Cell Surface Specializations
- Lateral surface
o Junctional complexes seen as “terminal bar” on LM
 Corresponds to zonula occludens, zonula adherens and macula
adherens in EM
o Zonula cccludens (tight junctions)
 Region where outer leaflets of adjacent PM fuse to form a zone
around the entire apical perimeter of cells
 Complexity and number of strands determines “leakiness”
 Prevents entrance or exit of substances into the intercellular space
from the lumen
o Zonula adherens (intermediate junctions; belt desmosome)
 Extends completely around perimeter, just basal to the ZOs
 Opposing PMs are reinforced on cytoplasmic surfaces by a mat of
actin filaments  extend into the terminal web
 Fasciae adherens are analogous structures found in the intercalated
discs between cardiac muscle cells
 Intercellular space often contains amorphous or filamentous
material
o Macula adherens (desmosomes)
 Focal, disc-shaped adhesive junction between adjacent cells
 Dense plaque on each cell’s cytoplasmic side
 Intermediate keratin filaments (tonofilaments) look in and out of
plaques
 Intercellular space between the two halves may have material that
aids in stabilization
o Gap junctions (nexus, communicating junction)
 Array of 6 subunits
 6 connexins make a connexion
 Common between cardiac, smooth muscle cells and in nervous
cells for communication
 Allow passage of ions between cells for communication and
coordination
o Lateral interdigitations
 Finger-like projections that lock together adjacent cells
-
Basal surface
o Basal lamina (basement lamina)
 Acellular supportive structure, visible only with EM
 Composed mainly of type IV collagen
 Two zones
 Lamina rare (lamina lucida) – low density
 Lamina densa – more dense, under rare
 2 zones + reticular lamina = basement membrane
 Reticular lamina part of dermis
o Hemidesmosome (half desmosome)
 Attachment basal surface of cells to underlying basal lamina
 Structure is half of a desmosome
o Basal plasma membrane infoldings
 Common in ion transporting epithelia
 Form deep invaginations that compartmentalize mitochondria
 Provide energy for ion pumps
-
Apical surface
o Microvilli
 Finger-like evaginations that extend into the lumen  increase SA
 Comprise brush border of kidney proximal tubule cells and striated
border of intestinal absorptive cells
 Glycocalyx on exterior surface
 Branched terminal oligosaccharides  brush border is PAS
positive
 Contain bundle of ~30 actin filaments that extend from the
microvillus core into the terminal web (between ZA)
o Stereocilia
 Not cilia  long microvili
 Found in epididymis of the male reproductive tract
o Cilia
 Actively motile of certain epithelia for transport of substances
along their surface
 Tracheobronchial epithelium, oviduct
 Covered by plasmalemma
 Each cilium cotains an axoneme
 9+2 doublet pattern of microtubules
 From one of the doublet microtubules, dyenin ATPase arm
exits unidirectionally
 At base of each cilium, cylindrical basal body
 Identical to structure of centriole
o 9+0 triplet pattern of microtubules