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Mammalian Reproduction • All mammals have sexual reproduction – Haploid gametes by meiosis – Control of gamete production dependent on hormones • Mammals are viviparous (except the Monotremes) – Placental mammals retain their young in utero – Most mammalian offspring require large amount of parental care – Males play a small role in parental care (in most cases) Mammalian Reproduction • Female reproductive cycles involve periodic release of a mature ovum – Ovulation • Most female mammals have estrous cycles – Females sexually receptive to males only at time of ovulation (“estrus”) • Primates (inc. humans) have menstrual cycles Human Reproductive System • Sex differentiation is determined genetically; presence of Y chromosome determines maleness • Developing embryo has wolffian ducts and müllerian ducts – Wolffian ducts – develop into male epididymus, vas deferans, and accessory organs (seminal vesicle and prostate) – Müellerian ducts – develop into vagina, uterus, cervix and oviducts Human Reproductive System • In males, genes located on Y chromosome initiate differentiation of epithelial cells into Sertoli cells • Sertoli cells produce anti-Müllerian hormone; causes the deterioration of the Müllerian duct (defeminization) • Mesenchyme cells differentiate into Leydig cells – Leydig cells produce the androgens (male sex steroids) Human Reproductive System • Androgens – Testosterone – primary male sex hormone, responsible for the development and differentiation of the Wolffian duct • Sertoli cells require testosterone for spermatogenesis – 5-Dihydrotestosterone – responsible for masculine development of external genitalia • Guevodoces – genetic condition, delays male sexual development until puberty Human Reproductive System • In females (XX), the absence of testosterone leads to feminization (default condition) • All mammals, regardless of sex, begin life with primordial (undifferentiated) gonads • No androgens for Wolffian ducts to develop, will degenerate. Müellerian ducts will develop due to secretion of estrodial • Gonads will become ovaries, vagina, uterus, and cervix Developmental disorders • XO, Turner’s syndrome – infertile female • XXY, Klinefelter’s – testes development (male), but sterile • Congenital Adrenal Hyperplasia, XX embryo – ovarian development; adrenal gland overproduces androgens; mascularization of genitalia; development of Wolffian ducts and Müellerian ducts (no sertoli cells!) – At puberty, can get secondary characteristics, ovary kicks in and secretes estrodial Human Male Reproductive System • When testes form in the male embryo, they develop highly-convoluted seminiferous tubules, the site of sperm production • In the wall of the seminiferous tubule, a spermatogonium divides by mitosis producing diploid cells • Diploid cell known as the primary spermatocyte undergoes meiosis producing secondary spermatocytes Human Male Reproductive System • Seminiferous tubules contain Sertoli cells • Sortoli cells help convert spermatocytes into spermatozoa (sperm) by engulfing their extra cytoplasm • Shortly before birth, the testes descend into the scrotum, because sperm need cooler temperature to develop Human Male Reproductive System • Sperm are delivered into the epididymis where they must remain for at least 18 hours for mobility to develop, then pass into the vas deferens • Seminal vesicles produce fructose-rich fluid, while the prostate gland produces a milky fluid semen Human Male Reproductive System Human Female Reproductive System • Ovaries develop more slowly than testes • Ovaries contain microscopic ovarian follicles • Each follicle contains a potential egg cell called a primary oocyte and smaller granulosa cells – Granulosa cells secrete estradiol (estrogen) Human Female Reproductive System • Primary oocytes begins meiosis, but arrested in 1st prophase (meiosis I) before birth; finite number of oocytes formed by mitosis • At puberty, granulosa cells proliferate and secrete estrogen – Triggers first menstrual cycle – Stimulates secondary characteristics Human Female Reproductive System • After puberty, oocyte completes first meiotic division (meiosis I) • However, the first meiotic division is very uneven – Produces large ‘secondary oocyte’ and a small polar body (disintegrates) – The secondary oocyte begins meiosis II, and is arrested in metaphase II – Fertilization stimulates continuation of meiosis The Menstrual Cycle Human Female Reproductive System • Development of a mature, or Graafian follicle occurs monthly (~28 days) during the menstrual cycle • Follicles remaining in ovary after ovulation develop into the corpus luteum • The corpus luteum secretes progesterone, which stimulates proliferation of the endometrium – usually short-lived unless oocyte gets fertilized – When corpus luteum dies, loss of progesterone results in shedding of endometreum Human Female Reproductive System • After fertilization, the developing embryo produces human chorionic gonadotropin – Maintains the corpus luteum – Prevents menstruation by keeping levels of estrodial and progesterone high – This hormone is tested for in pregnancy tests Developmental Biology • Fertilization – union of male and female gametes; consists of three events: – Sperm penetration and membrane fusion – Egg activation – Fusion of nuclei • Development involves: – Growth – Differentiation – Specialization Fertilization • A sperm must penetrate to the plasma membrane of the egg for membrane fusion to occur • Egg enveloped by one or more protective coats – Zona pellucida in mammalian eggs Fertilization • Membrane fusion activates the egg – When sperm makes contact with the egg’s plasma membrane, it triggers a release of Ca+2 from internal organelles starting at the point of sperm entry – Changes membrane potential of egg, prevents other sperm from fusing with egg – Enzymes from cortical granules remove sperm receptors – prevents polyspermy! Fertilization • Sperm triggers egg to complete meiosis (remember that the oocyte is arrested in metaphase II) • Restores the diploid state (haploid sperm nuclei fuses with haploid egg nuclei) • There are proteins on surface of the acrosome that will bind to receptors on the membrane of the egg – very specific match, secures species specificity Development • Meiosis – formation of male and female gametes • Fertilization • Cleavage – a period of rapid cell division • Gastrolation – cell division slows, but cells go through extensive rearrangement ectoderm, mesoderm, and endoderm • Organogenesis – develop of specific organs Development • Cleavage – the rapid division of the zygote into a larger and larger number of smaller and smaller cells called blastomeres • No overall increase in size of embryo • Animal pole and vegetal pole – Blastomeres of animal pole form external tissues of the body; blastomeres of vegetal pole form the internal tissues Development • A hollow ball of cells, the blastula, contains a fluidfilled cavity, the blastocoel Development • In bilateral phyla, all organisms have developmental stage involving 3 tissue layers: ectoderm (outside), mesoderm (middle), and endoderm (innermost layer) • In protostomes, the mouth develops first – All worms, arthropods, molluscs – Coelom formed by hollowing out of solid mass of cells that make up mesoderm – Undergo spiral cleavage – Determinant cleavage – fate of cells determined early Development • In deuterostomes, the mouth forms second – Echinoderms and chordates – Mouth forms from opening on opposite side of blastopore – Blastopore – opening to outside, first opening in development – Radial cleavage or rotational cleavage – Indeterminant development - fate of cells determined by interactions between other cells Development • Deuterostomes evolved from Protostomes >500 million years ago • Go back to Chapter 32 – pages 626-7 Development • The pattern of development is dependent on the amount of yolk Development • Egg contains yolk proteins, huge stores of mRNA, tRNA, and ribosomes (powers egg during cleavage), and morphogenic determinants (decide what cell becomes) • In eggs containing moderate to little yolk, cleavage occurs throughout the egg – holoblastic cleavage • In eggs containing large amounts of yolk, cleavage, incomplete or meroblastic cleavage occurs Development • Holoblastic cleavage – eggs without very much yolk; results in more evenly-sized cells, cleavage furrows extend throughout the egg • Meroblastic cleavage – cleavage forrows cannot make it through yolk; incomplete cleavage – In birds and reptiles, cleavage is restricted to region of concentrated cytoplasm Meroblastic cleavage Development • Compaction occurs at 8-cell stage • Outer cells develop into trophoblast – Becomes part of placenta – Inner cell mass develops into embryo – Random: no one cell is destined to be inner or outer – Blastocoel forms by secretion of fluid by trophoblastic cells Gastrulation • Gastrulation – migration and rearrangement of cells of the blastula • Results in formation of three germ layers: – Endoderm (internal organs) – Mesoderm (bones, heart, blood vessels, muscles, connective tissue, gonads) – Ectoderm (skin and nervous system) Gastrulation • Cells move during gastrulation, undergoing a variety of cell shape changes • Cells attached tightly at junctions will move as cell sheets • Invagination – cell sheet dents inward Gastrulation • In mammals, embryo develops from inner cell mass • A thickening of cells forms the primitive streak • In the middle of the primative streak, cells sink inward forming the primative groove – Cells entering the sides of the primative groove become mesoderm and endoderm – Cells that stay on outside become ectoderm Gastrulation in mammals Extraembryonic membranes • As an adaptation to terrestrial life, the embryos of reptiles, birds, and mammals develop within a fluid-filled amniotic membrane, or amnion • Chorion – fetal contribution to placenta • Yolk sac – critical role in the nutrition of birds and reptiles, present in mammals, but not nutritive • Allantois – in birds, fuses with chorion and fascilitates gas exchange; in mammals, it contributes blood vessels to developing umbilical cord Extraembryonic membranes Extraembryonic membranes Development • Neurulation – formation of the nervous system; the first ‘organs’ that develops in embryo • Folds of ectodermal tissue come together, forming the neural tube • Neural tube has to close off • Neural tube becomes spinal cord Development • Cell migration – cells migrate to different parts of the embryo to form distinct tissues – Example – cells of neural crest form sense organs • Organogenesis – formation of organs in their proper locations – Occurs by interaction of cells within and between the 3 germ layers – Follows rapidly on the heels of gastrulation Development • At some point, every cell’s fate becomes fixed; cell determination • All of the cells in an animal’s body, with the exception of a few specialized ones that have lost their nuclei, contain the same compliment of genetic information • To a large degree, a cell’s location in the developing embryo determines its fate (but this is only true up until a certain stage in development) Development • Ontogeny recapitulates phylogeny! • In the early 1800’s, van Baer noted that early embryos of all members of phylum or subphylum appear fairly similar • As development continues, it takes on more specific characteristics of classorderfamilygenusspecies • Human development goes through embryonic stages of other animals Development • Developmental patterns of more recently evolved groups are built on more primative patterns • Ernest Haeckel (late 1800’s) surmised that evolution of new species arise by adding on an additional step to the end of the previous species’ embryonic stage (we go through adult stages of all previous species) Plant Development • Development begins once the egg cell is fertilized Polar nuclei Egg Sperm Micropyle Pollen tube 3n endosperm 2n zygote Plant Development • The first division of the fertilized egg is asymmetrical – One daughter cell becomes embryo, small – Other larger cell becomes the suspensor, which links the embryo to the nutrient tissue of the seed – Cells near the suspensor become root – Cells at opposite end become shoots Plant Development • Tissue formation – Protoderm – develops into dermal tissue that protects plant (external surface of plant) – Ground Meristem – develops into ground tissue that stores food and water – Procambium – develops into vascular tissue that transports water and nutrients; xylem and phloem Plant Development • Cotyledons – One or two seed leaves (monocot or dicot, respectively) develop • May store, absorb food from endosperm – Seed coat forms • Seeds may exist in dormant state (for <thousands of years) • Resistant to harsh, undesirable conditions Plant Development • Seed formation – Early in the development of an angiosperm embryo, the embryo stops developing – In many plants, development is arrested following the differentiation of the meristems and cotyledons – Seeds protect the young plant, provides food for the embryo (until it can produce its own), and facilitates dispersal of the embryo Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Shoot apical meristem Seed coat (integuments) Procambium Root apical meristem Root cap Endosperm Cotyledons Plant Development • Germination cannot take place until water and oxygen reach the embryo • Some seeds only germinate when sufficient water is available to leach inhibitory chemicals from the seed coat • Other seeds germinate only after passing through the intestines of birds or mammals • Other seeds germinate only after being exposed to fire First leaves Plumule Epicotyl Cotyledon Hypocotyl Hypocotyl First leaf AdventiColeoptile Scutellum tious root Withered cotyledons Seed coat Primary roots Secondary roots a. Coleorhiza Radicle Primary root b. Plant Development • Fruits – Mature ovaries – During seed formation, the flower ovary begins to develop into a fruit