Locomotion of Cells

Locomotion of Cells :

By  far  the  most  important  type  of  movement  that  occurs in  the  body  is  that  of  the  muscle  cells  in  skeletal,  cardiac, and  smooth  muscle,  which  constitute  almost  50  percent  of  the  entire  body  mass. Two other  types  of  movement—ameboid  locomotion  and  ciliary movement—occur in other cells.

Ameboid Movement :

Ameboid  movement  is  movement  of  an  entire  cell  in relation  to  its  surroundings,  such  as  movement  of  white blood  cells  through  tissues.  It  receives  its  name  from  the fact  that  amebae  move  in  this  manner  and  have  provided an excellent tool for studying the phenomenon.

Typically,  ameboid  locomotion  begins  with  protrusion  of  a  pseudopodium  from  one  end  of  the  cell.  The pseudopodium  projects  far  out,  away  from  the  cell  body, and  partially  secures  itself  in  a  new  tissue  area.  Then  the remainder  of  the  cell  is  pulled  toward  the  pseudopodium. Figure  2-16  demonstrates  this  process,  showing  an  elongated  cell,  the  right-hand  end  of  which  is  a  protruding pseudopodium.  The  membrane  of  this  end  of  the  cell  is continually  moving  forward,  and  the  membrane  at  the left-hand  end  of  the  cell  is  continually  following  along  as the cell moves.

Mechanism  of  Ameboid  Locomotion :  

Figure  2-16 shows  the  general  principle  of  ameboid  motion.  Basically, it  results  from  continual  formation  of  new  cell  membrane at  the  leading  edge  of  the  pseudopodium  and  continual absorption  of  the  membrane  in  mid  and  rear  portions  of the  cell.  Also,  two  other  effects  are  essential  for  forward movement  of  the  cell.  The  first  effect  is  attachment  of  the pseudopodium  to  surrounding  tissues  so  that  it  becomes fixed  in  its  leading  position,  while  the  remainder  of  the cell  body  is  pulled  forward  toward  the  point  of  attachment.  This  attachment  is  effected  by  receptor  proteins  that line  the  insides  of  exocytotic  vesicles.  When  the  vesicles become  part  of  the  pseudopodial  membrane,  they  open so  that  their  insides  evert  to  the  outside,  and  the  receptors now  protrude  to  the  outside  and  attach  to  ligands  in  the surrounding tissues.

At  the  opposite  end  of  the  cell,  the  receptors  pull  away from  their  ligands  and  form  new  endocytotic  vesicles. Then,  inside  the  cell,  these  vesicles  stream  toward  the pseudopodial  end  of  the  cell,  where  they  are  used  to  form still new membrane for the pseudopodium.

The  second  essential  effect  for  locomotion  is  to  provide the  energy  required  to  pull  the  cell  body  in  the  direction  of the  pseudopodium.  Experiments  suggest  the  following  as an  explanation:  In  the  cytoplasm  of  all  cells  is  a  moderate to  large  amount  of  the  protein  actin.  Much  of  the  actin  is  in the  form  of  single  molecules  that  do  not  provide  any  motive power;  however,  these  polymerize  to  form  a  filamentous network,  and  the  network  contracts  when  it  binds  with  an actin-binding  protein  such  as  myosin.  The  whole  process  is energized  by  the  high-energy  compound  ATP.  This  is  what happens in the pseudopodium of a moving cell, where such a  network  of  actin  filaments  forms  anew  inside  the  enlarging  pseudopodium.  Contraction  also  occurs  in  the  ectoplasm  of  the  cell  body,  where  a  preexisting  actin  network  is already present beneath the cell membrane.

Types of Cells That Exhibit Ameboid Locomotion :

The most common cells to exhibit  ameboid  locomotion  in the  human  body  are  the  white  blood  cells  when  they  move out  of  the  blood  into  the  tissues  to  form tissue  macrophages. Other  types  of  cells  can  also  move  by  ameboid  locomotion  under  certain  circumstances.  For  instance,  fibroblasts move  into  a  damaged  area  to  help  repair  the  damage  and even  the  germinal  cells  of  the  skin,  though  ordinarily  completely  sessile  cells,  move  toward  a  cut  area  to  repair  the opening.  Finally,  cell  locomotion  is  especially  important  in development  of  the  embryo  and  fetus  after  fertilization  of an ovum. For instance, embryonic cells often must migrate long  distances  from  their  sites  of  origin  to  new  areas  during development of special structures.

Control of Ameboid Locomotion—Chemotaxis :

The  most  important  initiator  of  ameboid  locomotion is  the  process  called  chemotaxis.  This  results  from  the appearance  of  certain  chemical  substances  in  the  tissues.  Any  chemical  substance  that  causes  chemotaxis  to occur  is  called  a  chemotactic  substance.  Most  cells  that exhibit  ameboid  locomotion  move  toward  the  source of  a  chemotactic  substance—that  is,  from  an  area  of lower  concentration  toward  an  area  of  higher  concentration—which  is  called  positive  chemotaxis.  Some  cells move  away  from  the  source,  which  is  called  negative chemotaxis.

But  how  does  chemotaxis  control  the  direction  of  ameboid  locomotion?  Although  the  answer  is  not  certain,  it is  known  that  the  side  of  the  cell  most  exposed  to  the chemotactic  substance  develops  membrane  changes  that cause pseudopodial protrusion.

Cilia and Ciliary Movements :

A  second  type  of  cellular  motion,  ciliary  movement,  is  a whiplike  movement  of  cilia  on  the  surfaces  of  cells.  This occurs  in  only  two  places  in  the  human  body:  on  the  surfaces  of  the  respiratory  airways  and  on  the  inside  surfaces of  the  uterine  tubes  (fallopian  tubes)  of  the  reproductive tract.  In  the  nasal  cavity  and  lower  respiratory  airways, the  whiplike  motion  of  cilia  causes  a  layer  of  mucus  to move  at  a  rate  of  about  1  cm/min  toward  the  pharynx,  in this  way  continually  clearing  these  passageways  of  mucus and  particles  that  have  become  trapped  in  the  mucus.  In the  uterine  tubes,  the  cilia  cause  slow  movement  of  fluid from  the  ostium  of  the  uterine  tube  toward  the  uterus cavity;  this  movement  of  fluid  transports  the  ovum  from the ovary to the uterus.

As  shown  in  Figure  2-17,  a  cilium  has  the  appearance of  a  sharp-pointed  straight  or  curved  hair  that  projects  2 to  4  micrometers  from  the  surface  of  the  cell.  Many  cilia often  project  from  a  single  cell—for  instance,  as  many as  200  cilia  on  the  surface  of  each  epithelial  cell  inside the  respiratory  passageways.  The  cilium  is  covered  by  an outcropping  of  the  cell  membrane,  and  it  is  supported by  11  microtubules—9  double  tubules  located  around the  periphery  of  the  cilium  and  2  single  tubules  down the  center,  as  demonstrated  in  the  cross  section  shown in  Figure  2-17.  Each  cilium  is  an  outgrowth  of  a  structure that  lies  immediately  beneath  the  cell  membrane,  called the  basal body  of the cilium. 

The  flagellum  of  a  sperm  is  similar  to  a  cilium;  in  fact, it  has  much  the  same  type  of  structure  and  same  type  of contractile  mechanism.  The  flagellum,  however,  is  much longer  and  moves  in  quasi-sinusoidal  waves  instead  of whiplike movements.

In  the  inset  of  Figure  2-17,  movement  of  the  cilium is  shown.  The  cilium  moves  forward  with  a  sudden, rapid  whiplike  stroke  10  to  20  times  per  second,  bending  sharply  where  it  projects  from  the  surface  of  the  cell. Then  it  moves  backward  slowly  to  its  initial  position.  The rapid  forward-thrusting,  whiplike  movement  pushes  the fluid  lying  adjacent  to  the  cell  in  the  direction  that  the cilium  moves;  the  slow,  dragging  movement  in  the  backward  direction  has  almost  no  effect  on  fluid  movement. As  a  result,  the  fluid  is  continually  propelled  in  the  direction  of  the  fast-forward  stroke.  Because  most  ciliated  cells have  large  numbers  of  cilia  on  their  surfaces  and  because all  the  cilia  are  oriented  in  the  same  direction,  this  is  an effective  means  for  moving  fluids  from  one  part  of  the surface to another.

Mechanism  of  Ciliary  Movement :

 Although  not  all Chapter 2  The Cell and Its Functions aspects  of  ciliary  movement  are  clear,  we  do  know  the following:  First,  the  nine  double  tubules  and  the  two  single  tubules  are  all  linked  to  one  another  by  a  complex  of protein  cross-linkages;  this  total  complex  of  tubules  and cross-linkages  is  called  the  axoneme.  Second,  even  after removal  of  the  membrane  and  destruction  of  other  elements  of  the  cilium  besides  the  axoneme,  the  cilium  can still  beat  under  appropriate  conditions.  Third,  there  are two  necessary  conditions  for  continued  beating  of  the axoneme  after  removal  of  the  other  structures  of  the  cilium:  (1)  the  availability  of  ATP  and  (2)  appropriate  ionic conditions,  especially  appropriate  concentrations  of  magnesium and calcium. Fourth, during forward motion of the cilium,  the  double  tubules  on  the  front  edge  of  the  cilium slide  outward  toward  the  tip  of  the  cilium,  while  those  on the  back  edge  remain  in  place.  Fifth,  multiple  protein  arms composed  of  the  protein  dynein,  which  has  ATPase  enzymatic  activity,  project  from  each  double  tubule  toward  an adjacent double tubule.

Given  this  basic  information,  it  has  been  determined that  the  release  of  energy  from  ATP  in  contact  with  the ATPase  dynein  arms  causes  the  heads  of  these  arms  to “crawl”  rapidly  along  the  surface  of  the  adjacent  double tubule.  If  the  front  tubules  crawl  outward  while  the  back tubules remain stationary, this will cause bending.

The  way  in  which  cilia  contraction  is  controlled  is  not understood.  The  cilia  of  some  genetically  abnormal  cells do  not  have  the  two  central  single  tubules,  and  these  cilia fail  to  beat.  Therefore,  it  is  presumed  that  some  signal, perhaps  an  electrochemical  signal,  is  transmitted  along these two central tubules to activate the dynein arms.