Author: Chris Colby (colby@bu-bio.bu.edu) Title: Animal Development Overview of developmen

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====================================================================== Author: Chris Colby (colby@bu-bio.bu.edu) Title: Animal Development ====================================================================== Overview of development and phylogenetic perspective **************************************************** Multicellular life ------------------ Many groups of organisms are multicellular (ex. plants, animals, fungi, red algae, brown algae, some green algae and other assorted protists). Most multicellular life forms have separate germ (reproductive) and somatic cells. Two of these groups develop from structures called embryos -- plants and animals. The embryo of a plant and the embryo of an animal have independent evolutionary origins -- they are not homologous. In this lab we examine some patterns of embryology in animals. Animal Development ------------------ Development from a single cell to an adult organism entails growth, differentiation and morphogenesis. Development can be either direct (young organisms resemble small adults -- ex. placental mammals) or indirect (in animals, has a larval stage -- ex. frogs; or undergoes metamorphosis -- ex. insects). Differentiation is the specialization of a cell line. Cells go from totipotency to performing a specific function; usually this change is irreversible. Morphogenesis is the formation of the physical structure of the organism. This is also irreversible. There are two basic patterns of animal development -- protostomous and deuterostomous. Protostomes ----------- Development in protostomes is characterized by: spiral cleavage (first cleavage spirals right (dextrotropic cleavage), second cleavage spirals left (levotropic cleavage) -- and so on when viewed from the animal pole), determinate cleavage (cell differentiation begins at first cell division), schizocoelus coelom formation (coelom forms from split in mesoderm) and the blastopore forms the mouth of the organism. Representative protostomes are arthropods (ex. insects), molluscs (ex. octopuses) and annelids (ex. worms). Deuterostomes ------------- Development in deuterostomes is characterized by: radial cleavage, indeterminate cleavage (cells remain totipotent for first several cell divisions), enterocoelus coelom formation (coelom forms from a budding off of the archenteron) and the blastopore forms the anus of the organism. Representative deuterostomes are echinoderms (ex. sea urchins) and chordates. Some animals (ex. sponges or cnidarians) have simpler devlopmental patterns and do not fall into either the protostome or deuterostome categories. The sequence of development and comparative embryology ****************************************************** In this lab we will look at some early stages of development in three deuterostomes, sea urchin, frog and chicken. See cladograms for pattern of relationship. cladograms ---------- Animals --- Porifera ********************************************** * Cnidaria ****************************************** ******* * * Platyhelminthes ************************** ***** * * Mollusca ************************ ******* * ********** * * Annelida *********************** **** * * Arthropoda ****** * * Echinodermata ******** * *************************** Chordata ************ Molluscs, Annelids and Arthropods are protostomes Echinoderms and Chordates are deuterstomes Many minor (and some not so minor) lineages deleted Vertebrates --- birds ******** ******* crocodiles *** * ***** lizards, snakes **** * ****** turtles **************** * ****** mammals ********************* * ***** amphibians *********************** * **** lobe-fin fish ************************ * **** ray-fin fish **************************** * **** sharks ************************************* * ************ agnathans ************************************* Eggs ---- Eggs differ in the amount and location of yolk. Yolk contains stored nutrients the organism will use in development. Eggs can have yolk distributed evenly throughout the egg (isolecithal). In amphibians, the yolk concentration increases along an animal-vegetal orientation (this is called telolecithal). This is carried to an extreme in reptiles, birds and monotremes where the yolk is restricted to a membrane bound yolk sac. Placental and marsupial eggs do not contain yolk -- nutrients are delivered to the developing animal from the mother via the placenta. Fertilization ------------- Fertilization occurs when the sperm enters the egg -- it can be broken down into three stages: penetration, activation and fusion. Penetration is when the sperm and egg meet and the sperm makes it way into the egg cytoplasm. This is presented in most bio texts as the sperm tracking down the egg and digesting its way in -- in reality it is much more complex and the egg plays a more active role. In animals with yolk heavy eggs (including amphibians, reptiles, birds and monotremes), many sperm may enter the egg but only one fuses with the nucleus. The rest degenerate. In mammals, the first sperm penetration triggers a response that prevents further sperm entry. After penetration, the cell goes into high gear (activation) -- cell metabolism and protein production increases dramatically. In mammals, activation also results in the second mitotic division. In other animals, meiosis is complete at this stage. Fertilization is complete when the sperm and egg nuclei fuse. Cleavage -------- In yolk light cells, cleavage divides the entire cell (holoblastic cleavage). In isolecithal eggs , resulting cells are all roughly the same volume. Cells of varying volume result from cleavage of telolecithal cells. Larger cells (and often slower rates of division) are seen at the vegetal pole. Even holoblastic cleavage is seen in agnathans. Uneven holoblastic cleavage is seen in amphibians and some fish. Yolk heavy cells undergo incomplete (meroblastic) cleavage. This type of cleavage is seen in birds, reptiles and some fish. Mammals evolved from ancient reptiles. Because placental mammalian eggs have no yolk, they undergo holoblastic cleavage. Details of this (and later stages of development) differ, however, from holoblastic cleavage in fish and amphibians. Morula ------ When the developing animal reaches the 32 cell stage it is called a morula. At this stage it is either a solid ball of cells (in animals with holoblastic cleavage) or a solid disc (in animals with meroblastic cleavage). Blastulas and blastodiscs ------------------------- When the animal reaches 500 - 2000 cells it is called a blastula if the animal consists of a hollow ball of cells, or a blastodisc. A cavity called a blastocoel is present at this stage. In a blastula, the blastocoel is in the center of the ball; in a blastodisc it is between the disc and the yolk. Amphibians and most fish form blastulas. Reptiles, birds, monotremes and some fish form blastodiscs. Placentals form a blastodisc similar to reptiles. (I'm not sure about marsupials.) But, instead of the disc sitting on sac of yolk, it lies inside a hollow ball of cells called a trophoblast. Gastrula -------- Gastrulation is a process that ends in the production of a structure containing the three primary tissues layers (endoderm, mesoderm and ectoderm). From these three tissue layers, all the organ systems of the body develop. In sea urchins, one side of the blastula invaginates. The blastula collapses from a hollow ball to a two cell layered cup-shaped structure. The blastocoel disappears. The resulting space is called the archenteron, the "hole" in the space is called the blastopore. In amphibians, the blastopore migrates inward and upward, displaces the blastocoel downward and eventually collapses it. The resulting archenteron contains a yolk plug. In reptiles, birds and mammals, a fold in the blastodisc (the primitive streak) is the entry point for migrating presumptive mesoderm cells. The archenteron is formed when the flat embryo folds. Extra-embryonic membranes of reptiles, birds and mammals -------------------------------------------------------- In reptiles, birds, and monotremes extra-embryonic membranes form within the shelled egg -- the yolk sac, amnion, chorion and allantois. The yolk sac encompasses the yolk in the egg cell. Enzymes secreted from this sac digest the yolk for use by the animal. The amnion and the chorion are two membranes the enclose the embryo and the entire developing system, respectively. The amnion fills with fluid that helps to buffer the organism from physical shocks. The space between these two membranes is an extension of the coelomic cavity. The allantois, an outcropping of the gut, is a waste recepticle. During development, the yolk sac shrinks and the allantois swells. In placental mammals, these membranes are also present. The yolk sac, however, does not contain yolk. Instead, it is the area where blood cells first form. These later migrate into the embryo. In placentals, the allantois is modified and incorporated into the umbilical cord -- the organ where nutrients and gases are passed to the embryo and wastes (and gases) are passed from the embryo. Development of organ systems from three germ layers --------------------------------------------------- Vertebrates are essentially segmented organisms although later development hides this fact. In early development repeated blocks of tissue called somites form in the mesoderm. Each somite develops into a single vertebrae and corresponding muscles. See the fate map for organ systems that form from each of the three tissue layers. Ecology and evolution of development ************************************ Life history, reproductive strategy and development --------------------------------------------------- The life history and reproductive strategy of an organism influences the developmental pattern it undergoes. The eggs of many invertebrates, as well as those of amphibians and some fish are lightly yolked because initial development finishes when the animal reaches a larval stage. At this point, the larvae must acquire food on their own to enable them to develop into the adult, sexually mature form. Often, larval forms and adult forms exploit different food resources. Reptiles and birds, by contrast, have direct development and all the nutrients necessary must be supplied before the animal is able to gather food on it's own. Reptiles, which don't have larvae, evolved from amphibians, which have larvae. The "larval" stage of reptiles occurs within the egg. The "larvae" have been modified to remain sessile and structures for larval motility have been reduced. Mammals likewise have direct development. In monotremes, nutriment comes from egg yolk. In marsupials, a lightly yolked egg develops into a "larvae" that then crawls to the pouch. In the pouch, the young suckle for nutrition. In placentals, nutrients are supplied directly from the mother via the umbilical cord. Placentals differ from most other animals in that they are viviparous (bear live young). But, viviparity has evolved in other lineages. Sharks, for example, bear live young. In addition, some lizards and snakes are viviparous. In these lizards, the arrangement of the extra-embroyonic tissues are similar to mammals, but the details differ. In sharks, the details of viviparity are radically different. It appears that viviparity has evolved more than once. In some lineages the embryo is fed fed via a highly vascularized yolk sac that is continuous with the maternal bloodstream. Others have extensions of the uterus that extend into the mouth and gills of the young and secrete "milk". Still others feed their young via continued ovulation. The young feed off the eggs the mother sheds. Ontogeny and Phylogeny ---------------------- Everything in biology has a history of millions of years. All traits (including physical structures, biochemical pathways and behaviors) of extant organisms have been modified with descent to arrive at their present form. This is also true of developmental patterns. In development, organisms pass through stages of development that their ancestors passed through(*). This is because evolution does not proceed by making sweeping changes in body plans. Evolution can only "tinker" with existing traits. (*) organisms do _not_ pass through the adult form of all their ancestors. Later stages in development are more prone to be modified by natural selection. Early pathways in development are the basis of later pathways in development. Any change early on has a cascading effect, thus any mutation leading to such a change is likely to be detrimental. But, changes in the "periphery" of a developmental pattern may, on occasion, be favorable. This is seen in that early development in animals is highly conserved; but developmental patterns diverge (often greatly) towards the end of development. Of course, selection "sees" all stages of development, so even some early stages can be modified. By studying the patterns of development if animals, biologist obtain clues to their phylogeny.

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