Embryology PDF

“Inside the lump was my twin” A short scene from Nia Vardalos’ film: “My Big Fat Greek Wedding” in which Toula’s conservative in-laws-to-be are introduced to eccentric Aunt Voula. https://www.youtube.com/watch?v=mWispEM3900 How on earth is this relevant to our course? It touches on: Pattern forma...

“Inside the lump was my twin” A short scene from Nia Vardalos’ film: “My Big Fat Greek Wedding” in which Toula’s conservative in-laws-to-be are introduced to eccentric Aunt Voula. https://www.youtube.com/watch?v=mWispEM3900 How on earth is this relevant to our course? It touches on: Pattern formation and differentiation Embryogenesis Twinning Chimerism Twins Twins two or more offspring from a single pregnancy, either from a single zygote or multiple separate zygotes. There are 4 different types of twins: monozygotic, dizygotic, multiple, and vanishing twins. Zygote: (From the ancient Greek for “joined” or “yoked” It refers to the earliest stage of development – just after the sperm and egg have fused to create a diploid cell. In some books and literature, the term is also loosely applied to cover the first few cell division stages too.  Ex Fever. , comefrommygote 100se - connection Monozygotic membrane 2 ruptures sep separate embryos Twins: both derived from a single fertilized egg 1 1 egg sperm + Wolpertet al., p. 137 Box 3F, Fig 1 earlier splitting = Separate amnions chorions, , and placent as  2 ovaries shed at same time Dizygotic 2 Fertilization = events occur twins: diff + diff eggs sperm develop from separate fertilizations NOT IDENTICAL - different genetically Raising Children Network (Australia) Limited: https://raisingchildren.net.au/pregnancy/ health-wellbeing/twin-pregnancy/twins  diff experience diet , stress ↑ RARE Z - ↑ - multiple not for twins at same time good mom + ↓ d embryos metaboli Ente resources 2 indir growing > I looses out =. Ex Cells rise to. give vertebrae take cells insert into early embryo = - trace lineage Chimerism W which structures Chick vs. quail - occurs naturally Chimerism: genetic chimera occurs when a single organism forms comprising cells of more than one distinct genotype. Chimeras can be formed naturally by: either the fusion of embryos or exchange of cells; or by experimental manipulation Mix/March Experiments knock Out experiments press 2 Stick + E cumulate embryos into tog ether single mass Two separate embryos used as source materials to produce experimental combinations (mouse on left p. 24, fig 1.24; quail/chick on the right p. 119, fig. 3.30)  Chimerism : distinct cell lines ↳ an individual has two or more genetically Profound Chimerism of 2 - comprised The Curious Case of Lydia Fairchild: dif genomes Mother abandoned to raise 3 kids, with a 4th on the way Law requires paternity test to prove father owes support payments Spouse proved to be father of first 3 children, but Lydia appears not to be genetically related Court appoints observer and tests upon delivery of third, father’s contribution proven, Lydia’s not… Was she trying to swindle the welfare system to augment a surrogacy business? More tests done of Lydia (hair follicle, skin in addition to saliva, cervix, blood). She is chimeric (composed of two different genotypes) - - ↳ effect of 2 embryos fused  Profound Chimersim Washington Fertility clinic mix-up? Father’s genome represented only ~10% of offspring’s genome. sperm mix up - > - someone related to Should be 50% - a sperm sample mix-up? - him ? / Father was the product of a vanishing twin – brother’s DNA - no relatives contributed to his germ cell line (sperm). He was both father sperm and uncle to his children 2 prior were his, one was his in bank vanished brother)… Some of brother’s cells came to rest and formed sperm primordia in his developing testes (more of this aspect of development in a later class). - 2 ind in uterus contribute cells to - early stage embryo populate sperm lineage How prevalent is profound chimerism? - - Vanishing twins…. Unknown effects, unknown Important in rare blood transfusion problems, transfusion and transplant engraftment, rape cases, sports blood doping, House MD episodes  Stem Cells Fetus (delivered Mom aborted miscarried) - , , - her mother (older Siblings) Microchimerism Coexistance of Fetal and maternal cell lines happened when in the same organism early Stage. Alzheimer’s study low Frequency > - Found Y-containing cells (male cells) in female brain embryos Microchimersim detected in both healthy and diseased brains Other studies have found male cells in females’ kidney uterus, blood, etc. Where did cells come from? Bi-direction trans-placental trafficking between mother – fetus Mother/fetus placental blood barrier formerly thought to be - impermeable – turns out to be quite leaky to stem cells. 6. - > nutrients - Exchange stem cells gases exchanged , metabolites gifts & helps to mother repair remain reproductively to and viable Optimal stem cell load with 3-4 kids (fewer or more children reduced stem cell numbers) - - permeable to cells from mom and offspring Older brother => younger brother cells Microchimerism Fetus acquires stem cells from mother Mothers can pick up stem cells from several sources: Via her own mother who herself hosted male stem cells: - - Later children can pick up stem cells left in residence by former - - - inhabitants of the womb (elder siblings) – so mother born with - elder brother’s stem cells Mother can acquire stem cells from including All offspring - Fetus that later miscarried - Fetus that was later aborted - Microchimerism – Why??! Birthing is hard on mother Provision of stem cells aids in recovery and recuperation Contingent protection against some diseases? Example – Lupus - Some mothers have a susceptible genotypic allele Some fathers posses a resistant allele Mothers mated to resistant fathers may be slightly more resistant to developing Lupus. Presumably, alleles conveyed by resistant allele fetus colonize the mother and produce either marrow or circulating blood stem cells that carry the resistant paternal allele. Stem cells that contribute to metapoidic system/resistance Critical Problem of Developmental Biology: Differentiation How do cells know to become: The right cell type In the right place At the right time EG; osteoclasts, myocytes, keratinocytes, neurons etc., usually develop in the right place. How does the process start, and how is it regulated? This is the central question of embryology/development Stem Cells # Cells with the capacity to differentiate into other cell types # Examples: Freshly fertilized embryo/zygote Hematopoietic cells in bone marrow retinol Induced pluripotent stem cells (iPSC) > - degredation rescue Ex bling rendered going. Cancer stem cells totally blind Stem cells can have different capabilities to differentiate – the range can be narrow or broad, and this range of possibilities, of futures, is called potency. The greater the number of cell types - the stem cell can differentiate, the greater the potency. knock out tumour tumours aren't single > - - a cell type but stem cells from helerogenius (many diff cell types) which it rid of - arose hard to get  become of cells victim history Stem Cells bladders & pig corpse - decellularize (detergent) > - all left is ECM > - cultivate media lab constraint - > insert Stem cells - Ex Smooth. grow muscle > - transplant into host function as normal b ladder Constraint - to channels restrict development now packed tog - snape - From Wikipedia: https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Stem_cell_division_and_ differentiation.svg/512px-Stem_cell_division_and_differentiation.svg.png  Stem Cells periodicity Fig 8.14  Categories of Stem Cells Totipotent: These stem cells have the most potential and can become all toti-totally cell types within that organism. This includes the embryo’s body, but also total organisms its germ cells and to its extraembryonic structures such as the amnion, Stem cells become anything umbilicus and parts of the placenta. The first cells of an embryo have this potency. give rise to any cell type Pluripotent: These stem cells can give rise to any cell type within an plentifully organism (with the exception of placenta and other extra-embryonic Pluripotent = not placenta structures). An example of these type of cells would be iPS, or induced pluripotent stem cells currently being studied for therapeutic deployment in humans. A stem cell that can differentiate into any cell lineage contributing to the body is pluripotent. Multi = Multipotent: These stem cells have much less potency then the two stem Multiplayer cells mentioned. They can still differentiate into a wide variety of cells characterlbut, a restricted to a certain lineage of cells. An example would be can be any 6 to game a video hematopoietic stem cells which can derive into red blood cells, (lineage) lymphocytes, granulocytes, or macrophages. - - - - Unipotent: These stem cells have the least potency, and they serve as the - precursor for a single lineage. An example of this would be keratinocytes - which differentiate into mature keratinocytes throughout their lifespan. Oogonia and spermatogonia are also examples. - - 3 line stem cell germ epidermal (producing sperm) stem cell /producing swin) What Directs the trajectory of Differentiation? Constraints refer to factors/circumstances channel the trajectory of stem cell differentiation. Two major concepts that determine when and where stem cells differentiate are canalization and niches. Canalization refers to confining differentiation to a single path, although there are multiple potential paths that can be taken under different situations. Specific envir that. limits the potency Niches function as constraining microenvironments. These microenvironments may contain the conditions (pH, temp, , cell packaging, inter-cellular connectivity etc.) and cellular - - - - signals needed for differentiation. Can be viewed as accident - - of history and placement.  Example of Canalization/Contraints Teratocarcinoma (relating to monsters – cancer) & start primordial as Stem cells Probably derived from male mouse primordial germ cell (early stage stem cell to sperm) Tumours contain mixed up tissues – bits of hair lots of tissue/ follicle, tooth, smooth muscle, skin, intestine. organ types Can be transplanted between mice that are immuno-compromised (nude mice: inbred line) Can be cultured Cultured cells can be transferred to mouse hosts – tumour grows  Teratocarcinoma DON'T GIVE RISE TO GERM CELLS normal hariotype - body of early stage embryo mouse embryo ↓ normal' contribute lls embryo is chimeric terecarcinoma cells Fig 8.36 Teratocarcinoma # * Lesson: the context a cell is placed in helps to determine its differentiation future limited by Niche revisited: intestinal crypt cells crypt - epithelial extensions the wall covering of the small intestine Fig 8.25 Intestinal Crypt - The stem cells from which the intestinal epithelium Fig 8.26 is renewed are found at the base of the crypts. villus upwards protect - leave the crypt- > epithelial cell types & more shed from the tip - most routinely involved in renewing the epithelium. Fig 8.25 Human Pluripotent Stem Cells (ES Cells and iPSCs) giverise a - t type Select for cells that continue to proliferate The Challenge Induce differentiation in a specific manner – don’t want wrong cell type or a tumour to develop 1. Constraints? Provide adequate context to canalize Examples – de-cellularized scaffolds (usually organs or tissues - treated with detergent to kills and remove cells leaving only the - extracellular matrix behind): - Dog bladders (successful) Human skin/heart (studies) Human esophagus (fraudulent report) 2. No adverse effects –tumours - 3. Transgenic iPSCs means genetically modified humans – long term effects on cell genome, human susceptibility? Waddington – Epigenetic Landscape (1958) development that is not genes CPH , cell packing density) cell environment = at various the cell points (ball) can take specific permittedto trajectories different , leading outcomes or cell Fates Baedke, J (2013): Studies in History and Philosophy of Science Part C: Biological and Biomedical Sciences. December 2013, 44(Part B) Epigenetic Landscape Examined & imposes constraints progressively harder to get back up It is hard to reverse differentiation to Stem Cell Once committed, it is hard to alter fate Question: How do cells know where they are in order to differentiate into the right cell type in the right place at the right time? CELL SIGNALING  never transcription factors MAP What is a Morphogen? um used Signaling molecules to develop a Definition: SHAPE map t a plan A diffusible substance which distributes in a varied manner to endow different locations with different concentration values. In a very real sense a morphogen establishes a map according to which cells receive specific cues and can differentiate accordingly. molecule secreted outside the celle follow the leader A Spectacular Example: Dictyostelium discoideum (myxamoeba or slime mold) roods low s aggregate needasig a lar - Forage Unicellular amoeba respond to a specific cue (diffusing cAMP) to migrate together to form a multicellular slug (grex). This slug migrates, eats bacteria, other slugs, and then comes to rest to form a stalk and fruiting body. gives off - give rise to spores spores -> Chemotactic movement stimulated by secreted cAMP, = opposed by cell surface degradation by phosphodiesterases.  STARVING CELLS & CAMP Slime Mold Migration Secreted ↳ ↳ adjacent degraded cells Secrete CAMP by phosphodiesterase (free (a2t) ↓ # form Slug cAMP secreted by starving cells migrates , & eats backrig , Other slugs , comes to rest Recruit adjacent cells to also secrete cAMP and to migrate to ↳ forms a form grex (slug) - > single focus move to Stalk and Fruiting cAMP is degraded by phosphodiesterase, and this involves body gives off free Ca2+ spores Movement can be monitored either directly (watching cells move) or using Ca2+- sensitive fluorescent markers cAMP secretion, degradation, movement occurs in highly structured waves of activity See Amoeba aggregation video: https://www.youtube.com/watch?v=tpdIvlSochk See chemical wave activity (Ca2+ from cAMP activity): https://www.eurekalert.org/pub_releases/2019-02/ou- mpt020619.php MTS orientaled in all - : same direction Illuminates when there is a wave of Cad + light goes the same A Chemical Model: Belousov- Zhabotinsky reaction Belousov (1956) tried to model Kreb cycle using a simplified mix of chemicals pressence/absence of ions Add a pH indicator, and much to his surprise got a test tube X that oscillated between colours for quite some time (tried to 2 dark/clear get it published, but journal editors thought he must have made a mistake) Zhabotinsky (1970s) refined and repeated the experiment, but flattened into a Petri dish. Highly order waves of colour change seen. (publication success!) Signals that B reaction in 3D mixing volume form and disipate https://www.youtube.com/watch?v=dMF4RjiITGM highly in ordered Fashion BZ reaction in flattened “2D” static volume: https://www.youtube.com/watch?v=jRQAndvF4sM  Reaction Diffusion Models This type of oscillation, at its simplest, can be formed from two components: Activator (locally centred) that stimulates production of an inhibitor competinrisea Inhibitor is fast diffusing. EX. -CAMP into media react w cell inhibitor produce It turns off production of the activator, and give tostandingget indirectly, therefore, also the stimulus for itself. With inhibition falling off, the activator can then be synthesized again. Reaction Diffusion model is also called a Turing model after Alan Turing (father of computer science, portrayed by Benedict Cumberbatch in the film “The Imitation Game”). Published “The chemical basis of morphogenesis” in 1952. Modeled reaction diffusion system mathematically to explain holstein cow black/white coat colour, zebra stripes, butterfly wing spots etc.  Morphogens If slime mold is an example, then chemical cues can direct cells - into highly organized patterns of behavior. - Lewis Wolpert (author of your text), a famous developmental biologist formulated the French Flag Model to explain how cells respond to cues in developing systems: of tissue If a peice transplanted 62 from one region of an embryos embryo to another retains sells in its identity of origin , but differentiates to embryos not according its new positional instructions specialized ↳ took out a peice of each acts like reciprocal graft ↓ top right ↳ put in diff place acts like - bottom right French Flag Model ↳ emersed in media conc gradient morphogen & experience diff amount of morphogen ↳ differentiale on dep cone gradient onlyadistinguinas Morphogen Model Requirements Morphogens must be diffusible/spread externally excreted Cells must be competent to perceive slight differences in concentration temporal Window see and to to respond signal a Cells must be competent to respond by performing some specific activity (differentiate): Migrate Die Grow Change shape Secrete substances (matrix, hormones, other cues) Divide Go into quiescent state A Real Life Example: Planaria Flatworm lives under logs and feeds on bacteria Very simple body plan like a flattened sock with a small centrally located pocket (a gut that everts to secrete enzymes and absorb nutrient). ↳ enzymes pocket stomach on digest food Two “eye” spots are light sensitive remain in dark Masters of regeneration! can cut them and they will regenerate of the what is totality missing https://en.wikipedia.org/wiki/Planarian  Monotonic Gradient Model receive differential morphogen concentration [Head Morphogen] [Tail Morphogen] high head come I low tail cone Cells at plane of amputation: > - smouth a gradient info Experience a positional discontinuity missing Start by Proliferating - reconcentrating the gradient > - Try to smooth the gradient Planaria Regeneration Mario Ivankovic et al., 2019. Development. 146, dev167684. doi:10.1242/dev.167684 Other Test Systems: Cockroach Cheap to catch and rear Plentiful supply Go through several developmental molts Regenerate between each molt Can graft easily – chitinous exoskeleton binds with crazy glue!! Cut and paste grafting experiments peice of amputation glue onto stump Graft-Host Recombination Effects on Gradients and Regenerates: Bryant and Iten cells of amputation plane recog doa morphogen u and reconstitute regenerate gradient reconstituted proliferate reestablish morphogen gradient Intercalary Regeneration: Proximo-Distal discontinuity d respond On both sides of plane - between 2 amputation planes proliferate / Smooth discontinuity - ↑ gap reverse direction positional discontinuity fill shortest - way possible low to high Intercalary Regeneration Cells experience positional discontinuity Cells proliferate Increased mass of cells reconstitutes gradient Gradient directs cell differentiation pathways If two disparate values are juxtaposed, smoothening occurs to cover the distance by Intercalary Regeneration Intercalation occurs by the shortest route  Other Experimental Juxtapositions: Bryant’s Clockface Model (Polar Coordinate Model) Imagine limb as a cone, then Cut off graft and rotate or; Switch left graft to right host Cells in the limb field know their relative to the circumferential position and ( axes of the limb (ie. anterior/posterior dorsal/ventual or coordinates 1 - 12 After McCusker et al., 2015. Regeneration. doi: and along the limb axis (A-E) 10.1002/reg2.32 proximal-distal  New Kind of Juxtaposition or Discontinuity Created ↳ take of regenerating If circle a complete mound of cells from wrist positional information is present ↳ in a regeneration - competant regenerating cone on to environment , then a limb left host site Will form I can't have graft and all smooth create discontinuity & cells proliferate to and the try smooth difference make new fields of information and growth After McCusker et al., 2015. Regeneration. doi: 10.1002/reg2.32  Result? Supernumerary Limbs generate extra This is exemplified by the formation of supernumerary limbs when the stylopod from a left forelimb is rotated 180° and grafted to the stylopod of the right forelimb, which generates positional discontinuities that result in the intercalation of complete circles of McCusker et al., 2015. Regeneration. doi: positional values at the dorsal/ventral poles 10.1002/reg2.32  Flowers: Simple example of morphogen diffusion patterns at work > - large genomes Some things that differentiate Plants from Animals Plant cells encumbered/bound by a rigid cell wall Plant cells don’t migrate Plant cytokinesis at cell plate, not by constriction All somatic cells contain the capacity to generate germ cells (pollen and ovum – ie; sperm and egg), not set aside to specialized cells as in most animals Flowers emerge from shoot apical meristem. Morphogen gradients act upon this hemispheric bud of cells to activate downstream patterning genes – MADS transcription factors. not MADS do for plants what homeobox genes do for animals. a Simplistic MADS model (don’t bother with the gene names) morphogen Illustrates how the genes direct flower development. Arabidopsis: model system short generation time Cabbage and mustard family Smallish genome (for a plant - plant genomes tend to be large – why? They can’t move away from predators, environment, need to carry a larger set of genes against contingencies) Fully sequenced Can be easily mutated (x-rays, chemicals) Can make deletions and transgenics Short breeding cycle Can do genetics Cheap to rear in large numbers  Morphogens not itself # - morphogen MAD MADS Genes turn on own sweep of genes activate other genes (ex development) , MADS genes are not morphogens but the transcription factor genes that morphogens activate Transcription factors activate other genes and set an agenda for development – master regulatory switches Morphogens define discrete domains (map) Different combinations of genes are activated to set agenda for subsequent gene activations A AB male - reprod E B C only - Stimulated by MADS C morphogens Female each domain reprodu of activity parts activates diff downstream genes Fig 13.30 Flowers and the MADS genes Combined activity of MADS genes A+B contribute to the expression of flower petals. Combined activity of MADS genes B+C 13C Fig 1 contribute to the expression of the stamen (male plant reproductive organs). MADS gene delay in where/ when expressed can effect C contributes to the expression of the carpel (female plant reproductive organs). development Aristotle (384-322 B.C.) no and idea semen living clot were materials Student of Plato, teacher of Alexander the Great Coined terms like : viviparity (live birth), oviparity (egg hatching) Complex body forms appear to form from simple rudiments Father contributes semen, Mother contributes clot Early stage embryos display no hint of gender “Indifferent stage” of development among mammals – cloacal slit (vagina like opening). In birds, amphibians, and reptiles, waste and gametes come out through the same passage – the cloaca. In males, the slit sutures up and a phallus forms. Are males the more highly developed form of human? (remember he originated from a Greek City State where only men who owned property could vote – interestingly, the word for those who did not participate in public affairs was “idiot”) males more evolved humans ; Women represented arrest of development  nature of sex determines Lucretius (1 Century B.C.) st gender sement clot Aristotle’s observations by the Roman Lucretius (1st Century B.C) Clot and semen in opposition to determine gender. Male contribution is forceful, goal-oriented; female contribution have is placid, accommodating if sex invested not s male attributes dominate , male I Sex Quality and characteristic of coitus can influence whether clot of semen dominate, and this in turn determines gender. Different competing theories begin to evolve 1. Creationism: inanimate material comb (sperm clot) + god life i. Spontaneous generation interviewed to create ii. Preformationism: God installed into all humans Animalculists - > Sperm involved Ovists > - only egg 2. Epigenesis Starting w something simple becomes more complex over time Creationism: Spontaneous Generationism During coitus, God intervenes to spark life from inanimate matter Semen and uterine environment provide the material upon which this spark works to build an individual Refuted unsuccessfully by Spallanzani (a priest - more on him later) of science done by priests being a lot - time resources , Boiling kills microbes, and they cannot reanimate if sealed from don't free supply of contaminated air Seal microbes the glass -> grow doesnot spark spontaneously Only firmly proven by flask experiments of Louis Pasteur – air not the issue… life starts from life and doesn’t animate spontaneously  Creationism: Preformationism Preformationism – all generations present in the first gamete at the day God created humankind during Genesis and encapsulating all generations until the End of Time Is a form of reiterative encapsulation like Matryoshka dolls, one individual inside the other, inside the other, inside the other etc.,) Preformationism Ovists: believed egg is the source of individual, semen just adds fertilizer/growth support. Animalculists: believed sperm carried the individual who just needed to be planted in the womb to grow and thrive. If you were to look inside any of the above, at high enough magnification, you’d see a tiny person inside, and inside that person, another etc. Epigenesis Life builds gradually from seed, sperm, egg, in a gradual and step-wise fashion. add organ systems in a brain Starts with simple rudiments which grow in size and complexity Tissues, organs, and organ systems gradually develop and consolidate Preformationism: Ovists Malpighi (1672) chicks Prof. Bologne University. Observed early development of neural folds, and their hearts which included their primitive cardiac tubes, aortic arches, and somites (precursors to vertebrae and intercostal muscles). Embryo starts simple, with already pre-existing structures, and then gets complex. BUT…was not looking at the embryos early enough – laid eggs already fertilized and developed to several hundreds if not thousands of cells. Additionally, he was the first to artificially fertilize insect egg by bathing them with fertilizing fluid of male but saw the uninseminated eggs remained infertile. Fig 1.3 Ovists (continued) Bonnet (1700s) Correspondent of Spallanzani. Lawyer, deaf from childhood. Coined the terms phyllotaxis and was among first to use “evolution” studied hydra regeneration, but importantly, aphids which display asexual clonal reproduction, female parthenogenesis. - Aphids alternate generations by sexual, then clonal reproduction. Conclusion? Sperm may not necessarily be required. The division giving rise to aphid parthenogenesis - results from a simple mitosis process. Harvey (1628) pamphlet on the circulatory system in animals including key facts such as the differences between veins and arteries and that they are connected via capillaries. He coined the term “Ex ovo omnia” meaning from the egg comes all, denying spontaneous generation and situating himself as an ovist (though without any proof…).  Preformationism: Animaluculists Van Leeuwenhoek (1678) inventor of the microscope, coined the term “animalculists”. Saw sperm in ejaculate of executed prisoner who had syphilis. Thought that the motile sperm were parasites that caused syphilis. - didn't think were human cells Hartsoeker (1694) imagined a little person, homunculus in the head of the sperm. He thought that during fertilization, the tip of the sperm unites with the female egg and the animalcule inside the sperm becomes one with the female and the egg Fig 1.4 Epigenesis Spa KlanzaniERM pants SP ↳ Spallanzani (Priest late 1700s). Did first developmental biology experiments (newt limb regeneration) Studied toads and frogs when they go into amplexus. The females shed eggs and the males release sperm in the form of semen Made wax shorts for males and removed them after amplexus to collect semen. took shorts Waxed shorts prevented fertilization during amplexus. off pipette Semenal fund Collected semen fertilized eggs, eggs were fertilized failed to identify that sperm was active agent in fertilization and not a parasite. Epigenesis Continued Wolff (1760) chicks (taught von Kolliker, Pander, von Baer) Gut and circulatory system arise from very simple rudiments (rolling of sheets etc.). trajectory to Believed that there was some kind of vital force, termed entelechy, development that propels development. Embryo is built from a formless fluid and - shadows the existence of an “essential force” that leads to the organization of organic matter - - Egg Candling Pander (1820) student of Wolff Described the 3 germinal layers - ectoderm, mesoderm, and endoderm. outermostger Ectoderm gives rise to skin, lens, and central nervous system. e - - E Mesoderm gives rise to muscle, bone, and organs. - - Endoderm gives rise to the organ linings such as the endothelium of gut, and the endocardial cushion of the heart. - - roll up individually or together Epigenesis Continued Prevost/Dumas (1820) Catalogued mating success Sterile males often had immotile sperm (ie; mules) > no - reproduction Viable males have motile sperm Von Kölliker (1840) Cell theory: cells can only arise from other cells - Pig testes cross-section of seminiferous tubules – sperm are not parasites but develop incrementally - - Sperm start as as ↳ tiny base round cells ↳ change size and Shape as more from the base Spermiogenesis Lumen Interstitial Tissue Leydig Cells Sertoli Cells Spermiogenic Lineage Sperm and Egg Fuse Hertwig and Fol (1870s) conducted a research in Echinoderms such as starfish and sea urchins. (Eggs are clear and you can see sperm entering egg.) Fol discovered a filament between the joining sperm and egg. - ↳ two helps to pull the together Chambers (1920) higher resolution optics, used fine glass needle to break filament. Filament broke preferentially at the sperm end. The egg produced the filament and reeled the sperm in? (Not!!!) Dan (1950) studied sea urchins using polarized light microscopy: the - filament grows from the sperm cell’s head. Now known as the acrosome reaction. starts to dev head to it sperm sperm -> make contact wegge pulls egg Conclusion Both Sperm and Egg Necessary to Embryogenesis Epigenesis is Most Parsimonious Explanation (Occam’s Razor) William of Ockham (Occam) 1287 – 1347 C.E. English Franciscan Friar/Philosopher “entities should not be multiplied without necessity” or…. When presented with different scenarios, the simplest (most parsimonious) explanation is the most likely to be correct Where we go from here: Spermiogenesis Oogenesis Syngamy (sperm egg fusion) Sperm gating/selection