Chapter 47 Animal Development travismulthaupt com The Zygote

Chapter 47 Animal Development travismulthaupt. com

The Zygote l The question of how a zygote becomes an animal has been asked for centuries. l As of the 18 th century, preformation was the prevailing notion. The idea was that a preformed miniature infant (homuniculus) was contained within an egg or a sperm. travismulthaupt. com

The Zygote l More than 2000 years ago: Aristotle originally posed the idea of epigenesis to explain embryonic development. l This idea explained that an animal gained form gradually from a relatively formless egg. travismulthaupt. com

The Zygote l As microscopy improved in the 19 th century, embryologists began to see that embryonic development took place in a series of progressive steps and epigenesis became the favored explanation. l The genome of the zygote and differences in early embryonic cells determine an organism’s development. travismulthaupt. com

The Zygote l There are some important events that regulate animal development: l 1. Fertilization l 2. Cleavage l 3. Gastrulation l 4. Organogenesis travismulthaupt. com

1. Fertilization l Fertilization is when an egg and a sperm unite forming a zygote. l The main function of fertilization is to produce a diploid egg. l Egg activation occurs when the sperm contacts the egg’s surface and initiates metabolic reactions that trigger the onset of embryonic development. travismulthaupt. com

1. Fertilization l In general, when eggs meet sperm, an acrosomal reaction is triggered. travismulthaupt. com

1. Fertilization l The process begins when a vesicle at the tip of the sperm called the acrosome discharges hydrolytic enzymes. travismulthaupt. com

1. Fertilization l These enzymes dissolve the egg’s jelly coat enabling the acrosomal process to penetrate the it and attach to the egg’s surface. travismulthaupt. com

1. Fertilization l The sperm binding receptors on the egg’s surface are attached to the vitelline layer. travismulthaupt. com

1. Fertilization l This recognition of the egg’s surface receptors to sperm is what ensures only sperm from the same species can penetrate the egg. travismulthaupt. com

1. Fertilization l The binding of the sperm membrane to the membrane of the egg changes the state of the ion channels of the egg. travismulthaupt. com

Fast Block to Polyspermy l Within 1 -3 seconds, depolarization occurs where Na+ ions rush into the egg, and is known as the fast block to polyspermy. It prevents multiple sperm from entering the egg. travismulthaupt. com

Fast Block to Polyspermy l The fast block to polyspermy is short- lived and the membrane polarization only lasts about one minute. travismulthaupt. com

Slow Block to Polyspermy The fusion of the sperm and egg also triggers a series of changes in the egg that are more long-lived. l Sperm binding activates a signal transduction pathway which causes calcium ions to be released from the egg’s ER into the cytosol. l travismulthaupt. com

Slow Block to Polyspermy l Release of Ca 2+ from the ER occurs 1 st at the site of sperm entry and moves like a wave over the surface of the egg. travismulthaupt. com

Slow Block to Polyspermy The high concentration of calcium initiates the cortical reaction. l The reaction triggers the fusion of the egg’s plasma membrane with numerous vesicles lying just beneath the membrane. l travismulthaupt. com

Slow Block to Polyspermy l The contents of these vesicles enter the previtelline space. l Enzymes in these contents degrade the proteins holding the vitelline layer to the plasma membrane. travismulthaupt. com

Slow Block to Polyspermy l These changes transform the vitelline layer into the fertilization envelope. l No more sperm can enter the egg at this time. travismulthaupt. com

Slow Block to Polyspermy l This is referred to as the slow block to polyspermy. l The sharp rise in Ca 2+ also increases the metabolism of the egg. travismulthaupt. com

Slow Block to Polyspermy l After about 20 minutes, the egg nucleus fuses with the sperm nucleus creating a diploid zygote. l Fertilization has many common features among species and many differences. These differences are mainly with timing and various stages of meiosis. travismulthaupt. com

1. Fertilization--Sperm Activation In mammals, secretions in the female reproductive tract to help to activate sperm. l Additionally, the mammalian egg is cloaked by follicle cells through which the sperm must penetrate before reaching the zona pellucida. l travismulthaupt. com

1. Fertilization--Sperm Activation The zona pellucida is the external matrix of the egg and functions as a sperm receptor. l When sperm binds to this, it induces many of the same reactions as seen in the sea urchin. l One difference is that there is no fast block to polyspermy in mammals. l travismulthaupt. com

1. Fertilization--Sperm Activation l After the egg and sperm membranes fuse the whole sperm is taken into the egg. l The egg lacks a centrosome which the sperm has. l The centrosome will duplicate and assist in the creation of the mitotic spindle which will be used for the first cell division. travismulthaupt. com

1. Fertilization--Sperm Activation l In mammals, the nuclei do not immediately fuse. Instead, the 2 sets of c-somes share a common spindle apparatus during the 1 st mitotic division. l Only after the first division, as diploid nuclei form in the 2 daughter cells, do the chromosomes from the 2 parents come together in a common nucleus. travismulthaupt. com

1. Fertilization l Fertilization is quite slow in mammals. l Once fertilization is complete, rapid cell division ensues and the cells proceed through the M and S phases of the cell cycle virtually skipping G 1 and G 2. travismulthaupt. com

1. Fertilization l Thus, the embryo doesn’t enlarge much. Rather, it becomes an aggregate of blastomeres called a blastocyst (blastula). travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

2. Cleavage The color of the frog egg cells allows for easy following of cells during development. l Animal hemisphere is deep gray due to melanin. l The vegetal pole is yellow due to lack of melanin. l travismulthaupt. com

2. Cleavage The first 5 to 7 divisions produce what is called the morula. l Within the morula a fluid filled cavity called a blastocoel forms. When fully formed, the aggregate of cells is now called blastula. l travismulthaupt. com

2. Cleavage l Many animals, with the exception of possibly mammals, have zygotes with a definite polarity that determines the pattern of cleavage. travismulthaupt. com

2. Cleavage In classic examples such as the frog, the distribution of yolk in the egg determines cleavage patterns. l The vegetal pole contains a large amount of yolk, and cleavage through this is slow. l The animal pole contains very little yolk and cleavage occurs quickly through it. l travismulthaupt. com

2. Cleavage--3 Body Axes l The 3 body axes are established early on in development. l 1. Dorsal/ventral l 2. Right/left l 3. Anterior/posterior travismulthaupt. com

2. Cleavage l Using the frog as an example, fusion of the sperm and egg initiates a body axis. l At this point, the animal pole will move toward the vegetal pole passing through the site of sperm entry. travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

2. Cleavage l As a result of the cortical rotation, a light gray region known as the gray crescent forms in some species and the dorsal/ventral axis is formed in the zygote. travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

2. Cleavage l This light gray region forms the dorsal side of the embryo. l In frogs (animals with a thick vegetal pole), the blastocoel, is located in the animal pole because the yolk isn’t as thick. travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

2. Cleavage Yolk is very plentiful in many animals such as a bird. l In this example, the yolk is actually an egg cell swollen with yolk nutrients. A small white disk of actual cytoplasm is located on the animal pole. l travismulthaupt. com

2. Cleavage Albumen l The entire cell is surrounded by a protein rich solution of egg white which provides additional nutrients to the growing embryo. Chalaza travismulthaupt. com Yolk Disk of Cytoplasm

2. Cleavage l Cleavage is restricted to the small disk of yolk free cytoplasm. l Cleavage can’t proceed through the dense yolk. travismulthaupt. com

2. Cleavage http: //scienceblogs. com/pharyngula/2006/07/the_evolution_of_deuterostome. php l Incomplete division of the yolk rich egg is known as meroblastic cleavage. l In contrast, holoblastic cleavage results in complete division of the egg that lacks a lot of yolk, (or a moderate amount of yolk). travismulthaupt. com http: //scienceblogs. com/pharyngula/2006/07/the_evolution_of_deuterostome. php

2. Cleavage l The cleaving cells on the top of the yolk of a bird egg produce a cap of cells called the blastoderm. l The blastoderm then rests on the yolk and divides into 2 layers called the epiblast and the hypoblast. travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

2. Cleavage l The cavity between them is called the blastocoel. l This is the equivalent of the blastula stage of the frog/sea urchin. travismulthaupt. com Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

3. Gastrulation l Gastrulation is the rearrangement of cells of the blastula to form a 3 layered embryo with a primitive gut. travismulthaupt. com

3. Gastrulation l Gastrulation differs between animal groups, but it shares some general characteristics: n A. Changes in cell motility. n B. Changes in cell shape. n C. Changes in cellular adhesion. travismulthaupt. com

3. Gastrulation The result is that some cells at the surface of the blastula move inward establishing 3 cell layers. l The 3 layered embryo is called the gastrula. l The 3 layers are given the term embryonic germ layers and consist of: n 1. Endoderm--lines digestive l tract. n 2. Mesoderm--partially fills the space between endoderm and ectoderm. n 3. Ectoderm--which forms the outer layer of the gastrula. travismulthaupt. com

3. Gastrulation--Sea Urchin l l These 3 layers will form all cell layers and tissues in the adult animal. In the sea urchin, the blastula consists of a single layer of cells enclosing a blastocoel. Gastrulation begins at the vegetal pole. Here, migratory cells called mesenchyme cells enter the blastocoel. travismulthaupt. com

3. Gastrulation--Sea Urchin The cells which remain form the vegetal plate which eventually buckles inward by a process called invagination. l Invagination proceeds forming the archenteron (primitive gut). l The open end of the archenteron will become the anus, it is called the blastopore. l travismulthaupt. com

3. Gastrulation--Sea Urchin l The second opening forms as the archenteron touches the inside of the ectoderm on the other side. This is the mouth (in deuterostomes). l A rudimentary digestive tube is now formed. Ectoderm Blastocoel Mesenchyme Cells Vegetal Plate travismulthaupt. com http: //biology. kenyon. edu/courses/biol 114/Chap 14/PMC_SEM. gif

3. Gastrulation--Frog In the frog, gastrulation also produces a 3 layered embryo with an archenteron. l Gastrulation is more complex due to the yolk content of the cells, and the fact that the wall of the blastula is more than 1 cell-layer thick. l Gastrulation begins on the dorsal side of the blastula in a line along the region of the gray travismulthaupt. com crescent. l

3. Gastrulation--Frog l This invagination forms the dorsal lip which pinches inward around the cell. l The cells on the surface of the embryo continue to roll inward in a process called involution forming the future endoderm and mesoderm. travismulthaupt. com

3. Gastrulation--Frog Eventually, the blastocoel collapses as the archenteron replaces it. The archenteron is lined with endoderm. l As gastrulation completes, the blastopore encircles the yolk plug. l After the archenteron reaches the opposite side of the embryo, a mouth will form. l travismulthaupt. com

3. Gastrulation--Chick l The chick’s gastrulation is similar to that of a frog except the inward movement of cells during gastrulation is affected by the yolk. travismulthaupt. com

3. Gastrulation--Chick l Cleavage in the chick results in a blastoderm with an epiblast and a hypoblast. travismulthaupt. com

3. Gastrulation--Chick All cells that form the embryo will come from the epiblast. l During gastrulation, some of the epiblast cells move toward the blastoderm’s midline producing a thickening called the primitive streak. l travismulthaupt. com

3. Gastrulation--Chick l The primitive streak will form the bird’s anterior/posterior axis. l The primitive streak is the functional equivalent to the lip of the blastopore in the frog, but structures are aligned differently. l Inward moving cells displace hypoblast cells and form endoderm. travismulthaupt. com Scott F. Gilbert Developmental Biology

3. Gastrulation--Chick Other epiblast cells move laterally into the blastocoel and form mesoderm. l Epiblast cells remaining on the surface form ectoderm. l The hypoblast helps direct the formation of the primitive streak and forms part of the stalk that keeps the yolk mass connected to the embryo. l travismulthaupt. com

4. Organogenesis is the process by which parts of the 3 germ layers develop the rudiments of organs. l Organogenesis involves localized morphogenic changes in tissue and cell shape versus the large scale mass movement of cells seen in gastrulation. l travismulthaupt. com

4. Organogenesis Evidence of organogenesis is seen in the appearance of folds splits and dense clustering of cells. l Easy observations are made in chordates when the notochord, neural tube, etc. take form. l travismulthaupt. com

4. Organogenesis l The notochord is formed from dorsal mesoderm which condenses just above the archenteron. travismulthaupt. com

4. Organogenesis Signals sent from the notochord to the ectoderm cause the region of the ectoderm to form the neural plate. l The neural plate will curve inward forming the neural tube which runs along the anteriorposterior axis of the embryo eventually forming the CNS. l travismulthaupt. com

4. Organogenesis l In vertebrates, the neural crest forms along the border where the neural tube pinches off from the ectoderm. l Neural crest cells then migrate to various parts of the embryo giving rise to many structures such as peripheral nerves, travismulthaupt. com teeth, skull bones, etc.

4. Organogenesis l Lateral to the notochord are condensations of mesodermal cells arranged in the block called somites. l The somites give rise to cells that migrate to new locations forming vertebrae and muscles. travismulthaupt. com

Development l All vertebrates require an aqueous environment to grow and develop. l The evolution of structures that allow this to happen have occurred and are the shelled egg and uterus. These structures protect the embryo with an amnion which is a fluid filled sac. travismulthaupt. com

Extraembryonic Membranes l The development of mammals, birds, and reptiles includes the formation of 4 extraembryonic membranes. l These membranes include: n Yolk sac n Chorion n Allantois n Amnion travismulthaupt. com

Extraembryonic Membranes l The yolk sac covers the yolk and develops blood vessels that transplant nutrients from the yolk to the embryo. travismulthaupt. com

Extraembryonic Membranes l The chorion is involved in gas exchange with the surroundings. travismulthaupt. com

Extraembryonic Membranes l The allantois is a disposable sac for metabolic wastes produced by the developing embryo. It also functions with the chorion in respiration. travismulthaupt. com

Extraembryonic Membranes l The amnion protects the embryo in a fluid filled cavity. travismulthaupt. com