Cleavage to gastrulation
M.A.Kai-Kai
Learning Objectives.
Define major mechanisms of development.
 Describe the mammalian zygote.
 Understand the process of mitotic division/cleavage of
the zygote
 Understand process of blastulation
 Understand the process of gastrulation and the functions
i.e. formation of the embryonic three germ
layersectoderm, endoderm, mesoderm.
 Overview of the adult derivatives of the germ layers.

Mechanisms of Development
1.Mitosis and growth differential mitosis and growthbody form
--cell proliferation e.g.in cleavageincrease in number, cells smaller in size.
--growth by deposition of extracellular matrix,intracellular organelles and matrix.
--spread of epithelial sheets e.g. extraembryonic membranes.
2.Restriction and determinationtotipotent blastomeresrestricted potency
3.Gene activation and differential expression of functional genes.
4.Differentiatione.g.cytodifferentiation into specific phenotype.
5.Cell/tissue interactionmediate embryonic signals e.g.primary inductionnotochord.
6.Cell movementshort or long migrations, e.g. neural crest cells.
7. Pattern formationintrinsic blueprint directing development.
8.Foldinge.g.cephalic, caudal and lateral body foldsbody form
9. Morphogenesiscumulative mechanisms transform internal and external
body form.
Cell movement, changing cell interactions, cell division, changing patterns of gene
expression are all quite restricted in the adult but are rampant in the embryo.
CELL-CELL AND CELL
MATRIX INTERACTIONS
CONTROL OF GENE
EXPRESSION
EXTRACELLULAR
MATRIX
CONTROL OF
CELL DIVISION
STEM CELLS
CELL SIGNALLING
CELL MOTILITY
AND SHAPE CHANGE
EMBRYOLOGY
APOPTOSIS
REPRODUCTIVE
SYSTEM
LIMBS
NERVOUS
SYSTEM
CARDIOVASCULAR
SYSTEM
DIGESTIVE
SYSTEM
RESPIRATORY
SYSTEM
URINARY
SYSTEM
The
zygotea single cell formed at
fertilisation.
Structure.
--diploid nucleus from both parents. -cytoplasm is maternal.
--surrounded by zona pellucida
Cleavagemitotic division at
12hours, 2days and 3days in mouse,
pig and dog. Rate about one
division/day.
Zygote period lasts from fertilisation
to hatching of the blastocyst.
Nutrition/embryotroph
--mammalian zygotes is provided by
uterine secretions/histiotroph and the
zygotes own reserve.
--avian zygotes feed on the yolk.
The Zygote
Unique Features of Mammalian Cleavage
Cleavage is series of mitotic division of the zygote into progressively smaller cellular
units; blastomeres
 First cleavage synchronous.
 Differences in pattern of cleavage dependent upon amount of
yolkmeroblastic/incomplete and holoblastic/complete
 Totipotent early blastomeres(Hans Driesch
 1st cleavage is meriodal
 2nd cleavageone meriodional and one equatorial/termed rotational cleavage
 Second cleavage not synchronised in 2 blastomeres
 Compaction occurs at 8-cell stage.
--blastomeres flatten and form intercellular connections
--E-CAD(cadherin:glycoprotein) on cell surface adhesion.
--microvilli(actin) extend on surfaces of adjacent cells and anchor cells together.
--tight junctions prevent free exchange of fluid between the inside and outside
allowing accumulation of fluid inside blastomeres.
--the gap junctions couple all the blastomeres of the compacted embryo and permit
exchange of ions and small molecules from one cell to the next.

Morula;
16-cell stage, embryo
enclosed in the zona pellucida.
Late morula, first differentiation
event in mammalian development.
--cells aggregate into internal inner
mass cells(ICM) and external
trophoblast.
--at 64-cell stage ICM and
trophoblast form distinct populations.
Trophoblast/trophoectodermform
s ectoderm of chorion/ placenta.
Inner mass cells are pluripotent;
form embryo and partly
extraembryonic membranes.
The Morula
A TIMELINE - PIG DEVELOPMENT
The embryonic phase contains
both series and parallel
components
0
BLASTULATION
10
D
A
Y
S
20
30
GASTRULATION/
NEURAL TUBE
FORMATION
SEGMENTATION/
SOMITE FORMATION
FORMATION OF BODY
FOLDS AND EXTRAEMBRYONIC
MEMBRANES
Size of
embryo
5 mm
10 mm
EARLY ORGANOGENESIS
30 mm
50 mm
40
FETAL PHASE
114
IMPLANTATION
In the mid to large size domestic
mammals the embryonic phase
lasts 30-40 days and the fetal phase
varies with the size and maturity of
the neonate
Blastulation
Formation of the blastocyst
--A series of rapid cell divisions produce
a blastula of 64 cells with an inner
cell mass(ICM) and an outer layer of
trophoblasts.
--trophoblast secrete fluid into the
morula. Creates a cavity; the
blastocoele(A)
--ICM/embryo proper lies to one side
 Transition from morula to blastula
marked by:
--rapid enlargement of blastocoele
--differentiation of blastomeres into ICM
and trophoblast cells(B).
 Trophoblast cells induce special
changes in the uterine lining at
implantation.
 Trophoblast cells preferentially
express maternal genes, and
inactivate paternal genes.
 The blastula hatches from the zona
pellucida and implants in the uterus.
 Blastogenesis starts at 32-cell
stage.

ICM
segregation
Trophoblast
Stem cells
Properties of stem cells.
 Pluripotent embryonic stem adult stem cells.
 Embryonic stem cells(ESC) from inner mass cells,totipotent can give rise
to all embryonic cells except trophoblast.ectoderm, endoderm and
mesoderm.
 ESC form pluripotent stem cellcommitted stem cell then
progenitor/precursor cellsdifferentiate into a cell lineage.
Example haemangioblastmultipotent haematopoietic stem cellmyeloid
progenitor cellblood cells.
 Adult stem cells more lineage-restricted in their ability to differentiate
 Restriction on the potency of stem cells is determined by the
microenvironment
 Adult stem cells important for the continual production of tissues e.g
intestinal epithelium.
 Inner mass cells also used for creating chimeric animals.
 Expression of transcription factor e.g Oct-4 is required for the maintenance
of pluripotency.
 Murine embryonic stem cells useful models to elucidate the unique
properties of mammalian stem cells.
 Valuable therapeutic potential in treatment of degenerative diseases.
Formation of hypoblast
and epiblast.

1.
2.



Formation of hypoblast begins late in
blastulation
Segregation(A)
Delamination of ICM cells.Cells expand
beneath trophoblast form
hypoblast(B)/extraembryonic endoderm
Hypoblast tube(blastocoele) inside tube
of trophoblast(C)
Formation of hypoblast results in twolayered embryo(C)
Cells on surface ICM form epiblast(C)
Blastula: Hatching and Implantation.
Blastulation embryo arrives in uterus(4)
 Blastula hatches from zona pellucida and contact uterus.
 Blastocyst surrounded by ZP prevents premature implantation and ectopic
pregnancy.
 Hatching involvesA trypsin-like protease lyses of ZP
--Trophoblast cells secrete proteases which degrades the endometrial wall and
blastocyst embeds(5).

3
4
4
1
5
2
Gastrulation(1)
Gastrulation transforms flat two-layered
blastula(epiblast,hypoblast) into three-layered
gastrulaectoderm, endoderm, mesoderm.

Mechanism of gastrulation consists of:
1.
Formation of primitive streak(PS)
2.
Involution of PS
3.
Regression of PS.
Primitive node
1. Formation of primitive streak, marked by:
--expansion of epiblast cells and caudal
convergence(A).
--Primitive streak(PS) forms as longitudinal ridge
in midline(B)
--PS elongates, cranial tip widened as primitive
node(C)
Primitive streak
Gastrulation(2)
2. Involution of primitive streak(PS)
--epiblast cells leave PS
--primitive groove in midline
--first group of cells form intraembryonic
endoderm(A)
-- --second group of epiblast cells
intercalate between endoderm and
ectodermform mesoderm
3. PS regresses caudally
Enlarged tip Of PSHensen’s
node/primitive knot, moves to posterior
region(C).
4.Function of PS is to form three germ
layers
Cranial
A
AA
Caudal
B
B
B
C
C
Hensen’s node
The
Notochord
--a rod-shaped aggregate
of chordamesoderm cells
extending along entire
length of embryo.
--notochord cells formed of
migrating cells from
Hensen’s node
Functions of notochord.
1.Defines cranial-caudal
axis of embryo.
2. Serves as primary
inducer at
neurulation.Induces
formation of the neural tube
and somitogenesis.
3. Transient, remnants in
intervertebral disc as
nucleus pulposus.
Formation of the Notochord
FORMATION OF THE MAMMALIAN GASTRULA - 3
Migration through the primitive streak forms a three-layered embryo
Transverse through primitive streak
Somatopleure
Primitive
streak
The 3 germ
layers
Ectoderm
Mesoderm
Embryo proper
Endoderm
Junctional line
Splanchnopleure
Splanchnopleure
Trophoectoderm
Extraembryonic
Primitive
gut
Mesoderm
Endoderm/hypoblast
Extraembryonic coelom
Transverse section through a mammalian embryo(14days in pig)
Periods of Development of Conceptus

Embryo implants by end of blastogenesis

Embryogenesis is period of differentiation and organogenesis

Major organs and systems formed, morphogenesis is later.

Period ends with formation of the primordial nervous system, circulatory
system digestive and excretory systems and limb buds.

The embryo is a miniature adult of the species.

Period is most vulnerable to teratogenic malformations

High mortality rate due to nutritional deficiency, excessive ambient
temperature and infections.

Embryonic deaths occur, about 15% in the queen cat, bitch and horses
Summary
The zygote goes through different stages of development by specific
morphogenetic processes.
 Cleavage, a series of mitotic divisions produces a morula
 Blastulation forms the blastula which hatches from the zona pellucida and
implants in the wall of the uterus.
 Gastrulation transforms the flat blastula into a three-layered gastrula
 The process of gastrulation involves formation of the primitive streak by
convergence of epiblast cells towards the midline and involution of the streak
to form the endoderm and mesoderm. The surface epiblast cells form the
ectoderm.
 The notochord forms from mesoderm in Hensen’s node and acts as primary
inducer of neurulation.
 Organogenesis starts after neurulation.All adult organs are formed from the
three germ layers.
 Periods of development areblastogenesis, embryogenesis and the fetal
stages.The entrance and duration of each stages in the species.

References
Carlson, B. M., Foundations of Embryology (6th.Edition) 1996.
McGraw-Hill inc. London. Page 151 – 226
1.
2.
Gilbert, S.F., Developmental Biology (8th. Edition) 2006. Sinauer
Associates Inc. Sunderland, Massachuetts. USA. Page 348 - 354
McGeady, T.A., Quinn, P.J., Fitzpatrick, E.S., & Rayan, M.T., (2006).
Veterinary Embryology. Page 25 – 38.
3.
Noden, D.M., DeLaHunta, A., The Embryology of Domestic Animals.
1985, Williams & Wilkins. London. Page25 – 27, 33-40.
4.