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Reproduction and Development Gametogenesis Gametogenesis uses meiosis and produces haploid cells called gametes Human Reproductive Systems The primary reproductive organs are called gonads Males » testes Females » ovaries These also secrete hormones important for the development of secondary sexual traits Males Testes These start to form in the abdominal cavity They eventually descend into the scrotum The sperm require the cooler temperatures in order to develop properly Composed of many lobes Each lobe contains two or three seminiferous tubules where the sperm are produced It’s all about the hormones Spermatogenesis Primary spermatocytes are diploid cells, undergo meiosis I, and produce secondary spermatocytes Secondary spermatocytes undergo meiosis II, each with the haploid number of chromosomes Semen Sperm move from the testes into the epididymis where they mature They travel through the vas deferens, into the ejaculatory duct, through the urethra, and out the penis The sperm are continually being mixed with secretions from the seminal vesicles, the prostate gland, and the bulbourethral glands along the way to form semen These secretions provide nutrients, buffer, and lubricating fluids for the sperm Females Ovaries Located in the abdominal cavity Produce both estrogen and progesterone Oocytes are present in the ovaries The oviduct connects the ovary to the uterus The uterus is mainly muscle The uterine lining is called the endometrium The lower portion of the uterus is the cervix The connection to the outside is the vagina Oogenesis Human females are born with all their eggs already present in their ovaries These primary oocytes are arrested in meiosis I Oogenesis occurs once per cycle FSH triggers one primary oocyte to complete meiosis I to produce a secondary oocyte One secondary oocyte is produced per cycle The other cells resulting from meiosis become polar bodies The primary oocyte is located inside the follicle As the follicle matures, it moves towards the wall of the ovary closest to the oviduct (fallopian tube) The follicle ruptures, releasing the secondary oocyte (ovulation) The ruptured follicle develops into the corpus luteum, which secretes hormones that prepare the reproductive tract for pregnancy and that maintain it during the early phases of pregnancy If fertilization occurs, then the corpus luteum continues to produce hormones If fertilization does not occur, then the corpus luteum degenerates It’s really about the hormones The female reproductive cycle is actually two cycles in one: 1. The ovarian cycle controls the growth and release of an egg 2. The menstrual cycle prepares the uterus for possible implantation of an embryo Hormonal signals coordinate the two cycles, keeping them in synch If an embryo implants in the uterine wall, it will obtain nutrients from the endometrium, and the thickened lining will not be discharged Menstruation is uterine bleeding caused by the breakdown of the endometrium, the blood-rich inner lining of the uterus and is a sign that pregnancy has not occurred during the previous cycle Fertilization Sperm and egg have to meet They do so in the oviduct Development Once fertilization has occurred, development takes over The initial cell divisions are cleavage divisions - cell division without any major increase in cell volume The developing organism is called A zygote at conception An embryo from the zygote’s first division through the eighth week of pregnancy A fetus from the ninth week of pregnancy to the birth of the baby About 6–7 days after fertilization, the embryo has reached the uterus as a fluid-filled hollow ball of about 100 cells called a blastocyst Protruding into the central cavity on one side of the blastocyst is a small clump of cells called the inner cell mass, which will eventually form the fetus The blastocyst will implant in the endometrium About 9 days after conception, the cells of the early embryo begin an organized migration that produces the gastrula, with three main layers 1. The ectoderm eventually develops into the nervous system and outer layer of skin (epidermis) 2. The endoderm becomes the innermost lining of the digestive system and organs such as the liver, pancreas, and thyroid 3. The mesoderm gives rise to most other organs and tissues, such as the heart, kidneys, and muscles During the pregnancy, the placenta develops to protect, serve, and nourish the embryo The three trimesters of human pregnancies are marked by significant events First trimester About 5 weeks after fertilization, the embryo has developed a brain and spinal cord, four stumpy limb buds, a short tail, and primitive gill-like structures Overall, a month-old human embryo looks pretty much like a month-old embryo of any other vertebrate species About 9 weeks after fertilization, the embryo is called a fetus It has a clear sac, the amnion, around it It has all of its organs and major body parts, and the embryo’s limb buds have become tiny arms and legs with fingers and toes The sex of the fetus can be determined by an ultrasound exam Second trimester The main developmental changes during the second and third trimesters involve an increase in size and general refinement of the human features A 14-week-old fetus is about 6 cm (2.4 inches) long At 20 weeks, the fetus is about 19 cm (7.6 inches) long, weighs about half a kilogram (1 lb), and has the face of an infant, complete with eyebrows and eyelashes The fetal heartbeat is detectable with a stethoscope, and the mother can usually feel the fetus moving Third trimester The third trimester is a time of rapid growth as the fetus gains the strength At birth, a typical baby is about 50 cm (20 inches) long and weighs 3–4 kg (6–8 lb) Childbirth Near the end of the third trimester, birth occurs Occurs due to a series of rhythmic contractions of the uterus (labor) Estrogen is a key hormone It leads to an increase in the number of oxytocin receptors on the uterus Oxytocin and prostaglandins stimulate the uterine contractions