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| Hormones and Female Cycles |
| The ovarian cycle is hormonally regulated in two phases. The follicle secretes estrogen before ovulation; the corpus luteum secretes both estrogen and progesterone after ovulation. Hormones from the hypothalamus and anterior pituitary control the ovarian cycle. The ovarian cycle covers events in the ovary; the menstrual cycle occurs in the uterus.
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| Hormonal controls and the functioning of the female reproductive system. |
Menstrual cycles vary from between 15 and 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menstruation. During menstruation the uterine lining is broken down and shed as menstrual flow. FSH and LH are secreted on day 0, beginning both the menstrual cycle and the ovarian cycle. Both FSH and LH stimulate the maturation of a single follicle in one of the ovaries and the secretion of estrogen. Rising levels of estrogen in the blood trigger secretion of LH, which stimulates follicle maturation and ovulation (day 14, or midcycle). LH stimulates the remaining follicle cells to form the corpus luteum, which produces both estrogen and progesterone.
Estrogen and progesterone stimulate the development of the endometrium and preparation of the uterine inner lining for implantation of a zygote. If pregnancy does not occur, the drop in FSH and LH cause the corpus luteum to disintegrate. The drop in hormones also causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus.
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| Reproductive Technologies Can Enhance Fertility |
About 1 in 6 couples is infertile due to physical or physiological conditions preventing gamete production, implantation, or fertilization. Blocked oviducts (often from untreated STDs) are the leading cause of infertility in females. Low sperm count, low motility, or blocked ducts are common causes of male infertility.
Hormone therapy can cause increased egg production. Surgery can open blocked ducts. About 40 of the cases are due to male problems, 40 due to female problems and the remaining 20% are caused by some unknown agent(s). In vitro fertilization (test-tube babies) is a widely used technique to aid infertile couples.
| Fertilization and Cleavage |
Fertilization has three functions:
- transmission of genes from both parents to offspring
- restoration of the diploid number of chromosomes reduced during meiosis
- initiation of development in offspring
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| Sperm on the surface of a human egg. |
- Contact between sperm and egg
- Entry of sperm into the egg
- Fusion of egg and sperm nuclei
- Activation of development
Cleavage is the first step in development of ALL multicelled organisms. Cleavage converts a single-celled zygote into a multicelled embryo by mitosis. Usually, the zygotic cytoplasm is divided among the newly formed cells. Frog embryos divide to produce 37,000 cells in a little over 40 hours.
The blastula is produced by mitosis of the zygote, and is a ball of cells surrounding a fluid-filled cavity (the blastocoel). The decreasing size of cells increases their surface to volume ratio, allowing for more efficient oxygen exchange between cells and their environment. RNA and information carrying molecules are distributed to various parts of the blastula, and this molecular differentiation sets the stage for the layering of the body in the next phases of development.
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| Gastrulation |
Gastrulation involves a series of cell migrations to positions where they will form the three primary cell layers.
- Ectoderm forms the outer layer
- Endoderm forms the inner layer.
- Mesoderm forms the middle layer
Ectoderm forms tissues associated with outer layers: skin, hair, sweat glands, epithelium. The brain and nervous system also develop from the ectoderm.
The mesoderm forms structures associated with movement and support: body muscles, cartilage, bone, blood, and all other connective tissues. Reproductive system organs and kidneys form from mesoderm.
The endoderm forms tissues and organs associated with the digestive and respiratory systems. Many endocrine structures, such as the thyroid and parathyroid glands, are formed by the endoderm. The liver, pancreas, and gall bladder arise from endoderm.
Immediately after gastrulation, the body axis of the embryo begins to appear. Chordates have the cells that will form the nervous system fold into a neural tube (which will eventually form the spinal cord). The mesoderm forms the notochord (which will eventually form the vertebrae). The mesoderm at this time forms somites, which form segmented body parts, such as the muscles of the body wall.
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