Lesson 20

Introduction to animal development

  • development = zygote develops into a multicellular organisms

  • all complex and organized changes in development are driven by gene expression changes

major processes of development

  • cell division = makes more cell

  • cell differentiation = cells become specializes

  • pattern formation = determines where body parts develop

  • morphogenesis = actual construction of body plan

Cell division in early development

  • Early embryonic cell division ( cell cleavage )

    • rapid divisions

    • minimal cell growth between divisons

    • abbreviated cell cycle ( primarily S and M phases )

      • goes back and forth between S and M phases

      • so synthesis of DNA then mitotic division

    • original zygote cytoplasm into smaller volumes

  • Later development

    • cell cycle slows

    • G1 and G2 phases become established or lengthen

    • cells grow between divisions

Cell differentiation and stem cell s

  • makes specialized cells

  • as time goes by number of possible cell it can be gets smaller

  • so cells as an adult are going to be fully differentiated and specialized

  • Stem cells = unspecialied cells with potential to become differnt cell types/divide continually

    • catorgrized by degree of potency ( number of potential fates of a stem cell )

  • being more potent means less differentiated

  • stem cell = possibilities of becoming different cell

Process of differentiation

  • driven by changes in gene expression

  • Gradual process occurring over multiple cell division

  • progressively restricts possible cell fates

  • → cells with specific functions and characteristics

  • REMEMBER ALL CELLS IN AN ORGANISM HAVE THE SAME DNA SEQUENCE BUT DIFFERENT GENES ARE EXPRESSED IN DIFFERENT CELL TYPES     

  • Stem cells = unspecialized cells with potential to become different cell types/divide continually

    • categorized by degree of potency ( number of potential fates of a stem cell )

Stem cell potency ( MEMORIZE ) ( ordered from early development to later development )

  • Totipotent : can form all embryonic and extra embryonic tissues

    • zygote, early blastomeres

  • Pluripotent : can form all embryonic tissues, but NOT extra embryonic tissues

    • embryonic stem cells

  • Multipotent : differentiate into few different cell types

    • adult stem cells

  • Unipotent : can only differentiate into on cell type

    • Spermatogonia ( differentiate into sperm ) F


Pattern formation

  • body plan of organism

  • HOX genes. = specify where structures should be in body regions along the head-to-tail axis

    • highly conserved,

    • over years still see them from fruit flies to higher mammals

    • universal blueprint for almost all aninmals

    • control head-to-tail development

    • regulate basic fundemental aspects of body plan development

    • because they do this, if there are many changes it has deleterious(harmful) effects


  • Key concept : specific sets of expressed genes create discrete segments that become differnt body parts in teh correct sptial location

  • HOX genes in evolution

    • highly conserved

    • small changes in when, whee, or how strongly HOX genes are expressed can alter body structure and contribute to the evolution of different body parts

      • ex: frnot legs on lobsters is pincers, but other legs are normal

    • mutation can be harmful,

    • duplication can get

Morphogensis

  • actuala generation of an organisms body form

    • processes:

    • cell grwoth

    • cell divison

    • cell migration

    • change sin cell shape

    • apopotosis

cell migration in morphogeneiss

  • cell msust migrate to difffernet location for proper body development

  • migration occurs throuugh the exracellular matrix ( ECM )

Cell shape in morphogensis

  • changes in cell shape are imp. for proper ufnctoin

  • differetn cel types can adopt differnet shapes:

    • adipocyes ( fat cells )

    • muscle cell

    • sneurons

  • form = function

    • imporper shape means improper function

Apoptosis in morphogensis

  • programmed cell death

  • activated by “cell death pathway” genes

    • contents broken down and ingested and recycled

  • used ot ‘sculpt’ appendages or reduce cell #’s

  • necrosis = cell death due to injury [ not controlled ]

    • cell membrane ruptures and cellular contents are related into extra cellular space

Nuclear reprogramming

  • resetting gene expression patterns in differentiated cells so they behave like stem cells

  • requires reversal of epigenetic modifications that maintain cell identity

  • demonstrates that cell differentiation can be reversed under certain conditions

Application of nucleur reprogramming

  • reproductive cloning = creates genetically identical copies of organisms

  • Potential uses

    • reintroduction of extinct species

    • replication of livestock

  • Therapeutic cloning

    • produces patient specific tissues for medical treatments

    • potential to provide rejection-free organs and tissues

    • treatment for autoimmune diseases

SCNT in therapeutic cloning [ ? ]

  • somatic cell nuclear transfer ( SCNT )

    • transferring the nucleus and adding it to the donor ( egg/Oocyte without nucleus now becomes egg cell with transplanted somatic nucleus) , this new cell ( blastocyte ) can be cultured to produce pluripotent cell which can be used to make the tissue that you want

      • take inner cell mass from blastocyst (early-stage, pre-implantation mammalian embryo that forms roughly 5 to 7 days after fertilization ) , grow in petri dish to obtain pluripotent stem cells

      • the inner cell mass cells can be then transferred to the injured tissues

      • first successful with human cells in 2013


Induced Pluripotent Stem Cell ( iPSC )

  • Start with somatic cell ( ex: skin fibroblast )

  • introduce transcription factors

  • reprogram gene expression to produce a pluripotent stem cell

  • use specific signals to direct differentiation into desired cells types kinds of tissues into the cultured stem cells

    • chemical signals, environmental signals could be used to develop differnt

  • Pros: ethically, no eggs involved, only ever get pluripotent stem cell so less chance of making new organism