Lecture 5 Specialised Cells 2

Context & Scope

  • Lecture 3 surveys four specialized cell types in multicellular (especially vertebrate) bodies
    • Focus areas: sex cells, epithelial cells, connective-tissue cells, nerve cells
    • Muscle cells explicitly skipped (“everyone knows about muscles”)
  • Constant theme: how body-level specialization serves the gene line’s “immortality” and the long evolutionary struggle from single-cell to complex organisms
  • Extra-reading essays (optional but illustrative)
    • “Coming to grips with male nipples” → Why male mammals retain apparently useless mammaries
    • “The lengthening limits of life” → Whether ancient life can be prolonged or even resurrected (e.g., dinosaurs)

Sex Cells (Germ Cells)

  • Evolutionary backdrop
    • Early eukaryotes deploy diverse reproductive strategies (budding, clonal fission, sexual fusion)
    • Volvox colonies show first clear division into somatic vs. germ cells
    • Somatic layer “holding hands” protects interior sex cells
    • When mature, sex cells burst out, destroying nursery—pattern recurs in higher taxa
  • Sperm diversity & fossil record
    • Morphologies look similar yet encode radically different genomes (koala vs. wombat sperm examples)
    • 19\,\text{Ma} Miocene ostracod deposit (NW Queensland): soft tissues, cells, nuclei & spiral chromosomes preserved—oldest known fossil sperm
  • Transition to land ⇒ necessity for internal aqueous environment
    • Organisms “brought the pond inside”; sexual intercourse keeps gametes hydrated
    • Asian swallowtail butterfly sports a penile “eyeball” → visual targeting of female tract
  • Embryonic origin & migration
    • Primordial germ cells (spermatogonia & oogonia) originate on gut epithelium, then migrate to gonads
    • Spermatogenesis: sequential divisions → spermatids → \sim3\times10^{8} sperm / ejaculation
    • Oogenesis: ~5\times10^{5} primary oocytes per ovary peak; follicular “nursery” ruptures at ovulation, mirroring Volvox
  • Inefficiency & casualties
    • Only \approx1 of 3\times10^{8} sperm reaches an egg; stripped-down design (nucleus + flagellum) maximizes speed but limits cytoplasmic contribution
    • Embryo’s cytoplasm \approx99.9\% maternal; sperm likened to “viral DNA delivery” → provocative idea that males are a 1.5-billion-year-old STD
  • Anatomical imperfection: ovary–tube gap
    • Creates risk of ectopic pregnancy (tubal, ovarian, abdominal)
    • Abdominal implantation acts as “super-parasite,” forming placenta on peritoneum
    • Implies theoretical feasibility of male pregnancy (if embryo implants on abdominal wall)
  • Male nipples & latent lactation
    • Males retain milk ducts; sustained stimulation can yield colostrum (observed in Aka Pygmies)
    • Cultural nod: Robert De Niro’s “Männerary” gag in Meet the Fockers
    • Tocophobia = fear of childbirth now hypothetically relevant to men

Epithelial Cells

  • Definition: continuous sheets lining external & internal surfaces, plus derivatives (glands, teeth, etc.)
  • Primary roles
    1. Protection
    • Simple (single-layer) epithelia → thin, allow exchange; e.g., lung alveoli
    • Stratified/compound epithelia → thick, resist abrasion; e.g., soles, oral mucosa
    1. Secretion
    • Cells over-produce specific molecules → exocytosis
      • Lung epithelium: mucus (traps dust/bacteria); cilia (former symbiotic spiro-bacteria) sweep toward pharynx
      • Smoking immobilizes cilia → smoker’s cough
    • Gland evolution
      • Exocrine: epithelium invaginates; retains duct (salivary, sweat). Muscle around duct prevents constant drooling.
      • Endocrine: duct degenerates; secretions enter bloodstream (pancreas, thyroid, adrenals)
    1. Embryological fountainhead
    • Early blastula: all cells pluripotent; fate becomes fixed after gastrulation
    • Fate-swap experiments in chick embryos show omnipotence pre-determination
    • Gastrulation: epithelium “punches in” → two layers; in deuterostomes, first opening becomes anus, second mouth
  • Teeth as modified epithelium
    • Shark skin covered in dermal denticles (= true enamel–dentine teeth) used for prey-testing via abrasion
    • During vertebrate development, oral epithelium invades jaws, forming tooth germs; eruption parallels shark skin origin
    • Absence of anal teeth attributed to lack of selective advantage
  • Developmental mishaps → Dermoid cysts
    • Determined epithelial fragments misplaced; form cysts containing teeth, hair, bone, even mini-brains
    • Examples: ovarian dermoid with teeth, dog’s nasal teeth, boy with 526 teeth removed, hairy ocular cysts
  • Cancer susceptibility
    • \approx75\% of human cancers arise in epithelia
    • High mitotic rate (needed for wear-and-tear) may slip into uncontrolled proliferation
    • Direct exposure to environmental carcinogens (lung epithelium vs. liver)

Connective-Tissue Cells

  • Characteristic: importance of extracellular matrix (ECM) over cell body
    • Blood → fluid ECM (plasma) secreted by blood cells
    • Bone → osteocytes secrete mineral matrix, ultimately imprisoning themselves
    • Fibrous connective tissue → collagen (≈35\% of body mass) provides tensile strength
  • Molecular fossils & de-extinction prospects
    • Dinosaur bones yield preserved collagen & proteins → potential reverse-engineering of original DNA
    • Amber studies (≈40–100 Ma)
    • George Poinar recovers insect DNA matching fossil identity (e.g., fungus gnats)
    • Controversial claims of revivable bacteria from amber (evidence lacking)
    • Finds: hemoglobin in 46 Ma mosquito; tick entangled in dinosaur feather (\sim99 Ma)
    • “Living” vs. “dead” biomolecules
    • Poinar’s criterion: any entity able to grow, reproduce, or synthesize protein when placed in optimal conditions is alive
    • Demonstrations:
      • Tasmanian tiger (thylacine) DNA expressed in mouse, producing blue-green proteins
      • 31{,}000-year-old Silene seeds germinated
      • Schweitzer’s t-rex capillaries lacked DNA but sparked search momentum
    • Jack Horner’s “Chickenosaurus” program
    • Birds are living dinosaurs; modify bird genome to re-express ancestral traits
    • Experiments: chicken oral epithelium + mouse mesenchyme → hen’s teeth (curved, dinosaur-like)
    • Gene tweaks produce chickens with crocodile-type snouts
    • Pet dinosaurs deemed inevitable as gene-editing advances

Nerve Cells

  • Derived from epithelial lineage; specialized for rapid electro-chemical signaling (>100\,\text{m·s}^{-1})
  • Solve limitations of hormone-only coordination in large bodies (speed, on/off control)
  • Emergent brain complexity
    • Initially for motor coordination; evolved internal interactions → cognition, planning, “mind”
    • Creates tension: germ-line drives (sex cells) vs. executive brain decisions (psychozoic era)
  • Existential twist: technology & the Drake equation
    • All factors favor abundant intelligent life except L = average lifespan of a technological civilization
    • Mammal species persist 2–5\,\text{Ma}; Homo sapiens only \sim3\times10^{5} years old
    • Nuclear brinkmanship illustrates fragility; survival hinges on preventing self-inflicted extinction

Cross-Cutting Themes & Implications

  • Specialization trades cellular independence for organismal complexity, but introduces vulnerabilities (cancer, ectopics, developmental anomalies)
  • Evolution repurposes existing modules (skin teeth → mouth teeth, cilia → mucociliary escalator)
  • Fossil biomolecules blur line between past and present; resurrecting traits—or organisms—appears technically feasible
  • Intellectual emergence (nerve cells → brains) empowers but endangers the species; extending the “L-factor” may become biology’s ultimate project