1. Chapter 33: Growth and Development

2. INTRODUCTION Prenatal period: begins with conception and ends at birth
Postnatal period: begins with birth and continues until death
Human developmental biology: study of changes occurring during the cycles of life from conception to death

3. A NEW HUMAN LIFE
Production of sex cells: spermatozoa are produced by spermatogenesis; ova are produced by oogenesis
Meiosis (Figures 33-1 and 33-2)
Special form of cell division that reduces the number of chromosomes in each daughter cell to half of those in the parent cell
Mature ova and sperm contain only 23 chromosomes, half as many as other human cells
Meiotic division: two cell divisions that occur one after another in succession
Meiotic division I and meiotic division II
Both divisions consist of an interphase, prophase, metaphase, anaphase, and telophase

6. A NEW HUMAN LIFE: PRODUCTION OF SEX CELLS
During prophase I of meiosis, crossover occurs in which genetic material is “shuffled”
Daughter cells formed by meiotic division I contain a haploid number of chromosomes
Meiotic division II: essentially the same as mitotic division; reproduces each of the two cells formed by meiotic division I and forms four cells, each with the haploid number of chromosomes

7. A NEW HUMAN LIFE: PRODUCTION OF SEX CELLS (cont.)
Spermatogenesis: process by which primitive male sex cells transform into mature sperm; begins at approximately puberty and continues throughout life (Figure 33-3)
Meiotic division I: one primary spermatocyte forms two secondary spermatocytes, each with 23 chromosomes
Meiotic division II: each of the two secondary spermatocytes forms a total of four spermatids

9. A NEW HUMAN LIFE: PRODUCTION OF SEX CELLS (cont.)
Oogenesis: process by which primitive female sex cells transform into mature ova (Figure 33-4)
Mitosis: oogonia reproduce to form primary oocytes; most primary oocytes begin meiosis and develop to prophase I before birth and stay there until puberty
Once during each menstrual cycle, a few primary oocytes resume meiosis and migrate toward the surface of the ovary; usually only one oocyte matures enough for ovulation, and meiosis again halts at metaphase II
Meiosis resumes only if the head of a sperm cell enters the ovum

11. A NEW HUMAN LIFE (cont’d)
Ovulation and insemination
Ovulation: expulsion of the mature ovum from the mature ovarian follicle, into the abdominopelvic cavity, and then into the uterine (fallopian) tube
Insemination: expulsion of seminal fluid from the male into the female vagina; capacitation renders sperm able to fertilize; sperm travel through the cervix and uterus and into the uterine (fallopian) tubes

12. A NEW HUMAN LIFE (cont.)
Fertilization: also known as conception (Figure 33-5)
Most often occurs in the outer third of the uterine tube
Thermotaxis: sperm are attracted to warmth of uterine tubes
Chemotaxis: ovum attracts and “traps” sperm with special molecules
Acrosome reaction permits the release of enzymes that burrow through the outer layers of ovum (zona pellucida and corona radiata)
When one spermatozoon enters the ovum, the ovum stops collecting sperm on its surface
The sperm releases its nuclear chromosomes into the ovum; proteins and RNA from the sperm enter the ovum to assist with early development
23 chromosomes from the sperm head and 23 chromosomes in the ovum make up a total of 46 chromosomes
Zygote: fertilized ovum; genetically complete

14. PRENATAL PERIOD
Begins with conception and continues until the birth of a child
Cleavage and implantation: once zygote is formed, it immediately begins to divide (Figure 33-6)
Morula: solid mass of cells formed from zygote; takes approximately 3 days; continues to divide (Figure 33-7)
Blastocyst: by the time the developing embryo reaches the uterus, it has formed a hollow ball of cells, which implants into the uterine lining (Figure 33-8)
Approximately 10 days pass from fertilization until implantation in the uterine lining; ovum has a store of nutrients that supports this embryonic development until implantation has occurred

18. PRENATAL PERIOD: CLEAVAGE AND IMPLANTATION
Blastocyst has an outer layer of cells and an inner cell mass
Trophoblast: outer wall of the blastocyst
Inner cell mass: as blastocyst develops, yolk sac and amniotic cavity are formed (Figure 33-9)
In human beings, yolk sac’s functions are largely nonnutritive
Amniotic cavity becomes a fluid-filled, shock-absorbing sac (bag of waters) in which the embryo floats during development (Figure 33-10)
Chorion develops from trophoblast to become an important fetal membrane in the placenta

21. PRENATAL PERIOD: CLEAVAGE AND IMPLANTATION (cont.)
Placenta (Figure 33-11)
Anchors fetus to the uterus and provides a “bridge” for the exchange of nutrients and waste products between mother and baby
Also serves as an excretory, respiratory, and endocrine organ
Placental tissue normally separates maternal and fetal blood supplies
Has important endocrine functions; secretes large amounts of human chorionic gonadotropin, which stimulate the corpus luteum to continue its secretion of estrogen and progesterone (Figure 33-12)

24. PRENATAL PERIOD Periods of development (Figures 33-13 to 33-15)
Gestation period: approximately 39 weeks; divided into three 3-month segments called trimesters
Embryonic phase extends from fertilization until the end of week 8 of gestation
Fetal phase: weeks 8 to 39

28. PRENATAL PERIOD (cont.)
Stem cells
Stem cell: unspecialized cell that produces lines of specialized cells; has a certain level of potency (range of types it can produce)
Totipotent stem cell: can produce any type of cell; found in zygote
Pluripotent stem cell: embryonic stem cell that can produce a broad range of cell types; found in embryonic germ layers
Multipotent stem cell: adult stem cell found in some tissues can produce a few cell types and thus maintain functional populations of specialized cells

29. PRENATAL PERIOD (cont.)
Formation of the primary germ layers
Three layers of developmental cells arise early in the first trimester of pregnancy
Cells of embryonic disk differentiate and form each of the three primary germ layers
Each of the three primary germ layers gives rise to specific organs and systems of the body (Figure 33-16)
Endoderm: inside layer
Ectoderm: outside layer
Mesoderm: middle layer

31. PRENATAL PERIOD (cont.)
Histogenesis and organogenesis (Figure 33-16)
Histogenesis: process by which primary germ layers develop into different kinds of tissues
Organogenesis: how tissues arrange themselves into organs
Differentiation and development of the reproductive systems are examples
Reproductive tract (Figure 33-17)
Gonads attach to mesonephric (wolffian) ducts, which become the male reproductive tract
Gonads (unattached) and paramesonephric (müllerian) ducts develop into the female reproductive tract
External genitals (Figure 33-18)
In the male, the genital tubercle eventually becomes the glans of the penis and the folds become the penis shaft and scrotum
In the female, the genital tubercle becomes the clitoris and the folds become the labia

34. BIRTH, OR PARTURITION
Transition between prenatal and postnatal periods of life
Cortisol triggers labor by reducing human chorionic gonadotropin and thus also progesterone, removing the “brake” on oxytocin, which stimulates the uterine muscles to produce labor contractions (amplified by a positive feedback effect); prostaglandins enhance oxytocin’s effects

35. BIRTH, OR PARTURITION (cont.)
Stages of labor (Figure 33-20)
Stage one: period from onset of uterine contractions until cervical dilation is complete
Stage two: period from maximal cervical dilation until the baby exits through the vagina
Stage three: process of expulsion of the placenta through the vagina
Multiple births: birth of two or more infants from the same pregnancy; twins are most common (Figure 33-21)
Identical twins result from the splitting of embryonic tissue from the same zygote early in development
Fraternal twins result from the fertilization of two different ova by two different spermatozoa

38. POSTNATAL PERIOD
Begins at birth and continues until death; commonly divided into a number of periods (Figure 33-22)
Infancy begins at birth and lasts until approximately 18 months
Neonatal period: first 4 weeks of infancy; dramatic changes occur at a rapid rate (Figure 33-23)
Changes allow the infant to become self-supporting, especially the respiratory and cardiovascular systems (Figure 33-24)
Apgar score assesses general condition of a newborn infant

42. POSTNATAL PERIOD (cont.)
Childhood extends from end of infancy to sexual maturity, or puberty
Early childhood: growth continues at a rapid pace but month-to-month gains are less consistent
By age 6 years, child looks more like a preadolescent than an infant or toddler
Nervous and muscular systems develop rapidly during the middle years of childhood
Deciduous teeth are lost during childhood, beginning at approximately 6 years of age
Permanent teeth have erupted by age 14 years, except for the third molars (wisdom teeth)

43. POSTNATAL PERIOD (cont.)
Adolescence and adulthood
Adolescence is considered the teenage years (ages 13 to 19); marked by rapid and intense physical growth, resulting in sexual maturity
Puberty: stage of adolescence during which a person becomes sexually mature
Changes triggered by increases in reproductive hormones (Figure 33-25)
Primary sexual characteristics: maturation of gonads and reproductive tract
Secondary sexual characteristics: fat and hair distribution, skeletal changes, etc. (Figure 33-26)
Adulthood: characterized by maintenance of existing body tissues

46. POSTNATAL PERIOD (cont.)
Older adulthood and senescence
As a person grows older, a gradual decline occurs in every major organ system in the body
Gerontologists theorize a number of different aging mechanisms, all of which may be involved in the processes of aging
Limit on cell reproduction
Environmental factors
Viruses
Aging genes
Degeneration of mitochondria: perhaps associated with progressive damage by oxygen free radicals (Figure 33-27)

48. EFFECTS OF AGING
Common degenerative changes frequently characterize senescence (Figure 33-28)
Skeletal system (Figure 33-29)
Bone mineral density decreases, thus changing texture, degree of calcification, and shape of bones
Lipping occurs, which can limit range of motion
Decreased bone size and density lead to increased risk of fracture
Decreased bone mineral density can be avoided (at least partly) by exercise and adequate calcium intake

51. EFFECTS OF AGING (cont.)
Muscular system (Figure 33-30)
Muscle mass decreases to about 90% by age 50 years and 50% by age 80 years
The number of muscle fibers decreases as we age but can be offset by an increase in muscle fiber size through exercise
Ratio of “fast” to “slow” functioning in muscle fibers decreases, slowing the function of muscle organs

53. EFFECTS OF AGING (cont.)
Integumentary system (skin)
Skin becomes dry, thin, and inelastic
Pigmentation changes and thinning hair are common problems associated with aging
Urinary system
Number of nephron units in the kidney decreases by almost 50% between the ages of 30 and 75 years
Decreased blood flow through kidneys reduces overall function and excretory capacity
Diminished muscle tone in bladder results in decreased capacity and inability to empty, or void, completely

54. EFFECTS OF AGING (cont.)
Respiratory system
Costal cartilages become calcified
Respiratory efficiency decreases
Decreased strength of respiratory muscles
Cardiovascular system
Degenerative heart and blood vessel disease: one of the most common and serious effects of aging
Atherosclerosis: buildup of fatty deposits on blood vessel walls narrows the passageway for blood
Arteriosclerosis: “hardening” of the arteries
Hypertension: high blood pressure

55. EFFECTS OF AGING (cont.)
Special senses
Sense organs: gradual decline in performance and capacity with aging
Presbyopia: farsightedness caused by hardening of lens
Cataract: cloudy lens, which impairs vision
Glaucoma: increased pressure within the eyeball; if left untreated, often results in blindness
Decreased hearing
Decreased taste

56. EFFECTS OF AGING (cont.)
Reproductive systems
Mechanism of sexual response may change
Fertility decreases
In females, menopause occurs between ages 45 and 60 years
Benefits of aging: mostly improved brain functions: less anxious or fearful, less resistant to happiness, better interpretation of visual information, improved problem solving and wisdom

57. CAUSES OF DEATH
In developed countries such as the United States, heart disease, cancer, and stroke (cerebrovascular accident) are among the leading causes of death (Figure 33-31)
In developing countries, heart disease and stroke are also leading causes of death, along with infectious diseases such as HIV/AIDS, diarrheal disorders, and malaria