BSC2086 A&P II Final Exam Study Guide – Lesson 23 1 Lesson 23: Development Introduction to Development • Development: Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity • Fertilization (Conception): When male & female gametes fuse & development begins • Differentiation: Creation of different types of cells required in development o Occurs through selective changes in genetic activity o As development proceeds, some genes are turned off while others are turned on • Embryological Development: Occurs during first two (2) months after fertilization o Embryology: Study of the events that occur during embryological development • Fetal Development: Begins at start of 9th week o Continues until birth • Prenatal Development: Embryological & fetal development stages • Postnatal Development: Commences at birth o Continues to maturity – state of full development or completed growth • Capacitation must occur before spermatozoa can fertilize secondary oocyte o Contact with secretions of seminal glands o Exposure to conditions in female reproductive tract • Fertilization occurs in the uterine tube within a day after ovulation o Secondary oocyte travels a few centimeters o Spermatozoa must cover distance between vagina & ampulla Covers 5 inches/hour Takes ~2 hours for sperm to reach ampulla Fertilization • Fertilization – produces zygote containing 46 chromosomes o Occurs upon fusion of two haploid gametes – each contain 23 chromosomes • Spermatozoon – travels relatively large distance to deliver paternal chromosomes to fertilization site • Secondary Oocyte – provides cellular organelles, inclusions, nourishment, & genetic programming necessary to support development of embryo for a week • Acrosome of Sperm Cells o Releases: Hyaluronidase: Enzyme that breaks bonds between adjacent follicle cells Acrosin: Proteolytic enzyme o Penetrate corona radiata & zona pellucida (toward oocyte surface) Sperm binds to receptor found on zona pellucida Activates acrosome Releases enzymes to digest path through zona pellucidaBSC2086 A&P II Final Exam Study Guide – Lesson 23 2 • Oocyte Activation: Changes in oocyte metabolism triggered by contact and fusion of sperm & oocyte cell membranes o Causes: 1. Cortical Reaction – oocyte plasma membrane depolarizes & releases (exocytosis) enzymes that: • Inactivate sperm receptors • Harden zona pellucida o Both prevent polyspermy 2. Completion of meiosis II & formation of 2nd polar body 3. Activation of enzymes to increase cell’s metabolic rate • After oocyte activation & completion of meiosis II: o Amphimixis: Fusion of male & female pronuclei Female Pronucleus: Nuclear material remaining in ovum after oocyte activation Male Pronucleus: Swollen nucleus of spermatozoon o Now zygote has 46 chromosomes o Fertilization is complete • Cleavage: Series of cell divisions o Produces daughter cells o First cleavage occurs 30 hours after fertilization Produces 2 daughter cells – each ½ size of original zygote Daughter cells called blastomeres • Differentiation: Involves changes in genetic activity of some cells, but others o Influenced by cytoplasmic content o As the blastomeres divide, each receives a portion of cytoplasm that may differ in composition o Oocyte receives many molecules (nutrients & RNA from granulosa cellsBSC2086 A&P II Final Exam Study Guide – Lesson 23 3 Gestation • Induction: Cells release chemical substances that affect differentiation of other embryonic cells o Can control highly complex processes o Not error free – can disrupt normal development • Gestation: Time spent in prenatal development o Consists of three (3) integrated trimesters – each three (3) months long • First Trimester: Period of embryological and early fetal development o Rudiments of all major organ systems appear • Second Trimester: Development of organs & organ systems o Body shape & proportions change • Third Trimester: Rapid fetal growth & deposition of adipose tissue o Most major organ systems are fully functional First Trimester • First Trimester = most dangerous period in prenatal life o 40% of conceptions produce embryos that survive past first trimester • Includes Four (4) Major Stages: o Cleavage o Implantation o Placentation o EmbryogenesisBSC2086 A&P II Final Exam Study Guide – Lesson 23 4 • 1. Cleavage o Sequence of cell divisions begins immediately after fertilization o Zygote becomes a pre-embryo – develops into multicellular blastocyst o Ends when blastocyst contacts uterine wall o Blastomeres: Identical cells produced by cleavage divisions (mitosis) Subsequent divisions faster – every 10-12 hours o Morula: Stage after three (3) days of cleavage Pre-embryo is solid ball of cells resembling mulberry Reaches uterus on day 4 o Blastocyst: Hollow ball formed by blastomeres with an inner cavity (blastocoele) Blastomeres no longer identical in size & shape • Trophoblast: Outer cells o Develop into structures that provide nutrients to embryo o Secrete enzyme that erodes zona pellucida (hatching the blastocyst) • Inner Cell Mass: Located on one side inside trophoblast o Insulated from outside o Will form embryoBSC2086 A&P II Final Exam Study Guide – Lesson 23 5 • 2. Implantation o Occurs seven (7) days after fertilization o Begins with attachment of blastocyst to endometrium of uterus o Blastocyst adheres to uterine lining Inner cell mass side faces the endometrium o Trophoblast cells divide rapidly at contact point, creating several layers Cellular Trophoblast – cells closest to interior of blastocyst remain intact • Cells next to endometrium fuse – forms multinucleated syncytial trophoblast o Erodes path through uterine epithelium by secreting hyaluronidase o Maternal epithelial cells divide & migrate to enclose the blastocyst by day 10BSC2086 A&P II Final Exam Study Guide – Lesson 23 6 o Erosion of endometrium by syncytial trophoblast releases nutrients for blastocyst Erosion occurs at capillary walls – destroys them, releasing blood Blood pools in trophoblastic channels called lacunae o Villi extend away form the trophoblast into endometrium Accelerating breakdown of endometrial blood vessels & blood flow to
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