induction and morphogenesis

8/3/2019 Induction and Morphogenesis

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Induction and Morphogenesis

Readings

y Chapter 15 Section 15.1y Chapter 43 Section 43.3y Chapter 19 P.437-439

Introduction

y Cells in the organizer express and secrete proteins that influencethe behaviour (cell movement) and identities (commitment) of

nearby cells

y Induction is a mechanism for influencing the behaviour and/oridentity of nearby cells.

y A protein(s) is translated and secreted by some cells to inducenearby cells to change

Signals can be both paracrine and autocrine

Not all cells can respond to a particular signal. A cell must be competentto respond.


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General features of signal transduction pathways

Outside Cell Ligand (proteins, small

molecules)

Cytoplasm

Receptor

(proteins)

Transducer(s)

+ Second Messengers

(enzymes)

(small molecules)

Nucleus

Effector

(transcription factors)

Target

(genes)

Two Modes ofInductive Signaling

A relay mechanism induces multiple cell fates by using multiple signals


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Morphogens are special signaling proteins that are involved in patterning embryos via

gradient signaling

y Properties of morphogenso Induce different cell fates at different concentrationso Can act at a distanceo Have a direct effect on the target cell

Different cellular response at different morphogen concentrations

A morphogen acts directly on a distant target cell


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and cells respond differently depending upon the concentration of morphogen they

encounter

We cant distinguish between these different signaling modes on the basis of cell

behaviour

Cell Responses to Inductive Signals

y Animal cells:o Divideo Growo Change shapeo Stick togethero Moveo Differentiateo Die

Morphogenesis is driven by induce changes in cellular behaviour


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Cell Behaviour: Adhesion

The adhesive properties of individual cells keeps the embryo organized


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Neurulation: Making the Nervous System

y Neurulation begins during gastrulationy Cells that migrate early over the dorsal lip of the blastopore from

notochordy The notochord induces the overlying ectoderm to become neural

ectoderm and subsequently thicken, form parallel ridges, and fold in

on itself to form a neural tube below the epidermal ectoderm

y The central nervous system develops from the neural

Two Major Cell Arrangements


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Epithelial

y Regular cell shapey Tightly packedy Many cell-cell contactsy Simple or stacked sheets

Mesenchymal

y Irregular cell shapey Loosely packed withiny Extracellular matrixy Few cell-cell contact

Large scale changes in embryos and tissues result from small but coordinated changes

in cell shape


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Complex organs like the vertebrate eye from by a series of inductive events


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Somitogenesis: Segmenting the Vertebrate Body

y During neurulation, mesoderm along the sides of the notochordcondenses into blocks of cells called somites

Somites are patterned by signals from adjacent tissues


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Homeotic genes direct the morphogenesis of tissues that are appropriate to a specific

region of the body

induction and morphogenesis

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