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