development+-+rachel

Human Life Cycle 16.1 p. 320 INSERTING FIGURES IS ALWAYS A GOOD THING. A zygote has 46 chromosomes, both the egg and sperm contribute 23 chromosomes each. Male meiosis occurs in the testes; female occurs in the ovaries. Sperm is much smaller than an egg cell. While swimming to the egg sperm only carry chromosomes, the egg has both chromosomes and cytoplasm awaiting for the zygote. Mitosis (duplication division) is cell division that occurs during growth and repair of tissues so the number of chromosomes stays the same at 46. Meiosis (reduction division) takes place when sex cell are produced so the number of chromosomes reduce from 46 to 23.
 * (1) How many chromosomes does a mother contribute to a new individual? A father?**
 * (2) Where does meiosis occur in males? in females?**
 * (3) Compare and egg cell to a sperm cell.**
 * (4) What is the difference between meiosis and mitosis.**

Development Chapter 17 17.1 During fertilization, a single sperm is drawn into the egg by microvilli of its plasma membrane (micrograph). With the help of enzymes from the acrosome, a sperm makes its way through the zona pellucida. After ta pserm binds to the plasma membrane of the egg, changes occur that prevent other sperm from entering the egg. Fertilization is complete when the sperm pronucleus and the egg pronucleus contribute chromosomes to the zygote. 1. several sperm penetrate the corona radiata 2. only one sperm is able to enter the egg (penetrate the zona pellucida) 3. sperm head binds tightly to the zona pellucida 4. acrosome releases digestive enzymes that open a path through the zona pellucida for the sperm 5. the sperm enters the egg 6. sperm nucleus and egg nucleus fuse together 7. egg’s plasma membrane depolarizes upon impact with sperm (prevents binding of any other sperm) 8. cortical granules release enzymes causing the zona pellucida to become an impenetrable fertilization membrane Polyspermy is when more than one sperm cell enters the egg. This is prevented by the egg plasma’s depolarization, which prevents other bindings with sperm cells. In addition to depolarization the vesticles (cortical granules) release enzymes that transform the egg and sperm into fertilized membrane than cannot be penetrated. 17.2 Cleavage – zygote divide, each cell receives a full complement of chromosomes and genes (no size increase occur) Growth – cell division is accompanied by an increase in size of the daughter cells Morphogenesis – the embryo begins to assume various shapes Differentiation – cells take on specific structure and function (the nervous system is first)
 * (5) Describe in detail how fertilization occurs**
 * (6) What is polyspermy and how is prevented during fertilization?**
 * (7) Describe the processes involved in development.**

Extraembryonic membrane are outside the embryo and have four functions: 1. chorion – develops the fetal half of placenta, blood vessels within the chorionic villi are continuous with the umbilical blood vessels. 2. Allantois – extends away from the embryo, its blood vessels become the umbilical blood vessels 3. Yolk sac – first embryonic membrane to appear (firste site of blood cell formation) 4. Amnion – contains fluid to cushion and protect the embryo and then as it enlarges and develops into a fetus Pre-emnbryonic development encompasses the events of the first week. 1. Olvuation 2. Fertizlation 3. Cleavage - Passing down the oviduct to the uterus the zygote divides repeatedly 4. Morula - the morula, compact ball of embryonic cells becomes a blastocyst 5. Blastocyst - cells of the blastocyst arrange themselves so there’s an outer layer of cells (eventually the chorion) surrounding an inner cell mass (eventually the embryo) 6. Implantation - the blastocyst implacts itself in the uterine lining Embryonic development takes place between the second week and end of the second month. At first only tissue present, the amniotic cavity is above the embryonic disk and the yolk sac is below; chorionic villi are present. After 20 – 25 days the allantois and yolk sac are positioned inside the body stalk becoming the umbilical cord. After 35+ days, embryo has both head and tail regions. The umbilical cord takes blood vessels between the embryo and placenta.
 * (8) What are the extraembryonic membranes and what is their role?**
 * (9) What happens during pre-emrbyonic development?**
 * (10) What happens during embryonic development?**

Gasrulation is an example of morphogenesis during which cells move or migrate, in this case to become tissue layers called the primary germ layers. Primary level structure 1. ectoderm (outer layer) – epidermis of skin; epithelial lining of oral cavity and rectum; nervous system 2. mesoderm (middle layer) – skeleton; muscular system; dermis of skin; cardiovascular system; urinary system; reproductive system; outer layers of respiratory and digestive systems 3. endoderm (inner layer) – epithelial lining of digestive tract and respiratory tract; associated glands of these systems; epithelial lining of urinary bladder
 * (11) What are the primary germ layers and what body structures come from each layer?**