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Date Created: 02/18/16
Chapter 23 ● How do you go from a fertilized egg to a functioning adult? ● Embryology: looking at the physical aspects of development. ○ Where did that cell go? What derived from that initial cell? What are the tissue layers? ○ Last 20 years integration of molecular biology and developmental biology. Evolutionary biologist have been interested in molecular differences during development as well. ○ Aristotle first known scientist to study development in the 4th century. He noted differences in how animals were born and types of cell division. When embryos have cell division they occur differently. ● Zebrafish development ○ First 18 hours of development ○ After fertilization cells begin to divide at a stage known as cleavage. ■ head and tail emerges ■ yolk (filled with nutrients) is wanted inside the embryo, want gut tissue to surround yolk. This stage is known as gastrulation. ■ Organogenesis is when major organs develop. ● Fertilization ○ Two haploid gamete come together and form a diploid zygote. ● Cleavage ○ First stage of development ● Gastrulation ○ Cell moves around and end up in specific places. ● Organogenesis ○ Divided by trimesters in humans. ● Gametogenesis: The formation of gametes in the reproductive organs of adult organisms. ○ Sperm and egg each contribute the equal number of chromosomes ○ In general gametes are haploid meaning they have one set of chromosomes ○ Egg contains more cytoplasm than sperm. In it we will find will contain yolk, mRNA, proteins, and anything in cytoplasm ● Van Leeuwenhoek discovered sperm, originally thought to be parasitic animals but later believed each contained a preformed embryo. Thought that serm was seed and females were the nutrient rich soil. in 1876 sperm and egg were decided that they needed to fuse to form an embryo. ○ Sperm: ■ Head: nucleus and acrosome (Genetic information) ■ Neck: centriole (cell mitosis) ■ Midpiece: Mitochondria (energy for movement) ■ Tail: Flagella (act as propeller pushing towards egg) ○ Eggs: Large and non motile (if it is moving it is due to aqueous environment with currents) Large in size compared to sperm. ■ Greater energy investment than sperm, as it contains the nutrients required for the embryo’s early development. Mammalian embryos receive nutrients from placenta. Those that are laid get nutrients from the yolk. Contains mRNA, proteins ■ Sea Urchin Egg ● Jelly layer in sea urchin provides protection, broadcast spawners (receive environmental signal where they spew egg and sperm), external fertilization, need aqueous environment to allow movement. ■ Vitelline envelope: connected to plasma membrane. Made up of glycoproteins. In mammals it is thick and called zona pellucida. ■ Cortical Granules: small vesicles filled with enzymes that are activated during fertilization. acrosomes in a way that they contain enzymes ■ Cytoplasmic Determinants: maternal factors (mRNA and protein) that guides early development. ● Fertilization: Egg and sperm of single species come together, where the sperm releases nucleus into egg and fuse their membrane, the nuclei comes together, chromosomes line up, and cell division starts. ○ Acrosomal Reactions : when sperm head contacts the jelly layer, acrosomal reaction begins. Sperm releases enzymes into egg, “eat” through the jelly layer eventually getting to vitellin envelope and plasma membrane where the membranes fuse where nucleus is released and comes together with the nucleus of egg. ● How do we keep multiple sperm from releasing nucleus into egg? ○ Polyspermy: More than one sperm entering egg ○ In sea urchins cortical granules release enzymes that cause the vitelline to move away from plasma membrane. Release calcium that starts at high levels and goes over entire egg after sperm entrance. Calcium larger, Vitelline envelope moves away. What was once vitellin envelope is now fertilization envelope and plasma membrane. Take all sperm that have not entered and push it way. This creates a barrier. Cortical granules release Enzymes will sometimes break down sperm. ● How do we know we have the same species together? Frank Lillie attempted to answer this. Found that if you take egg cells and treat them with an protease, isolating the eggcell receptor that binds to sperm. Breaks down protiens, all the sperm clumped together. Protein identified as bindin. ○ Question: If bindin is the protein on the surface of sperm that acts like a key, what is the “lock”? ○ Hypothesis: The lock is a receptor protein on the surface. Bindin will bind to the receptor. ○ Results: Found protein fragment which binded specifically to the sperm bindin called fertilizin. A species specific bindin. Fertilizin is a lock protein which binds to bindin, preventing cross species development ○ there must be a method of cross species, chromosomes will not match up and development stops. Frank Lille found that ● Fertilization in mammals ○ Occurs internally, we do not have species specific receptor proteins. ○ Mammals have to block polyspermy when it hits zona pellucida ○ There are two nuclei in egg (one sperm and one egg) during fertilization. ○ In males you have 1 diploid cell and turns to 4 haploid, In females you have 1 diploid divided in 4 haploid but one is viable as it has more nutrients becoming the egg while the others become polar bodies (trash). The last polar body is not released until sperm enters body. ● Cleavage : rapid cell division (First step in embryogenesis) ○ each cell called blastomere, at end of cleavage we have mass of blastomeres called blastula. Cleavage takes initial cytoplasm and divides it. Growth does not happen during cleavage. Embryo is just subdividing with initial cytoplasm. ● The purpose of cleavage is to take the fertilized egg and divide up the cytoplasmic determinants. In mammals cleavage occurs in fallopian tube or oviduct, the structure that connects ovary to uterus. As cells divide only a subset of cells will have the cytoplasmic determinant. Egg is released during ovulation, and fertilized right after (cannot survive more than 24 hours) and day 7 is when it will implant into uterine wall. Trophoblast cells will be extra embryonic membranes (placenta) and inner cell mass which will eome the embryo. ● Patterns of Cleavage: ○ Radial cleavage is when cells divide at right angles, cells are the same size. ○ Spiral cleavage cells divide at oblique angles to one another. (Looks like snowball) and end up with different sized cells. ■ Both cases with very little yolk. More yolk= more resistance/ viscosity. ○ Discoidal cleavage cell division in only small area on top (Zebra,Chicks) ○ Superficial cleavage cell division of mitosis is not followed by cytokinesis. (multi nucleic embryo) (Fruit flies) ● Sea shells have different coils (left and right), these organisms have spiral cleavage and direction dependent on spirality. Direction of cleavage is the same as direction of coiling. Left=counterclockwise, right=clockwise. It is a maternal effect gene, dependent on proteins/mRNA in the egg of mother. Cell division slows at the end of cleavage. ● Gastrulation extensive and highly organized cell movements radically rearrange the embryonic cells into a structure called the gastrula. It is critical because that is when the cells get to the right place to make your body. ● Form the three major tissue layers (germ layer) ○ Ectoderm (outside) ■ Forms the outer covering of the adult body and nervous system. ○ Mesoderm (middle) ■ gives rise to muscle, most internal organs and connective tissues such as bone and cartilage. ○ Endoderm (Inside) ■ Produces the lining f the digestive tract or gut, along with some of the associated organs. ● Space in middle of blastula is , blastopore which eventually becomes the anus and the body elongates while the germ layers form, in sea urchin cells move independently during gastrulation. Blastocoel allows yellow cells to move in then red then blue. Yolk plug eventually becomes anus. The major body axes also form (anteriorposterior, dorsalventral) ● Organogenesis ○ The process of tissue and organ formation that begins once gastrulation is complete and the embryonic germ layers are in place. Cells proliferate and differentiate. ■ Differentiated means that cells become specialized. They differentiate when they begin to produce specific proteins for specific cell types. ● Notochord: ○ Appears in dorsal mesoderm, rod like structure, runs length of body, many organisms replace it (transit structure). It functions as a key organizing element. Important to beging of life (cartilage pad in between vertebrate) as it sends signals to tell the ectoderm to tell them to fold. Folds do not form if notochord isn't there. ● Neural Tube ○ forms precursor of brain and spinal cord. Mesodermal cells organized into blocks of tissue called somites.As they form it makes a hollow tube known as neural tube. (Specific for group of animals called chordates (humans)). Give rise to spinal cord and brain and send signals to mesoderm to form blocks of tissues to go down the body called somites. (also transite) ● Somites: ○ Form variety of structures on either side of the neural tube. ive rise to an array of tissue: muscle on back, connective tissue in skin, limbs, bone building cells. They are not initially determined ( do not know path they have to go down) take cells from one part and place somewhere else and see what they become. As they mature they become irreversibly determined and will eventually differentiate into a specific cell type based on their location within the somite. Depending on combination of signals, it will determine a cell's determination. ● Myoblasts a cell that is determined to become a muscle cell but has not begun producing muscle specific proteins. The Search for a Gene That Causes MuscleCell Differentiation.Researchers found that MyoD was the protein that causes muscle cell differentiation. MyoD is a regulatory transcription factor that binds to enhancers upstream of musclespecific genes. ● Pattern Formation ● Morphogenesis ● Determine ● Differentiate Chapter 50 (Partial) 2/10/15 ● Reproduction is an unconscious desire of all organisms, a fundamental attribute to life ● Reproduction can be asexual or sexual ○ Asexual: reproduction through mitosis such that offspring will be genetically identical to the parent. Benefit: Faster and no need to find a mate. ○ Sexual: Gametes come together (meiosis) in which offsprings are different from parents. Benefits: Organisms can adapt and find new traits providing genetic variability, can allow them to evolve with change in environment. ○ Some organisms can be both, usually at different times of the year ● Asexual: ○ Budding ■ An offspring begins to form within or on a parent, the process is complete once the bud breaks free and begins to grow on its own. The offspring is a miniature version of the parent. (No Gender) ○ Fission ■ An individual simply splits into two or more descendants. one individual that splits into 2 or more (no gender) ○ Parthenogenesis ■ Female offspring develop from unfertilized eggs. These offsprings are genetically identical to mother. (Always Female) ● Some species can store sperm for long periods of times in reproductive tract. ● Daphnia (crustaceans): Can reproduce both sexually and asexually. ● Spring and summer they produce diploid offspring by parthenogenesis. Eggs are contained in brood pouch and shed into water where they hatch. ● Late summer and early fall, females start to produce male offsprings. Males will produce haploid sperms which will fertilize haploid eggs that are produce in winter. ○ Proximate causation is the how, Ultimate causation is the why ○ Experimental ■ Water Quality (whether or not it was crowded): Crowded ■ Food Concentration: Low ■ Day Length Short: (winter) More Daphnia in same space will most likely produce sexually. This is because days are shorter so weather is colder, crowded water means more competition and produce less offsprings sexually than by parthenogenesis. As environment worsens more sexual reproduction because if species are all the same then they will all die out. ● Gametogenesis: ○ Spermatogenesis: ■ Start with diploid cell (Spermatogonia) ■ Undergoes mitosis to produce primary spermatocyte ■ Undergoes Meiosis I (Diploid) producing secondary spermatocyte. ■ Meiosis II Produce 4 haploid cells, spermatids which contain the nuclear material ○ Oogenesis: ■ Start with diploid cell (Oogonium) ■ Undergoes mitosis to produce primary oocyte ■ Undergoes Meiosis I ( Diploid) producing secondary oocyte and a polar body (polar body may divide again but not common) ■ Meiosis II Produce 4 haploid cells, One becomes the ootid and the others become polar body. Polar body is just genetic material, the egg is extra cytoplasm. ● Fertilization: External or internal (external is still sexual reproduction). Join sperm and egg to form a diploid zygote. ● ALL FERTILIZATION IS SEXUAL ○ External fertilization (it is still sexual): gametes come together outside female body, or broadcast spawner. Release based on environmental changes, most live in aquatic environment and produce a huge amount of gametes. This is because higher chance in survival and will not dry out. ○ Internal fertilization: Egg and sperm come together inside female. Occurs in most terrestrial organisms and some aquatic organisms. Salamanders produce packets of sperm (spermatophore) which vary in types. ○ Sperm Competition: When female mate twice what happens between first and second male? Second male mating displaces sperm from first male mating. Some species eject sperm from undesirable male (cryptic female choice) ● Wide variety of sexual reproduction. Females will eat the male after copulation, some a penis will break off and plug the female preventing secondary male production. Can have giant sperm, males will produce a variety in size in order to clog up the reproductive area in female. False penis in birds and a lot of infidelity. Hermaphrodites line up backwards with partners (testes and ovaries line up) fire love dats with sperm whee they both get fertilized. Serial hermaphrodites( not male and female at same time). Hypodermic enis will inject into female similar to neede. ○ Self fertilization is still sexual reproduction but not much genetic variation 2/12/16 ● The offspring can “born” in a variety of ways. ○ Internal Fertilization ■ Oviparity: lay eggs ■ Viviparity: live birth ■ Ovoviviparity: eggs are retained within female body and then gives birth to the hatch embryo. Offspring are nourished by nutrientrich yolk stored in egg. Often in colder places because rate of defects increase and it needs to be kept warm. ■ Laid ● Cons: Cannot defend their self, Exposed to environment while being developed so can have effects on it. ● Pros: Less investment ■ Live ● Cons: Gestation periods get difficult with time ● Pros: Mobile so can move from danger, having a sense of defense. Located in constant environment.
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