development I and II Flashcards
what is development?
all changes that occur during a life cycle, embryonic, after birth/hatching
what are model organisms?
they are species specifically chosen for research because they can easily be studied
what is fertilization?
sperm (usually flagellated, motile) + ovum (large, nonmotile = zygote
what are the consequences of fertilization?
restores the 2n (diploid), often determines sex, activates egg and stimulates reactions promoting development
what are the major steps in fertilization?
the sperm dissolves the protective layers around the egg and binds to surface receptors to ensure compatibility between the sperm and egg of the same species. at the same time to egg undergoes changes to its surface to prevent polyspermy.
what surrounds the egg?
the egg has a plasma membrane and one or more coverings that aid in fertilization. these coverings act as barriers to interspecific fertilization, which is particularly important for species that undergo external fertilization. in mammals, the egg is surrounded by coverings both internally and externally, and fertilization typically occurs internally.
what is the acrosomal reaction?
this is a process in fertilization where the sperm penetrates the protective layers of the egg. the sperm pushes through follicle cells and binds to receptor in zona pellucinda (ZP3), the acrosome (strucutre at the tip of sperm) busts and releases enzymes, helps the sperm through the zona pellucinda, this penetration, known as plasmogamy, leads to the cortical reaction
what is the cortical reaction?
follows the acrosomal reaction in the fertilization process, Ca2+ is released in the egg from smooth ER, a wave of Ca2+ spreads through the egg, cortical granules fuse with plasma membrane, enymes are released and trigger a slow block
what is a slow block in mammals?
- an enzyme catalyzes changes in the zona pellucida -> hardens + alters sperm receptors
- blocks polyspermy from occurring.
what is the activation of the egg?
- follows cortical reaction
- triggered by the increase in calcium ions (Ca2+) in the cytoplasm
- leads to increased respiration, activation of maternal enzymes and proteins, and increased protein synthesis using the mRNA already present in the egg.
- nuclear fusion also occurs, the secondary oocyte that was arrested at metaphase II resumes the process and meiosis is completed (fertilization triggered this)
what is nuclear fusion and what is the result?
microtubules guide the sperm nucleus to the egg. in humans, several hours after the sperm enters the egg, this process results in the formation of a zygote, which is diploid and totipotent, meaning it has the potential to give rise to all cell types.
when does fertilization end?
when the zygote undergoes its first division, for mammals that is about 12-26 hours in
what is cleavage?
the second step of embryonic development where the zygote becomes a multicellular embryo, there are rapid divisions with no cell growth
what does yolk do to the pattern of division?
yolk is a mixture of proteins, phospholipids, and fats that serves as food for the developing embryo. the amount and distribution of yolk vary between animal groups and depend on the needs of the embryo. Embryos with little yolk develop faster and exhibit equal cleavage, while those with lots of yolk develop more slowly and show unequal cleavage.
what are the stages of cleavage?
zygote (1 cell) -> embryo (2+ cells) -> blastula: a hollow embryo (with blastocoel) at least 128 blastomeres
what are the patterns of cleavage?
equal holoblastic, unequal holoblastic, and mesoblastic
what is equal holoblastic?
a pattern of cleavage where complete division occurs and the cells are equal sizes. there is a central blastocoel. EX: echnioderms, mammals, and annelids
what is unequal holoblastic?
asymmetric cleavage caused by the yolk distribution, the yolk slows down cytokinesis, this includes amphibians
what is meroblastic?
where only regions without yolk divide completely
examples: birds, reptiles, many fishes, insects
what occurs after cleavage?
After the normal cell cycle resumes and cell division slows, morphogenesis, a cellular and tissue-based process by which the animal body begins to take shape, occurs during the next two stages of development: gastrulation and organogenesis.
what is gastrulation?
a morphological process by which hollow blastula becomes layered embryo (gastrula) with the three embryonic germ layers, ectoderm (outside), endoderm, and mesoderm
how does gastrulation in sea urchins work?
During gastrulation, various processes occur:
1. Cells at the vegetal pole (bottom) migrate into the blastocoel and later secrete calcium carbonate to form a simple skeleton.
2. The vegetal plate forms, and its cells flatten and fold inwards, creating the archenteron.
3. Cells at the tip of the archenteron extend to the opposite wall and make contact, eventually fusing the archenteron with the blastocoel wall to form the digestive tube.
4. This process results in the formation of three germ layers: the ectoderm, endoderm, and mesoderm, which give rise to structures like the mouth, anus, and cilia
how does gastrulation in birds work?
In vertebrates, the process of germ layer formation is significantly different from that in echinoderms due to the presence of a large amount of yolk, which complicates the typical process. Vertebrate embryos undergo meroblastic cleavage, resulting in two primary layers: the upper epiblast (from which the embryo derives) and the lower hypoblast, both situated on top of the yolk mass.During a process called involution, epiblast cells migrate towards the midline. Some remain on the surface, contributing to the formation of the ectoderm. Others move inwards towards the yolk, then laterally, giving rise to the mesoderm. Alternatively, some cells move downward, displacing the hypoblast and forming the endoderm. Additionally, a primitive streak, known as the blastopore, forms during this process. Hypoblast cells play a role in forming a sac around the yolk and serve as a connection between the yolk and the embryo but do not directly contribute to the embryo itself.
how does gastrulation occur in humans?
After cleavage, the blastocyst forms, which is the mammalian version of the blastula. It reaches the uterus about six days after fertilization and consists of approximately 100 cells surrounding a central cavity. The blastocyst has two main parts: the trophoblast, an outer single layer of cells, and the inner cell mass, a cluster of cells that becomes the embryo and is the source of the embryonic cell line. During implantation, the trophoblast contacts the uterine lining, secretes enzymes, and erodes an area of the endometrium. The embryo then penetrates this area, thickens, and extends projections into the endometrium. Gastrulation, which takes about one week and occurs during the 2nd-3rd week of pregnancy, is when the inner cell mass forms a flat disk with two layers. Some epiblast cells remain in place, forming the ectoderm. Others migrate in between the layers to become the mesoderm, and some migrate and mix with the hypoblast to become the endoderm. The human gastrula also develops extraembryonic membranes: the chorion, amnion, yolk sac, and allantois. These membranes hold liquid, form blood vessels, and facilitate gas exchange and the disposal of waste products.
