Lecture 4 History of Reproduction.pptx

Where do we come from? What’s Old is new Again Lecture 4 Aristotle (384-322 B.C.) 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, Moth...

Where do we come from? What’s Old is new Again Lecture 4 Aristotle (384-322 B.C.) 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”) Lucretius (1 Century st B.C.) 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 is placid, accommodating 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: i. Spontaneous generation ii. Preformationism: Animalculists Ovists 2. Epigenesis 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 (more on him later) Boiling kills microbes, and they cannot reanimate if sealed from free supply of contaminated air 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 Epigenesis Life builds gradually from seed, sperm, egg, in a gradual and step-wise fashion. Starts with simple rudiments which grow in size and complexity Tissues, organs, and organ systems gradually develop and consolidate Preformationism: Ovists Malpighi (1672) chicks. 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. studied 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. 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 Spallanzani (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. Waxed shorts prevented fertilization during amplexus. Collected semen fertilized eggs, failed to identify that sperm was active agent in fertilization and not a parasite. Epigenesis Continued Wolff (1760) chicks Gut and circulatory system arise from very simple rudiments (rolling of sheets etc.). Believed that there was some kind of vital force, termed entelechy, that propels development. Embryo is built from a formless fluid and shadows the existence of an “essential force” that led to the organization of organic matter Egg Candling Pander (1820) student of Wolff Described the 3 germinal layers - ectoderm, mesoderm, and endoderm. Ectoderm gives rise to skin, lens, and central nervous system. 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. Epigenesis Continued Prevost/Dumas (1820) Catalogued mating success Sterile males often had immotile sperm (ie; mules) 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 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 joined sperm and egg. 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. 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