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 1-8 Morula 16-64 Blastula 128-15,000 Gastrula >15,000 Neurula Neural tube formation Embryo Period of organogenesis (2-8 weeks)* Fetus 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 Holoblastic meroblastic

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 Holoblastic meroblastic

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 Neural tube 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

Lacunar stage of trophoblast

Cytotrophoblast + amnion mesoderm = extraembryonic somatic mesoderm  somatopleure (ectoderm + mesoderm) Yolk sac endoderm + amnion mesoderm = extraembryonic splanchnic mesoderm  splanchnopleure (endoderm + mesoderm)

Cytotrophobast + syncitiotrophoblast = primary villa (precursor of chorionic villi)

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

In amphibians

In humans

Nucleus pulposus