1. Development, Pregnancy, and Heredity

2. Overview of the Prenatal Period
Begins with fertilization
Secondary oocyte and sperm unite
Ends 38 weeks later with birth
Composed of three shorter periods
Pre-embryonic period
Embryonic period
Fetal period

3. Prenatal period (continued)
Pre-embryonic period
First 2 weeks after fertilization
Zygote, cell produced by fertilization, becomes spherical multicellular structure blastocyst
Ends when blastocyst implants in uterine lining

4. Prenatal period (continued)
Embryonic period
3rd through 8th weeks of development
Rudimentary versions of major organs appear
Now called an embryo
Fetal period
Remaining 30 weeks prior to birth
Organism is now called a fetus
Continues to grow and increase in complexity
Embryogenesis, developmental period of pre-embryonic and embryonic periods

5. Figure 29.1

6. Fertilization Fertilization Two gametes fuse to form new diploid cell
Contains genetic material derived from both parents
Restores diploid number of chromosomes
Determines the sex of the organism
Initiates cleavage
Occurs in widest part of uterine tube, ampulla
Oocyte viable for 24 hours following ovulation
Sperm remain viable for 3 to 4 days

7. Capacitation
Physiological conditioning undergone by sperm to become capable of fertilizing the secondary oocyte
Occurs in female reproductive tract
Glycoprotein coat and some proteins
Removed from sperm plasma membrane
Lasts several hours

8. Sperm Millions deposited in vagina during intercourse
Few hundred with a chance at fertilization
Attracted to oocyte by chemicals it releases
Bound by progesterone released by cumulus cells around oocyte
Causes influx of Ca2+
Necessary for capacitation, acrosome reaction, fertilization
Attempt fertilization when reaching secondary oocyte
Only one sperm able to fertilize

9. Corona radiata penetration
First phase of fertilization
Sperm reaching secondary oocyte
Initially prevented entry by corona radiata and zona pellucida

10. Zona pellucida penetration
Acrosome reaction
Release of digestive enzymes from acrosomes
Allows sperm to penetrate zona pellucida
After penetration of secondary oocyte
Immediate hardening of zona pellucida
Prevents other sperm from entering this layer
Ensures only one sperm fertilizes the oocyte
Polyspermy, if two sperm enter simultaneously
Immediately fatal with 23 triplets of chromosomes

11. Fusion of sperm and oocyte plasma membranes and fusion of sperm and ovum pronuclei
Contact of sperm and oocyte plasma membranes
Immediately fuse
Only sperm nucleus enters oocyte
Secondary oocyte completing second meiotic division
Forms an ovum
Nucleus of sperm and ovum, pronuclei
Each with haploid number of chromosomes
Fuse to become diploid nucleus
Zygote, the single diploid cell formed

12. Cleavage Cleavage Series of mitotic divisions of zygote
Increases cell number but not overall size of structure
Size only increases after implantation in uterine wall
Before 8-cell stage
Cells not tightly bound together
Become tightly compacted after third cleavage divisions
Compaction, process by which contact between cells is increased to the max

13. Cleavage (continued) Morula, 16-cell stage
Cells of morula continue to divide
Develops fluid-filled cavity, blastocyst cavity
At this stage, pre-embryo is a blastocyst
Trophoblast, outer ring of cells surrounding cavity
Will form the chorion
Embryoblast, packed cells within one side of blastocyst
Will form embryo proper
Cells pluripotent, able to develop into any tissue

14. Cleavage in the Pre-Embryo

16. Implantation Implantation steps
Blastocyst enters lumen of uterus by end of first week
Zona pellucida around blastocyst breaks down
Blastocyst burrows into the endometrium, implantation
Begins by about day 7
Trophoblast subdividing into 2 layers
Cytotrophoblast, inner layer
Syncytiotrophoblast, outer layer
By day 9, blastocyst completely burrowed into uterine wall
Contacts nutrients in uterine glands

17. Human chorionic gonadotropin (hCG)
Produced by syncytiotrophoblast
Signals reproductive system that implantation occurred
Promotes maintenance of corpus luteum
Produces estrogen and progesterone to build uterine lining
Detected in urine by end of 2nd week
Basis of most pregnancy tests
Levels high for first 3 months of pregnancy
Then decline, causing corpus luteum degeneration
By then placenta producing own estrogen to maintain pregnancy

19. Hormone Levels During Pregnancy
Figure 29.6

20. Formation of the Bilaminar Germinal Disc and Extraembryonic Membranes
Changes to embryoblast
By day 8
Cells of embryoblast starting to form two layers
Hypoblast layer adjacent to blastocyst cavity
Epiblast layer adjacent to amniotic cavity
Together form flat disc, bilaminar germinal disc

21. Extraembryonic membranes
Formed by bilaminar germinal disc and trophoblast
Mediate between them and environment
Protects embryo
Assist in nutrition, gas exchange, and removal of waste
Yolk sac
1st extraembryonic membrane to develop
Continuous with hypoblast layer
Does not store yolk (as it does in birds and reptiles)
Important site for early blood cell and blood vessel formation

22. Extraembryonic membranes (continued)
Amnion
Membrane continuous with epiblast layer
Eventually encloses entire embryo in fluid-filled sac, amniotic cavity
Protects membrane from drying out
Specialized to secrete amniotic fluid bathing embryo

23. Extraembryonic membranes (continued)
Chorion
Outermost extraembryonic membrane
Formed from cytotrophoblast cells and syncytiotrophoblast
Cells blend with functional layer of endometrium
Eventually form placenta
Site of nutrient exchange between embryo and mother

24. Development of the Placenta
Highly vascular structure
Functions:
Site of exchange of nutrients, wastes, and respiratory gases between maternal and fetal blood
Transmits maternal antibodies to developing embryo or fetus
Produces estrogen and progesterone
Maintains and builds the uterine lining
Begins to form during 2nd week

25. Placental components Fetal portion developing from chorion
Maternal portion from functional layer of uterus
Connecting stalk
Connects early embryo to placenta
Eventually contains umbilical arteries and veins
Precursor to future umbilical cord

26. Placental components (continued)
Chorionic villi
Fingerlike structures formed from chorion
Contain branches of umbilical vessels
Gas and nutrient exchange
Functional layer of endometrium with maternal blood vessels
Maternal blood does not mix with fetal blood
Bloodstreams so close that nutrients and gases mix
O2 diffusing from maternal blood to fetal blood
CO2 diffusing from fetal to maternal blood

28. Placental characteristics
Most growth during fetal period
Adheres firmly to wall of uterus
Expelled from uterus after the baby is born
Afterbirth
Selectively permeable structure
E.g., respiratory gases passing freely
Microorganisms and certain maternal hormones prevented

29. Placental characteristics (continued)
Some harmful substances able to cross
E.g., viruses, bacteria, drugs, alcohol, toxins
May cause birth defects or death
Dose and timing affecting fetus susceptibility
Pregnant women urged to quit smoking, refrain from taking drugs and drinking alcohol

30. Gastrulation Critical period of development; occurs during third week
Epiblast forms three primary germ layers
Cells from which all body tissues develop
Ectoderm, mesoderm, endoderm
Three-layered structure called an embryo

31. Gastrulation and Formation of the Primary Germ Layers
Gastrulation (continued)
Begins with formation of primitive streak
Thin depression on surface of epiblast
Primitive node
Cephalic end of streak
Consists of elevated area surrounding small primitive pit
Invagination
Cells detaching from epiblast layer
Migrate through primitive streak between epiblast and hypoblast layer

32. Gastrulation (continued)
Epiblast, source of 3 primary germ layers
All body tissues and organs derived from these layers
Endoderm
Epiblast cells that displace hypoblast
Mesoderm
New primary germ layer of cells formed by epiblast cells
Ectoderm
Cells remaining in epiblast

33. The Role of the Primitive Streak In Gastrulation: Early Week 3, Superolateral View
Figure 29.8a

35. Differentiation of ectoderm
On external surface of cylindrical embryo
Responsible for forming nervous system tissue
Neurulation
Forms
Epidermis, sense organs, pituitary gland, adrenal medulla, enamel of teeth, lens of eye

36. Differentiation of Mesoderm
Figure 29.12

37. Differentiation of mesoderm Five categories of mesoderm
Notochord
Formed by tightly packed midline group of mesodermal cells
Basis for central body axis and axial skeleton
Induces formation of neural tube
Paraxial mesoderm
Found on both sides of neural tube
Forms somites, blocklike masses
Forms axial skeleton, muscle, and cartilage, dermis, and connective tissues

38. Differentiation of mesoderm (continued)
Intermediate mesoderm
Lateral to paraxial mesoderm
Forms most of kidneys, ureters, and reproductive system
Lateral plate mesoderm
Most lateral layers of mesoderm
Forms spleen, adrenal cortex, and cardiovascular system
Serous membranes and connective tissue of limbs
Head mesenchyme
Forms connective tissues and musculature of face

39. Differentiation of endoderm
Becomes innermost tissue after transverse folding
Forms
Linings of GI, respiratory, urinary, and reproductive tracts
Tympanic cavity, auditory tube
Liver, gallbladder, pancreas, palatine tonsils, thyroid and parathyroid glands, thymus

40. Organogenesis Organogenesis Organ development
Begins once layers have formed and folding complete
By week 8
Upper/lower limbs have adult shape and most organ systems have rudimentary form
Particularly sensitive to teratogens during this time
Substances causing birth defects or death
Include: alcohol, tobacco, drugs, and some viruses

41. Organogenesis (continued)
“Peak development” periods at different times
Teratogens most dangerous during peak development of particular system

42. Fetal Period Fetal period From beginning of 3rd month to birth
Maturation of tissues and organs
Rapid growth of body
Weight increase most striking during last two months

43. The Course of Pregnancy
Pregnancy divided into trimesters
First trimester
First 3 months of pregnancy
Zygote becoming embryo and then early fetus
Second trimester
Months 4 to 6 of pregnancy
Growth of fetus and expansion of maternal tissues
Third trimester
Months 7 to 9 of pregnancy
Fetus growing most rapidly
Mother’s body preparing for labor and delivery

44. Hormonal Changes Estrogen and progesterone
Produced by corpus luteum during first trimester
Mostly produced by placenta in 2nd, 3rd trimesters
High levels suppressing FSH and LH secretion
Ovarian cycle and follicular development arrested
Facilitate
Uterine enlargement, mammary gland enlargement, and fetal growth
Faster-growing nails, fuller hair
Relaxation of ligamentous joints
Uterus functional layer growth due to progesterone

45. Corticotropin-releasing hormone (CRH)
Relaxin
Secreted by corpus luteum and placenta
Promotes blood vessel growth in uterus
Corticotropin-releasing hormone (CRH)
Secreted from placenta in large amounts
Role in length of pregnancy and timing of childbirth
Responsible for aldosterone rise in mother
Promotes fluid retention and edema

46. Human chorionic thyrotropin (HCT)
Secreted by placenta
Stimulates the thyroid gland
Increases woman’s metabolic rate
Human placental lactogen (HPL)
Secreted from placenta
Affects how pregnant woman metabolizes certain nutrients
Mother metabolizing more fatty acids instead of glucose
Inhibits effects of insulin; more glucose available for fetus

47. Prolactin
Increased levels (10x) produced by anterior pituitary
Ensures lactation occurs after giving birth
Oxytocin
Increased levels produced by hypothalamus
Involved in uterine contractions
Involved in milk expulsion from mammary glands
Increase in second and third trimesters
In response to rising estrogen levels

48. Uterine and Mammary Gland Changes
Uterine expansion
Begins to enlarge once implantation occurs
By 12 weeks
Uterus just superior to pubic symphysis
Impinges on space of urinary bladder
Causes more frequent urination
Especially during first and third trimesters
Most of enlargement due to
Muscle hypertrophy, hyperplasia, placental growth, and amniotic fluid

49. Uterine and Mammary Gland Changes
Mammary glands
Tender during first trimester
Due to increasing levels of estrogen and progesterone
Melanocyte-stimulating hormone
Secreted by placenta
Darkening of areola and nipples
Darkens linea alba, now linea nigra
Growth of mammary glandular tissue

50. Initiation of True Labor
True labor: uterine contractions that increase in intensity and regularity, result in changes to the cervix
Mother’s hypothalamus secrets increasing levels of oxytocin
Fetus’s hypothalamus also secreting oxytocin
Both sources stimulate placenta to secrete prostaglandins
Fatty acids and hormonelike substances
Stimulate uterine muscle contraction
Soften and dilate the cervix
Combined maternal and fetal oxytocin initiates true labor

51. Characteristics of true labor
Increase in frequency over time
Increase in intensity as labor progresses
Pain radiating from upper abdomen to lower back
Pain not going away in response to movement
Contractions facilitate cervical dilation and expulsion of fetus/placenta

52. False labor: uterine contractions not resulting in 3 stages of labor
Braxton-Hicks contractions
Irregularly spaced and do not become more frequent
Relatively weak and do not increase in intensity
Pain limited to lower abdomen and pelvic region
Pain sometimes stops with movement
Do not lead to cervical changes

53. Stages of True Labor Dilation stage 1st stage of labor
Begins with onset of regular uterine contractions
Ends when cervix is effaced (thinned) and dilated to 10 cm
Longest of 3 stages
Greatest variability
Nulliparous women (who have not given birth) experience longer dilation stage, 8 to 24 hours
Parous women (who have given birth) may be in this stage for 4 to 12 hours

54. Dilation stage (continued)
Starts with regularly spaced uterine contractions
Increases in intensity and frequency
Baby’s head against cervix causes effacing and dilation
Rupture of amniotic sac and release of amniotic fluid
“Water breaking”
Manually ruptured if necessary

55. Stages of True Labor and Childbirth: Dilation Stage
Figure 29.15a,b

56. Expulsion stage Begins with complete dilation of cervix
Ends with expulsion of fetus
Usually 30 min to several hours
Nulliparous women with longer stage
Uterine contractions help push fetus through vagina
Facilitated if woman “bears down”
Uses Valsalva maneuver to increase abdominal pressure

57. Expulsion stage (continued)
Crowning
When first part of baby’s head distends vagina
Head followed by rest of the body
Episiotomy sometimes necessary
Perineal muscles surgically incised
Creates wider opening for body
Umbilical cord clamped and tied off

58. Stages of True Labor and Childbirth: Expulsion Stage
Figure 29.15c
(c) ©D. Van Rossum/Science Source;

59. Placental stage Occurs after baby is expelled
Uterus continuing to contract
Compresses uterine blood vessels; displaces placenta from uterine wall
Afterbirth
Placenta and remaining fetal membranes
Expulsion completed within 30 minutes
Carefully examined to make sure all expelled
Complications may occur if fragments are left

60. (d) ©INSADCO Photography/Alamy
Figure 29.15d
(d) ©INSADCO Photography/Alamy

61. Milk letdown (continued)
Prolactin spikes occur in prolactin production each time baby breastfeeds
Promotes new breast milk production
As infant feeds, dopamine release inhibited by hypothalamus
Inhibition stimulates large amounts of prolactin secretion
Figure 29.16b

62. Overview of Human Genetics
Genetics definitions
Heredity
Transmission of genetic characteristics from parent to child
Genetics
Field of biology studying heredity and transmission patterns
Karyotype
Display of chromosomes pairs, ordered and arranged by size and similar features
Homologous chromosomes
Paired chromosomes with genes for equivalent biological characteristics
Autosomes
Twenty-two pairs of chromosomes without genes determining sex

63. Karyotype
Figure 29.18
©CNRI/Science Source

64. Overview of Human Genetics
Genetics definitions (continued)
Sex chromosomes
Last two chromosomes containing genes that specify sex
Locus
Specific space where each gene is located on a chromosome
Alleles
Variants of one gene found at some locus on homologous chromosomes
E.g., alleles determining type A or type O blood
Dominant allele
Expresses, or physically shows, the trait
Represented by capital letter

65. Genetics definitions (continued)
Recessive allele
Trait is masked
Expressed only if present on both homologous chromosomes
Represented by lowercase letter
Punnett square
Box showing specific gene combinations resulting from two parents
Gives probability that a particular gene combination can occur
Homozygous
If identical alleles present

66. Genetics definitions (continued)
Heterozygous
Both dominant and recessive allele present
But only dominant allele expressed
Expression of the recessive allele may appear to skip generations
Its phenotype is masked by the dominant allele
Genotype
Genetic makeup of an individual
Phenotype
Physical expression of genotype

67. Dominant Versus Recessive Alleles
Figure 29.19
(Widow's peak) ©Aaron Haupt/Science Source; (Straight hairline) ©Brownstock/Alamy RF

68. Patterns of Inheritance
Strict dominant-recessive inheritance
Mendelian inheritance
Dominant allele always expressed in the phenotype
Relatively few traits follow this pattern
Most involving interaction of multiple genes
May be affected by environmental factors

69. Incomplete dominance Two heterozygous alleles E.g., sickle cell trait
Phenotype is intermediate between homozygous dominant or recessive
E.g., sickle cell trait
Most individuals with two identical alleles, A
Code for normal hemoglobin A in erythrocytes
Sickling allele (s) produces abnormal hemoglobin (S)
Erythrocytes brittle and sickle-shaped
Sickle-cell disease if two homozygous recessive alleles
Heterozygous individuals carrying sickle-cell trait
Under low oxygen conditions some erythrocytes may develop sickle shape

70. Codominant inheritance
Two alleles equally dominant
Both alleles expressed in the phenotype
E.g., ABO blood group
Blood types A and B codominant
A allele from one parent, B allele from other parent
Leads to AB blood type
Third allele, i, is recessive
ii results in O blood type

71. Polygenic inheritance
Multiple genes interacting to produce phenotypic trait
Genes on same or different chromosomes
Most human traits result from this
E.g., eye color, height, skin color, predispositions to many diseases

72. Sex-linked traits Traits expressed by genes on X or Y chromosomes
900–1400 genes on X chromosome
Most not involved in sex determination
70–200 genes on Y chromosome
Mostly for male development
Sex-linked traits most often involve X chromosome

73. X-linked recessive traits
Always expressed in a male
Has only one X chromosome
Expressed in a female only if she has two recessive alleles
Low probability
Carrier, woman with one X-linked recessive allele only
Does not exhibit phenotypic effects
May pass X-linked allele to children
If passed to female, also a carrier
If passed to a male, will express X-linked trait

74. X-linked recessive traits (continued)
E.g., color blindness
Individual has trouble distinguishing red and green
Women rarely color-blind
Requires recessive allele from both mother and father
More often carriers
If man inherits allele
Lacks normal allele to counteract color-blindness
Will be color-blind