Animal Development Chapter 53 Fertilization In all sexually-reproducing
Animal Development Chapter 53 Fertilization In all sexually-reproducing animals, the first step is fertilization union of male and female gametes Fertilization itself consists of three events: -Sperm penetration and membrane fusion -Egg activation -Fusion of nuclei 2 Fertilization Sperm penetration and membrane fusion -Protective layers of egg include the jelly layer and vitelline envelope in sea urchins,
and the zona pellucida in mammals -The acrosome of sperm contains digestive enzymes that enable the sperm to tunnel its way through to the eggs cell membrane -Membrane fusion permit sperm nucleus to enter directly into eggs cytoplasm 3 Fertilization Egg activation -Membrane fusion triggers egg activation by the release of Ca2+ which initiates changes in the egg -A block to polyspermy occurs -Changes in eggs membrane potential -Alteration of eggs exterior coats -Enzymes from cortical granules
membrane Vitelline envelope First polar body Nucleus Cytoplasm of egg Cortical granules a. Cortical granules
Cytoplasm b. 5 Fertilization Egg activation -Sperm penetration has three other effects 1. Triggers the egg to complete meiosis 2. Triggers a cytoplasmic rearrangement 3. Causes a sharp increase in protein synthesis and metabolic activity in general 6 Fertilization
Primary Oocyte First Metaphase of Meiosis Second Metaphase of Meiosis Diploid nucleus Meiosis Complete Polar bodies Polar body Female pronucleus (haploid) Roundworms (Ascaris)
7 Fertilization Fusion of nuclei -The haploid sperm and haploid egg nuclei migrate toward each other along a microtubule based aster -They then fuse, forming the diploid nucleus of the zygote 8 1. Sperm penetrates between granulosa cells. Fertilization
2. Some of the zona 4. The sperm nucleus 3. Sperm and egg pellucida is degraded dissociates and plasma membranes by acrosomal enzymes. enters cytoplasm. fuse. Plasma membrane Granulosa cells Zona pellucida
Cortical granules 6. Additional sperm can no longer penetrate the zona pellucida. 5. Cortical granules release enzymes that harden zona pellucida and strip it of sperm receptors. Hyalin attracts water by osmosis. 7. Sperm and egg pronuclei are enclosed in a
nuclear envelope. 9 Cleavage Cleavage is the rapid division of the zygote into a larger and larger number of smaller and smaller cells called blastomeres -It is not accompanied by an increase in the overall size of the embryo In many animals, the two embryo ends are: -Animal pole = Forms external tissues -Vegetal pole = Forms internal tissues 10 Cleavage The outermost blastomeres in the ball of cells
become joined by tight junctions Innermost blastomeres pump Na+ into the intracellular spaces -Create osmotic gradient, which draws water The result is a hollow ball of cells, the blastula, containing a fluid-filled cavity, the blastocoel 11 Cleavage Patterns Cleavage patterns are highly diverse -Influenced by amount of yolk in the egg Sea Urchin Frog Chicken
Animal pole Nucleus Cytoplasm Cytoplasm Nucleus Air bubble Nucleus Shell Plasma membrane Albumen
Yolk Yolk a. Vegetal pole Yolk b. c. 12 Cleavage Patterns Eggs with moderate to little yolk undergo holoblastic (complete) cleavage -In sea urchins, a symmetrical blastula is produced, surrounding spherical blastocoel
-In amphibians, an asymmetrical blastula is produced, with a displaced blastocoel -Because egg contains much more yolk in one hemisphere than the other 13 14 Cleavage Patterns Eggs with large amounts of yolk undergo meroblastic (incomplete) cleavage -In eggs of reptiles and birds, the clear cytoplasm is concentrated at one pole called the blastodisc -Cleavage is restricted to this area -Resulting embryo is not spherical 15
Cleavage Patterns 16 Cleavage Patterns Mammalian eggs contain very little yolk, and so undergo holoblastic cleavage -Form a blastocyst, which is composed of: -Trophoblast = Outer layer of cells -Contributes to the placenta -Blastocoel = Central fluid-filled cavity -Inner cell mass = Located at one pole -Forms the developing embryo 17 Cleavage Patterns
ICM Blastocoel Blastodisc Yolk Trophoblast 18 Fate of Blastomeres In mammals, early blastomeres do not appear to be committed to a particular fate -The earliest patterning events occur at the eight-cell stage -Outer surfaces of blastomeres flatten
against each other in a process called compaction -Produces polarized blastomeres, which then divide asymmetrically 19 Gastrulation Gastrulation is a process involving a complex series of cell shape changes and cell movements that occurs in the blastula -It establishes the basic body plan and creates the three primary germ layers -Ectoderm Exterior -Mesoderm Middle -Endoderm Inner 20
Gastrulation 21 Gastrulation Cells move during gastrulation using a variety of cell shape changes -Cells that are tightly attached to each other via junctions will move as cell sheets -Invagination Cell sheet dents inward -Involution Cell sheet rolls inward -Delamination Cell sheet splits in two -Ingression Cells break away from cell sheet and migrate as individual cells 22 Gastrulation Patterns Also vary according to the amount of yolk
Gastrulation in sea urchins -Begins with formation of vegetal plate and ingression of primary mesenchyme cells (future mesoderm cells) into blastocoel -Remaining cells of vegetal plate invaginate into blastocoel forming the endoderm -Archenteron (future digestive gut) -Cells staying at surface form ectoderm 23 Animal pole Ectoderm Future ectoderm Ectoderm
Blastocoel Primary mesenchyme cells (PMCs) Vegetal pole a. Filopodia Archenteron PMC Future endoderm
Blastopore b. Anus c. 24 Gastrulation Patterns Gastrulation in frogs -Cells from the animal pole involute over the dorsal lip of blastopore into the blastocoel -Cells eventually press against far wall -Eliminate blastocoel, producing the archenteron with yolk plug
-These movements create two layers -Outer ectoderm and inner endoderm 25 -Mesoderm forms later in between Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal pole Dorsal lip Ectoderm Ectoderm Mesoderm Archenteron Endoderm Ectoderm
Archenteron Blastocoel Blastocoel Mesoderm Vegetal pole a. Blastocoel Yolk plug Dorsal lip of blastopore Ventral lip
b. c. Neural plate Neural fold Neural plate 26 d. e. Gastrulation Patterns Gastrulation in birds -Avian blastula consists of a disc of cells, the blastoderm, sitting atop large yolk mass
-First, blastoderm delaminates into two layers, with blastocoel cavity in between -The upper layer produces all 3 germ layers -Cells that migrate through primitive streak form endoderm or mesoderm -Cells that remain form ectoderm 27 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blastoderm Yolk Blastocoel Yolk
Primitive streak Mesoderm Ectoderm Endoderm Yolk 28 Gastrulation Patterns Gastrulation in mammals -Proceeds similarly to that in birds -Embryo develops from inner cell mass -ICM flattens and delaminates into 2 layers -A primitive streak forms
-Cell movements through it give rise to the three primary germ layers 29 Gastrulation Patterns Inner cell mass Primitive streak Amniotic cavity Ectoderm Ectoderm Mesoderm Formation of yolk sac
Trophoblast a. b. Endoderm c. Endoderm d. 30 Extraembryonic Membranes
As an adaptation to life on dry land, amniotic species developed several extraembryonic membranes -Nourish and protect the developing embryo These membranes are formed from embryonic cells 31 Extraembryonic Membranes 1. Amnion = Encloses amniotic fluid 2. Chorion = Located near eggshell in birds -Contributes to the placenta in mammals 3. Yolk sac = Food source in bird embryos -Found in mammals, but it is not nutritive 4. Allantois = Unites with chorion in birds, forming a structure used for gas exchange -In mammals, it contributes blood vessels
32 to the developing umbilical cord Extraembryonic Membranes Chick Embryo Mammal Embryo Chorion Amnion Chorion Yolk sac Amnion Umbilical
blood vessels Yolk sac Villus of chorion frondosum Allantois Maternal blood a. b. 33
Organogenesis Organogenesis is the formation of organs in their proper locations -Occurs by interaction of cells within and between the three germ layers -Thus, it follows rapidly on the heels of gastrulation -Indeed, in many animals it begins before gastrulation is complete 34 Organogenesis To a large degree, a cells location in the developing embryo determines its fate At some stage, every cells ultimate fate becomes fixed cell determination
A cells fate can be established in two ways: 1. Inheritance of cytoplasmic determinants 2. Interactions with neighboring cells -Cell induction 35 Organogenesis in Drosophila Salivary gland development -The sex combs reduced (scr) gene is a homeotic gene in the Antennapedia complex -Prior to organogenesis, it is expressed in an anterior band of cells -At the same time, Decapentaplegic protein (Dpp) is released from dorsal cells -Forms a gradient in the dorsal-ventral direction 36
Organogenesis in Drosophila Salivary gland development -Dpp inhibits formation of salivary gland rudiments -Thus, during organogenesis, salivary glands develop in areas where Scr is expressed and Dpp is absent 37 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prior to Organogenesis Dpp a.
During Organogenesis Salivary gland Labium b. 38 Organogenesis in Vertebrates Organogenesis in vertebrates begins with the formation of two structures unique to chordates -Notochord -Dorsal nerve cord -Its development is called neurulation
39 Development of Neural Tube The notochord forms from mesoderm -Region of dorsal ectodermal cells situated above notochord thickens to form the neural plate -Cells of the neural plate fold together to form a long hollow cylinder, the neural tube -Will become brain and spinal cord 40 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neural plate
Mesoderm Endoderm b. Neural tube Ectoderm Neural crest Mesoderm Endoderm Somite 41 c.
Generation of Somites Mesoderm sheets on either side of notochord separate into rounded regions called somitomeres -These separate into segmented blocks called somites -Form in an anterior-posterior wave with a regular periodicity -Ultimately give rise to skeleton, muscles and connective tissues 42 Generation of Somites Mesoderm in the head region remains connected as somitomeres -Form muscles of the face, jaws and throat Some body organs develop within a strip of
mesoderm lateral to each row of somites -Remainder of mesoderm moves out to surround the endoderm completely -Mesoderm separates into two layers -Coelom forms in between 43 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chordamesoderm Notochord Intermediate mesoderm Kidney
Gonads Circulatory system Linings of body cavities Lateral plate mesoderm Extraembryonic Head Paraxial mesoderm Cartilage
Somite Skeletal muscle Dermis 44 Neural Crest Cells Neurulation occurs in all chordates However, in vertebrates it is accompanied by an additional step -Just before the neural groove closes to form the neural tube, its edges pinch off, forming a small cluster of cells called the neural crest -These cells migrate to colonize many different regions of developing embryo
45 Neural Crest Cells Neural crest cells migrate in three pathways -Cranial neural crest cells are anterior cells that migrate into the head and neck -Trunk neural crest cells are posterior cells that migrate in two pathways -Ventral pathway cells differentiate into sensory neurons and Schwann cells -Lateral pathway cells differentiate into melanocytes of the skin 46 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Epidermis
Neural tube Posterior Lateral Pathway Cells take a dorsolateral route between the epidermis and somites Neural crest cells Anterior Aorta Notochord a.
Posterior somite Anterior somite Ventral Pathway Cells travel ventrally through the anterior half of each somite Ventral Pathway Cell Fates Lateral Pathway Cell Fates Dorsal root ganglia Ventral root Schwann
cells Melanocytes Sympathetic ganglia Adrenal medulla b. 47 Neural Crest Cells A mutation in a gene that promotes survival of neural crest cells produces white spotting on ventral surfaces of human babies & mice
48 Neural Crest Cells Many of the unique vertebrate adaptations that contribute to their varied ecological roles involve neural crest derivatives -For example gill chambers provided a greatly improved means of gas exchange -Allowed transition from filter feeding to active predation (higher metabolic rate) -Other changes = Better prey detection, and rapid response to sensory information 49 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chordates Vertebrates Zygote Pharynx Lining of respiratory tract Lining of digestive tract Endoderm Blastula Gastrula
Ectoderm Neural crest Gill arches, sensory ganglia, Schwann cells, adrenal medulla Liver Mesoderm Outer covering of internal organs Lining of
thoracic and abdominal cavities Dorsal nerve cord Epidermis, skin, hair, epithelium, inner ear, lens of eye Major glands Pancreas Brain, spinal cord,
Kidney Dermis Skeleton Striated muscles 50 Vertebrate Axis Formation Hans Spemann & Hilde Mangold transplanted cells of the dorsal lip of one embryo into the future belly region of another -Some of the embryos developed two notochords: a normal dorsal one, and a second one along the belly
-Moreover, a complete set of dorsal axial structures formed at the ventral transplantation site in most embryos 51 Vertebrate Axis Formation Donor embryo Recipient embryo Primary neural fold Primary notochord, somites, and neural development Dorsal lip
Secondary neural fold Secondary notochord, somites, and neural development Primary embryo Secondary embryo 52 Organizers An organizer is a cluster of cells that release
diffusible signal molecules, which convey positional information to other cells -The closer a cell is to an organizer, the higher the concentration of the signal molecule (morphogen) it experiences -Different morphogen concentrations stimulate development of different organs 53 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Organ A Concentration of morphogen Organizer cells secreting morphogen
Organ B Organ C Distance from secretion site Embryo Decreasing morphogen concentration gradient 54 Organizers Creation of the Spemann organizer -In frogs, as in fruit flies, the process starts during oogenesis in the mother
-Maternally-encoded dorsal determinants are localized at the vegetal pole of the unfertilized egg -At fertilization, rearrangements in the cytoplasm cause this determinant to shift to the future dorsal side of the egg 55 Animal pole Pigmented cortical cytoplasm Microtubule array Inner cytoplasm Microtubules
Vegetal pole Diffuse black pigment Clear cortical cytoplasm Dorsal determinants a. Point of sperm entry Gray crescent Shifted dorsal determinants b.
Organizer Dorsal mesoderminducing signal Mesoderminducing signals (TGF- family proteins) c. Nieuwkoop center 56 Organizers The maternally-encoded dorsal determinants are mRNAs for proteins that function in the intracellular Wnt signaling pathway -Wnt genes encode a large family of cellsignaling proteins -Affect the development of a number of
structures in both vertebrates and invertebrates 57 Organizers Function of the Spemann organizer -Dorsal lip cells do not directly activate dorsal development -Instead, dorsal mesoderm development is a result of the inhibition of ventral development 58 Organizers The bone morphogenetic protein 4 (BMP4) is expressed in all marginal zone cells (the
prospective mesoderm) of a frog embryo -BMP4 is a morphogen that at high levels specifies ventral mesoderm cell fates The Spemann organizer functions by secreting BMP4 antagonists -Bind to BMP4 and prevents its binding to its receptor 59 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal pole Mesoderm Epidermal ectoderm Neural ectoderm Endoderm
Ventral Dorsal Organizer molecules: Chordin, Noggin, and others 60 Vegetal pole Organizers Evidence indicates that organizers are present in all vertebrates -In chicks, a group of cells anterior to the primitive streak called Hensens node functions like the Spemann organizer
-Secrete molecules that inhibit ventral development -Same as those in frog embryos 61 Induction Primary induction occurs between the three primary germ layers -Example: Differentiation of the central nervous system during neurulation Secondary induction occurs between tissues that have already been specified to develop along a particular pathway -Example: Development of the lens of the vertebrate eye 62
Induction Wall of forebrain Ectoderm Optic cup Lens vesicle Neural cavity Optic stalk Lens invagination Lens
Optic nerve Lens Sensory layer Pigment layer 63 Retina Human Development Human development from fertilization to birth takes an average of 266 days, or about 9 months
-This time is commonly divided into three periods called trimesters 64 First Trimester First month -The zygote undergoes its first cleavage about 30 hr after fertilization -By the time the embryo reaches the uterus, 6-7 days after fertilization, it has differentiated into a blastocyst -Trophoblast cells digest their way into the endometrium in the process known as implantation 65
First Trimester First month -During the second week, the developing chorion and mothers endometrium engage to form the placenta -Mom and babys blood come into close proximity, but do not mix -Gases are exchanged, however 66 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chorion Amnion Yolk sac
67 First Trimester First month -One hormone released by the placenta is human chorionic gonadotropin (hCG) -Maintains mothers corpus luteum -Gastrulation occurs in the second week -Neurulation occurs in the third week -Organogenesis begins in the fourth week -Embryo is 5 mm in length 68 First Trimester Second month -Miniature limbs assume adult shape -Major organs within abdominal cavity
become evident -Embryo grows to about 25 mm in length -Weighs about 1 gm, and looks distinctly human 69 First Trimester Third month -The ninth week marks the transition from embryo to fetus -Nervous system develops -Limbs start to move -Secretion of hCG by the placenta declines, and so corpus luteum degenerates -Placenta takes over hormone secretion 70
Increasing Hormone Concentration Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. hCG Estrogen Progesterone 0 1 2 3
4 5 6 Months of Pregnancy 7 8 9 71 Second Trimester
The basic body plan develops further -Bones actively enlarge in fourth month -Rapid fetal heartbeat can be heard by a stethoscope By the end of the sixth month, fetus is over 30 cm long, and weighs 600 gm 72 Third Trimester A period of growth and organ maturation Weight of the fetus doubles several times Most of the major nerve tracts in the brain are formed -Brain continues to develop and produce neurons for months after birth 73
Birth Estrogen stimulates mothers uterus to release prostaglandins, and produce more oxytocin receptors -Prostaglandins begin uterine contractions -Sensory feedback from uterus stimulates oxytocin release from posterior pituitary -Oxytocin and prostaglandins further stimulate uterine contractions 74 Birth Strong contractions, aided by the mothers voluntary pushing, expel the fetus -Now called a newborn baby, or neonate After birth, continuing uterine contractions expel the placenta and associated
membranes -Collectively called the afterbirth 75 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestine Placenta Umbilical cord Wall of uterus Cervix Vagina
76 Nursing Milk production (lactation) occurs in alveoli of mammary glands when stimulated by the anterior pituitary hormone prolactin -Milk is secreted into alveolar ducts During pregnancy, progesterone stimulates development of mammary alveoli -And estrogen stimulates development of alveolar ducts 77 Nursing After birth, anterior pituitary secretes prolactin -Sensory impulses associated with babys suckling trigger the posterior pituitary to
release oxytocin -Stimulates contraction of smooth muscles surrounding alveolar ducts -Milk is ejected (milk let-down reflex) The first milk produced after birth, colostrum, is rich in nutrients & maternal antibodies 78 Postnatal Development Growth of the infant continues rapidly after birth -Babies typically double their birth weight within 2 months Different components grow at different rates -Allometric growth 79 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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