Cleavage and Gastrulation (Esther Bell)
• To describe the mechanism of cell division leading to formation of the blastula indifferent
Vertebrates
The development of multi-cellular organisms begins from a single-celled zygote, which
undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division
are termed cleavage.
- No growth no size change just divisions.
- blastula: a 6-32-celled hollow structure that is formed after a zygote undergoes cell
division
o a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-
filled cavity (the blastocoel)
• Understand what the term cleavage means, and the different ways cleavage can be
achieved
Cleavage: Immediately follows fertilisation -
series of rapid cell divisions, without growth,
divides embryo into numerous small cell
Holoblastic: division goes through
whole cell
Absence of yolk
At end of roational there is a
blastocyte
Cell divisions at an angle
In frog
32 cell stage embryo, no size diff.
Animal pole is pigmented region
Vegetal is heavier because they contain yolk for
nourishment
,In mouse
Rotational cleavage
16 cell- morula which becomes blastocyst which then goes uterus
In human
Early cell divisons occur in oviduct
Blastocyst embeds in uterin wall
- 24 h for first cell division
- More 24h for second
- Day 4 you get morula
- Day 5 you get blastocyst
Meroblastic
discoidal
Embryo is on top of yolk and cleavage can’t go all
the way through
superficial
Because cell has a lot of yolk nuclei migrate to
edges of the cell and egg with multiple nuclei
formed is called syncytium.
No cell membrane at this stage
Chick - cleavage furrow- blastoderm -area pellucida opaca posterior marginal zone (where signals
are)
Maternal determinants = mRNAs + proteins synthesised by mother, required for early
patterning
E.G. Xenopus: Coco expressed in vegetal pole, required for endoderm and mesoderm
expression Drosophila: nanos expressed in posterior part of embryo, determines posterior
patterning of fly [protein diffuses + forms conc grad whilst embryo is in syncitium]
,MOUSE HUMAN blastocyst
Fish frog Blastula
Fish blastoderm
Blastoderm, blastocyst,
blastula are all equivalent
stages of development just
different name depending on
species
• Know that the outcome of cleavage is formation of a blastula with cells having already
reached some level of specification
In frog and fish- cells can specialize before gastrulation
Ectoderm: neural tissue and epidermis (skin)
Mesoderm:
Endoderm: respiratory and digestive tract
Mouse and frog and human holoblastic cleavage
Chicken and zebrafish meroblastic cleavage
• Understand what gastrulation means
Gastrulation moves basic 3 tissue layers (endoderm mesoderm ectoderm) to form body plan (head
body tail)
, • Understand the changes in spatial organisation from blastula to gastrula embryo
Gastrulation moves basic 3 tissue layers (endoderm mesoderm ectoderm) to form body plan
(head body tail)
- Humans gastrulate like the chick
Gastrulation in chick begins with formation of the primitive streak and cells have to
go through this streak (organizer in blastopore sends signals for this to begin)
• To describe gastrulation and introduce the
concept of cell movement and cell signalling
In frog and fish cells can specialize before
gastrulation
Ectoderm: neural tissue and epidermis (skin)
Mesoderm: somites, body kidneys spinal column
Endoderm: respiratory and digestive tract
cell movement:
involution – movement of a sheet of cells towards the
inside of the embryo by rolling in under itself
epiboly - when ectoderm stretches and moves around the
embryo
• To describe the mechanism of cell division leading to formation of the blastula indifferent
Vertebrates
The development of multi-cellular organisms begins from a single-celled zygote, which
undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division
are termed cleavage.
- No growth no size change just divisions.
- blastula: a 6-32-celled hollow structure that is formed after a zygote undergoes cell
division
o a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-
filled cavity (the blastocoel)
• Understand what the term cleavage means, and the different ways cleavage can be
achieved
Cleavage: Immediately follows fertilisation -
series of rapid cell divisions, without growth,
divides embryo into numerous small cell
Holoblastic: division goes through
whole cell
Absence of yolk
At end of roational there is a
blastocyte
Cell divisions at an angle
In frog
32 cell stage embryo, no size diff.
Animal pole is pigmented region
Vegetal is heavier because they contain yolk for
nourishment
,In mouse
Rotational cleavage
16 cell- morula which becomes blastocyst which then goes uterus
In human
Early cell divisons occur in oviduct
Blastocyst embeds in uterin wall
- 24 h for first cell division
- More 24h for second
- Day 4 you get morula
- Day 5 you get blastocyst
Meroblastic
discoidal
Embryo is on top of yolk and cleavage can’t go all
the way through
superficial
Because cell has a lot of yolk nuclei migrate to
edges of the cell and egg with multiple nuclei
formed is called syncytium.
No cell membrane at this stage
Chick - cleavage furrow- blastoderm -area pellucida opaca posterior marginal zone (where signals
are)
Maternal determinants = mRNAs + proteins synthesised by mother, required for early
patterning
E.G. Xenopus: Coco expressed in vegetal pole, required for endoderm and mesoderm
expression Drosophila: nanos expressed in posterior part of embryo, determines posterior
patterning of fly [protein diffuses + forms conc grad whilst embryo is in syncitium]
,MOUSE HUMAN blastocyst
Fish frog Blastula
Fish blastoderm
Blastoderm, blastocyst,
blastula are all equivalent
stages of development just
different name depending on
species
• Know that the outcome of cleavage is formation of a blastula with cells having already
reached some level of specification
In frog and fish- cells can specialize before gastrulation
Ectoderm: neural tissue and epidermis (skin)
Mesoderm:
Endoderm: respiratory and digestive tract
Mouse and frog and human holoblastic cleavage
Chicken and zebrafish meroblastic cleavage
• Understand what gastrulation means
Gastrulation moves basic 3 tissue layers (endoderm mesoderm ectoderm) to form body plan (head
body tail)
, • Understand the changes in spatial organisation from blastula to gastrula embryo
Gastrulation moves basic 3 tissue layers (endoderm mesoderm ectoderm) to form body plan
(head body tail)
- Humans gastrulate like the chick
Gastrulation in chick begins with formation of the primitive streak and cells have to
go through this streak (organizer in blastopore sends signals for this to begin)
• To describe gastrulation and introduce the
concept of cell movement and cell signalling
In frog and fish cells can specialize before
gastrulation
Ectoderm: neural tissue and epidermis (skin)
Mesoderm: somites, body kidneys spinal column
Endoderm: respiratory and digestive tract
cell movement:
involution – movement of a sheet of cells towards the
inside of the embryo by rolling in under itself
epiboly - when ectoderm stretches and moves around the
embryo
