MESODERM INTRODUCTION

(epimere) (hypomere) (mesomere) (nephrotome) Major mesodermal regions Amphibians

(epimere) (mesomere) (hypomere) Amniotes 33 hr chick 48 hr chick

Components and derivatives of the somite Dermatome - dermis of dorso- lateral body wall Myotome - skeletal muscles of the body Sclerotome - axial skeleton Myocoel - transient embryonic structure, not present in adult

1. Nephric/excretory system a. pronephros (adult kidney in primitive fish) b. mesonephros (adult kidney in amphibians) c. metanephros (adult kidney in amniotes) 2. Components of genital system a. gonads (everything except germ cells) b. ducts associated with gonads *male - dutuli efferentes, epididymis, vas deferens *female - oviducts, uterus, part of vagina Derivatives of the intermediate mesoderm (mesomere):

Components of the lateral plate/ventral mesoderm Circulatory system heart blood vessels

Limb morphogenesis 1.Activating factor (source unknown, possibly the somites) causes somatic mesoderm in Wolffian ridge region of fore- and hind-limbs to proliferate --> forms mass of mesenchyme below epidermis that bulges outward to form the limb bud Figure on page 394 in your text (Carlson).

2.Overlying ectoderm organizes to form the apical ectodermal ridge 3.Both the AER and the underlying mesoderm are critical for formation of the limb -- how? Figure on page 403 in your text (Carlson).

Interesting limb experiments: 1.Remove AER, limb stops developing 2.Transplant AER to a region of the Wolffian ridge where limbs don’t normally develop --> causes a limb to form 3.Spit AER or transplant an additional AER to limb bud - two limbs develop 4.AER transplants to wingless chicken embryos a. Wingless chicken has it’s own wing AERs early in development; however, these degenerate and, as a result, no wings develop. b. If an AER from a normal winged chicken embryo is transplanted to the wing bud of a wingless chicken embryo after it’s own AER degenerates, the transplanted AER will also degenerate - i.e. you can’t “rescue” the wings of wingless chicken embryos with transplanted AERs WHAT DOES ALL THIS SUGGEST? Figure included in.pdf from web link below.

1.Used chick embryos 2.Removed limb buds, separated limb ectoderm (AER) and limb mesoderm 3.Did experiments recombining these in different ways. 4.Wing mesoderm + leg AER --> wing Wing AER + leg mesoderm --> leg WHAT DOES THIS TELL YOU? Experiments of Zwilling

1.Muscle - from somite myotome. 2.Skeleton/connective tissue - from embryonic somatic mesoderm of lateral plate. 3.Blood vessels - from embryonic splanchnic mesoderm of lateral plate. Origin of limb parts: 4.Motor nerves - from spinal cord motor neurons (axons from these cells grow out into limb and synapse on muscles). 5.Sensory nerves - from neural crest cells --> neurons in spinal ganglia. 6.Schwann cells and pigment cells - from neural crest cells that migrate into forming limb Mesodermal components: Ectodermal components: Figure included in.pdf from web link below.

Morphogenesis of the hand Figure on P. 407 in your text (Carlson). Figure on P. 414 in your text (Carlson).