Cleavage, blastula, gastrula, neurula

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

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