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Published on November 15, 2007

Author: Elliott

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Animal Development:  Animal Development Larry S. Katz Animal Sciences 732-932-7426 Katz@aesop.rutgers.edu Lecture Topics:  Lecture Topics Development Fertilization Cleavage The germ layers Organogenesis Evolution of the vertebrate head The extraembryonic membranes Human gestation The neonate and the environment From fertilization to death What is Development?:  What is Development? Includes all stages in the life of an individual Here, we focus on the early stages – fertilization to birth What happens during this time? Fusion of egg and sperm forms the zygote The zygote undergoes division to increase numbers of cells It becomes more complex It grows in size The cells undergo changes in gene expression and so change with time/place in the embryo; this is called differentiation Differentiation occurs via cell determination Determination is the process of commitment of a cell line to a particular developmental pathway Some cells do not change; these are stem cells Differentiation :  Differentiation The ongoing differentiation results in the development of form and structure (i.e. morphogenesis) in the embryo Occurs via changes in Cell protein expression Signaling between cells Cell migration Interactions with the extracellular matrix Controlled death (apoptosis) of cells Fertilization :  Fertilization The first step in development of a new individual is fertilization Is the union of a (usually) tiny sperm and an (usually) enormous ovum to form a zygote Determines the sex of the offspring in mammals and many other animals Stimulates changes in the egg that allow development May be divided into four steps Fertilization: Four Major Steps:  Fertilization: Four Major Steps Sperm contacts the egg Sperm or its nucleus enters the egg Egg becomes activated and developmental changes begin Sperm and egg nuclei fuse Here, most of the discussion centers upon echinoderm fertilization; is very well understood Effects of Capacitation on Sperm:  Effects of Capacitation on Sperm Increased rate of metabolism Flagellum beats more rapidly; Result: Sperm are more motile (hyperactivated) Changes in sperm glycoproteins Allow sperm-egg binding Pro-Acrosin (inactive) is converted to acrosin (active) Able to digest zona pellucida proteins Capacitation:  Capacitation First: Contact and Recognition:  First: Contact and Recognition The plasma membrane of the sea urchin egg is surrounded by the vitelline layer and the thicker, outer jelly coat (zona pellucida in mammals) The acrosome reaction is the release of proteolytic enzymes from the acrosome of the sperm Acrosome digests a path through the external coverings If sea urchin gametes are of the same species, a protein called bindin on the acrosome adheres to a specific receptor on the vitelline membrane In sea urchins, sperm are immediately motile upon release to the sea water, due to changes in internal pH that occurs when the sperm encounter the sea water Sperm are attracted to the egg by chemotaxis In mammals, sperm must undergo capacitation, which is a maturation process which occurs in the female reproductive tract In mammals, specific interaction between sperm head proteins and zona pellucida www.sidwell.edu/us/science/vlb5/Labs/Urchin_Lab/:  www.sidwell.edu/us/science/vlb5/Labs/Urchin_Lab/ Pronuclear fusion:  Pronuclear fusion Fertilization: Sea Urchins:  Fertilization: Sea Urchins The microvilli of the egg membrane form a fertilization cone Sperm is drawn into the cone (right) Gamete plasma membranes fuse and the sperm is drawn into the egg cell Blocks To Polyspermy:  Blocks To Polyspermy Fast block to polyspermy involves egg plasma membrane action potential, prevents additional sperm from entering The slow block to polyspermy involves the cortical reaction Calcium ion entry causes fusion of large cortical granules with plasma membrane that release digestive enzymes The acellular vitelline membrane (not a plasma membrane but extracellular matrix) lifts away from the plasma membrane due to the digestive activity and local increase in ionic strength from the cortical granule contents Forms a fertilization envelope that hardens In mammals, the zona pellucida sperm receptors are modified to prevent further entry of sperm Some organisms do not block polyspermy. Some amphibians degrade the supernumery sperm http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html:  http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html Slow Block to Polyspermy:  Slow Block to Polyspermy The left image shows the approach of the sperm at about 2 o'clock and the rising of the vitelline membrane. Intracellular Ca++ is monitored by an indicator that becomes more fluorescent when it binds Ca++. Fertilization Activates the Egg:  Fertilization Activates the Egg Aerobic respiration increases Enzyme systems become activated A burst of protein synthesis begins In most animals, the nucleus undergoes the second division of meiosis In many eggs it is possible to physically stimulate an egg to undergo activation, and even division Such parthogenetic – and haploid eggs usually go through limited division series Naturally parthogenetic species have special mechanisms to retain the 2N state of their genes Pronuclear Fusion:  Pronuclear Fusion Once the second polar body is ejected the female pronucleus can fuse with the male pronucleus This is the genetic beginning of a new organism The haploid genetic complements of the two pronuclei form a 2N nucleus, which prepares the nucleus, and cell, for cleavage Cleavage:  Cleavage During cleavage the zygote divides, giving rise to many cells The ovum contributes the majority of the zygote cytoplasm Both gametes contribute equal numbers of chromosomes Cleavage is a series of rapid mitotic division not accompanied by significant cell growth The zygote forms a two celled embryo, and continues divisions to form a ball of 32 cells called the morula The morula continues divisions to form the hollow blastula with up to several hundred cells The cells are called blastomeres The cavity of the blastula is the blastocoel Patterns Of Cleavage:  Patterns Of Cleavage The pattern of cleavage is affected by the yolk Isolecithal eggs have a uniform yolk distribution Simple chordates and most invertebrates have isolecithal eggs Isolecithal eggs typically have holoblastic cleavage The daughter cells completely separate during cleavage Radial cleavage is typical of deuterostomes: echinoderms and Amphioxus First division is vertical; second division is at right angles Third division is horizontal at right angles to the first and forms an 8 cell embryo with 4 above and 4 cells below the last division plane Spiral cleavage is typical of protostomes: annelids and molluscs After first two divisions the plane of cleavage tilts and diagonal to the polar axis Cleavage and Gastrulation in Amphioxus:  Cleavage and Gastrulation in Amphioxus Radial cleavage Very similar to the sea star Spiral Cleavage in an Annelid Embryo:  Spiral Cleavage in an Annelid Embryo Yolk Content Is Important:  Yolk Content Is Important Yolk provides energy for egg development The more metabolically active end of the cell is the animal pole, which contains less yolk Amphibian eggs contain moderate amounts of yolk; called mesolecithal Undergo holobastic cleavage but the divisions are concentrated in the animal end of the egg (below, frog) Telolecithal Eggs:  Telolecithal Eggs Telolecithal eggs have much yolk concentrated at the vegetal pole of the egg Eggs of reptiles and birds are highly telolecithal Cell division takes place in the blastodisc Division is meroblastic; cells do not completely separate from each other and remain attached, at least initially, to the yolk mass In birds and some reptiles, the blastodisc splits into the epiblast (upper) and hypoblast (lower, nearest the yolk), separated by the blastocoel Cleavage in a Bird Embryo:  Cleavage in a Bird Embryo Developmental Determinants:  Developmental Determinants Cleavage may distribute developmental determinants in addition to changing the yolk distribution Cleavage provides building blocks for development The unequal distribution of cytoplasm of the zygote results in blastomeres with different cytoplasmic composition Mosaic development is a rigid developmental pattern Regulative development is a result of homogeneous cytoplasm, and cells produced by cleavage are equivalent Most animals have developmental patterns somewhere between these two extremes Cytoplasmic Determinants In the Frog Egg:  Cytoplasmic Determinants In the Frog Egg Determinants that are held in the egg are transferred to the zygote In amphibians, fertilization causes a movement of the cortical cytoplasm, revealing lighter underlying cytoplasm The lighter gray cytoplasm is called the gray crescent This region is bisected by the first division, causing left-right sidedness of the future embryo Cells developing from the area of the gray crescent become the dorsal portion of the embryo If cleavage is experimentally forced to exclude the grey crescent, no dorsalization Gastrulation:  Gastrulation The blastula develops a hole in one end and cells start to migrate into the hole; this forms the gastrula The process is called gastrulation The gastrula is a three-layered embryo The pattern of gastrulation is affected by the amount of yolk The cells at the vegetal pole invaginate, initiating gastrulation The opening of the archenteron is the blastopore The vegetal pole invaginates and meets the opposite wall, obliterating the blastocoel The archenteron is the newly formed cavity The blastopore is the opening of the archenteron, and becomes the anus in deuterostomes Gastrulation in a Frog Embryo:  Gastrulation in a Frog Embryo Gastrulation in Birds:  Gastrulation in Birds In birds, the epiblast cells form the primitive streak, with the primitive groove serving as the functional equivalent of the blastopore Cells migrate centrally, and dive into the streak, to turn under the outermost layer of cells and move laterally and anteriorly inside the developing embryo No archenteron is formed Hensen’s node is the site of cells that will form the mesodermally-derived notochord Cells sink into the interior and move under the epiblast to form an extension from the node Gastrulation in Birds:  Gastrulation in Birds Organogenesis:  Organogenesis Organogenesis is the formation of the organs Arises from the layering of cells that occurs during gastrulation The layers are germ layers; they have specific fates in the developing embryo: Endoderm The innermost layer Goes on to form the gut Mesoderm In the middle Goes on to form the muscles, circulatory system, blood and many different organs Ectoderm The outermost Goes on to form the skin and nervous system Organogenesis Begins With Development of the Nervous System:  Organogenesis Begins With Development of the Nervous System The nervous system is the first organ system to develop The notochord grows and induces overlying ectoderm to form the neural plate Cells of the neural plate fold to form the neural groove and the surrounding neural folds The neural folds fuse, forming a hollow neural tube The anterior portion forms the brain; the rest forms the spinal cord Right, neural fold formation in the human embryo The Neural Crest:  The Neural Crest The neural crest is a critical structure that guides formation of several organ systems The neural crest forms on either side of the point of fusion Its cells migrate to form the dorsal root ganglia, the postganglionic sympathetic neurons, many sense organs and all pigment-forming cells Blocks of mesoderm called somites form on the outside of the neural tube, become the vertebrae and associated parts of the segmented body axis Other organs are stimulated to form as a result, in part of neural crest cell movements and differentiation The trachea grows from the gut and lungs develop from it The pharyngeal pouches grow laterally from the pharynx Branchial grooves meet the pharyngeal pouches and form branchial arches, important in many structures of the head The pharyngeal and branchial grooves form the gill slits and gills in aquatic vertebrates Formation of the Human Heart:  Formation of the Human Heart The heart is derived from the fusion of blood vessels in the early embryo At first it is merely a single atrium and single ventricle Torsion brings the atrium anterior to the ventricle and partitions form that divide the atrium and ventricle into left and right chambers Formation Of the Visceral Organs:  Formation Of the Visceral Organs Digestive tract formation starts as a simple tube separated in the middle by the yolk stalk Separates into foregut and hindgut Liver, pancreas and trachea are hollow outgrowths from the gut Human, 5th week embryo Section Through Head of 5th Week Embryo:  Section Through Head of 5th Week Embryo The developing brain ventricle appears; the spinal cord begins to form above the notochord; the dorsal root ganglia form The blood vessels form The branchial arches form – these give rise to facial features The pharyngeal pouches give rise to the inner ear, eustachian tube and associated structures Evolution Of the Vertebrate Head:  Evolution Of the Vertebrate Head Pre-vertebrate animals lacked a true head, with all the complex bone structure, little sensory apparatus and poorly developed brain Amphioxis is a good example Such organisms lack neural crest cells, which separate from the dorsal neural tube and migrate to produce special cells that develop into unique head features Neurogenic placodes are ectodermal thickenings that are found only in the head – studied extensively in mouse and chick Produce different structures at different positions as revealed by differential hox gene expression Hox Genes and the Head:  Hox Genes and the Head Manzanares, of the MRC/London, introduced Hox regulatory element genes from Amphioxis into cells of vertebrate embryos Each hox gene was fused to a ‘reporter element’ that produced a dark stain Thus, the transcription of the gene showed its presence Each gene showed a unique pattern of staining in the hindbrain, neural crest, and neurogenic placodes in the head of both mouse and chick embryos That the Amphioxis Hox genes could respond to position in the more advanced vertebrate embryos supports the concept that modern vertebrates arose from a common ancestor with Amphioxis Hox genes probably were modified to suit the needs of the developmental role in the vertebrate to build a head Extraembryonic Membranes:  Extraembryonic Membranes Protect and nourish the embryo Terrestrial vertebrates have four extraembryonic membranes Develop from the germ layers, but are not part of the embryo and are lost at birth The chorion and amnion enclose the embryo The chorion surrounds the entire embryo The amnion encloses the embryo and forms an open volume between the embryo & the amnion called the amniotic cavity The amniotic cavity fills with amniotic fluid, which envelops the embryo and cushions it The amniotic fluid can be sampled to test for developmental abnormalities The allantois is an outgrowth of the gut In reptiles and birds, it stores nitrogenous wastes The yolk sac encloses the yolk in vertebrates with yolk-rich eggs In humans, there is no yolk sac, but the yolk aids in formation of red blood cells Extraembryonic Membranes:  Extraembryonic Membranes Human Prenatal Development :  Human Prenatal Development Gestation lasts 266 days from fertilization to birth Development begins in the oviduct About 24 hours after fertilization, the zygote has divided to form a 2-celled embryo The embryo passes down the oviduct by cilia and peristalsis The zona pellucida has dissolved by the 5th day, when the embryo enters the uterus The embryo floats free for several days, nourished by fluids from glands in the uterine wall At this point, it is called a blastocyst The trophoblast forms the chorion and amnion The inner cell mass forms the embryo itself Implantation :  Implantation The embryo implants in the wall of the uterus on about the 7th day of development Trophoblast secretes enzymes that locally erode the uterine wall 12-day Human Embryo:  12-day Human Embryo The Placenta:  The Placenta The placenta is the site of nutrient, gas, and waste exchange Secretes hormones that maintain pregnancy Trophoblast cells release human chorionic gonadotropin (hCG) which signals the corpus luteum to enlarge and produce progesterone The placenta develops from the embryonic chorion and maternal uterine tissue Chorionic villi are formed from the chorion, and project into the endometrium of the uterus The umbilical cord, containing two umbilical arteries and one umbilical vein connects the embryo and the placenta Development of the Placenta:  Development of the Placenta Organ Development:  Organ Development Begins during the first trimester Gastrulation occurs during the 2nd and 3d weeks, followed by neurulation (formation of the neural tube) The heart beats spontaneously after 3.5 weeks After the first two months of development, the products of conception are called a fetus At the end of the first trimester (first 3 months of development) Fetus can be recognized as a human ~56 mm long, and ~14 g The sexes can be differentiated Ears, eyes becoming well-developed, Skeleton starting to develop Notochord replaced with the developing vertebral column Moves, ‘breathes’, makes sucking motions with thumb Human Fetus at Ten Weeks:  Human Fetus at Ten Weeks 2nd and 3rd Trimesters :  2nd and 3rd Trimesters In the second trimester Fetus moves freely Heart can be heard with a stethoscope ‘Quickening’ movements felt by the mother If born at 24 weeks, the fetus has a 50% chance of survival Brain not yet able to support breathing Kidneys and lungs are immature In the third trimester Fetus grows rapidly Final differentiation of organs and tissues Grasping and sucking reflexes active If born before 37 wks, is considered premature, but has a good chance of surviving if born after 30 weeks Full-term baby weighs about 6000 g and is 52 cm long Multiple Births:  Multiple Births Sometimes the embryo splits at the two-cell stage Each is totipotent – can give rise to an individual If both live, give rise to identical, or monozygotic, twins Occasionally give rise to conjoined twins which share one or more body parts Fraternal, or dizygotic, twins are the result of two eggs ovulating and being fertilized Fertilized by different sperm: are different genetically Different sex is possible: not so with identical twins (why?) Similar situations can occur triplets Multiple births due to in vitro methods are NOT identical siblings Neonate Adapts Rapidly at Birth:  Neonate Adapts Rapidly at Birth In the uterus the fetus was provided all it needed When born, the neonate must adapt to its new environment and start to provide some of what it needs, itself The initial breathing response of the neonate is initiated by the accumulation of carbon dioxide Breathing initiates with the rapid generation and release of pulmonary surfactant, which lowers the surface tension, allowing the first breath to be taken Breathing increases blood flow through the pulmonary circuit Blood from the right ventricle flows through the large pulmonary circuit blood vessels Blood originally (in utero) bypassed the pulmonary circuit by passing through a hole in the heart wall between the right atrium and left atrium, called the foramen ovale and via a shut from the pumonary artery and aorta The foramen ovale and the shunt close shortly after birth Environmental Factors:  Environmental Factors Environmental factors affect the embryo Prenatal development is greatly affected by anything circulating in the maternal blood The embryo is most susceptible to harm during the first trimester Alcohol abuse causes distinct changes in the fetus, is a distinct set of effects, called fetal alcohol syndrome Thalidomide was a tranquilizer used widely in the 1950s until it was learned that it had serious side-effects and caused fetal abnormalities About 5% of newborns in the United States have a clinically significant birth defect, accounting for about 15% of deaths of newborns Sonograms may help diagnose defects and position of the fetus Aging:  Aging Aging is not a uniform process Results in decreased function in the organ systems Different organ systems age at different rates The homeostatic response to stress decreases during aging The model of cellular ageing describes the loss of ability to divide seen in older cells This process may be due to the loss of the production of telomerase Apoptosis, which is genetically programmed cell death, may also contribute to the changes seen during aging It is not the apoptosis process itself that leads to ageing; defects in apoptosis may lead to abnormalities of function

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