{"id":241,"date":"2017-08-16T10:35:12","date_gmt":"2017-08-16T10:35:12","guid":{"rendered":"http:\/\/americanboard.org\/Subjects\/biology\/?page_id=241"},"modified":"2017-09-22T17:52:48","modified_gmt":"2017-09-22T17:52:48","slug":"embryogenesis","status":"publish","type":"page","link":"https:\/\/americanboard.org\/Subjects\/biology\/embryogenesis\/","title":{"rendered":"Embryogenesis"},"content":{"rendered":"<div class=\"twelve columns\" style=\"margin-top: 10%;\">\n<div class=\"advance\"><a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/organization\">\u2b05 Previous Lesson<\/a>\u00a0<a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/comparitive-physiology\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/systems-and-interactions\">Next Lesson \u27a1<\/a><\/div>\n<p><!-- CONTENT BEGINS HERE --><\/p>\n<h1 id=\"title\">Embryogenesis<\/h1>\n<h4>Objective<\/h4>\n<p>In this lesson, you will review embryonic development.<\/p>\n<h4>Previously Covered:<\/h4>\n<ul>\n<li>The cell is the smallest unit of life.<\/li>\n<li>Cells are organized into tissues, tissues into organs, and organs into organ systems.<\/li>\n<li>The individual organism is the highest level of organization.<\/li>\n<\/ul>\n<section>Embryonic development in mammals proceeds by several recognizable stages. The formation and development of an embryo is a process called <abbr title=\"the process of the formation and development of the embryo in both plants and animals; involves the fertilization of the ovum, which produces a zygote that undergoes mitotic divisions (cleavage) and cellular differentiation, which leads to the development of the embryo\">embryogenesis<\/abbr>.<\/p>\n<h3>Fertilization<\/h3>\n<p><img decoding=\"async\" class=\"padding u-pull-right\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/fertilization.jpg\" alt=\"Fertilzation\" \/><\/p>\n<p>Pregnancy refers to the entire process of embryonic development. A pregnant placental mammal is carrying one or more developing embryos in the uterus. Pregnancy is preceded by conception, the moment when a sperm cell penetrates an egg cell, or ovum. This <abbr title=\"the process of a sperm penetrating an egg, marking conception, and the fusion of genetic materials, the beginning of a pregnancy\">fertilization<\/abbr> usually occurs in the <abbr title=\"a short, thick-walled tube in the female reproductive system that carries the egg from an ovary to the; also called fallopian tube\">oviduct<\/abbr>. Chemical changes within the fertilized egg quickly block entry by other sperm.<\/p>\n<h3>Zygote<\/h3>\n<p>The resulting <abbr title=\"the single cell that results from fertilization of an ovum by a sperm; After dividing several times, it implants in the uterus; It continues to divide, producing more cells and passing through the stages of embryo and fetus\">zygote<\/abbr> immediately begins rapid division by <abbr title=\"the mitotic process of early embryonic cell division of one cell to two, two into four, four into eight, etc.;\">cleavage<\/abbr>\u2014first into two cells, then four, then eight, doubling the number of cells every few minutes. In three or four days, when the zygote has become a ball of cells, it reaches the uterus. Once implanted in the <abbr title=\"the tissue layer lining the uterus\">endometrium<\/abbr>, the zygote becomes the <abbr title=\"a plant or animal in the early stage of development\">embryo<\/abbr>.<\/p>\n<h3>Embryo<\/h3>\n<p>After cleavage has produced 128 cells, <abbr title=\"the stage after cleavage produces 128 cells and produces the blastula or blastocyst\">blastulation<\/abbr> begins and the embryo becomes a <abbr title=\"a spherical layer of cells that surrounds a yolk- or fluid-filled cavity\">blastula<\/abbr>\u2014a hollow sphere of cells. Blastulation in mammals leads to the formation of the blastocyst. The outer layer of cells on the blastocyst will mingle with the cells of the inner layer of the uterine wall and become the <abbr title=\"a membranous organ that lines the uterine wall, envelopes the fetus, is attached to the umbilical cord, and exchanges nutrients, wastes, and gases between fetal and maternal blood\">placenta<\/abbr>. It is through this special organ that the developing embryo receives nutrients, exchanges gases, and disposes wastes. The <abbr title=\"a bubble-like, thin-walled sac that surrounds the fetus, contains amniotic fluid, and regulates temperature\">amniotic sac<\/abbr> also develops at this time. This sac forms from two thin membranes, the <abbr title=\"the outer membrane surrounding the embryo\">chorion<\/abbr> and the <abbr title=\"the membranous sac enclosing the fetus\">amnion<\/abbr>, and contains <abbr title=\"the protective, clear fluid (mostly water) that fills the amniotic sac and envelopes the fetus\">amniotic fluid<\/abbr>. This fluid maintains a constant temperature and absorbs shock from movements.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/EarlyEmbryonicDev.jpg\" alt=\"Early embryonic development - cleavage and blastulation\" \/><\/center><\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>A hollow sphere of cells during embryonic development is called a<\/p>\n<ol>\n<li>zygote.<\/li>\n<li>amniotic sac.<\/li>\n<li>ovum.<\/li>\n<li>blastula.<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct answer is D. A zygote is a cell resulting from fertilization. The amniotic sac consists of two membranes that hold the embryo, then fetus. An ovum is an egg\u2014a female sex cell or gamete.<\/p>\n<\/section>\n<h3>Germ Layers<\/h3>\n<div class=\"figures u-pull-right padding\">\n<p><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/blastula-gastrula.jpg\" alt=\"1 - Blastula; 2 - Gastrula; Ectoderm is orange, endoderm is red.\" \/><\/p>\n<p class=\"figcaption\">1\u2014Blastula. 2\u2014Gastrula. Ectoderm is orange; endoderm is red.<\/p>\n<\/div>\n<p>The next stage of embryogenesis is <abbr title=\"the process of embryonic development that follows the blastula stage, which establishes the three germ layers: ecto-, endo-, and mesoderm\">gastrulation<\/abbr>. During gastrulation the embryo&#8217;s morphology undergoes restructuring by cell migration. Depending on the organism, the cells migrate to form two or three germ layers. In <abbr title=\"having two primary cellular tissue layers: ecto- and endoderm\">diploblastic<\/abbr> organisms, the ovum gives rise to two germ layers: <abbr title=\"the outermost germ layer that forms the skin and the nervous system\">ectoderm<\/abbr> and <abbr title=\"the innermost layer of cellular tissue that forms the alimentary canal and digestive glands\">endoderm<\/abbr>. In <abbr title=\"having three primary germ layers: ecto-, endo-, and mesoderm\">triploblastic<\/abbr> organisms, the ovum gives rise to three germ layers: ectoderm, endoderm, and <abbr title=\"the middle layer of cellular tissue that forms bone, muscle, and connective tissue\">mesoderm<\/abbr>. The ectoderm layer forms the epidermis and nervous system. The endoderm layer forms the gastrointestinal tract, respiratory tract, and endocrine glands. The mesoderm layer forms the muscles, connective tissue, bones, and many of the organs.<\/p>\n<p>After the definition of these germ layers, organ systems begin to develop. This is called <abbr title=\"the process of organ formation and development\">organogenesis<\/abbr>. Beyond organogenesis, there seems to be no common pattern among the different taxa.<\/p>\n<h3>Fetus<\/h3>\n<p>At the end of the first trimester, the embryo becomes known as the fetus. During the second trimester, the vertebrate fetus grows very rapidly and becomes very active. The mother may feel the fetus moving and movements may be visible through the abdominal wall. During the third trimester, the fetus grows even more rapidly. Fetal activity may actually decrease as the fetus fills all the available space and positions itself for birth.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/fetus.jpg\" alt=\"The Fetus\" \/><\/center><\/p>\n<h3>Differentiation<\/h3>\n<p>The cells in the various layers appear to be similar, but probes for cell-specific proteins reveal that different groups of cells have already started on specific paths of future development. However, since all of the cells contain the exact same set of genetic information, what causes differentiation?<\/p>\n<ul>\n<li>A fertilized egg is much larger than the normal cells of an animal&#8217;s body. The frog egg has a volume 1.6 million times larger than a typical frog cell. The eggs of mammals are smaller, but even they are larger than their descendant cells will be.<\/li>\n<li>The cytoplasm of the fertilized egg is heterogeneous. It contains uneven distributions of mRNAs and proteins deposited in the egg by the mother.<\/li>\n<li>Cleavage of the fertilized egg partitions it into thousands of cells of normal size. Each contains a nucleus descended from the zygote nucleus. Yet, each nucleus finds itself surrounded by cytoplasm containing a particular mix of mRNAs and proteins that may be different from other cells.<\/li>\n<li>Transcription of its nuclear genes begins when the blastula contains thousands of individual cells. Each of these cells is now similar in size to a normal animal cell. But the genes that are expressed by the nucleus in a given cell are regulated by the varying mix of molecules, mostly protein transcription factors, found in the cytoplasm surrounding that nucleus.<\/li>\n<li>Once a cell is started down a path toward differentiation, that cell may release molecules that regulate the genes of nearby cells.<\/li>\n<\/ul>\n<h3>Fetal Tolerance<\/h3>\n<p><img decoding=\"async\" class=\"padding u-pull-right\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/baby.jpg\" alt=\"A baby smiling\" \/><\/p>\n<p>Pregnancy represents an immunological conundrum. Why does the mother&#8217;s body not automatically reject this obviously foreign body? The explanation is unclear, but the best hypothesis seems to be that paternal antigens are created. The mother&#8217;s white blood cells react to these antigens and start to prepare an immune response. This in turn triggers the production of a special suppressor white cell in the uterus that prevents the other white blood cells from mounting an attack. According to this hypothesis, if the initial immune response is weak, then no suppressor white cells are made. As a result, the continued attack by the mother&#8217;s white cells, while weak, may eventually trigger a spontaneous abortion. This may account for women who have multiple miscarriages for no apparent reason.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>During which trimester does organogenesis occur?<\/p>\n<ol>\n<li>First<\/li>\n<li>Second<\/li>\n<li>Third<\/li>\n<li>Fourth<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">A is the correct answer. The first trimester is characterized by development of rudimentary forms of all adult structures. Choice B is the trimester of growth. Choice C is the trimester of rapid growth. Choice D is incorrect because there is no fourth trimester.<\/p>\n<\/section>\n<h3>Summary<\/h3>\n<ul>\n<li>Embryonic development begins when the zygote divides by cleavage many times.<\/li>\n<li>Once implanted in the uterus, the zygote becomes an embryo.<\/li>\n<li>After 128 cells are produced from cleavage, the embryo becomes a blastula.<\/li>\n<li>At the end of the first trimester, the embryo becomes a fetus.<\/li>\n<\/ul>\n<\/section>\n<p><!-- CONTENT ENDS HERE --><\/p>\n<div class=\"advance\"><a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/organization\">\u2b05 Previous Lesson<\/a>\u00a0<a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/comparitive-physiology\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/systems-and-interactions\">Next Lesson \u27a1<\/a><\/div>\n<p><a class=\"backtotop\" href=\"#title\">Back to Top<\/a><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>\u2b05 Previous Lesson\u00a0Workshop Index\u00a0Next Lesson \u27a1 Embryogenesis Objective In this lesson, you will review embryonic development. Previously Covered: The cell is the smallest unit of life. Cells are organized into tissues, tissues into organs, and organs into organ systems. The individual organism is the highest level of organization. Embryonic development in mammals proceeds by several [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-241","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/241","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/comments?post=241"}],"version-history":[{"count":7,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/241\/revisions"}],"predecessor-version":[{"id":1155,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/241\/revisions\/1155"}],"wp:attachment":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/media?parent=241"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}