eporters gathered in Aries, France, on March 1, 1995, to
R celebrate a very special birthday. Jeanne Calment, a woman with the distinction (at the time) of being listed in The Guinness Book of Records as "oldest living human," was turning 120 years old. Although she was able to ride her bicycle daily up until her 100th birthday, after that Jeanne began to feel some physical effects of her many years. Her hearing diminished, and she lost her vision to cataracts. She hadn't been able to walk since breaking her hip in her 115th year. But this "oldest living human" still had a sharp wit. When reporters asked her what type of future she anticipated, she answered, "A very short one."
People over the age of 85 are the largest growing population group in many nations. Medical researchers are finding that many of these "oldest old" people are extraordinarily healthy. The reason—the most common disorders strike before this age, essentially "weeding out" much of the population. Heart disease and cancers tend to occur between the fifties and the eighties, and people who have developed Alzheimer disease or other neurological disorders usually succumb by their eighties.
The lucky "oldest old," researchers suspect, probably inherit combinations of genes that maintain their health as they age and enable them to fight off free radicals, repair DNA adequately, prevent cholesterol buildup in arteries, and resist certain cancers. But the "oldest" old may have more in common than lucky genes. Studies show that they handle stress well; live a moderate lifestyle; exercise regularly; and are well educated. Jeanne Calment explained her longevity: "I took pleasure when I could. I acted clearly and morally and without regret. I'm very lucky." She died in 1997, at age 122.
Humans grow, develop, and age. Growth is an increase in size. It entails increase in cell numbers as a result of mitosis, followed by enlargement of the newly formed cells and of the body.
Development, which includes growth, is the continuous process by which an individual changes from one life phase to another (fig. 23.1). These life phases include a prenatal period (pre-na'tal pe're-od), which begins with the fertilization of an egg cell and ends at birth, and a postnatal period (post-na'tal pe're-od), which begins at birth and ends with death.
The contrast between a human embryo at 28 days (a) and a six-month-old fetus (b) shows evidence of profound changes in development.
The prenatal period of development usually lasts for thirty-eight weeks from conception and can be divided into a period of cleavage, an embryonic stage, and a fetal stage.
Conception occurs when the genetic packages of sperm and egg merge, forming a zygote (zi'got). Thirty hours later, the zygote undergoes mitosis, giving rise to two new cells (fig. 23.2). These cells, in turn, divide to form four cells, which then divide to form eight cells, and so forth. These divisions take place rapidly with little time for the cells to grow. Thus, with each subsequent division, the resulting cells are smaller and smaller. This distribution of the zygote's cytoplasm into progressively smaller cells is called cleavage (klev'ij), and the cells produced in this way are called blastomeres. The ball of cells that results from these initial cell divisions is also called a cleavage embryo. Clinical Application 23.1 describes how genetic tests are conducted on blastomeres.
A couple expecting a child can estimate the approximate time of conception (fertilization) by adding 14 days to the date of the onset of the last menstrual period. They can predict the time of birth by adding 266 days to the fertilization date. Most babies are born within 10 to 15 days of this calculated time.
Obstetricians estimate the date of conception by scanning the embryo with ultrasound and comparing the crown-to-rump length to known values that are the average for each day of gestation. This approach is inaccurate if an embryo is smaller or larger than usual due to a medical problem.
(a) A light micrograph of a human egg cell surrounded by follicular cells and sperm cells (250x). (b) Two-cell stage of development (750x).
The tiny mass of cells moves through the uterine tube to the uterine cavity, aided by the beating of cilia of the tubular epithelium and by weak peristaltic contractions of smooth muscles in the tubular wall. Secretions from the epithelial lining bring nutrients to the developing organism.
The trip to the uterus takes about three days, and by then, the structure consists of a solid ball, called a morula, of about sixteen cells (fig. 23.3). The morula remains free within the uterine cavity for about three days. Cell division continues, and the solid ball of cells gradually hollows out. During this stage, the zona pellucida of the original egg cell degenerates, and the structure, now hollow and called a blastocyst, drops into one of the tubules in the endometrium. By the end of the first week of development, the blastocyst superficially implants in the endometrium (fig. 23.4).
Within the blastocyst, cells in one region group to form an inner cell mass that eventually gives rise to the embryo proper (em'bre-o prop'er)—the body of the developing offspring. The cells that form the wall of the blastocyst make up the trophoblast, which develops into structures that assist the embryo.
Sometimes two ovarian follicles release egg cells simultaneously, and if both are fertilized, the resulting zygotes can develop into fraternal (dizygotic) twins. Such twins are no more alike genetically than any brothers or sisters. Twins may also develop from a single fertilized egg (monozygotic twins). This may happen if two inner cell masses form within a blastocyst and each produces an embryo. Twins of this type usually share a single placenta, and they are identical genetically. Thus, they are always the same sex and are very similar in appearance.
About the sixth day, the blastocyst begins to attach to the uterine lining, aided by its secretion of proteolytic enzymes that digest a portion of the endometrium (fig. 23.5). The blastocyst sinks slowly into the resulting depression, becoming completely buried in the uterine lining. At the same time, the uterine lining is stimulated to thicken below the implanting blastocyst, and cells of the trophoblast begin to produce tiny, fingerlike processes (microvilli) that grow into the endometrium. This process of the blastocyst nestling into the uterine lining is called implantation (im-plan-ta'shun) and it begins near the end of the first week and is completed during the second week of development (fig. 23.6).
The trophoblast secretes the hormone hCG, which maintains the corpus luteum during the early stages of pregnancy and keeps the immune system from rejecting the blastocyst. This hormone also stimulates synthesis of other hormones from the developing placenta. Light micrograph of a human morula (500x).
Stages in early human development.
Stages in early human development.
Preimplantation Genetic Diagnosis
Six-year-old Molly Nash would probably have died within a year or two of Fanconi anemia had she not received a very special gift from her baby brother Adam — his umbilical cord stem cells. Adam was not only free of the gene that causes the anemia, but his cell surfaces matched those of his sister, making a transplant very likely to succeed. But the parents didn't have to wait until Adam's birth in August, 2000, to know that his cells could save Molly—they knew when he was a mere 8-celled cleavage embryo (fig. 23A).
When the Nashs learned that time was running out for Molly because they could not find a compatible bone marrow donor, they turned to preimplantation genetic diagnosis (PGD). Following in vitro fertilization, described in Clinical Application 22.3, researchers at the Reproductive Genetics Institute at Illinois Masonic Medical Center removed a
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