CLEAVAGE, BLASTULA,
GASTRULA, NEURULA
Lecture 2
Cleavage
• Mitosis
• Duplication of cells
• 1  2  4  8  16  etc.
• zygote  morula  blastula  gastrula 
neurula  embryo  fetus
• Yolk
• Contains nutrients for the zygote
Cleavage
Name
Number of cells
Zygote
Morula
Blastula
Gastrula
1-8
16-64
128-15,000
>15,000
Neurula
Embryo
Fetus
Neural tube formation
Period of organogenesis (2-8 weeks)*
Period of growth (2-9 months)*
* In humans
Distribution of yolk
• Oligolecithal (ex. Amphioxus, star fish, sea urchins
and mammals)
• Eggs with little yolk
• Cleavage produces cells of roughly the same size
• Mesolecithal (ex. Amphibians)
• Moderate amount of yolk
• Yolk impedes cleavage formation
• Produces cells of unequal size
• Holoblastic cleavage
• Telolecithal (ex. Reptiles or Birds)
• Large amount of yolk
• Cell division occurs only at one area
• Meroblastic cleavage
Amphioxus
Cleavage
• First division begins at the animal pole
• Second division perpendicular to the first
• And so on… forming blastomeres.
Cleavage in different yolk distributions
Blastula formation in Amphioxus
• Formation of a fluid filled cavity within the developing
embryo, called blastocoele, through a Na+ pump
Blastula formation in Amphibians
Blastula formation in Birds
Blastula formation in Birds
Blastula formation in Mammals
• Blastula or blastocyst
• Similar to initial cleavage in amphioxus or sea urchins,
then follows cleavage similar to birds
• At the morula stage  blastula stage  becomes
• Process called compaction
Morula Stage
Blastula Stage
Develops into
Inner cell mass
Embryoblast
Embryo proper
Outer cell mass
Trophoblast
Placenta
Blastula formation in Mammals
Blastula and implantation
Blastula formation
Formation of Germ layers
• Gastrulation = formation of 3 primary germ layers and the
primitive gut or archenteron
Formation of Germ layers
• Mechanisms of Development
• Cytoplasmic specification (pre-determined)
• Conditional specification (specific development through
interactions with the surrounding cells/environment or
its position in the developing embryo)
• Primary organizer
Ectoderm
• Outer epithelium of body and derivatives
• Hair, nails, epithelial glands, lining of mouth, enamel of teeth, lens
of eyes, inner ear, nasal and olfactory epithelium
• Neural tube
• Brain, spinal cord, motor nerves
• Neural crest
• Sensory ganglia and nerves, adrenal medulla, sympathetic ganglia,
skull, gill arches, dentine of teeth
Mesoderm
• Notochord  vertebrae
• Lining of thoracic and abdominal cavities
• Circulatory system
• Blood, bone marrow, endothelium, lymphatics
• Somites
• skeleton and muscle, dermis, connective tissue
• Urogenital system
• Smooth muscle and connective tissue of digestive tract
Endoderm
• Epithelium of respiratory tract
• Pharynx
• Epithelium of gut
• Liver, pancreas
• Inner lining of urinary bladder
• Gut tube
Gastrulation
• Further differentiation into 3 germ cell layers
• Formation of the blastopore or primitive streak
• Dorsal lip of the blastopore or Spemann organizer
• Dominant organizing region of the embryo
• Homologous to the Hensen’s node in birds and mammals
• Directs differentiation of cells into specific germ layers or organs
Gastrulation in Amphioxus
Gastrulation in Amphibians
Gastrulation in Birds
Gastrulation in Birds
1 = Epiblast (forms the ectoderm), 2 = Blastocoel, 3 = Hypoblast (forms
the endoderm), 4 = Subgerminal cavity, 5 = Yolk
Gastrulation in mammals
• Similar to birds
• Inner cell mass  embryoblast, forms 2 layers
• Epiblast  forms a cavity  amniotic cavity
• Epiblast + cytotrophoblast = amnioblast
• Hypoblast  forms primitive yolk sac
• Outer cell mass  trophoblast, forms 2 layers
• Syncitiotrophoblast  secretes beta HCG (human chorionic
gonadotropin)
• Cytotrophoblast
• Forms the placenta
Neurula
• Neurulation: Formation of the neural tube
• Period of development starting with the first traces of
formation of the neural plate and ending with the
closure of the neural tube
• Initiates formation of the central nervous system
• Formation of notochord from the endoderm or mesoderm
• Acts as initial organizer of the nervous system
(stimulates formation of the vertebrae and spinal cord)
and creates the basis of the body axis (head and tail)
• Eventually disappears and forms the nucleus pulposus
in mammals
Neurulation
• Ectoderm thickens to form the neural plate
• Edges of neural plate become raised forming a neural
fold, with a depression in the middle, called the neural
groove
• Anteriorly, neural plate is broadened and flattened
• Posteriorly, neural plate becomes narrow and cylindrical
Neurula formation in Birds
Neurulation
• Formation of the neural tube
• Neural folds grow and meet each other
• Closure begins in the middle and proceeds cephalad
and caudad
• Formation of the anterior and posterior neuropores,
which will eventually close
• Mesoderm thickens
Nucleus pulposus