Supplemental Lecture (97/03/02 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Animal Developmental Biology
    1. A list of vocabulary words is found toward the end of this document
  2. Gamete
    1. Haploid cells which, in animals, consist of the male produced sperm and the female produced egg (ovum).
    2. Fertilization:
      1. The primary role of gametes is fuse in a process called fertilization in order to restore the diploid state.
      2. Note that in animals and other multi-celled organisms, it is generally the case that gametes do not undergo mitosis.
  3. Ovum [pl. ova, egg]
    1. The female gamete.
    2. The single fertilized ovum supplies to the zygote all aspects (particularly cytoplasm and any outer structures) except one-half of the genetic material.
  4. Yolk
    1. Egg-supplied nourishment:
      1. A substance supplied in varying degrees by ova.
      2. Yolk is a nourishing substance.
      3. Typically, of course, more yolk is supplied when eggs have fewer sources of external nourishment such as the surrounding abiotic medium or directly from the mother.
    2. The amount and distribution of yolk supplied per ova impacts on the strongly on the early steps of embryonic and especially pre-embryonic development.
  5. Sperm
    1. The male gamete
    2. The single fertilizing sperm supplies to the zygote no aspects except one-half of the genetic material.
    3. Suppliers of genetic material:
      1. Sperm, however, are particularly good at supplying one-half of the genetic material found in zygotes.
      2. Indeed, the primary selective pressure on sperm is the supplying of this material.
      3. Consequently, sperm represent paired down, cheap, mobile, essentially DNA moving "trucks," and typically there are lots of them (i.e., excesses on the order of millions and billions).
      4. Particularly, in organisms with very promiscuous mating systems the competition for ova typically is won by males which supply the most and the best sperm, and less a consequence of various other characteristics associated with these males.
  6. Fertilization
    1. Fertilization is the fusion of gametes to form a diploid zygote.
    2. Fertilization is the means by which haploid gametes restore the diploid form.
    3. Generally in multi-celled organisms it is only upon fertilization (and hence the restoration of the diploid state) that growth via mitotic cell divisions is allowed to occur.
  7. Zygote
    1. First cell:
      1. A one-celled product of fertilization.
      2. Generally, in multi-celled organism, zygote formation is followed by mitotic cell division thus resulting in the development of a multi-celled embryo.
  8. Development
    1. Development consists of the complex series of mitotic cell divisions and other events which start with a one-celled product of fertilization (zygote) and end, more or less, with a sexually mature adult organism.
    2. Development occurs through various stages. In mammals, for example, these stages are either called or may be described as:
      1. pre-embryonic development
      2. embryonic development
      3. fetal development
      4. "post-birth" development.
    3. In this lecture, below, we will consider only the first three periods of development, with emphasis placed particularly on pre-embryonic development.
  9. Pre-embryonic development
    1. Zygotes through gastrulation:
      1. Pre-embryonic development begins with zygote formation and ends with gastrulation.
      2. Generally pre-embryonic development includes:
        1. cleavage
        2. blastula formation
        3. differentiation of the inner cell mass from the trophoblast
        4. formation of the placenta (in mammals)
        5. gastrulation
  10. Cleavage
    1. The first phase of pre-embryonic development is the mitotic division of the zygote from one cell into many cells.
    2. Little growth:
      1. Typically, this division occurs in the absence of much cell (particularly cytoplasmic) growth.
      2. Thus, the pre-embryo tends to increase in cell number, but not much in size.
    3. Cleavage results in the formation of a non-hollow ball of cells.
    4. Typically, the pattern of cleavage is significantly impacted upon by the amount and distribution of yolk.
    5. Blastula formation:
      1. Finally, in many animals, cleavage ends with the formation of a hollow in this ball of cells to form the blastula.
      2. Again, just what this hollow looks like is dependent upon the amount and distribution of yolk.
  11. Blastula
    1. A hollow, fluid filled ball of cells which constitutes the end-product of cleavage.
    2. Location is everything:
      1. Note that developmental events subsequent to this blastula stage are impacted particularly by positional information within this blastula.
      2. Two early positional considerations include the division of cells into ones which will develop into the embryo and ones which will develop into support structures (e.g., the placenta).
  12. Inner cell mass
    1. The cells which are destined, more or less, to develop into the embryo.
    2. These cells may consist simply as a lump positioned within the hollow, against the wall of cells that makes up the blastula, or helping define part of that wall. That is, found integrally a part of but nevertheless in one relatively small section of the blastula.
  13. Trophoblast
    1. Pre-placenta:
      1. The cells which are destined to become the embryonic support structures.
      2. Particularly these are nutrient supplying structures.
      3. One such structure, found in most mammals, is the placenta.
    2. The trophoblast, generally, are the cells which constitute the wall of the hollow ball at the blastula stage.
  14. Placenta
    1. Organ of interface:
      1. A mammalian organ which serves as the nutrient and waste exchanging interface between the mother and embryo or fetus.
      2. The placenta develops in part from the pre-embryonic trophoblast and in part from the maternal tissue.
  15. Basic mechanisms of development [cell migration, cell replication, programmed cell death, apoptosis, concentration gradients, differentiation]
    1. In addition to being the right place at the right time, there exist additional basic mechanisms of development including:
      1. cell migration
      2. cell replication
      3. programmed cell death
      4. concentration gradients
      5. differentiation
    2. The movement of cells which were born in one place to a different place in the embryo or fetus.
    3. The addition of new cells of the same type to existing sites.
    4. Programmed cell death:
      1. The removal of cells from existing sites.
      2. Note that very often structures are produced within larger cellular structures, and completion of the structure requires some means of removal of the excess cells. For example, the development of fingers involves the removal of the tissue (webs) initially found between the forming digits.
      3. Programmed cell death is also called apoptosis.
    5. A variation on right place-right time. Chemical concentration gradients help determine relative position.
    6. Differentiation:
      1. The changing of the protein expression pattern of cells in the course of their individual development and division.
      2. Differentiation very often is irreversible.
      3. That is, embryonic cells typically may become more specialized, but similarly typically more specialized (differentiated) cells are not well equipped to become less specialized.
  16. Gastrulation [deuterostome, protostome]
    1. Germ layer formation:
      1. The three germinal layers are created as a consequence of cell migration.
      2. This process is called gastrulation.
      3. Gastrulation is basic to the development of most animals.
    2. Basics:
      1. Gastrulation involves three basic steps:
        1. an invagination which forms the mouth or anus
        2. formation of a hole which will serve as either the mouth or the anus (whichever was not first created during the initial invagination)
        3. migration of cells to between the two tubes
      2. "Middle" cell migration may occur at various times relative to the other listed processes of gastrulation.
    3. The initial opening formed by the invagination occurring during gastrulation.
    4. Basic difference:
      1. Note that a basic phylogenetic division among animals involves whether the blastopore ultimately forms the mouth or the anus.
      2. For one group, the deuterostomes, the blastopore becomes the anus. Chordates are included among the deuterostomes.
      3. For the other group, the protostomes, the blastopore becomes the mouth. A large fraction of invertebrates are protosomes.
  17. Germinal (germ) layers
    1. Products of gastrulation:
      1. It is the germ or germinal layers which are formed during gastrulation.
      2. It is from these germinal layers that tissues, organs, and organ systems eventually arise.
    2. Three layers:
      1. There are a total of three germinal layers. They are called:
        1. endoderm
        2. mesoderm
        3. ectoderm
      2. These correspond to the gastrulation products, respectively, including the:
        1. inner tube cell layer
        2. cells which migrated to between the two tubes
        3. outer tube cell layer
  18. Primary tissues
    1. Four primary tissues:
      1. The three germinal layers give rise in the course of development to four primary tissues.
      2. These include:
        1. nervous tissue
        2. muscular tissue
        3. connective tissue
        4. epithelial tissue
    2. There is no one-to-one correspondence between germinal layer origin and primary tissue type. However, what correspondence does exist will be discussed below.
  19. Endoderm
    1. The inside tube of the now tube within a tube, i.e., the inside of the invagination.
    2. Endoderm gives rise primarily to epithelial tissue.
    3. The endoderm specifically gives rise to the following structures in humans:
      1. digestive tube
      2. lungs
      3. liver
      4. urinary bladder
      5. germ cells
  20. Mesoderm
    1. Between the tubes:
      1. The cells which migrate to between the tubes constitutes the mesodermal layer.
      2. This is particularly to within the cavity of blastula (i.e., the hollow which defines the blastula and which ultimately becomes a hollow between the endoderm and ectoderm layers).
    2. Mesooderm gives rise to muscle and connective tissue, but also some epithelial tissue.
    3. The mesooderm specifically gives rise to the following structures in humans:
      1. skeleton
      2. skeletal muscles
      3. some aspects of skull and jaw
    4. Earlier in development the mesoderm also gives rise to the somites, from which these later structures develop.
  21. Somites
    1. Body segments:
      1. Body segments which divide bodies into cross sections which may be stacked, for example, from head to toe.
      2. In vertebrates the somites are originally found adjacent to the notochord and then give rise to such things as ribs and skeletal muscles.
  22. Ectoderm
    1. Outside tube:
      1. The outside of the now tube within a tube.
      2. That is, this is the layer of the inner cell mass from which the endodermal and mesodermal layers are derived in the course of their invagination and/or migration.
    2. Ectoderm gives rise primarily to nervous tissue as well as some epithelial tissue.
    3. The ectoderm specifically gives rise to the following structures in humans:
      1. brain
      2. spinal cord
      3. peripheral nervous system
      4. skin
      5. hair
      6. eyes
    4. Earlier in development the mesoderm also gives rise to the epidermis, neural tube, and neural crest, from which these later structures develop.
  23. Embryonic development
    1. Begins with end of pre-embryonic development:
      1. Pre-embryonic development basically ends with the completion of gastrulation, i.e., the formation of both a mouth and an anal opening.
      2. In mammals this additionally corresponds to formation of a cavity, called the amniotic cavity, within which the developing organism will grow. I.e., this is the space enclosed by the amnion, the tissue membrane which surrounds the developing organism and which also encloses the amniotic fluid.
    2. Organ formation but not completion:
      1. Embryonic development basically takes the basically rudimentary organism found at the end of pre-embryonic development, and produces and a recognizable organism complete with all of its developing organ sytem.
      2. Much of organ development, however, is left to the post-embryonic developmental period known as fetal development.
      3. Thus, a human embryo nearing completion of the embryo stage of development (about 8 weeks) already looks human, but is no where near the point where survival outside of the womb (or womb-like environment--i.e., science fiction) is in any way possible.
    3. Embryo:
      1. An embryo is simply an organism currently undergoing embryonic development, but which has not yet completely, rudimentarily formed.
      2. Note that a human embryo at the end of its embryonic development is no more than about an 2.5 cm (an inch) in length and about a gram in mass.
  24. Neurulation
    1. Embryonic invagination:
      1. For vertebrates (and, in fact, all chordates), a key step in embryonic development is neurulation
      2. Neurulation resembles in the sense that it involves an invagination of tissue (in this case ectodermal tissue in an longitudinal invagination).
      3. However, neurulation results in closing off of this invagation, rather than the formation of a second opening as occurs during gastrulation.
    2. The key products of neurulation are the nerual tube and crest.
  25. Neural tube
    1. The primary product of the nueral tube, formed during neurulation, is the brain, the spinal cord, and various aspects of the peripheral nervous system.
  26. Neural crest
    1. The nueral crest cells, through cell migration, go on to form various aspects of the peripheral nervous system, the skull, and the jaw.
  27. Organogenesis
    1. The making of organs:
      1. The formation of the products of the neural tube, neural crest cells, the somites, and all of the rest of the cells which develop from the three germinal layers is called organogenesis.
      2. That is, organogenesis is the especially initial formation of organs starting with less differentiated tissues found in the early embryo.
    2. Websters:
      1. "The origin and development of bodily organs."
  28. Morphogenesis
    1. Shaping organs:
      1. Similar to organogenesis but referring specifically to the generation of shape and differentiation of cells and tissues.
      2. Don't worry about trying to differentiate the organogenesis and morphogenesis, but do remember that the two terms are not defined identically.
    2. Websters:
      1. "The formation and differentiation of tissues and organs."
  29. Fetal development
    1. Fetal development includes all of the rest of pre-birth development.
    2. Particularly, this involves further differentiation and maturation of organs and tissues.
    3. Fetal development also involves significant increases in organ size.
    4. In addition, organs begin to make their transition from physiological states suitable for pre-birth conditions to those which are suitable to the post-birth environment.
  30. Links
    1. The Virtual Embryo
  31. Vocabulary
    1. Blastula
    2. Cell migration
    3. Cell replication
    4. Cleavage
    5. Concentration gradients
    6. Deuterostome
    7. Development
    8. Differentiation
    9. Ectoderm
    10. Egg
    11. Embryonic development
    12. Endoderm
    13. Fertilization
    14. Fetal development
    15. Gamete
    16. Gastrulation
    17. Germinal layers
    18. Inner cell mass
    19. Mesoderm
    20. Morphogenesis
    21. Neural crest
    22. Neural tube
    23. Neurulation
    24. Organogenesis
    25. Ovum
    26. Placenta
    27. Pre-embryonic development
    28. Primary tissues
    29. Programmed cell death
    30. Protostome
    31. Somites
    32. Sperm
    33. Trophoblast
    34. Yolk
    35. Zygote
  32. Practice questions
    1. Which of the following do ova supply no more of than do sperm? (circle one correct answer) [PEEK]
      1. cytoplasm.
      2. mitochondria.
      3. DNA.
      4. nuclear DNA.
      5. nutrients.
      6. plasma membrane.
    2. Define Zygote (less than 10 word answer) [PEEK]
    3. Gastrulation results in the direct and immediate formation of __________ (name at least three things; be specific) [PEEK]
    4. The neural tube arises from which germinal layer? [PEEK]
    5. _________ the organogenesis begins for all organ systems, but morphogenesis is not completed? (circle one correct answer) [PEEK]
      1. During pre-embryonic development.
      2. During embryonic development.
      3. During fetal development.
      4. After birth
      5. all of the above
      6. none of the above.
  33. Practice question answers
    1. iv, nuclear DNA.
    2. The first diploid cell found during development of a multi-celled organism; the product of fertilization which goes on to form a multi-celled organism.
    3. mouth, anus, tube-within-a-tube, mesoderm, endoderm (and I suppose ectoderm is also a reasonable answer though one could argue that the ectoderm was present prior to gastrulation and, indeed, is what gave rise to the endoderm and mesoderm); I'm not so sure I would accept any of the four primary tissues; germinal layers would count as one answer.
    4. Ectoderm.
    5. iii, During embryonic development.
  34. References
    1. Benjamin, C.L., Garman, G.R., Funston, J.H. (1997). Human Biology. McGraw-Hill, Inc., New York. pp. 474-497.
    2. Morgan, J.G., Carter, M.E.B. (1996). Investigating Biology. Second Edition. The Benjamin/Cummings Publishing Co., Inc., Menlo Park, California. lab topic 23.
    3. Postlethwait, J.H., Hopson, J.L. (1995). The Nature of Life. Third Edition. McGraw-Hill, Inc., New York. pp. 308-331.