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Cell Membrane or Plasmalemma
Trilaminar in routine EM comprises a double layer of lipid molecules, in which proteins are distributed asymmetrically in a mobile mosaic pattern. Some proteins span the width of the membrane, and may vary rates of transport by changes in their conformation. Others, as enzymes, receptors, or adhesion molecules, etc. have active domains at the surface held in correct position by intramembranous domains imbedded in the lipid layer, and intracellular domains to engage in events inside the cell.

A sometimes fuzzy-looking coat of glycoprotein – glycocalyx – sticks to the external face of the membrane.

Functions of the membrane are:
Firm attachment to other cells or a basal lamina; membrane specializations for this are:
  1. junctional complexes
  2. gap junctions/nexuses,
  3. desmosomes,
  4. hemi-desmosomes,
  5. intercalated discs, and
  6. membrane interdigitations
Movement of materials outside the cell by the activity of cilia, e.g., ciliated epithelia of the respiratory tract and uterine tube. The wide-spread occurrence of solitary cilia (flagella), e.g., on neurons, adrenal cells, smooth muscle, may involve a vestigial body or one still functional. The stereocilia of the male reproductive tract are non-motile, clumped, long microvilli, probably absorptive.

Transport of materials in and out of the cell served by:
  1. Permeability (selective) of the membrane,
  2. Active transport through the membrane,
  3. Endocytosis, and its more scaled-up forms - pinocytosis and phagocytosis,
  4. Exocytosis; and increased exchange surface area by
  5. Microvilli (thousands on a cell), and
  6. Infoldings of membrane. Communication
  7. Gap junctions permit ions and excitation to spread from cell to cell, and unify and synchronize actions of many cells/cell assemblies.
Molecules: Wherever such actions are described, special molecules are acting, by binding to each other, changing their conformation or some other means. Examples are:
  1. Spectrin/fodrin provides a subplasmalemmal skeleton attached to the cell membrane by ankyrin, and to actin of the cytoskeleton, to permit control of the membrane’s shape and movement.
  2. Cell adhesion molecules (CAMs) allow cells to attach to only certain cell types or substrates.
  3. Integrins are cell-surface-membrane dimeric molecules (an alpha with a beta), by which cells choose to which extracellular matrix (ECM) components they wish to fasten, e.g., laminin.
  4. Connexins are proteins that combine as hexamers to form connexons – the gap-junction channels, allowing ions and small molecules to pass between cells. Connexins and the transports allowed vary among liver cells, neurons, etc.
  5. Occludins are responsible for the seal preventing passage of materials past inter-epithelial tight junctions.
Cytoskeletal (Filamento-Tubular) System
In absorptive and secretory epithelia, actin filaments are a major component of the apical terminal web inserting into junctional complexes and running up inside the microvilli. The actin is responsible for the movement involved in endocytosis and exocytosis.

Microtubules: Each microtubule is built of 13 tubulin filaments. The dimers constructing the filaments confer a repeated polarization along the tubule so that one end is ‘plus’, the other ‘minus’ - a difference that tells the attached microtubule motor proteins which end to head for.

Microtubules, not in a doublet or triplet formation, are supporting or cytoskeletal elements used, for instance:
  1. to give some shape to platelets,
  2. to cause and orient the elongation of cells, e.g., ameloblasts,
  3. to allow neurons and glia to grow long processes and keep them patent ;
  4. microtubules are responsible for axoplasmic flow.
  5. Microtubules direct certain materials to regions of the cell membrane, e.g., basolateral, creating cell polarity. Microtubules are dynamic structures in themselves and their construction and disassembly from tubulin dimers are under constant control.
Centrioles are cylinders open at one end with a wall composed of microtubules disposed longitudinally as nine triplets. The centrioles and additional microtubules direct and move the chromosomes along the mitotic spindle in cell division.

Cilia have, at their base, basal bodies, each identical with a centriole and often developing from one. Of the triplet, two microtubules extend up inside the cilium which thus has a core of nine peripheral doublets, plus an additional central pair. The doublets, by a sliding-tubule action using dynein arms, move the cilium to beat strongly in one direction by a bending-wave action.

Flagellum (tail) of the spermatozoon uses a cilium-like array of microtubules, aided by very thick fibres, for powerful swimming.

Sensory cilia containing microtubules occur on cells in the special sense organs, e.g., olfactory, and play some role in sensory transduction.

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