10 key points: #1) Contrast asexual reproduction to sexual reproduction: Contrast the two three very different kinds of asexual reproduction a) Embryonic development of unfertilized oocytes (egg cells): "Parthenogenesis", some lizards, many insects, & many other examples The oocyte keeps 2 sets of chromosomes, and just goes through the ordinary stages of embryonic development b) Budding: As in sea squirts, bryozoans, and what about corals? Fission in flatworms? or does that belong in the 3rd class? c) Rearrangements of cells that were already differentiated: Hydra (& corals?) Sponges & I don't know how many other examples #2) Fertilization; fusion of egg cell (oocyte) with sperm In sea urchins and many other animals, sperm and eggs are just released out into the sea water, to find each other if they can. (that is one category of external fertilization) Frogs, many fish, etc. also have external fertilization , but the male and female copulate; with sperm being released next to eggs. In (all) mammals, birds, reptiles, and some fish & sharks, they have internal fertilization; sperm inserted into oviduct of female. A few worms, bedbugs, etc. have hypodermic impregnation, in which sperm is injected into the female's body! (there are many strange phenomena in biology) #3) Early embryonic development: Different cleavage patterns, specific for each kind of animal. In some organisms, cleavage patterns are the same every time. Roundworms are an extreme example. Each member of a given species of roundworm has exactly the same number of cells (like 953 cells) etc. and every embryo has the same cell lineage. In other kinds of animals, cleavage patterns are irregular and differ in each individual case, apparently with no effects. Mammals have very irregular cleavage patterns, and so there IS NO consistent cell lineage in humans or other mammals. #4) Cleavage produces a multicellular ball of ~100-1000 cells This is the blastula stage of development; The blastula is hollow in many kinds of animals. #5) Many kinds of animals have very yolky eggs, and cleavage occurs only on one side. Examples are birds, reptiles, most fish, octopus, etc. In these, the equivalent of the blastula stage is a thin sheet of cells on the surface of a large mass of yolk. #6) The cells of the blastula stage next undergo active rearrangements called gastrulation and then neurulation. Cells that had been on the surface of the blastula stage fold or crawl actively into the interior, away from the surface. #7) These and other "morphogenetic cell movements" progressively subdivide the embryo into three "germ layers", and then subdivide each germ layer in to different parts. Gastrulation subdivides the embryo into the 3 germ layers Ectoderm: the cells of which later form the brain and skin Mesoderm: whose cells form skeleton, muscles, kidneys etc. Endoderm: whose cells form stomach, lungs etc. Neurulation subdivides the ectoderm into 3 divisions: Nervous system Neural crest Skin #8) The cells that will become the future egg and sperm cells are not part of any of these 3 germ layers; but form at different locations in different kinds of animals, and then crawl by "amoeboid locomotion" to colonize the sites of the testes and ovaries. These are called the primordial germ cells: if these are killed an animal will be sterile; if these are transplanted from one embryo to another, guess what happens! #9) The human body is made of about 250 differentiated cell types; each of which transcribes a certain sub-set of the genes (DNA) For example, the hemoglobin genes are only transcribed in red blood cells during their differentiation. Most "Developmental Biologists" (~= embryologists) try to explain the formation of anatomy by the spatial control of cell differentiation (stimulating transcription in this set of genes in the cell located here etc. etc. for each different cell type..) They tend to ignore the extent to which development is also partly a matter of active rearrangement of differentiating cells. (analogous to cell sorting, in which being a skin cell somehow causes cells to move to the outside, etc. for each cell type) Notice that gastrulation and neurulation wouldn't be expected, nor should cell sorting occur, nor many other phenomena, if each cell type just differentiated at its proper location. #10) Reptiles evolved 4 "extraembryonic membranes" to protect the embryo, get food from the yolk, and store urine. Birds and mammals still form these 4 extraembryonic membranes (one of which is the "amnion" = the "bag of waters" In mammals, these form the placenta, which absorbs food and oxygen from the mother's uterus, and also surround and protect the fetus until birth. ------- Questions that you might be asked on an exam: a) What is the distinction between parthenogenesis and budding? b) Does either one require fertilization of an egg cell? c) Which does involve an egg cell, and which does not? d) What is an example of a kind of animal that can reproduce parthenogenetically? e) What is a kind of animal that can reproduce by asexual budding? *f) Which of these two kinds of asexual development can produce colonies of interconnected animals, like corals? g) What is it called when a sperm fuses with an egg cell, and its nucleus enters the cytoplasm of the egg cell? h) Where does fertilization occur, relative to the location of the bodies of the parents, when sea urchin eggs are fertilized? i) What are some kinds of animals in which fertilization occurs inside the body of the mother animal? j) Are there any animals that do copulate, but have external fertilization? k) In the early mitotic divisions of cells in fertilized eggs, are these divisions in the same geometric patterns every time, but different from one species to another, or are they different from one individual to another? Or does THAT differ from one taxonomic group to another? [hint: yes] l) Are there any kinds of animals in which embryonic development occurs by exactly the same cell lineage, every time? What is an example? **m) What are some advantages to concentrating research on species of animals that happen to have a very consistent cleavage pattern and cell lineage? Conversely, in what ways might the results of such studies be very misleading, especially as regards medical applications to human development? n) Describe the blastula stage of embryonic development? o) In what way is the blastula stage different in species of animals (like birds and fish), in which the oocyte contains a large amount of inert stored food, with the cytoplasm concentrated in a layer along one side? p) The cells of the blastula actively rearrange themselves to form the 3 primary germ layers in the process called ____________? q) These 3 primary germ layers are called the ----derm, the ----derm and the ----derm. r) The nervous system and the outer layers of the skin are formed by cells of which of these 3? s) The skeleton and what other organs are formed by the ----derm? t) The remaining germ layer forms the digestive tract, and what else? u) The next major morphogenetic movement subdivides the ectoderm into 3 sub-parts, and is called what? v) The embryonic cells that will later differentiate into egg cells and sperm are part of which of these 3 germ layers? (if ANY! hint hint) w) What would be the result if someone grafted primordial germ cells from the embryo of a Rhode Island Red chicken to the embryo of a New Jersey Brown chicken? x) About how many differentiated cell types are there in humans? What are two or three specific examples of differentiated cell types? **y) Would you guess that sponges and Hydra have more differentiated cell types than humans, or fewer? Or would they have differentiated cell types at all? [Hint: yes] z) What are the two different kinds of mechanisms by which the various differentiated cell types get positioned in the proper anatomical locations? *!) Suppose that a certain differentiated cell types are found to be able to sort out to its correct anatomical location in the body, even after being dissociated and randomly mixed with other cells: would that suggest anything about the mechanisms that cause their location in a normal embryo? [Hint: suppose that it did suggest something; then what might that be?]. @) What is a specific example of an extraembryonic membrane, one that is formed in human development and also in bird development? #) Do developing bird and reptile embryos have anything that is analogous to the placenta in mammals? [Hint: yes]

 

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