Jeff Kerr; Week 1 MED 1011; Anatomy
If development is abnormal in utero, leads to congenital abnormalities. In postnatal life, abnormal cell behaviour may produce pathological changes that can be benign or malignant (tumours). Many tissues can naturally regenerate/replenish or regenerate if damaged.
There are three major domains: bacteria, archaea and eukarya (protists, plantae, fungi and animalia). Prokaryotes (bacteria and archaea) are single cels without a nucleus and other internal compartments found in all other cells. Eukaryotic cells make up all other organisms- contain nuclei and internal cellular compartments.
Prokaryotes (such as bacteria) are enclosed in a single membrane with conversions of energy and matter called metabolism; they lack a nucleus, do not have internal organelles or a cytoskeleton. Prokaryotes quickly divide by splitting in two (fission), which allows exponential growth of strong strains. Genetic material is circular.
- Enterococcus in colitis; E. coli in diarrhoea; salmonella in food poisoning
Archae live in unusual habitats such as hot springs, high salinity, low oxygen or high/low pH environments (inside cow stomach, inside volcanic vent).
Eukaryotes are 'cells within cells', thought to have come from cells integrating into larger hosts. Genetic material is confined to the nucleus and other components in organelles.
Multicellularity is cells clumped together, some became specialised to perform distinct functions such as those close to surface, others in central regions are less specialised- some began to become specialised for sex (passage of genes)
- Examples are volvox green algae, slime mold, sponges
Cells are designed to generate order out of chaos; everything tends towards disorder (2nd law of thermodynamics), it takes energy to generate order out of chaos, by using a lot of energy cells see that the right molecules get to the right places at the right times.
Loss of rigid prokaryotic cell wall allows plasma membrane to fold inwards, creates more surface area for biochemical reactions and creates chemical conditions confined to particular membrane compartments (organelles)
Differential gene expression is fundamental mechanism for cell specialisation, achieved through crossing over of DNA and chromosomes allowing for genetic exchange which is preserved when dividing. Differentiation defines the structure and function of a cell; brain and gut cells exhibit different parts of the genome. Stages of development and growth rely on varying expression of the genotype
Cytoskeleton is made up of microtubules, which are rigid protein cylinders for cell shape and to guide organelles/molecules to destinations; microfilaments assist contraction and stabilise shape; intermediate filaments resist tension and connect cells through special junctions called desmosomes.
- Microfilaments are actin, many and thin, dispersed around whole cell widely (green on fluorescence)
- Microtubules are tubulin protein, thicker and more web-like (orange on fluorescence)
Microtubules are important in mitosis, attach chromosomes and pull them into daughter cells. They can lengthen and shorten with ease.
Amoeba can move by using the actin in microfilaments; actin fibres assemble at one end and dissasemble at the other; has attachment to substrate via integrin
Surface specialisations are linked to cytoskeleton- microvilli are slender extensions of plasma membrane; cilia are hairlike and motile at the surface, flagella is a much longer cilium for propulsion
Special junctions unite parts of the cell membrane of adjacent cells. There are 4 types of special junction: spot weld/desmosome (macula adherens holds cells together, has a large intercellular space, has cadherin transmembrane proteins that bridge the space between epithelial cells, help to resist shearing forces, found in muscle tissue), attaches intermediate filaments like keratin in skin stratum basale, spinosum, granulosum, tight junctions (zonula occludens), forms an impenetrable barrier to fluid, efficacy of the junction increases with the number of sealing strands which anchor strands to actin (microfilaments) of the cytoskeleton; gap junctions are composed of 2 connexins which connect across intercellular space, also known as nexus; zonula adherens have a thick zone of adhesion, generally occurs in epithelial tissues, linked with actin filaments, forms a belt of adhesion.
Tight junctions effectively seal two apposing cell membranes, a barrier is
created which no molecules can move through; looks a bit like two sheets of corrugated iron welded together.
Desmosomes are common in stratum spinosum, multiple desmosomes give very strong overall binding, important for skin cells. Intermediate filaments run through desmosomes and join to other desmosomes, resist stresses placed on cells by linking them, and form an extensive cytoskeletal network.