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Developmental Aspects of Cells and Tissue Development All cells come from one cell The fertilized egg or “zygote Begins mitosis 24 hours post fertilization Undergoes mitosis to create the trillions of cells that make up one human being Blastocyst Stage – Day 7 Consists of many cells Divided into two parts inner cell mass trophoblast Has attached to the endometrium of the uterus Endometrium of Uterus Blastocyst Trophoblast Fluid filled inner compartment Inner Cell Mass Located on side of blastocyst Creates the amniotic sac and the three primary tissue germ layers Inner Cell Mass = Future Embryo but also Stem Cells • http://www.pbs.org/newshour/health/stem_c ells.swf New information! • What if you could reprogram cells back to stem cells? • http://www.teachersdomain.org/asset/nsn08 _vid_stemcell2/ Three primary germ layers Germ means “initial” Includes the ectoderm, mesoderm and endoderm All three are responsible for cell specialization and tissue formation Once cells specialize, they lose the ability to do other functions Formed during gastrulation Picture of three germ layers • Ectoderm • Mesoderm • Endoderm Gastrulation • "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986) • During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism. During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location. • The primary germ layers (endoderm, mesoderm, and ectoderm) are formed and organized in their proper locations during gastrulation. Gastrulation • Formation of the internal “gastric” system or gastrointestinal system • In deuterostomes (animals with backbones and starfish) the anus forms first (deutero – second, stome – mouth) • In protostomes, the mouth forms first Gastrulation • Gastrulation occurs after implantation, around days 14-16 after fertilization in human embryogenesis • Occurs prior to neurulation Gastrulation • http://bcs.whfreeman.com/thelifewire/conte nt/chp20/2002001.html • In addition to forming the internal GI tract, it also leads to the formation of the three germ tissue layers Ectoderm (outer skin) Forms the Outer epithelium – skin, hair, lining of mouth Tooth enamel Neural tube – spinal cord and brain Neural crest – sensory ganglia, skull, gill arches and dentine Mesoderm (middle skin) • Notochord • Lining of thoracic and abdominal cavity • Circulatory system • Skeleton (except skull) • Skeletal muscles • Dermis • Connective Tissues • Urogenital system Endoderm (inner skin) • Gut • Respiratory tract • Liver • Pancreas • Epithelium of urogenital system Neurulation • Formation of the spinal cord and brain tissue (neural crest and neural tube formation) • Occurs around day 18, post fertilization • Not sure what causes problems – Genetics? – Maternal high fever during neurulation phase – Maternal use of valproic acid to control epilepsy Anencephaly “without a cephalic region” • The fetus on the following page was born without the top of it’s skull or it’s cerebrum. • Although the fetus maintains a heartbeat and breathing, it has lost it’s “suck reflex” and will die from infection due to the open head • This is an example of a neural tube defect occuring as an embryo. Spina Bifida • Neurulation stops along the spinal cord, forming a meningomyelocele (expulsion of meninges and spinal cord through unfused area) Developmental Fate • Determined by cell position in the developing embryo • Chemical messages given off will determine cell fate (what it becomes) • Stem cells divide and give off chemical messages that tell the other cells what to be determined by placement of the cell in the growing embryo • Determined by homeobox genes, which turn other genes on or off to allow development! Homeobox genes • Copy and past the URL into the bar and view the video • http://www.pbs.org/wgbh/evolution/library/ 03/4/l_034_04.html Homeobox Genes • Also called protooncogenes • Important in cell/tissue/organ development in embryos • Turn genes “off” or “on” so that enyzmes and proteins are created for tissue and organ development • Can be affected by alcohol and drugs taken during the first months of pregnancy Thalidomide • Drug given as a sedative in Europe during the 1950’s and 1960’s • Found to cause profound defects of developing arms and legs of embryos during their formation • Banned in U.S. after discovery of this problem by Frances Oldham Kelsey in 1962. Animation of Development Tissues • Each of the four tissues observed muscles (movement), connective (support), covering (epithelial) and control (nervous) come from these three germ layers Cells that become amitotic • • • • Liver cells (hepatocytes) Cardiac cells Mature nervous cells Pancreatic tissues, such as the Isles of Langerhans which produce insulin Aging and cell death • Begins at physical maturity • May be due to –Toxic chemicals –Oxidating chemicals – Poor diet – Genetics –Xrays/UVA –Alcohol Aging • Epithelial tissue become thinner, easily damaged, drier due to decreased sebaceous gland output, loss of elasticity • Decreased estrogen, progesterone, testosterone output Aging • Connective tissue aging permits sagging of skin • Decreased bone formation leads to osteoporosis • Muscles and nerves atrophy – slow down, slower to react • Aging circulation = decreased nutrients/oxygen to tissues • Hyperplasia – increased tissue growth or enlargement – Examples include breast enlargement during pregnancy; earlobe enlargement and nose tissue enlargement as you age Other tissue changes