Early Vertebrate Development• Early Development of Mammals• Early Development of Birds…if we have time.
Long and Winding Road…• Both Egg and Sperm have to
overcome physical barriers.• Ampulla- site of fertilization.
Cleavage in Mammals• Rotational cleavage– 1st Meriodionally– 2nd one divides
meriodionally, one divides equatorially
• Asynchrony• No real Mid-Blastula
Transition.
Compaction• Division of early embryo up to 8 cell stage is loosely arranged.• At 8 cell stage, cells form tight interactions and divide into 16
cell morula.• Internal Cells= Internal Cell mass (ICM)• Most external cells become trophoblast.
Early Cells in Mammalian Embryo• Totipotent• Pluripotent
Cavitation.• Trophoblast cells secrete fluid into Morula: blastocoel• Trophoblast Na+ pumps are activated by oviduct cells.
Hatching of Mammalian Blastula• Zona pellucida prevents embryo from attaching to oviduct
wall. • Trypsin like protease: releases embryo from ZP.
Mammalian Gastrulation• Mother provides
nutrients• The reliance of embryo
on Mother has resulted in fetal organs– Chorion– Hypoblast/epiblast– Germ disc– Amniotic cavity
Mammalian Gastrulation: Primative Groove
Extraembryonic Membranes• Trophoblast cells make up cytotrophoblast and
syncytiotrophoblast.• Trophoblasts and Uterine tissue: Placenta
Axis specification in Mammals• Two signaling centers
– Node– Anterior Visceral Endoderm(AVE)
• Work together to form anterior region.
• Notochord: dorsal infolding of small ciliated cells
• AVE: visceral endoderm migrates forward.
• Antagonists of Nodal– Lefty-1; Cerberus– Nodal activates expression of
posterior genes; Lefty-1 and Cerberus blocks this activity
Axis Specification: Anterior-Posterior positioning• FGF and Retinoic acid gradients.• Ultimately leads to differential expression of Hox genes
Anterior-Posterior Axis: Hox Code
Retinoic Acid and Hox Gene Expression
Dorsal-Ventral and Left-Right Axes• DV axis formation: Poorly characterized.• Left-right axis formation: Nodal and Pitx2
Early Development of Birds: Cleavage
Gastrulation: Birds• The Hypoblast• The primative streak
The Primitive Streak
Primitive Streak: Molecular mechanisms of migration
Regression of Primitive streak
Axis specification in Birds• Gravity and PMZ
Left Right Axis
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