoblast) that surround a group of inner cells (inner cell mass – ICM)

Trophoblasts secrete fluid into morula (cavitation) to form blastocoel

ICM is positioned on one side of the ring of trophoblast cells - forms blastocyst

i.e. mammalian blastula

41
Q

What is the ICM and what does it give rise to?

A

Inner cell mass (ICM) are pluripotent internal cells that give rise to

  • The embryo
  • Yolk sac: enables nutrient uptake and development of circulatory system
  • Allantois: develops on posterior end of the embryo, stores waste products
  • Amnion: “Water sac” – allows the embryo to float in a fluid environment and prevents shock and desiccation
42
Q

What are trophoblast cells (trophectoderm)?

A

Trophoblast cells (trophectoderm) are external cells that form extraembryonic tissues

Chorion: contains blood vessels that exchange gasses with outside environment and is used for nutrition

  • Secretes hormones that cause the mother’s uterus to retain the fetus
  • Produces immune system regulators –prevents mother’s body from rejecting the fetus

Embryonic portion of placenta

43
Q

What activates genes that specify the outer cells to become trophoblasts?

A

Transcription factor Tead4 promotes transcription of Cdx2

44
Q

As the embryo moves through oviduct towards uterus, where does the blastocyst expand?

A

blastocyst expands within the zona pellucida

45
Q

What prevents the expanding blastocyst from adhering to the oviduct walls?

A

zona pellucida

ZP prevents it from adhering to the oviduct walls

46
Q

What is an ectopic pregnancy?

A
  • embryo adheres to oviduct walls

- medically critical condition

47
Q

Embryo needs to ‘hatch’ from ZP once it reaches the uterus

A

Mediated by trophoblasts —> secrete proteases –> digest a small hole in ZP

48
Q

How does the blastocyst hatch?

A

Blastocyst ‘hatches’ by digesting a small hole in ZP and squeezing through it as blastocyst expands

Once outside, the blastocyst makes direct contact with the uterus

49
Q

Implantation in the uterus

What alters the epithelial lining of the uterus?

A

estrogen and progesterone alter the epithelial lining of the uterus

After initial binding, several adhesion systems come into play to hold the embryo to the uterine lining

50
Q

What is the epithelial lining of the uterus called?

A

endometrium

extensive extracellular matrix that is ready to ‘catch’ the blastocyst

51
Q

Trophoblasts synthesize integrins that bind to _____, _____, and _____

A

Trophoblasts synthesize integrins that bind to collagen, fibronectin and laminin

P-cadherins on trophoblasts and endometrium help with adhesion

Wnt pathways get activated -trophoblasts secrete a set of proteases (e.g. collagenase)

Proteases digest the ECM

Blastocyst ‘buries’ itself in uterine wall

52
Q

Implantation in the uterus

There is an ‘implantation window’ for the blastocyst to adhere to the endometrium

A

There is an ‘implantation window’ for the blastocyst to adhere to the endometrium e.g. LIF expression goes up and pinopodes (apical protrusions of endometrium epithelium) appear  blastocyst has LIF receptors

53
Q

How long does fertilization to implantation take?

A

About 7 days

BIOC19 Animal Developmental Biology (6 decks)

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