Animal Development

• Conception in humans occurs when a sperm fuses with an egg (mature oocyte) in the oviduct or fallopian tube.
• Fertilization, also known as conception in humans, occurs when a sperm fuses with an egg (mature oocyte) in the oviduct or fallopian tube.
• Gametes are produced in a given time period.
• Different cell types are produced by meiosis.
• During human gestation, rudiments of all organs develop in the first trimester.
• While the embryo is in the oviduct, different cell types are produced by meiosis in the second trimester.
• During the blastocyst stage, different cell types are produced by meiosis in the third trimester.
• About 24 hours after fertilization, the resulting zygote begins dividing, a process called cleavage.
• Suckling stimulates the secretion of a posterior pituitary hormone, oxytocin, which triggers release of milk from the mammary glands.
• The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.
• Contraception, the deliberate prevention of pregnancy, can be achieved in a number of ways.
• Some contraceptive methods prevent the release of mature eggs and sperm from gonads, prevent fertilization by keeping sperm and egg apart, and prevent implantation of an embryo.
• Recent scientific and technological advances have made it possible to address many reproductive problems, including genetic diseases and infertility.
• It is now possible to diagnose many genetic and congenital abnormalities while the fetus is in the uterus using noninvasive techniques such as ultrasound imaging.
• Amniocentesis and chorionic villus sampling are invasive techniques in which a needle is used to obtain amniotic fluid or fetal cells for genetic analysis.
• A newer noninvasive method uses the fact that maternal blood contains fetal blood cells that can be tested.
• Hormone therapy can increase sperm and egg production.
• Surgery can correct blocked oviducts.
• Many infertile couples turn to assisted reproductive technologies (ART), procedures that generally involve surgically removing eggs (secondary oocytes) from a woman's ovaries after hormonal stimulation, fertilizing the eggs, and returning them to the woman's body.
• Unused eggs, sperm, and embryos from such procedures are sometimes frozen for later pregnancy attempts.
• With in vitro fertilization (IVF), the most common ART procedure, the oocytes are mixed with sperm in culture dishes and inserted in the woman’s uterus at the eight-cell stage or beyond.
• After another 2-3 days, the embryo typically arrives at the uterus as a ball of 16 cells.
• By about 1 week after fertilization, cleavage has produced an embryonic stage called the blastocyst, a sphere of cells surrounding a central cavity.
• During the final trimester, the fetus grows to about 3 - 4 kg (6 ½ - 8 ½ lbs.) in weight and 50 cm (19 - 20 inches) in length.
• This is the period of the most rapid growth of the fetus.
• Fetal activity may decrease as the fetus fills the available space.
• Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth).
• As the fetus grows and the uterus expands around it, the mother's abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages, and strain in the back muscles.
• Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition).
• A complex interplay of local regulators ( prostaglandins ) and hormones (chiefly estradiol and oxytocin) induces and regulates labor.
• Estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin.
• At the end of the third trimester, oxytocin causes the smooth muscle in the uterine wall to contract.
• Prostaglandins are released into the uterus by the placenta, increasing the contractions.
• A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin.
• As more smooth muscle cells are recruited, the contractions increase in intensity and force.
• There are three stages of labor: the opening up and thinning of the cervix, ending with complete dilation; expulsion, or delivery, of the baby; and the delivery of the placenta.
• An alternative to labor and delivery is the surgical delivery of the baby through a procedure called a Caesarian section.
• Lactation is an aspect of postnatal care unique to mammals.
• In response to suckling by the newborn, as well as changes in estradiol levels after birth, the hypothalamus signals the anterior pituitary to secrete prolactin, which stimulates the mammary glands to produce milk.
• The developing embryo must implant into the wall of the uterus (endometrium) within seven (7) days, or it will deteriorate and die.