Developmental Biology - Biology 4361Role of Cells in Development / GametogenesisSeptember 13, 2005Germ line concept - dual origin of gonads- germ line: lineage of cells from which gametes arise (v. somatic cells; soma = body)includes zygote, blastomeres that give rise to gametes (and somatic cells)- germ line concept: slide - gametes are essentially immortal, other (somatic cells) -eventually die- many animals (insects, roundworms, vertebrates) - clear and early separation of germcells from somatic cells- others (cnidarian, flatworms, tunicates, all plants)- somatic cells can become germ cellseven in adult organismsNOTE - This brings up an important point: there is tremendous variation in alldevelopmental systems; many similarities, but variations in fine points; also manyfundamental differences. We will try to deal with important or well-understood systems,note some of the variations.Primordial germ cells (PGCs; cover in later section) - associate with somatic cells to form gonads- different species - different mechanisms- PGCs arise outside of presumptive gonad and migrateMeiosis - slide point of gametogenesis: production of haploid cellsmeiosis - separation of homologous chromosome pairs and formation of haploid daughtercells containing only one chromosome (one allele) from each pairtwo successive divisions without DNA replication = four haploid cells1st meiotic division: pairs of homologous chromosomes segregated between twodaughter cells2nd meiotic division: separates two chromatids of each chromosome - creates fourhaploid cellsMeiotic prophase I: chromosomes coil and shorten - resembles mitotic prophase - fundamental differences: 1. twin chromatids of each chromosome held together along entire length (axialproteins)2. homologous chromosomes join together in pairs - connected by centralelements (longitudinal and transverse proteins) = synapsissynaptonemal complexcrossing overrecombination nodule - enzymes necessary for cutting and splicingMeiotic metaphase I:chromatids visible; crossed at chiasmata (NOTE - two chromatids within chromosome held together at centromere)kinetochore fibers extending from each chromosome oriented towards oppositespindle polesMeiotic anaphase Ihomologous chromosomes pulled apart at centromereschiasmata resolveMeiotic telophase Itwo daughter nuclei formed - ones set of duplicated chromosomesnon-identical chromatids because of crossing-overMeiosis IIeach centromere sends kinetochore fibers to both spindle poleschromatids separated as in mitosisformation of four haploid cellsNOTE - genetically non-equivalent - independent assortmente.g. in humans 23 chromosome pairs, therefore, 223 (~8 million) differenttypes of haploid cells formed from genome of single person; alsocrossing-over further increases genetic diversitySlide - meiosis in spermatogenesis and oogenesisSpermatogenesisseminiferous tubulesstructured environment - ordered array of cell types (and developmental stages)epithelium - Sertoli cellsSertoli cells - structural and functional; - form extensions connected by tight junctions = blood-testis barrier - interior compartment (spermatocytes, spermatids, sperm) sequestered fromblood and lymph; - environment controlled by Sertoli cell secretions plus germ cells- germ cells lodged within Sertoli cellsSertoli cells: 1) inhibit spermatogenesis prior to puberty2) stimulate spermatogenesis post-puberty3) provide nutrients4) send and receive hormonal signals- fetal development - release anti-Mullarian duct hormone - inhibits developmentof female reproductive ducts- respond to testosterone produced by interstitial cells (in seminiferous tubules)spermatogonia = reproductive stem cells (%) 1. Undifferentiated2. Capacity for self-renewal3. Give rise to committed progenitor cells (undifferentiated, but committed to formcertain cells)committed progenitor cells - Primordial Germ Cells (PGCs) amplifying divisions (e.g. spermatogonia A1 - A4)then commit to spermatocytes - enter into meiosiscommitted progenitor cells - Primordial Germ Cells (PGCs) go throughamplifying divisions before becoming differentiated cells (other examples- blood blast cells)- amplifying divisions - spermatogonia A1 - A4- then divide once to primary spermatocytes - commit tomeiosis/differentiationspermatogonia and maturing sperm connected by cytoplasmic bridges - incomplete cytokinesis post-meiotic division) - syncytium- ~1 µm diameter- purpose? bridges allow gene products from spermatogonial X and Ychromosomes to interact with developing sperm - cells haploid butfunctionally diploidspermiogenesis: sperm maturation/differentiation- acrosome formation - modified Golgi apparatus- flagellum- mitochondrial sheath- nuclear condensation - transcription in early spermatids; translation delayed several days- histones replaced by protamines- protamines - small arginine-rich (~60%); complete shutdown oftranscription; DNA assumes almost crystalline structure- residual bodies - cytoplasmic dropletshumans - 65 daysmouse - 34.5 daysNOTE - many species do not follow pattern described above; e.g. roundworm = ameboidOogenesis- oogenesis starts much earlier than spermatogenesis; takes longer to complete- mechanisms of oogenesis vary among species more than those of spermatogenesis- patterns of reproduction vary so greatly among species; e.g. thousands of eggs or one tovery few eggs- in species with many thousands of ova, oogonia are self-renewing stem cells that endurefor lifetime of organism - in species with fewer eggs = oogonia divide to form limited number of egg precursorcellse.g. human- ~1000 oogonia divide rapidly from 2nd to 7th month of gestation- form ~ 7 million germ cells - after 7th month number of germ cells drops precipitously; most oogonia die; - remaining enter first meiotic division = primary oocytes; - progress through first meiotic prophase until diplotene stage; - maintained there until puberty; - at onset of adolescence, groups of oocytes periodically resume meiosisSummary: first part of meiosis begins in embryo, signal to resume meiosis not givenuntil roughly 12 years later; some oocytes maintained in meiotic prophase fornearly 50 years; only about 400 primary oocytes mature through woman’slifetime- requirements of eggs differ from sperm- incorporate one sperm (block the rest)- provide molecular building blocks- structural information - protective envelopes for embryo- eggs - contain a great deal of maternal RNA, proteins- incorporated into oocyte during meiotic arrest (growth