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What is embryology?
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It is the study of the development of a unicellular zygote into a complete multicellular organization
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What is cleavage?
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A series of rapid mitotic divisions in early embryonic development
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What do these divisions lead to?
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An increase in cell number without a corresponding growth in cell protoplasm
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What happens to the total volume of cell cytoplasm during cleavage?
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It stays constant
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What does cleavage result in?
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Progressively smaller cells, with an increasing ratio of nuclear-to-cytoplasmic material
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What does cleavage do to the surface-to-volume ratio, and why?
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It increases it for each cell, improving gas and nutrient exchange
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What is an indeterminate cleavage?
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One that results in cells that maintain the ability to develop into a complete organism
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Are identical twins the result of an indeterminate cleavage?
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Yes
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What is a determinate cleavage?
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It results in cells whose future differentiation pathways are determine at an early developmental stage
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What is differentiation?
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A specialization of cells that occurs during development
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When does the first complete cleavage occur?
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Approximately 32 hours after fertilization
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When does the second cleavage occur?
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After 60 hours
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When does the third cleavage occur?
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At 72 hours
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After the third cleavage, the 8-celled embryo reaches where?
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The uterus
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As cell division continues, what is the solid ball of embryonic cells called that forms?
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A morula
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What is blastulation?
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It is what brgins when the morula develops a fluid-filled cavity called the blastocoel
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By the fourth day, what happens?
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The blastocoel becomes a hollow sphere of cells called the blastula
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What is the mammalian blastula called?
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The blastocyst
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How many cell groups does the blastocyst have, and what are they?
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2, the inner cell mass and the trophoblast
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What is the inner cell mass?
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It protrudes into the blastocoel
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What is the trophoblast?
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It surrounds the blastocoel and later gives rise to the chorion
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So, in a mammalian blastocyst, which of the two makes up the border?
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The trophoblast
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Where is the inner cell mass?
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On the inside of the trophoblast
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When does the embryo implant in the uterine wall?
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During blastulation
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When is blastulation?
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Approximately 5-8 days after fertilization
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What hormone prepares the uterus for implantation?
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Progesterone
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What does progesterone actually do?
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It causes glandular proliferation in the endometrium
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What is the endometrium?
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The mucosal lining of the uterus
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What do the embryonic cells secrete?
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Proteolytic enzymes that enable the embryo to digest tissue and implant itself in the endometrium
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At this site what eventually occurs?
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Maternal and fetal blood exchange materials at this site
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What does this location later become?
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The placenta
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Once implanted, cell migrations transform the single cell layer of the blastula into what?
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The gastrula
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How many layers does it have?
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3 layers
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In the sea urchin, what does gastrulation begin with?
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The appearance of a small invagination on the surface of the blastula
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What forms next?
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An inpocketing forms as cells continue to move toward the invagination, eventually eliminating the blastocoel
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What is the result?
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A two-layered cup, with a differentiation between an outer cellular layer and an inner cellular layer
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What is the outer cellular layer called?
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The ectoderm
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What is the inner cell layer called?
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The endoderm
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What is the newly formed cavity of the two-layered gastrula called?
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The archenteron
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What does this later transform into?
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The gut
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What is the opening of the archenteron called?
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The blastopore
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In deuterostomes such as humans, the blastopore is the site of what?
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The future anus
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For protosomes on the other hand, the blastopore is the site of what?
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The future mouth
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Proliferation and migration of cells into the space between the ecroderm and the endoderm gives rise to a third cell layer called what?
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The mesoderm
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These are known as what?
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The three primary germ layers
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What does the ectoderm differentiate into?
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Integument, the lens of the eye, and the nervous system
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What does the endoderm differentiate into?
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The epithelial linings of the digestive and respiratory tracts, and parts of the liver, pancreas, thyroid, and bladder
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What does the mesoderm differentiate into?
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The musculoskeletal system, circulatory system, excretory system ,gonads, connective tissue throughout the body, and portions of digestive and respiratory organs
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How is most of differentiation accomplished?
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Through selective transcription of the genoma
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What is induction?
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The influence of a specific group of cells sometimes called the organizer on the differentiation of another group of cells
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