Study Guides (247,931)
Canada (121,169)
Biology (1,099)
BIOL 211 (14)

Development Extra notes.doc

43 Pages
64 Views
Unlock Document

Department
Biology
Course
BIOL 211
Professor
Richard Ennis
Semester
Fall

Description
Figure  4­2 shows  three  basic  ways  that  such  inductions  occur.  The  first  method  is by close   cell­to­cell contact,  in a fashion  analogous  to (but  much  less  complex  than)  the  way  that  neurons   connect  with  other  neurons  or muscles  (see  Chapters  10  and  13).  The  second  method  is by  diffusion  of small, typically  soluble  morphogens,  in a fashion  analogous  to the  way  that  circulating   hormones  can  influence  cells  at  a distance  (see  Chapter  15). When  such  signaling  molecules   diffuse  directly  through  intervening  tissues  or extracellular  matrix  before  acting  on  the  target  cells,   they  are  said  to be  a paracrine  secretion.  In contrast,  if signaling  molecules  are  delivered  to  their  target  cells  via the  circulatory  system,  they  are  spoken  of as  endocrine  secretions.  Most   morphogens  important  in early  development  act  as  paracrine  secretions.  The  third  method  is by  local  changes  in the  composition  of the  extracellular  matrix  molecules  around  a cell, which  then   influence  the  differentiation  of adjacent  cells.  This latter  method  may  remain  important  in the  self­ organization  and  repair  of connective  tissues  throughout  life. Much  of embryogenesis  appears  to be  regulated  by series,  or cascades,  of inductions.  Cells and   tissues  respond  to inductive  influences  only during  a limited  period,  when  they  are  said  to be   competent  to respond.  As tissues  differentiate,  they  express  some  of the  genes  associated  with   their  final fate  (Fig. 4­3). As differentiation  proceeds,  cells  lose  their  competence  to respond  to  some  inductive  influences  but  not  to others.  Their potential  fates  are  said  to have  been   restricted.  Eventually,  the  fate  of most  cells  is determined  by subsequent  inductive  interactions.   This final restriction  in fate  is followed  by expression  of the  many  genes  associated  with  that   particular  cell type  and  eventual  achievement  of the  morphology  typical  for that  cell type.   Developmental  regulation  may  ultimately  be  viewed  as  a special  case  of the  broader  field  of  homeostatic  regulation,  by which  all parts  of an  organism  can  respond  to changes  while   continuing  to function  as  an  integrated  whole.  Studies  of developmental  regulation  will surely   prove  to be  among  the  most  important  aspects  of comparative  developmental  biology  because   changes  in the  regulation  of developmental  events  likely provided  mechanisms  for morphological   diversification  during  vertebrate  evolution.  An example  of this  is the  role  of Hox  genes  in axial   patterning  of vertebrates,  which  is discussed  at  the  end  of this  chapter. To emphasize  the  importance  of induction  and  developmental  regulation,  comparative   embryologists  often  speak  about  epigenetics  (Gr., epi 5 upon  or on  top  of 1 genetics  5 in this   case,  the  genome  level).  As a term,  epigenetics  refers  to cell fates  and  the  patterns  of interactions   and  changes  that  occur  at  the  cellular  level  during  embryogenesis  (Fig. 4­4). At the  bottom  of  Figure  4­4 is the  genetic  level,  which  is a traditional  focus  of molecular  and  cellular  biology.  Genes   code  for proteins,  such  as  structural  proteins,  enzymes,  morphogens,  or other  regulatory   molecules.  Some  of these  proteins  regulate  the  expression  of other  genes.  Other  proteins,  chiefly  structural  proteins  and  enzymes,  endow  cells  with  their  specific  cell  properties.  The  term  cell  properties  covers  an  array  of possible  functions,  from  a cell’s ability to adhere  to other  cells;  to its  ability to synthesize  and  secrete  morphogens  that  alter  the  function  or state  of differentiation  of  adjacent  cells;  to cell death,  which  is a normal  developmental  fate  for a surprisingly  large   percentage  of embryonic  cells. Exceedingly  complex  cell–cell interactions  and  morphogenetic  movements  can  occur  at  the   epigenetic  level.  For example,  if all of the  cells  in a flat  sheet  of epithelium  change  shape   simultaneously,  then  the  epi­thelium  rolls up  or folds  into  a tube  or sphere  that  may  serve  as  the   precursor  to an  organ.  The  outcome  of cell interactions  at  the  epigenetic  level  also  can  regulate   gene  expression.  In a mouse  embryo,  clusters  of migrating  neural  crest  cells  (discussed  later)   come  to lie beneath  portions  of the  skin  covering  the  jaw, where  they  participate  in reciprocal   inductive  relationships  with  the  overlying  skin  cells  in the  differentiation  of a tooth.  The  skin  cells   determine  what  type  of tooth  will form  (e.g.,  an  incisor  or a molar),  and  the  neural  crest  cells  then   take  over  control  of tooth  morphogenesis  (see  Chapter  16).  As another  example,  an  alpha  motor   neuron  that  fails to establish  contact  with  a muscle  fiber  will begin  to express  genes  for cell death   and  die.  As a concept,  epigenetics  is useful  chiefly because  it focuses  our  attention  on  the   regulatory  and  cellular  processes  of development,  which  are  a necessary  filter  between  the   genome  and  the  completely  constructed  adult.  In practice,  however,  we  have  much  to learn  about   the  specifics  of most  epigenetic  interactions  in most  tissues  of the  vast  majority  of vertebrates,   leaving  plenty  of room  for basic  research  in this  fascinating  area.
More Less

Related notes for BIOL 211

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit