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The Human Reproductive Human Development Objectives: • Identify the four phases of the menstrual cycle. • Describe fertilization. • Describe the function of the placenta. • Outline the life cycle after birth. Terms you need to know: • • • • • • • Menstruation Ovulation Follicle Implantation Zygote Placenta Fetus -Gastrulation -Embryonic stem cells -Differentiation The Menstrual Cycle • During the menstrual cycle, an egg develops and is released from an ovary. • The release of a mature egg from the ovary is called ovulation. • In addition, the uterus is prepared to receive a fertilized egg (progesterone) • If the egg is fertilized after ovulation, it is implanted in the uterus and embryonic development begins. • If it is not fertilized it is discharged along with the lining of the uterus. The Menstrual Cycle The menstrual cycle has four phases: 1. Follicular Phase 2. Ovulation 3. Luteal Phase 4. Menstruation The Menstrual Cycle -Begins when estrogen level in blood is low 1. Follicular Phase: (10 days) egg develops. • Pituitary gland to secretes Follicle Stimulating Hormone (FSH). • FSH causes a follicle to develop to maturity. Follicle: Cluster of cells surrounding egg to help it develop. Follicle produces estrogen, which causes thickening in lining of uterus. The Menstrual Cycle 2. Ovulation: (3-4 days) egg released into fallopian tubes. • High estrogen levels decrease secretions of FSH (feedback!!!) • Pituitary now secretes Leutinizing Hormone (LH)which causes follicle to rupture and release a mature egg into the fallopian tubes. The Menstrual Cycle 3. Luteal Phase (corpus luteum): (10-14) days. • After ovulation, LH causes broken follicle to fill with cells forming the corpus luteum. • Corpus luteum secretes progesterone which further thickens uterine lining. • Chances of fertilization are highest. • If fertilized zygote will float down fallopian tubes and implant itself in the lining of the uterus The Menstrual Cycle 4. Menstruation: (3-7 days) • If fertilization does not occur, secretion of LH decreases and the corpus luteum breaks down. • As corpus luteum breaks down, estrogen and progesterone levels drop causing the lining of the uterus to detach from the uterine wall. • This tissue, along with blood and an unfertilized egg are discharged through the vagina. Fertilization • Fertilization: Process by which sperm (n) meets an egg (n) in the Fallopian tubes of a female’s reproductive system to form a zygote (2n). Early Development: Cleavage • After fertilization, zygote begins a series of mitotic divisions called cleavage. • Cleavage and Fertilization take place in the fallopian tubes. • During cleavage, cells increase in number but decrease in size. • Early divisions of cleavage result in a solid ball of cells called a morula. • As cells divide further they form a sphere filled with fluid called a blastula. The fluid inside is called blastocoel Early Development Cleavage Early Development: Cleavage • Morula • Blastula Early Development: Implantation • About 6-7 days after fertilization, blastula attaches itself to the lining of the uterine wall. • The developing embryo secretes an enzyme that digests a path into the into the soft tissue • This process is known as implantation. • Implantation marks the beginning of pregnancy. Early Development: Implantation Early Development: Gastrulation • A cluster of cells gradually forms within the blastocyst. • This cluster sorts itself out into three layers of tissue in a process called gastrulation. • In gastrulation, the embryonic stem cells begin differentiating into the three cell layers: endoderm, mesoderm, ectoderm. • All organs and tissues of the embryo will be formed by these three layers. Gastrulation • Each of these germ layers will have special roles to play in building the complete animal. Some are listed in the table . Ectoderm Mesoderm Endoderm Skin Notochord Lining of gut Early backbone Brain Muscles Spinal cord Blood Lining of lungs Lining of bladder Liver Neurons Bone Sense receptors Sex organs Pancreas Gastrulation Similarities in embryonic development • Human embryonic development is very similar to embryonic development in other vertebrates such as reptiles, birds, fish and other mammals. • These similarities are REAL EVIDENCE that we share an evolutionary history with these organisms Similarities in development reveal common ancestry Early Development: Placenta • By the end of the third week, the nervous and digestive systems have begun to form. • The developing embryo needs a supply of nutrients and oxygen. It also needs a way to eliminate CO2 and metabolic wastes. • This is achieved by the placenta which serves as a connection (via the umbilical cord) between the mother and the developing embryo. • The placenta is the embryo’s organ of respiration, nutrition, and excretion. Question: • Why do humans and other mammals have the placenta? Why not have the blood supply of the embryo join up with the blood supply of the mother? • Write you answer here. Early Development: The Placenta • The placenta allows for the blood of the mother and the blood of the embryo to pass each other, but not to mix. • They are separated by the placenta; across this thin barrier, gases are exchanged CO2 and O2 and food and waste products diffuse. • Almost everything that the mother takes into her body passes through the placenta to the embryo. Later Development • After 8 weeks of development, the embryo is called a fetus. • By the end of 3 months, most of the major organs and tissues of the fetus are fully formed. During this time the umbilical cord forms and connects the fetus to the placenta. • 6 months after fertilization, the tissues of the fetus become more complex and specialized. • More tissues begin to function. Later Development • After 6 months, the fetus may be able to survive outside the uterus if life-supporting machinery is available. • Chances of survival are much better after three more months in the uterus. • Here fetus will double in mass, and the lungs and other organs undergo a series of changes to prepare for life outside the uterus. • Premature babies (before 8 mo.) often have breathing problems due to incomplete lung development. The Developing Fetus Embryo at 6 weeks Fetus at 14 weeks Fetus at 38 weeks Childbirth • About nine months after fertilization, the fetus is ready for birth. • A complex set of factors affect the onset of childbirth. • One factor is the release of the hormone oxytocin from the pituitary gland. • Oxytocin stimulates the rhythmic muscle contractions known as labor. • Oxytocin is also important in the bonding of mother and newborn baby. Childbirth • Labor contractions become more frequent and more powerful. • The opening of the cervix expands until it is large enough for the head of the baby to pass through it. • At some point the amniotic sac breaks and the fluid it contains rushes out of the vagina (water breaks). • Contractions of the uterus (and lots of pushing from mom) force the baby (usually head first) out through the vagina Changes in Baby After Childbirth • After birth, the baby may cry or cough to rid the lungs of fluid. • Breathing starts almost immediately and the blood supply to the placenta begins to dry up. • The umbilical cord is clamped and cut off with a small piece attached to the baby. This soon dries off and leaves scar called belly button. • Body systems quickly adapt to life outside of uterus. Supply their own oxygen, excrete their own wastes, maintain own body temperature. Changes in Mother After Childbirth • Within a few hours after birth, the pituitary has released a burst of oxytocin to allow for strong bonding between mom and baby. • The pituitary also releases a hormone called prolactin that stimulates the production of milk in the breast tissues of the mother. • Nutrients in the milk contain everything the baby needs for growth and development during the first year or so of life. • Breast milk also contains antibodies from the mother to help fight off pathogens. A couple years later and they look a bit like this Internal vs. External Reproduction • Internal Fertilization: • External Fertilization: Some aquatic animals Male releases sperm directly into the female’s gather in large groups during their breeding body where the egg is season. fertilized. • Females release eggs • Most terrestrial (land into water. dwelling) animals use internal fertilization. • Males release sperm over the eggs. • Fertilization takes place outside of the organism’s body Evolution often involves trade-offs • Organisms that fertilize internally often invest in fewer numbers of offspring. • The offspring have a relatively higher chance of survival than the offspring of external fertilization. • Rather than putting energy into a high number of offspring, the investment is in increasing the rate of survivability of the few offspring. • Parental care is often (not always) involved to increase the chances that the offspring will survive. Evolutionary trade-offs • Organisms that fertilize externally hedge their bets by producing large numbers of offspring. • Although only a very small percentage of these offspring survive, there is still a good chance that many will make it to adulthood. • In these organisms, resources and energy are invested in producing large numbers of organisms and not in the development and care of offspring. Male Fish Externally Fertilizing Eggs Galapagos Tortoises Fertilizing Internally