Cleavage, blastula, gastrula, neurula

May 9, 2018 | Author: Anonymous | Category: Science, Health Science, Histology
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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


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


Embryo proper

Outer cell mass



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

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