Commitment and Specification

Biology 4361 – Developmental Biology

June 11, 2009

Overview

How do cells organize themselves into tissues and organs?

How do forces in the embryo cause the differentiation of cells?

Differentiation - definition

Specification, commitment, and determination - concepts

Types of specification

Morphogens and morphogen gradients

Stem cells and commitment

The embryonic environment: external and internal

What forces affect embryonic development?

The Embryonic EnvironmentWhat is the “embryonic environment”?

Internal influences (e.g. intrauterine):- chemicals (e.g. maternal hormones, caffeine, nicotine)- competitors (e.g. litter-mates)

Environmental regulation pathway:- external stimulation triggers signaling event in embryo- signal stimulates an embryonic pathway (e.g. endocrine),

that changes the developmental pathway.

External influences:- light- temperature- humidity- predators - competitors- intraspecific signals

Differentiation

Differentiation – development of cellular specialization

Differentiation is a process; multi-step/multi-phase…

- preceded by the commitment of cell to certain fate(s)

1) specification

2) determination

Commitment is also a staged process:

Commitment Stages

undifferentiated differentiated

specification determination

- reversible

Capable of differentiating autonomously when

placed in a neutral environment; not when placed in

non-neutral environment.*

1. Specification.

- essentially irreversible

Capable of differentiating autonomously

even when placed into another embryonic region.*

2. Determination.

*Functional definition

Specification Types

I. Autonomous (mosaic)

- cells develop only according to early fate

e.g. Ceanorhabditis elegans

II. Syncytial

- cell fate dependent on exposure to cytoplasmic

determinants in a syncytium

e.g. insects

III. Conditional (regulative)

- fate depends on context

e.g. vertebrates

Autonomous Specification

- cells are specified by differential distribution of cytoplasmic

components during cleavage of the egg and early embryo.

- proteins

- RNA

Fabio Piano (Cornell University)

Autonomous Specification

Tunicate (sea squirt)

blastomere separation

Autonomous Specification - 2

Blastomeres are committed

at a very early stage in

mosaic development

Each blastomere contains

positional information

in the form of specific

proteins and genes

dissociated blastomeresIf split, each dissociated

blastomere pair forms

original structures

Removal Experiment - Mosaic

mitosis

early embryo

later embryo

Syncytial Specification

Syncytium – nuclear division without cell division; results in

cytoplasm with many nuclei

Drosophila

Cleavage

FELICE FARBER

nuclei & cytoplasm form

syncytial blastoderm

Syncytial Specification through

Morphogen Gradients

Drosophila egg

bicoid – anterior determinant

nanos – posterior determinant

Maternal messages: U Irion & D St Johnson

Syncytial Specification through

Morphogen Gradients

bicoid – anterior

nanos – posterior

Maternal messages:

1) each region has a distinct Bicoid:Nanos ratio

1:0 10:1 1:1 1:5

2) Bicoid:Nanos determines anterior-posterior identity

Bicoid & Nanos proteins =

morphogens

Each morphogen

establishes a gradient

throughout the

embryo (like a

diffusion gradient)

Syncytial Specification through

Morphogen Gradients

Cells identity depends

on their position in

multiple gradients

Bicoid Protein = Head

Bicoid Manipulation

= morphogen gradient

Conditional Specification

Conditional Specification

Cell fate depends on

interactions with

neighboring cells

Embryonic cells can change

fates to compensate for

missing parts = Regulation

Conditional specification produces

Regulative Development

Removal Experiment

differentiated cell

mitosis

early embryo

later embryo

?

Conditional SpecificationLegs and antennae: structurally-related; different morphologies.

Experiment: Transplant embryonic cells that would produce

proximal leg (close to the body) to an area that would

ordinarily produce antenna tip.

claws

proximal leg

distal antenna?

I. Duncan

antenna

claw

Morphogen Gradients

e.g. cells respond to protein concentration by turning different colors

Cell commitment and

differentiation are

programmed by various

morphogen gradients.

Conditional Specification

Cell commitment and

differentiation are

programmed by various

morphogen gradients.

Transplants of flag “cells”

shows that they retain

their identity (nationality),

but grow according to

the cells around them.

Transplanted leg cells keep “leg” identity

- but modify development from their original location (proximal

to the body), to that of their new location (the distal-most point).

Morphogen gradient started at the body (source) specifies proximal structures.

I. Duncan

Conditional Specification

As the morphogen concentration decreases more distal structures form.

Therefore, while transplanted leg cells kept their identity, they were

“conditioned” by the low morphogen concentration at the tip (sink) to

form the most distal leg structures – claws.

Leg-antenna transplant

Stem Cells and Commitment

Pluripotent* – uncommitted; makes many types of cells.

Totipotent* – ability to make all cell types; embryo and

trophoblast (fetal portion of the placenta).

Multipotent* – committed; makes several different types of cells

*all stem cells regenerate copies of themselves

* *

Stem Cell Derived Blood Cells

Overview

How do cells organize themselves into tissues and organs?

How do forces in the embryo cause the differentiation of cells?

Differentiation - definition

Specification, commitment, and determination - concepts

Types of specification

Morphogens and morphogen gradients

Stem cells and commitment

The embryonic environment: external and internal

What forces affect embryonic development?

Morphogenesis and Cell Adhesion

How are tissues formed from populations of cells?

How are organs constructed from tissues?

How do organs form in particular locations and how do

migrating cells reach their destinations?

How do organ and their cells grow, and how is their growth

coordinated throughout development?

How do organs achieve polarity?

Cell Interactions

Cell membrane protein components bind cells together; e.g.

Cadherin

Cells interact with each other either through paracrine signaling

at some distance, or through direct contact.

Calcium-dependent adhesion

- multiple forms

Cadherin-Mediated Cell Adhesion

actin

microfilament

system =

anchoring and

movement

Different cells

have different

cadherins.

Different cadherins

have different

affinities for

each other.

Thus, cell types can

segregate

themselves

based on

membrane components.

Ca2+-dependent

binding:

Ca2+ can control

both strength

and reversibility

of binding