Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 47 Animal Development

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Development is determined by the zygote’s genome and differences between embryonic cells Cell differentiation is the specialization of cells in structure and function Cytoplasmic determinants, the uneven distribution of maternal substances within the unfertilized egg, cause early embryonic cells to differentiate in some species. Morphogenesis is the process by which an animal takes shape

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 47.1: After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis Important events regulating development occur during fertilization and the three stages that build the animal’s body – Cleavage: cell division creates a hollow ball of cells called the blastula. – Gastrulation: produces a 3-layered embryo called the gastrula. – Organogenesis: generates rudimentary organs from which adult structures grow.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fertilization Fertilization brings the haploid (1 set of chromosome) nuclei of sperm and egg together, forming a diploid (2 sets of chromosomes) zygote The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development (“activates the egg”)

LE 47-3 Sperm-binding receptors Jelly coat Acrosome Actin Sperm head Basal body (centriole) Sperm plasma membrane Sperm nucleus Contact Acrosomal reaction Acrosomal process Contact and fusion of sperm and egg membranes Entry of sperm nucleus Cortical reaction Fertilization envelope Egg plasma membrane Vitelline layer Hydrolytic enzymes Cortical granule Fused plasma membranes Perivitelline space Cortical granule membrane EGG CYTOPLASM

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Acrosomal Reaction The acrosomal reaction is triggered when the sperm meets the egg. This process begins when a specialized vesicle at the tip of the sperm called the acrosome, discharges hydrolytic enzymes. – This reaction releases hydrolytic enzymes that digest material surrounding the egg – This allows the acrosomal process (elongated sperm structure) to penetrate the jelly coat.

Molecules of a protein on the tip of the acrosomal process adhere to molecules of a specific receptor proteins on the egg’s surface. This gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy (multiple sperm). – Depolarization occurs when the ion channels open on the egg’s plasma membrane allowing sodium ions to flow into the egg cell and change the membrane potential. – Depolarization occurs within 1-3 seconds after a sperm binds to an egg.

Fusion of egg and sperm also initiates the cortical reaction This reaction induces a rise in Ca 2+ from the egg’s ER into the egg’s cytosol. This causes cortical granules in the egg to fuse with the plasma membrane and discharge their contents. This leads to swelling of the perivitelline space, hardening of the vitelline layer, and clippling of sperm binding receptors. These changes cause formation of a fertilization envelope that functions as a longer-term slow block to polyspermy. – Does occur in vertebrates (fishes and animals)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Activation of the Egg The sharp rise in Ca 2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell With these rapid changes in metabolism, the egg is said to be activated Sperm cells do not contribute any materials required for activation. The unfertilized eggs of many species can be artificially activated by the injection of Ca 2+ or by a variety of mildly injurious treatments, such as temperature shock.

LE 47-5 Binding of sperm to egg Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) Increased intracellular calcium level Cortical reaction begins (slow block to polyspermy) Formation of fertilization envelope complete Increased intracellular pH Fusion of egg and sperm nuclei complete Increased protein synthesis Onset of DNA synthesis First cell division 1 Seconds Minutes

Fertilization in Mammals Fertilization in other species share the same timing as the sea urchin in the previous slide. However, timing differs with species. – Sea urchins meiosis is already completed when the egg is released from the female. – In humans, the unfertilized egg stays at metaphase of meiosis II. Meiosis is NOT completed until they are fertilized in the female reproductive tract. Fertilization is generally internal.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Secretions in the mammalian female trace alter certain molecules on the surface of sperm cells and also increase sperm motility. The mammalian egg is cloaked by follicle cells released along with the egg during ovulation. The sperm must migrated through this layer of follicle cells before it reaches the zona pellucida. In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy Fertilization in Mammals continued

LE 47-6 Follicle cell Acrosomal vesicle Egg plasma membrane Zona pellucida Sperm nucleus Cortical ganules Sperm basal body EGG CYTOPLASM 1. Sperm migrated through the coat of follicle cells and binds to receptor molecules in the zona pellucida of the egg. 2. This binding induces the acrosomal reaction, in which the sperm released hydrolytic enzymes into the zona pellucida. 3. Breakdown of the zona pellucida by these enzymes allows the sperm to reach the plasma membrane of the egg. Membrane proteins of the sperm bind to the receptors on the egg membrane, and the two membranes fuse 4. The nucleus and other components of the sperm cell enter the egg. 5. Enzymes released during the cortical reaction harden the zona pellucida, which now functions as a block to polyspermy.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fertilization in Mammals continued After the egg and sperm membrane fuse, the whole sperm, tail and all is taken into the egg. The egg lacks a centrosome. The basal body of the sperm’s flagella now acts as the centrosome and wraps itself around the centriole. This will allows mitotic spindles to form for the first cell division. Fertilization is much slower in mammals. The first cell division occurs hours after sperm binding in mammals.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cleavage cell division creates a hollow ball of cells called the blastula. Fertilization is followed by cleavage, a period of rapid cell division without growth Cells undergo S and M phases of the cell cycle but skip Gap 1 and Gap 2. Little or now protein synthesis occurs. – The embryo does not enlarge during this period of development. Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres. Each with its own nucleus.

LE 47-7 First 5-7 divisions from a cluster of cells known as the morula A fluid-filled cavity called the blastocoel begins to form within the morula and is fully formed in the blastula, a hollow ball of cells.blastocoel During cleavage, different regions of the cytoplasm end up in separate blastomeres. These regions may contain different cytoplasmic determinants, in many species this partitioning sets the stage for subsequent developmental events. Fertilized eggFour-cell stageMorulaBlastula

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gastrulation Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut. Varies from one animal to another, the process is driven by change in cell motility, changes in cell shape, and changes in cellular adhesion to other cells and to molecules of the extracellular matrix. This results in the three cell layers.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The three layers produced by gastrulation are called embryonic germ layers – The ectoderm f orms the outer layer – The endoderm l ines the digestive tract – The mesoderm p artly fills the space between the endoderm and ectoderm Eventually, these three cell layers develop into all the tissues and organs of the adult animal. Video: Sea Urchin Embryonic Development Video: Sea Urchin Embryonic Development

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organogenesis During organogenesis, various regions of the germ layers develop into rudimentary organs Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mesoderm lateral to the notochord forms blocks called somites Lateral to the somites, the mesoderm splits to form the coelom The neural plate soon curves inward, forming the neural tube Many structures are derived from the three embryonic germ layers during organogenesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Developmental Adaptations of Amniotes Because all vertebrate embryos required an aqueous environment for development, embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell (birds & reptiles) or the uterus (marsupials & eutherian) Organisms with these adaptations are called amniotes In these organisms, the three germ layers also give rise to the four membranes that surround the embryo

LE Embryo Amniotic cavity with amniotic fluid Allantois Amnion Albumen Yolk (nutrients) Yolk sac Chorion Shell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mammalian Development Fertilization takes place in the oviduct, and the progresses as the embryo completes its journey down the oviduct to the uterus. The eggs of placental mammals – Are small and store few nutrients – Exhibit holoblastic cleavage (complete cell division of egg, having little or moderate amount of yolk) Gastrulation and organogenesis resemble the processes in birds and other reptiles Early cleavage is relatively slow in humans and other mammals

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings At completion of cleavage, the blastocyst forms The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells As implantation is completed, gastrulation begins The extraembryonic membranes begin to form By the end of gastrulation, the embryonic germ layers have formed

LE 47-18a Blastocyst reaches uterus. Endometrium (uterine lining) Maternal blood vessel Blastocyst implants. Inner cell mass Trophoblast Blastocoel Hypoblast Trophoblast Epiblast Expanding region of trophoblast

LE 47-18b Hypoblast Chorion (from trophoblast Epiblast Amniotic cavity Amnion Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Extraembryonic membranes start to form and gastrulation begins. Amnion Chorion Endoderm Mesoderm Ectoderm Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four extraembryonic membranes. Allantois Expanding region of trophoblast

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way