Developmental Biology 2

• A.    The yolk is transported into the bottom portion of the egg.

  B. The male frog grabs the female’s back and fertilizes the eggs as the female frog releases them.

  C. Eggs

  D. Fertilization causes the cytoplasm of the egg to move such that different parts of the cytoplasm find themselves in a new locations.

  E-H. During cleavage, the volume of the fog egg stays the same, but it is divided into tens of thousands of cells.
• In most frogs, gametogenesis and fertilization are seasonal as they are dependent upon their immediate environment and on the temperature of air and water.
• If the frog is mature, the pituitary gland secretes hormones that stimulate the ovary to make estrogen. Another ovarian hormone, progesterone, signals the egg to resume its meiotic division.
• In most species of frogs, fertilization is external. The male frog grabs the female’s back and fertilizes the eggs as the female frog releases them.
• Fertilization activates those molecules necessary to begin cell cleavage and development. The sperm and egg die quickly unless fertilization occurs.
• Organogenesis begins when the notochord rod of mesodermal cells in the most dorsal portion of the embryo tells the ectodermal cells above it that they are not going to become skin. At this stage, the embryo is called a neurula. The cells that had connected the neural tube to the epidermis become the neural crest cells. They give rise to the pigment cells of the body (the melanocytes), the peripheral neurons, and the cartilage of the face.
• The mesodermal tissue adjacent to the notochord becomes segmented into somites, the precursors of the frog’s back muscle, spinal cord, and dermis (the inner portion of the skin). The embryo develops a mouth and an anus, and it elongates into the typical tadpole structure.
• In amphibians, metamorphosis is initiated by hormones from the tadpole’s thyroid gland, and these changes prepare an aquatic organism for a terrestrial existence.
• For locomotion, the hindlimbs and forelimbs differentiate as the paddle tail recedes. The cartilaginous skull of the tadpole is replaced by the predominantly bony skull of the young frog. The horny teeth the tadpole use to tear up pond plants disappear as the mouth and jaw take a new shape, and the fly-catching tongue muscle of the frog develops.
• The speed of metamorphosis is carefully keyed to environmental pressures.
• Since the bottom half of the egg usually contains the yolk, it divides more slowly (because the large yolk deposits interfere with cleavage). This portion is the vegetal hemisphere of the egg. Conversely, the upper half of the egg usually has less yolk and divides faster. This upper portion is called the animal hemisphere of the egg.
• Every living organism develops. Development can be seen even among the unicellular organisms.
• Nuclear control of cell morphogenesis and the interaction of nucleus and cytoplasm are beautifully demonstrated in studies of Acetabularia.
• 1. Hammerling found that when he transferred the nucleus from one species into the stalk of another species, the newly formed cap eventually assumed the form associated with the donor nucleus.Acetabularia development.
  2. Thus, the nucleus was seen to control Acetabularia development.
  3. These studies suggest that (1) the nucleus contains information specifying the type of cap produced, and (2) material containing this information enters the cytoplasm long before cap production occurs.
• In the 1930s, J. Hammerling took advantage of these unique features and exchanged nuclei between two morphologically distinct species, mediterranea and crenulate.
• One current hypothesis proposed to explain these observations is that the nucleus synthesizes a stable mRNA that lies dormant in the cytoplasm until the time of cap formation.
• The portion with the nucleus eventually formed a new cap, as expected; so did the apical tip of the stalk. However, the intermediate portion of the stalk did not form a cap. Hence, the expression of the cap is controlled not only by the nuclear transcription, but also by the translation of the cytoplasmic RNA.
• ·       Paramecia, for instance, reproduce by fission, but sex is accomplished by conjugation.

  ·       When two paramecia join together, they link their oral apparatuses and form a cytoplasmic connection through which they can exchange genetic material.

  ·       Therefore, no reproduction has occurred, only sex.
• Chlamydomonas is usually haploid, having just one copy of each chromosome. The individuals of each species, however, are divided into two mating types: plus, and minus.

  When a plus and a minus meet, they join their nuclei fuse to form a diploid zygote.Chlamydomonas cells. This is true sexual reproduction, for chromosomes are reassorted during the meiotic divisions and more individuals are formed.

  This zygote is the only diploid cell in the life cycle, and it eventually undergoes meiosis to form four new
• ·       The first path involves the orderly division of the reproductive cell and the subsequent differentiate of its progeny into different cell types.

  ·       This path to multicellularity can be seen in a remarkable series of multicellular organisms collectively referred to as the family Volvocaceae, or the Volvocaceans.

  ·       In these organisms, then, a very important developmental principle has been worked out: the ordered division of one cell to regenerate a number of cells that are organized in a predictable fashion.
• The next two genera of the volvocacean series exhibit another important principle of development: the differentiation of cell types within an individual organism. The reproductive cells become differentiated from the somatic cells.
• Although not all animals set aside the reproductive cells from the somatic cells, this separation of germ cells from somatic cells early in development is characteristic of many animal phyla.
• Although all the Volvocaceans, like their unicellular relative Chlamydomonas, reproduce predominantly by asexual means, they are also capable of sexual reproduction, which involves the production and fusion of haploid gametes.
• Isogamous – haploid gametes that meet similar in size, structure, and motility.
• Heterogamy – swimming gametes of every different sizes are produced by the different mating types.
• Oogamy – specialized form of heterogamy which involves the production of large, relatively immotile eggs by one mating type and small, motile sperm by the other.
• Dictyostelium discoideum in its asexual cycle, solitary haploid amoebae (called myxamoebae or “social amoebae” to distinguish them from amoebae species that always remain solitary) live on decaying logs, eating bacteria, and reproducing by binary fission.
• Two myxamoebae can fuse to create a giant cell, which digests all the other cells of the aggregate.
• When it has eaten all its neighbors, it encysts itself in a thick wall and undergoes meiotic and mitotic divisions; eventually, new myxamoebae are liberated.
• Initially, identical cells are differentiated into one of two alternative cell types, spore, and stalk.
• Dictyostelium discoideum is also an organism wherein individual cells come together to form a cohesive structure composed of differentiated cell types, akin to tissue formation in more complex organisms.
• There is no directed movement that occurs during the first 4 to 5 hours following nutrient starvation. During the next 5 hours, however, the cells can be seen moving at about 20 um/min for 100 seconds.
• This movement is due to chemotaxis: where the cells are guided to aggregation centers by a soluble substance. This substance was later identified as cyclic adenosine 3’,5’-monophosphate (cAMP).
• Aggregation is initiated as each of the cells begins to synthesize cAMP.
• Neighboring cells respond to cAMP in two ways: they initiate a movement toward the cAMP pulse, and they release cAMP of their own. After this, the cell is unresponsive to further cAMP pulses for several minutes.
• The anterior cells normally become stalk, while the remaining, posterior cells are usually destined to form spores.
• This ability of cells to change their developmental fates according to their location within the whole organism and thereby compensate for missing parts is called regulation.
• While growing mitotically on bacteria, Dictyostelium cells do not adhere to one another.
• The initial cell-cell adhesion is mediated by a 24,000-Da (24-kDa) glycoprotein that is absent in myxamoebae but appears shortly after division ceases. This protein is synthesized from newly transcribed mRNA and becomes localized in the cell membranes of the myxamoebae.
• If myxamoebae are treated with antibodies that bind to and mask this protein, they will not stick to one another, and all subsequent development ceases.