Animal Development

1. Animal Development Chapter 42

2. Principles of Animal Development? • Development – where multicellular organisms grow and increase in organization and complexity • Development begins with a fertilized egg and ends with a sexually mature adult • Three principle mechanisms: • Individual cells multiply • Some of their daughter cells differentiate, or specialize in both structure and function • As they differentiate, groups of cells move and become organized into multicellular structures

3. From identical to differentiated • All of the cells of an individual animal’s body are genetically identical in an animal’s body, • How can they differentiate into different structures with distinct functions? • The solution is that different genes in different places and at different times in an animal’s body, are active.

4. Indirect and Direct Development • Baby mammals and reptiles are miniature versions of the adults of their species, undergoing direct development • The majority of animals species undergo indirect development - the newborn has a very different body structure than the adult

5. Indirect Development • Animals undergo a radical change in body form • Amphibians - frogs and toads, most invertebrates • Females produce huge numbers of eggs, each containing a small amount of food reserve called yolk • The yolk nourishes the developing embryo until it hatches into a small, sexually immature feeding stage called a larva • Parents provide these vulnerable offspring with neither food or protection from predators, most die in their larval stage • After feeding for weeks to years, a handful of survivors undergo a revolution in body form - metamorphosis, and become sexually mature adults

6. Butterflies and Larvae • Most larvae look very different from adults, but also play different roles in their ecosystems • Most adult butterflies sip nectar from flowers and unintentionally pollinate the flower in return • Their caterpillar larvae munch on leaves, often of specific host plants

7. Cicadas • We regard the adult form as the “real animal” and the larval stage as “preparatory” • Most of the life span of some animals – insects - is spent in the larval form • North American periodical cicadas spend 12- 16 years as underground larvae, sucking juices from plant roots, and only 4-6 weeks as adults, mating and laying eggs

8. Direct Development • Newborn animals resemble miniature adults • Snails and fish, all mammals, reptiles (including birds), undergo direct development • As the young animal matures, it may grow bigger, but does not fundamentally change its body form

9. Two Direct Development Strategies • Juveniles are typically much larger, so they need more nourishment before emerging into the world • Two strategies have evolved that meet the embryo’s food requirement: • Birds, most reptiles, and a few fish produce eggs that contain large amounts of yolk • Mammals, snakes, and a few fish have little yolk in their eggs, embryos are nourished within the mother’s body

10. Thank your mom • Providing food for directly developing embryos places great demands on the mother • Many offspring, - of birds and mammals - require additional care and feeding after birth, placing additional demands on one or both parents • Relatively few offspring are produced, but a higher proportion reach adulthood, because the parents devote more resources to each individual • Most of the mechanisms of development—the control of gene expression to allow differentiation of individual cells and entire body parts—are fundamentally similar in vertebrates and invertebrates, and in animals with indirect or direct development

11. Cleavage of the Zygote • Cleavage of the zygote begins development • The formation of an embryo begins with cleavage – a series of mitotic cell divisions of the fertilized egg or zygote • The zygote is a large cell

12. Morula • During cleavage, there is little or no cell growth between cell divisions • As cleavage progresses, the available cytoplasm is split up into ever smaller cells that gradually approaches the size of cells in the adult • Eventually a solid ball of small cells, the morula, is formed

13. Blastula • A cavity opens within the morula and the cells become the outer covering of a hollow structure, the blastula • The details of cleavage differ by species and are partly determined by the amount of yolk, which hinders cytokinesis • Eggs with large yolks (hen’s egg) don’t divide all the way through; nevertheless • In birds and other reptiles, the blastula is flattened on top of the yolk

14. Gastrula • The location of cells on the surface of the blastula forecasts their developmental fate in the adult • Gastrulation begins when a dimple - the blastopore - forms on one side of the blastula • Surface cells migrate through the blastopore in a continuous sheet • The resulting indentation enlarges to form a cavity that will become the digestive tract

15. Gastrulation • The migrating cells form three tissue layers in the embryo • Cells that move through the blastopore to line the future digestive tract are called endoderm, it also forms the respiratory tract lining, liver and pancreas • The cells remaining on the outside form the ectoderm and form surface structures such as skin, hair, nails, and nervous system • Cells that migrate between the endoderm and ectoderm form the third layer, - mesoderm - which forms structures between these two, the muscles, skeleton, and circulatory systems

16. Gastrulation in the Frog

17. Animation: Overview of Animal Development

18. Organogenesis • Adult structures develop during organogenesis • Organogenesis - development of adult structures from embryonic tissue layers • Two major processes: • A “master switch” genes turn on and off, each controlling the transcription and translation of the genes involved in producing each structure • Organogenesis prunes away excess cells

19. Organogenesis • This sculpting requires death of excess cells • Embryonic vertebrates have more motor neurons than adults • Embryonic motor neurons are programmed to die unless they form a synapse with a skeletal muscle, which releases a chemical that prevents the death of the neuron • All amphibians, reptiles, and mammals pass through embryonic stages in which they have tails and webbed fingers and toes • In humans, these stages appear during the fourth to seventh weeks of development • A few weeks later, the cells of the tail and webbing die; the tail disappears, and the hands and feet have separated fingers and toes

20. Extraembryonic Membranes • Development in reptiles and mammals depends on extraembryonic membranes • Fish live and reproduce in water, by spawning • Amphibians spend adult lives on land, lay their eggs in water • In both, the embryo obtains nutrients from the yolk of the egg, oxygen from water, and releases its wastes into water • Terrestrial vertebrate life was not possible until the evolution of the amniotic egg • First in reptiles, persists today in reptiles, birds and their descendents, the mammals • It allows these groups to complete their development in their own “private pond”

21. The Amniotic Egg • The amniotic egg is characterized by four extraembryonic membranes – • The chorion, amnion, allantois, yolk sac • In reptiles - • The chorion lines the shell and exchanges O2 and CO2 between the embryo and the air • The amnion encloses the embryo in a watery environment • The allantois stores and isolates wastes • The yolk sac contains the yolk

23. Placental Mammals • In placental mammals—except marsupials—the embryo develops within the mother’s body until birth • Marsupials - Kangaroos and monotremes (platypuses) • All four extraembryonic membranes are essential for development

25. Differentiation of Cells • A zygote contains all genes needed to produce an entire animal • Every cell of the body contains all of these genes—that makes cloning possible • In any given cell some genes are expressed, others are not • The differentiation of cells during development happens because of differences in gene expression

26. Controlling Gene Expression • Cells have ways of controlling gene expression, ie regulating which genes are transcribed into mRNA • Transcription factors bind to DNA near the promotor regions, where gene transcription begins • Different transcription factors bind to different genes and turn their transcription on or off • Which genes are transcribed determines the structure and function of the cell • This leads to one of the central questions about development: What causes different cells to transcribe different genes?

27. Animal Development • Molecules positioned in the egg and produced by nearby cells control gene expression during embryonic development • In animal embryos, the differentiation of individual cells and the development of entire structures are driven by one or both of two processes: • The actions of gene-regulating substances inherited from the mother in her egg • Chemical communication between the cells of the embryo

28. Maternal Molecules in the Egg • May direct early embryonic differentiation • All of the cytoplasm in a zygote was in the egg before fertilization, sperm only contributed a nucleus • In most invertebrates and some vertebrates, specific mRNA and protein molecules are concentrated in different places in the egg’s cytoplasm during oogenesis • Some of these proteins are transcription factors that regulate which genes are turned on and off

29. How cells divide is important • During the first cleavage divisions after fertilization, the zygote and daughter cells divide at specific places and orientations • As a result, these cells receive different maternal mRNAs and transcription factors • So, different cells transcribe different genes, start differentiating into distinct cell types and ultimately give rise to specific structures • The positioning of maternal molecules in some eggs so strongly controls development, that the egg can be mapped according to the major structures that will be produced by daughter cells inheriting each section of cytoplasm

30. A “Fate Map” of the Sea Squirt Egg

31. Mammal Embryos • In mammal embryos, current evidence indicates that all cells formed during cleavage are functionally equivalent • Which cells give rise to which parts of the embryo appears to be a matter of chance, depending where the cells happen to be located during the transition from the morula to the blastocyst

32. Chemical Communication • Regulates embryonic development • Induction - in animal embryos, the developmental fate of each cell is determined by chemical interactions between cells • Cells release chemical messengers that alter the development of other, nearby cells • Specific groups of genes are selectively activated in the recipient cells, causing them to differentiate • In amphibian embryos, a cluster of cells near the blastopore, called the organizer, determines whether nearby cells will become ectoderm or mesoderm and where the head and nervous system will form

33. Organizer Proteins • Organizer proteins interact with other messengers to stimulate or repress the expression of genes in nearby cells, often genes that encode transcription factors and that therefore exert widespread effects on gene expression • Which genes are expressed determines the structures and functions of the cells • As these cells differentiate, they release other chemicals that alter the fate of still other cells, in a cascade that culminates in the development of the tissues and organs of the adult body

34. Homeobox Genes • Regulate development of entire segments of the body • Although their functions differ in different animals, homeobox genes code for transcription factors that regulate the transcription of many other genes • Each homeobox gene has major responsibility for the development of a particular region in the body • Homeobox genes were discovered in fruit flies, where specific mutations cause entire parts of the body to be duplicated, replaced, or omitted • For example, one mutant homeobox gene causes the development of an extra body segment, complete with an extra set of wings • Homeobox genes are arranged in a head-to-tail order

35. Fruit Fly Homeobox Genes • Early in development, homeobox genes are transcribed in a specific sequence within the animal body • “head” homeobox genes are transcribed in the anterior part of the embryo, “tail” homeobox genes are transcribed in the tail

36. How Do Humans Develop? • Controlled by the same mechanisms as in the development of other animals • Differentiation and growth are rapid for the first 2 mo. • A human egg is usually fertilized in the uterine tube and undergoes a few cleavage divisions there, becoming a morula on its way to the uterus • By the 5th day after fertilization, the zygote has developed into a hollow ball of cells, known as a blastocyst (mammalian version of a blastula) • A blastocyst is an outer layer of cells surrounding a cluster of cells called the inner cell mass; the outer cell layer becomes the chorion • The outer cell layer attaches to, then burrows into the endometrium of the uterus,= implantation

37. The Journey of the Egg day 2 day 3 (b) An egg within the uterine tube day 4 day 1 4 cells 2 cells morula blastocyst day 7 zygote inner cell mass of blastocyst sperm Fertilization occurs within the uterine tube embryo day 0 The blastocyst implants in the uterus ovary ovulated egg muscle layer uterine wall endometrium (a) The first week of development

38. Animation: Fertilization

39. Week 1 • The chorion and endometrium form the placenta • The chorion secretes chorionic gonadotropin (CG), which prevents the death of the corpus luteum • The corpus luteum sustains pregnancy by secreting progesterone and estrogen for the first couple of months, until the placenta takes over • All the cells of the inner cell mass have the potential to develop into any type of tissue • This allows the inner cell mass to produce the entire embryo and the three remaining extraembryonic membranes • The inner cell mass is also the source of human embryonic stem cells, may be used to replace damaged adult tissues

40. A Blastocyst Implants outer cell layer (future chorion) cavity inner cell mass endometrium (uterine lining) (a) Early blastocyst embryonic disk (future embryo) yolk sac endometrium chorion amniotic cavity amnion (b) Late blastocyst

41. Week 2 • After implantation, two cavities form and gastrulation occurs • During the 2nd week, the inner cell mass grows and splits, forming two fluid-filled sacs that are separated by a double layer of cells called the embryonic disk • One layer of cells is continuous with the yolk sac, in placental mammals it contains no yolk • The second layer of cells is continuous with the amnion

42. Week 3 • Gastrulation begins near the end of the 2nd after fertilization, and is usually complete by the end of the 3rd week • The two layers of the embryonic disk separate slightly,and a slit forms in the cell layer on the “amnion side” of the disk • Cells migrate through the slit into the interior of the disk, forming mesoderm, endoderm, and the fourth extraembryonic membrane, the allantois • The cells remaining on the surface become ectoderm

43. 3-4 weeks • 3rd week, the embryo begins to form the spinal cord and brain • The heart expands and becomes muscular, starts to beat at the beginning of the 4th week • During the 4th week, embryo bulges into the uterine cavity, bathed in fluid contained within the amnion • The umbilical cord forms from the fusion of the yolk sac to the embryonic digestive tract • The body stalk contains the allantois, which contributes the blood vessels that will become the umbilical arteries and vein • The umbilical cord now connects the embryo to the placenta, which has formed from the merger of the chorion of the embryo and the lining of the uterus

44. Human Development in the 4th Week location of the developing embryo in the uterus chorion embryo placenta chorionic villi body stalk form the umbilical cord yolk stalk yolk sac amnion

45. 4-7 weeks • During the 4th and 5th weeks, the embryo develops a tail and pharyngeal (gill) grooves • These structures are reminders that we share ancestry with other vertebrates that retain their gills in adulthood • In humans they disappear as development continues • By the 7th week, the embryo has rudimentary eyes, a rapidly developing brain; the webbing between fingers and toes is disappearing

46. A 5-Week-Old Human Embryo pharyngeal grooves 8 mm arm bud heart eye leg bud umbilical cord brain tail

47. 2 months • At the end of the 2nd month, nearly all major organs have begun to develop • Structures like the arms and legs are recognizably human • The gonads appear and develop into testes or ovaries • The first two months of pregnancy are a time of rapid differentiation and growth for the embryo and a time of danger • Although vulnerable throughout development, the rapidly developing organs are extra sensitive to toxic substances • Sex hormones are secreted. These hormones affect future development of the embryo - the reproductive organs, brain and other parts of the body

48. An 8-Week-Old Human Embryo amniotic sac 30 mm umbilical cord placenta

49. The last 7 months • Growth and development continues • The brain continues to develop rapidly and the head remains disproportionately large • The lungs, stomach, intestine, and kidneys enlarge and become functional, although they will not be used until after birth • As the brain and spinal cord grow, they begin to generate behaviors • As early as the 3rd month of pregnancy, the fetus can move and respond to stimuli • Some instinctive behaviors appear, such as sucking

50. Premature Births • Nearly all fetuses 32 weeks or older can survive outside the womb with some medical assistance • Heroic measures can often save infants born as early as 26 weeks, although more mature fetuses have a much greater chance of healthy survival