Development Introduction Early Stages of Development Quiz part 1 Suites of Developmental Characters Quiz part 2

In this tutorial, you will learn: The basics of early development from fertilization through gastrulation. Major differences in early development between taxonomic groups. Credits: Figures and images by N. Wheat unless otherwise noted. Starfish embryo photos courtesy of K. Wynne. Sea urchin gastrulation, and frog cleavage & gastrulation videos courtesy of TheDeepSci’s YouTube channel. Chick gastrulation video courtesy of elishanathan’s YouTube channel. Blastocyst image from Wikipedia. Funded by Title V-STEM grant P031S090007.

Introduction Development describes the changes in an organism from its earliest beginnings through maturity.

Fertilization Fertilization is the initial event in development in sexual reproduction. Union of male and female gametes. Recombination of paternal and maternal genes. Restoration of the diploid number (two sets of chromosomes).

Zygote The diploid cell resulting from fertilization is now called a zygote. Photo courtesy of K. Wynne Photos in the following slides illustrate development in the starfish (Phylum Echinodermata).

Cleavage Cleavage – rapid cell divisions following fertilization. Very little growth occurs while the cells are dividing. Each cell called a blastomere. 2 cell stage 4 cell stage Photos courtesy of K. Wynne

Can’t view the video? Link To YouTube: Cleavage This video shows cleavage in a frog embryo: Can’t view the video? Link To YouTube: http://www.youtube.com/user/TheDeepSci#p/u/5/UPJJwVNzaHw

Morula Morula – the name given to the solid ball of cells that results from cleavage. First 5-7 divisions. Photo courtesy of K. Wynne

Blastula As divisions continue, a fluid filled cavity, the blastocoel, forms within the embryo. The resulting hollow ball of cells is now called a blastula. early blastula late blastula Photos courtesy of K. Wynne

Gastrulation The morphogenetic process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut (archenteron). early gastrula late gastrula Photos courtesy of K. Wynne

The Blastopore The blastopore is the first opening in the embryo – the point of invagination during gastrulation. The blastopore will eventually become either the mouth or the anus. One end of the gut-tube or the other. The space that forms during this time is the primitive gut, the archenteron. Blastopore Archenteron Photo courtesy of K. Wynne

Gastrulation The three tissue layers produced by gastrulation are called embryonic germ layers. The ectoderm forms the outer layer of the gastrula. Outer surfaces, neural tissue The endoderm lines the embryonic digestive tract. The mesoderm partly fills the space between the endoderm and ectoderm. Muscles, reproductive system

Gastrulation – Sea Urchin Gastrulation in a sea urchin produces an embryo with a primitive gut (archenteron) and three germ layers. Can’t view the video? Link To YouTube: http://www.youtube.com/user/TheDeepSci#p/u/3/Lgb4wMsZwZA

Can’t view the video? Link To YouTube: Gastrulation - Frog Result is again an embryo with gut & 3 germ layers. Gastrulation in the frog is more complicated due to the moderate amount of yolk present in the egg. Blastula wall more than one cell thick. Can’t view the video? Link To YouTube: http://www.youtube.com/user/TheDeepSci#p/u/0/WSouNeWnZ_0

Can’t view the video? Link To YouTube: Gastrulation - Chick Gastrulation in the chick is affected by the large amounts of yolk in the egg. Embryo essentially sits on top of large mass of yolk. Can’t view the video? Link To YouTube: http://www.youtube.com/watch?v=u52pTXtg75s Primitive streak – a groove on the surface along the future anterior-posterior axis. Functionally equivalent to blastopore lip in frog.

Gastrulation - Mammal In mammals the blastula is called a blastocyst. Inner cell mass will become the embryo while trophoblast becomes part of the placenta. Inner cell mass Trophoblast

Gastrulation - Mammal Gastrulation in mammals involves the inner cell mass and is similar to that of the chick due to the fact that mammalian ancestors and early mammals laid eggs. The large mass of yolk may be gone, but the developmental pattern remains.

Question 1 Shortly after fertilization, the zygote undergoes a series of rapid cell divisions. This is called Meiosis Gastrulation Organogenesis Cleavage

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During cleavage, each cell in the embryo is called a Question 2 During cleavage, each cell in the embryo is called a Blastula Morula Gastrula Blastomere

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The solid ball of cells that results from cleavage is called Question 3 The solid ball of cells that results from cleavage is called Blastula Morula Gastrula Blastomere

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Question 4 A fluid filled space called the blastocoel forms inside the embryo - at this point it is a Blastula Morula Gastrula Blastomere

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The stage that results in three germ layers and a primitive gut is the Question 5 The stage that results in three germ layers and a primitive gut is the Blastula Morula Gastrula Blastomere

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Question 6 Which of the following is not one of the three primary germ layers formed during gastrulation? Endoderm Myoderm Mesoderm Ectoderm

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Question 7 The difference in gastrulation found in different types of animals is due primarily to The amount of yolk present The size of the blastomeres Development in different species is completely unrelated None of the above

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Suites of Developmental Characters Two major groups of triploblastic animals: Protostomes include flatworms, annelids and molluscs. Deuterostomes include echinoderms and chordates.

Protostomes & Deuterostomes Protostomes & deuterostomes are differentiated by: Spiral vs. radial cleavage Mosaic vs. regulative cleavage Blastopore becomes mouth vs. anus Schizocoelous vs. enterocoelous coelom formation.

Spiral vs. Radial Cleavage Spiral cleavage – occurs in most protostomes. Some ecdysozoans show radial or superficial (insects) cleavage. Radial cleavage – is found in most deuterostomes. Tunicates and mammals have specialized cleavage patterns.

Mosaic vs. Regulative Development Mosaic development – cell fate is determined by the components of the cytoplasm found in each blastomere. An isolated blastomere can’t develop. Protostomes Regulative development – the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. A blastomere isolated early in cleavage is able to from a whole individual (e.g. twins). Deuterostomes

Fate of the Blastopore Protostome means “first mouth”. Blastopore becomes the mouth. The second opening will become the anus. Deuterostome means “second mouth”. The blastopore becomes the anus and the mouth develops as the second opening. Blastopore Photo courtesy of K. Wynne

Coelom Formation The coelom is a body cavity found in many triploblastic organisms that is completely surrounded by mesoderm. Not all protostomes have a true coelom. Pseudocoelomates have a body cavity between mesoderm and endoderm. Acoelomates have no body cavity at all other than the gut.

Coelom Formation In protostomes that have a coelom, a mesodermal band of tissue forms before the coelom is formed. In the process of coelom formation called schizocoely, this mesoderm splits to form a coelom. In enterocoely, the coelom forms as outpocketing of the gut. Typical deuterostomes have coeloms that develop by enterocoely. Vertebrates use a modified version of schizocoely.

Radial cleavage is found primarily in which group? Question 8 Radial cleavage is found primarily in which group? Protostomes Deuterostomes Both groups show radial cleavage Neither group shows radial cleavage

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This figure shows Radial cleavage Spiral cleavage Question 9 This figure shows Radial cleavage Spiral cleavage A four-cell stage embryo A blastula

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Question 10 Early in cleavage, a blastomere becomes isolated. It grows into an entire separate individual. This is an example of __________ development found in ____________. Mosaic, protostomes Mosaic, deuterostomes Regulative, protostomes Regulative, deuterostomes

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The blastopore will become the mouth in Question 11 The blastopore will become the mouth in Protostomes Deuterostomes All animals Neither – it forms something else

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A proliferation of cells A cavity or space Question 12 “Coel” is a word-part that we see frequently. The blastocoel is the space inside the blastula, the coelom is a mesoderm-lined body cavity. What will you find associated with “coel” words? An organ like the heart A proliferation of cells A cavity or space It indicates a developmental stage

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Question 13 Most protostomes show this type of coelom formation where a band of mesoderm splits to form the coelom. Enterocoely A modified version of enterocoely Schizocoely A true coelom is never found in protostomes

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