Fertilization and Preembryonic Development

Spermatozoa cannot fertilize an egg until they complete the process of [1], in which the plasma membrane of the sperm head becomes more fragile. When a spermatozoon contacts an egg, it exhibits a/an [2] reaction in which it releases enzymes that penetrate the egg surface. One of these enzymes is [3], which loosens up the granulosa cells around an egg, and another is the trypsin-like [4]. As soon as a sperm penetrates the oocyte membrane itself, the oocyte membrane depolarizes; this effect, called the [5], inhibits any more sperm from attaching to the egg surface. It is followed by a process called the [6] in which the cortical granules of the egg undergo exocytosis and their secretion produces a [7] impenetrable by any additional sperm. The purpose of these mechanisms is to prevent [8], the fertilization of an egg by two or more sperm.

A fertilized oocyte completes meiosis II, the sperm and egg pronuclei release their chromosomes, and the now-diploid fertilized egg is called a/an [9]. Starting about 30 hours later, the egg undergoes a series of mitotic divisions called [10], producing a multicellular berrylike body called the [11]. In about [12] hours, the conceptus reaches the uterus. It remains unattached for another 4 to 5 days, during which time the zona pellucida dissolves and the conceptus becomes a/an [13]–a hollow sphere with an inner cell mass, the [14], which will become the embryo, and an outer cell mass, the [15], which will soon attach the conceptus to the uterine wall. This process of attachment, called [16], begins with that outer cell mass growing rootlike extensions that penetrate into the endometrium until the conceptus is entirely buried in it. During this process, the inner cell mass differentiates into three primary germ layers, the [17], [18], and [19]. This is completed by the end of the second week, and the individual is then considered to be a/an [20].

 1._________________________11._________________________

 2._________________________12._________________________

 3._________________________13._________________________

 4._________________________14._________________________

 5._________________________15._________________________

 6._________________________16._________________________

 7._________________________17._________________________

 8._________________________18._________________________

 9._________________________19._________________________

10._________________________20._________________________

Embryonic and Fetal Development

For the first 8 weeks of development, a conceptus is nourished mainly by the [21] mode of nutrition, in which it digests [22] cells of the endometrium. Starting at 2 weeks and dominating by 8 weeks, [23] nutrition gradually takes over and the conceptus is nourished by diffusion of nutrients from the mother's bloodstream. Placentation begins as outgrowths of the syncytiotrophoblast called [24] become surrounded by pools of maternal blood that eventually merge and form a blood-filled [25]. 23 nutrition becomes more and more significant as the membrane between the maternal and fetal blood becomes thinner and its permeability, or [26], increases. In addition to the placenta, four embryonic membranes develop: the [27], a transparent sac filled with fluid in which the fetus floats; the [28], where the first blood and germ cells develop; the [29], around which the umbilical cord develops; and the [30], which forms part of the placenta and secretes the major hormones of pregnancy.

From 2 through 8 weeks, the conceptus develops all of its [31], and when these are present the individual is considered a fetus. The circulatory pathway shows substantial differences before and after birth. Blood bypasses the liver by way of a shunt called the [32] and bypasses the lungs by way of the [33] between the right and left atria of the heart and the [34] between the pulmonary trunk and aorta. After birth, these shunts close and the lungs and liver take over certain functions previously served by the placenta. The appendages begin as small [35], and later the fingers and toes appear as [36], which separate in the second month. In the fifth month, the skin is covered with a greasy cheeselike secretion called [37] and exhibits fine, downy hair called [38]. The intestines accumulate fetal feces called [39]. In the seventh month, the fetus turns into a head-down [40] position.

21._________________________31._________________________

22._________________________32._________________________

23._________________________33._________________________

24._________________________34._________________________

25._________________________35._________________________

26._________________________36._________________________

27._________________________37._________________________

28._________________________38._________________________

29._________________________39._________________________

30._________________________40._________________________

The Neonate

A baby is considered a neonate ("newborn") for the first [41] weeks postpartum. In its first 6 to 8 hours, it goes through a [42] in which the heart rate, respiratory rate, and body temperature fluctuate and then stabilize. The first few breaths are difficult, as the neonate must inflate the [43] of its lungs. The cardiovascular shunts close over the ensuing months, with the foramen ovale leaving a depression in the atrial wall called the [44]. Infants weighing less than [45] are considered to be premature. The most common cause of death in premature infants is hyaline membrane disease, also called [46]; it results from an insufficiency of a lung secretion called [47]. Premature infants also have difficulty with thermoregulation because of the immaturity of the [48], and they show edema, bleeding, and jaundice, all attributable to the immaturity of the [49].

Any abnormality present at birth is called a/an [50] anomaly. These may result from exposure to infectious diseases, to agents called [51] that cause anatomical deformities, to agents called [52] that alter DNA or chromosome structure, or to [53], the failure of two homologous chromosomes to separate at anaphase I of meiosis. This failure of chromosomal separation causes [54], the absence of a chromosome or the presence of an extra one. For example, it can cause the [55] syndrome in which a girl has three X chromosomes (XXX), [56] syndrome in which a girl has only one X chromosome, or [57] syndrome in which a male has XXY. The most common autosomal abnormality is [58] syndrome in which there are three copies of chromosome 21. The risk of this syndrome increases in proportion to the age of the mother.

41._________________________50._________________________

42._________________________51._________________________

43._________________________52._________________________

44._________________________53._________________________

45._________________________54._________________________

46._________________________55._________________________

47._________________________56._________________________

48._________________________57._________________________

49._________________________58._________________________

Aging and Senescence

In and beyond middle age, the hair often becomes thinner because of the reduced rate of [59] and the skin and hair become drier because of atrophy of the [60]. Many people exhibit [61], patches of dilated capillaries visible through the skin of the face. In proportion to a person's lifetime exposure to sunlight, the skin exhibits additional degenerative changes called [62]. Senescence of the skin accelerates senescence of the skeletal system because of the declining synthesis of [63]. The loss of bone mass common in aging is called [64] or, when severe, [65]. The most common joint disorder of old age, experienced by nearly everyone to some degree, is [66]. The muscular system also atrophies in old age, perhaps partly as a matter of [67] atrophy stemming from the death of motor neurons. Atrophy of the nervous system begins around age 35. The most common neurodegenerative disease of old age is [68].

Inadequate exercise and nutrition commonly lead to [69] in old age, which in turn reduces the oxygen transport capacity of the blood and contributes to the senescence of organs everywhere. Another contributing factor in 69 is atrophy of the stomach, resulting in less secretion of [70] and therefore less absorption of vitamin B₁₂. Blood pressure rises with age because the blood vessels become stiffened by [71]. A decline in the amount of red bone marrow and lymphatic tissue results in a decline in the [72] and therefore less resistance to infection and cancer. The COPDs, or [73] diseases, are more common in old age as a lifetime of smoking catches up with many people. The kidneys atrophy markedly, showing a 30% to 40% decline in the number of [74] by age 85. Lung and kidney degeneration severely impairs the homeostatic control of water, electrolytes, and [75]. Older men often experience greater difficulty emptying the bladder because of hyperplasia of the [76], whereas older women, especially those who have had multiple children, are increasingly subject to [77] because of weakening of the pelvic muscles. Tooth loss in old age frequently results from [78], inflammation of the tissues around the teeth. Heartburn is more common because of weakness of the [79] sphincter between the stomach and esophagus.

The average age at death, or [80], has increased significantly over the past century, but more as a result of reduced infant and child mortality than as a result of increased [81], the maximum attainable age. There are many hypotheses on the causes of senescence. One is that it results from the "programmed cell death," or [82], of cells that have completed their function or reached a limit in the number of times they can divide. Cells lose a little DNA from the [83] at the end of each chromosome every time they divide, and may become incapable of dividing anymore when the 83 is too short. Some of the degenerative changes of age may result from increased [84] between protein molecules, causing stiffening of the joints, loss of elasticity of the skin and lungs, and so forth. Highly reactive chemical particles called [85] may also contribute to degenerative diseases as [86], chemicals that protect cells against them, become less abundant in older cells. [87] diseases, in which the immune system mistakes the body's own tissues for foreign antigens and attacks them, also become more common in old age. This is the cause of [88] arthritis, for example. Death was once defined by the absence of a spontaneous heartbeat and respiration, but is now defined in terms of [89]–a complete lack of brain waves for a period of time defined by various state laws.

59._________________________75._________________________

60._________________________76._________________________

61._________________________77._________________________

62._________________________78._________________________

63._________________________79._________________________

64._________________________80._________________________

65._________________________81._________________________

66._________________________82._________________________

67._________________________83._________________________

68._________________________84._________________________

69._________________________85._________________________

70._________________________86._________________________

71._________________________87._________________________

72._________________________88._________________________

73._________________________89._________________________

74._________________________

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.