Development begins when the haploid sperm and ovum unite in the process of fertilization (or conception), to form a diploid zygote. After ovulation, a layer of cells surrounds the ovum. At least 100 sperm are needed to release an enzyme that creates gaps in this layer. Then one of the sperm may penetrate the cytoplasm of the ovum. This triggers reactions that render the ovum impermeable to additional sperm. In addition, the ovum completes meiosis II. The zygote then divides by mitotic division without an increase in the size of the conceptus. This division is called cleavage. After one or more cleavages, or even in the blastocyst stage, the cells may separate and develop independently, resulting in genetically identical twins. Fraternal twins result when two ovulations occur and are fertilized by different sperm. In the United States about one pregnancy in 90 produces twins, and 70% of twins born are fraternal.
From fertilization until all of the organs are formed, the developing individual is called an embryo. In humans this stage lasts two months. For the rest of the prenatal (before birth) development the individual is called a fetus. Table 9.8 lists the events of this development.
The amnion is a membrane that grows to completely enclose the fetus, which floats in the amniotic fluid to protect it from shock. Surrounding the amnion is another membrane called the chorion. Eventually, as the fetus and amnion grow, the amnion fuses with the chorion, forming the single membrane whose rupture discharges the amniotic fluid, signaling the start of labor. The portion of the chorion that is in contact with the endome-trium is called the placenta. The fetus is connected to the placenta by the umbilical cord. The placenta has fingerlike projections into the endometrium called chorionic villi. These provide surface area for the exchange of nutrients and waste products between the fetus and the mother.
The placenta also has important endocrine functions. Soon after implantation, chorionic gonadotropin (CG) is produced. This maintains the corpus luteum so that progesterone production is continued. Progesterone maintains the thick endometrium. After month three or four of the pregnancy, CG drops sharply and the placenta produces progesterone itself; the corpus luteum is allowed to degrade. The placenta also produces large amounts of estrogens, especially estriol, from testosterone produced by the fetal adrenal glands. The high levels of progesterone and estrogen are thought to contribute to the "morning sickness'' experienced by some pregnant women. The hormone human placental lactogen, placental prolactin, plus maternal hormones prolactin and thyroid hormones prepare the mammary glands for milk production and has other effects similar to growth hormone. The peptide hormone relaxin prepares for birth by causing dilation of the cervix and suppressing oxytocin production by the hypothalamus.
TABLE 9.8 Events of First-Trimester Development in the Human
Time Since Fertilization
7-10 days 10-16 days
Week 3 Week 6 Week 13
First cleavage produces two-cell stage.
The embryo has developed into a solid ball of cells, called a morula.
The ball becomes hollow, and is called a blastocyst. The outer layer is the trophoblast, the smaller inner cells will form the embryo.
The embryo implants itself into the uterine endometrium.
By process of gastrulation the embryo forms first two, then three distinct germ layers: ectoderm ultimately gives rise to the skin, epithelium of mouth, nose, and anus, the nervous system, pituitary gland, and adrenal medullae; mesoderm, produces muscle, cardiovascular system, lymphatic system, kidneys, adrenal cortex, gonads and ducts, bone, linings of body cavities, all connective tissues; endoderm forms the lungs, thymus, thyroid, pancreas, most of the digestive tract including the liver, and stem cells that produce gametes.
Formation of extraembryonic membranes from germ layers and the trophoblast: yolk sac, amnion, allantois, and chorion. The part of the chorion that connects with the endometrium is called the placenta.
Neural tube forms, distinguishing dorsal, ventral, right and left sides.
Fetal heart begins to pump.
End of first trimester, in which organogenesis (organ formation) is begun. External genitalia become distinctive of their sex.
By the end of the first trimester, all the major organ systems have become initiated. It is during this time that the fetus is most sensitive to agents, such as drugs or other toxins, that can cause birth defects (teratogens). Development of sex organs is quite interesting. At the end of the first trimester the embryo possesses gonads and external genitals, but the genitals have not differentiated into male or female. Furthermore, the embryo possesses two sets of ducts. One is destined to become the vas deferens of males, the other the uterus, etc., of females. Which one developes is determined by a single gene on the Y chromosome that stimulates formation of the testes. The testes then produce testosterone, which stimulates development of the male ducts and external genitalia. The testes also produce a hormone that blocks development of the female ducts. The absense of testosterone results in development of female characteristics. Occasionally, hormonal disorders result in individuals who are genetically of one sex developing external genitalia of the other sex. These individuals may grow, apparently normal, to adulthood without knowing that they are genetically of the opposite sex, until they find out that they are unable to have children.
In the second trimester, development continues. In the third trimester most organ systems become ready to perform their normal functions. A number of changes occur in the mother to support the pregnancy: Respiration rate and tidal volume increase, blood volume increases, nutrient and vitamin requirement increases 10 to 30%, causing increased hunger, glomerular filtration in the kidney increases 50% along with urine production; the uterus increases in size; and mammary glands increase in size and begin secretory activity.
After 280 days (40 weeks) the fetus is said to be full term and is ready for birth. Medical technology makes it possible to deliver premature babies safely as early as the twenty-seventh week. The entire process of birth is called parturition.
Estrogen levels increase throughout the pregnancy, making the uterine muscle increasingly sensitive and likely to start contractions. Progesterone has an inhibiting effect, but estrogen production increases faster just before birth. Estrogen also increases the sensitivity of uterine muscle to oxytocin. Oxytocin is produced by the hypothalamus but released by the posterior pituitary. Its release is stimulated by increasing estrogen levels and by distortion of the uterine cervix by the weight of the fetus. Estrogen and oxytocin stimulate production of prostaglandins in the endometrium that also stimulates muscle contractions. Once these factors reach a critical level, parturition begins and is maintained by a positive feedback loop. Parturition occurs in three stages. (1) In the dilation stage the cervix dilates completely, contractions occur at increasing frequency, and the amniochorionic membrane ruptures, releasing the amniotic fluid (the "water break''). This stage usually lasts 8 hours or more. (2) In the expulsion stage, which lasts less than 2 hours, contractions occur at maximum intensity, and the newborn infant, or neonate, is delivered through the vagina. (3) In the placental stage the uterus contracts to a much smaller size, tearing the placenta from the endometrium. Within an hour of delivery the placenta is delivered in what is called the afterbirth.
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