jimtrue.com : school : BSC2010 : CH 47: Animal Development

Posted by Jim True on April 22, 2004 6:44 AM. Last Updated October 22, 2006 9:23 PM

Disclaimer for all material noted here is at the bottom of this web page.

CH 47: Animal Development

Epigenesis

• Gametes, especially the egg, have been recognized for centuries as the starting point of new multicellular individuals. (Which came first the chicken or the egg?)
• Preformation -- the concept that eggs or sperm contained a complete but very tiny individual who simply grew when activated to do so. Homonculous, complete perfect, microscopic individual inside the egg. This was OBVIOUSLY an old belief.
• Epigenesis -- Development of an organism is progressive, from a formless egg. Another concept that was also considered, and actually is the correct one.

Development

• Development -- change in shape, structure and function of cells. Growth is increase in number or size. Development is change in cell location, function and structure.
• It occurs because as cells divide during development, they received DIFFERENT cytoplasm and organelles, thus the DNa in the nuclei receive different "triggers" to activate different genes for development.
• When we examine development in the earliest stages of life in animals or plants, we are studying a developing embryo. We use this term for both developing plants and animals. Embryology for animals only will be concentrated on in this course.
• The most complex process of development is found among vertebrates, animals with a rigid internal backbone.
• We can divide vertebrate development into the following stages:
  1. Fertilization
  2. Cleavage
  3. Gastrulation (aka Differentiation) (prior to this point you have growth not development)
  4. Organogenesis
     • Neurulation
     • Neural Crest Formation
• ALL DEVELOPMENT IS FROM MITOSIS and CYTOKINESIS!

Fertilization

• The process of fertilization actually involves several stages:
  • Sperm Penetration(Figure 47.2, p1000) -- Oocyte (2n) or egg (n) (oocyte is diploid; egg is haploid) is surrounded by a variety of protective cells plus an external membrane.
    • The first sperm to penetrate the surrounding cells and make contact with oocyte releases enzymes from a special tip, the acrosome, which fuses the sperm and oocyte membranes. Haploid nucleus, power supply and motility (flagella).
    • This causes a rapid depolarization of the egg cell membrane, rendering it impermeable to additional sperm. Allow sperm nucleus inside the cell and prevents other sperms from getting into the egg.
    • A second, denser barrier to sperm penetration occurs when special cortical granules, vesicles just inside the egg cell memebrane, fuse with the cell membrane and release their contents.
    • This creates a thick fertilization envelope, which bars additional sperm entry.
    • After sperm penetration, the cytoplasm of the ovum or oocyte is rearranged, which determines axes of cell division and can determine the body organization of the animal. This rearrangement of cytoplasm may, in later divisions, determine what those cells are going to be.
  • Activation of Egg -- (The triggering of meiosis to create the haploid egg nucleus). The contact and fusion of sperm with the egg causes a variety of internal metabolic changes to be "switched on". This will trigger meiosis itself.
    • The activation of developmental changes is initiated in the egg cytoplasm by mRNA already present.
    • It is possible to induce activation without sperm and also without a cell nucleus! Specialized form of asexual meiosis.
    • parthenogenesis -- Activation without sperm can lead to the production of an asexually produced multicellular offspring. All the offspring will be identical (clones) and female.
    • Because the sperm essentially contains nothing more than paternal (father's) chromosomes, it is the maternal DNA that controls production of such things as mitochondria and centrioles. Sperm does not contribute cytoplasm, ONLY chromosomes.
  • Fusion of Haploid Gamete Nuclei -- The actual fertilization event, resulting in a single diploid cell, the zygote haploid to haploid creates diploid, a zygote. (figure 47.5, p 1002) From this point forward, the process of growth is INCREDIBLY rapid.

Cleavage

• Cleavage ("splitting") (figures 47.6 & 47.7, p.1003; 47.8, p.1004
• Starting with the zygote, it is:
  • Rapid mitotic division (growth stages are short circuited... no increase in size, just lots and lots of divisions; end of cleavage may be 2000 cells, but the overall zygote dimensions will not increase.
  • Produces increasing number of cells (blastomeres)
  • Blastomeres are progressively smaller;
  • Embryo size remains unchanged
• There are several patterns of cleavage among different animal groups, mainly due to the concentration of yolk. The food source for the developing embryo.
• Yolk -- Yolk is the nutritive substance that provides the energy for early development.
• It is very dense and slows the rate of cleavage or even halts it. This will define how the arrange of mitosis will occur.
  • Microlecithal -- little yolk present.
  • Mesolecithal -- moderate amount of yolk.
  • Macrolecithal -- large amount of yolk present.
• Microlecithal eggs are typically isolecithal as well (yolk evenly distributed).
• Meso-- and macrolecithal eggs have telolecithal distribution (yolk more conentrated in one region than another).
• Mesolecithal eggs are moderately telolecithal.
• Vegetal hemisphere (VH) (vegetal pole) -- Yolk-rich. Vegetal or vegetate means to feed. This is the primary food supply for the developing embryo and will primarily develop in this region.
• Animal hemisphere (AH) (animal pole) -- Yolk-poor.
• Mitosis is more rapid in AH, therefore cells will be smaller than those in VH.
• Blastomeres of AH are micromeres.
• Blastomeres of VH are macromeres.
• Macrolecithal eggs are HIGHLY telolecithal. No cell division is possible in yolk region.
• Examples of egg types:
  • Isolecithal -- invertebrates (sea star), human (placental mammals)
  • Mesolecithal -- frog (almost all amphibians and fishes)
  • Telolecithal -- birds, reptiles
• Based on yolk concentration and distribution, there are several cleavage patterns:
  • Holoblastic -- The entire embryo undergoes cleavage.
    • Holoblastic equal -- Typical in MOST isolecithal eggs. All blastomeres divide at an even rate, so all are approximately the same size.
    • Holoblastic unequal -- Occurs in mesolecithal eggs. Mitosis rates and blastomere sizes differ in AH and VH.
  • Meroblastic -- occurs mainly in telolecithal eggs. Restricted development to small area near the yolk.
    • Embryo development is restricted to one small part of the ovum called the blastodisc, a cluster of cells on top of the dense yolk.
    • Placental mammals (e.g. humans) are unusual in that they have isolecithal eggs but exhibit meroblastic cleavage.
• Regardless of the pattern exhibited, cleavage proceeds in the same geometric progression of division stages:
  • Zygote (1 cell) --> 2 cell stage --> 4 cell stage --> 8 cell stage --> Up to this point, blastomeres can be counted.
  • 16-32 cell stage -- forms a solid ball of cells called a morula (greek word meaning "berry") -->
  • 64-2000 + -- an increasingly hollow ball of cells called the blastula. Between morula and blastula, zygote is dropped into the uterus and attaches.
  • In placental mammals (vertebrates in which the embryo is nourished from the mother's circulatory system), the structure is a blastocyst, an outer layer of cells with an inner cell mass.
  • The "hollow" in any blastula stage is liquid filled and is called the blastocoel. (Blasto - 'seal' )
  • Blastula is the last stage in cleavage.

Gastrulation

• As cells continue to divide, they begin to migrate inward, pushing from surface of blastula into blastocoel.
• The embryo is now changing size and shape, and is called a gastrula. Now entering development.
• Also, cells begin to differ in structure and function, so the process of gastrula development (gastrulation) is also called differentiation. (Figures 47.9, p.1005, 47.10, p.1006).
• As cells migrate into blastocoel, they create an opening, the blastopore, which will become the future mouth or anus of the animal.
• Cells continue inward growth, producing a tube, the archenteron ("ancient gut" - AR- KEnter - ON), the earliest development of the digestive system.
• These cells now differ from outer cells.
• These two different layers of cells (plus one more that is produced between them) make up germ layers. Also called Germative layers.
• Germ Layer -- group of cells that give rise to tissues (group of similar cells that together perform a specific function) and organs (group of tissues that perform a specific function).
• There are 3 germ layers:
  • Ectoderm (blue) ("outer skin") -- The outer layer of germ cells. Produces the skin and nervous system.
  • Endoderm (yellow) ("inner skin") -- The innermost layer of germ cells. Produces the digestive tract and the lining of most interior "tubes". Circulatory, reproducitive, urinary, excretory, respiratory.
  • Mesoderm (red) ("middle skin") -- Produced by cells moving from ecto- and endoderm. Cells migrate between endo and ecto; muscle tissue, bones.
• Mesodermal cells produce muscles, internal skeletal system, parts of other systems such as circulatory system.
• Most animals all three germs layers and thus are considered triploblastic.
• Diploblastic animals possess only two germ layers. Jelly fish, coral, sea urchins.
• One group, the sponges, has NO/zero layers.
• Differences in egg and cleavage pattern also affects gastrulation.
• There are four variations in the general Gastrulation Pattern:
  1. Occurs in animals with microlecithal, isolecithal eggs (except placental animals) -- This is found in most invertebrates ("lacking vertebrae" -- animals without backbones).
     • Ecto- and endoderm form as archenteron develops.
     • Mesoderm (if present) develops from cells moving inward from ecto- and endoderm.
  2. Occurs in mesolecithal eggs with holoblastic unequal cleavage. (Figure 47.10, p.1006).
     • Includes most fishes and amphibians.
     • Micromeres of the animal hemisphere are produced more rapidly.
     • Epiboly -- the micromeres overgrow the vegetal hemisphere, enclosing it and the yolk.Growing over and enclose the vegetal hemisphere. Creates the yolk plug stage.
     • Micromeres then push inward (involution), forming the blastopore and archenteron (endoderm forms).
     • Where micromeres move inward is called the dorsal lip of the blastopore.
     • This marks where the dorsal surface (back) of animal will develop.
     • The blastocoel decreases in size and may or may not completely disappear.
     • Additional cells develop in the dorsal lip region and push between the ectoderm and endoderm, forming the mesoderm.
  3. Occurs in highly telolecithal eggs with meroblastic cleavage (reptiles and birds).
     • Blastodisc on top of yolk is arranged like a cap or thin sheet of cells.
     • Cells begin to fold inward along the primitive streak, a thickened cellular region which marks the long axis of body.
     • As they fold inward, they form a primitive groove, the functional equivalent of the blastopore (no blastopore formed).
     • These infolding cells form mesoderm in the middle of the disc and endoderm along the bottom.
     • At what will be the anterior (head) end of the primitive streak is a thick knot of cells called Hensen's node.
     • These cells are mesodermal in nature and will produce a long rod of fibrous cells called the notochord.
     • The notochord is the precursor to the vertebral column (spine).
  4. Occurs in microlecithal, isolecithal egg with meroblastic cleavage, i.e., placental mammals.
     • At the end of cleavage, the outer cell layer of the blastocyst forms the trophoblast, cells that eventually become the placenta, which connects the circulatory system of the mother and the embryo.
     • The inner cell mass becomes the embryo, in much the same fashion as in birds and reptiles.

Organogenesis

• The germ layers initially produce tissues, groups of cells that work together to perform a specific function.
• In more complex animals, tissues in turn form organs, groups of tissues that together perform specific functions.
• Organogensis is the process of organ development.
• We will examine ONLY one specific case.
• Neurolation -- the development of the central nervous system (brain and spinal cord).
• Occurs in all animals belonging to a group called the phylum Chordata (all fishes, amphibians, reptiles, birds and mammals & 2 other groups).
• Develops from ectodermal cells lying alongside the primitive streak, which continue to divide.
• Cells form thickened layers on either side of the streak called neural folkds or neural ridges.
• Lying between the neural folds is the neural groove.
• Cells of the neural fold continue dividing until they join together above the groove.
• This forms a long hollow tube of cells, the neural tube.
• the neural tube lies parallel to and directly above the notochord. Dorsal hollow nerve cord.
• Lining this tube is a hollow tube of cells that are expanded in the head region.
• This is the dorsal hollow nerve cord (DHNC). It is the precursor to the spinal cord in vertebrates (animals with a rigid internal backbone, which is developed from the notochord).
• The DHNC and notochord are two of four characteristics shared by ALL chordates.
• In one group of chordate animals, the vertebrates, an additional structure forms during neuralation.
• Ectodermal cells along the neural folds in the region where the folds fuse to form the neural tube form the neural crest.
• Cells from this region migrate inwards to form parts of various sense organs, sensory nerve structures of the spinal cord, and pigment producing cells.

Disclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.