An Introduction to Development and Inheritance

1. An Introduction to Development and Inheritance • Development • Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity • Inheritance • Transfer of genetic material from generation to generation

2. 29-1 Development • Differentiation • Creation of different types of cells required in development • Occurs through selective changes in genetic activity • As development proceeds, some genes are turned off, others are turned on • Fertilization • Also called conception • When development begins

3. 29-1 Development • Embryological Development • Occurs during first two months after fertilization • Study of these events is called embryology • Fetal Development • Begins at start of ninth week • Continues until birth

4. 29-1 Development • Prenatal Development • Embryological and fetal development stages • Postnatal Development • Commences at birth • Continues to maturity, the state of full development or completed growth

5. 29-1 Development • Inheritance • Transfer of genetically determined characteristics from generation to generation • Genetics • Study of mechanisms responsible for inheritance

6. 29-2 Fertilization • Fertilization • Fusion of two haploid gametes, each containing 23 chromosomes • Produces zygote containing 46 chromosomes • Spermatozoon • Delivers paternal chromosomes to fertilization site • Travels relatively large distance • Is small, efficient, and highly streamlined

7. 29-2 Fertilization • Gamete • Provides: • Cellular organelles • Inclusions • Nourishment • Genetic programming necessary to support development of embryo for a week

8. 29-2 Fertilization • Fertilization • Occurs in uterine tube within a day after ovulation • Secondary oocyte travels a few centimeters • Spermatozoa must cover distance between vagina and ampulla • Capacitation • Must occur before spermatozoa can fertilize secondary oocyte • Contact with secretions of seminal glands • Exposure to conditions in female reproductive tract

9. 29-2 Fertilization • Acrosomes • Release hyaluronidase and acrosin, enzymes • Penetrate corona radiata, zona pellucida, towardoocyte surface • Oocyte Activation • Contact and fusion of cell membranes of sperm and oocyte • Follows fertilization • Oocyte completes meiosis II, becomes mature ovum

10. 29-2 Fertilization • Polyspermy • Fertilization by more than one sperm • Prevented by cortical reaction • Cortical Reaction • Releases enzymes that: • Inactivate sperm receptors • Harden zona pellucida

11. 29-2 Fertilization • Female Pronucleus • Nuclear material remaining in ovum after oocyte activation • Male Pronucleus • Swollen nucleus of spermatozoon • Migrates to center of cell

12. 29-2 Fertilization • Amphimixis • Fusion of female pronucleus and male pronucleus • Moment of conception • Cell becomes a zygote with 46 chromosomes • Fertilization is complete

13. 29-2 Fertilization • Cleavage • Series of cell divisions • Produces daughter cells • Differentiation • Involves changes in genetic activity of some cells but not others

14. Figure 29-1a Fertilization A secondary oocyte and numerous sperm at the time of fertilization. Notice the difference in size between the gametes.

15. Figure 29-1b Fertilization Oocyte at Ovulation Ovulation releases a secondary oocyte and the first polar body; both are surrounded by the corona radiata. The oocyte is suspended in metaphase of meiosis II. Corona radiata First polar body Zona pellucida

16. Figure 29-1b Fertilization Fertilization and Oocyte Activation Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis. Second polar body Fertilizing spermatozoon

17. Figure 29-1b Fertilization Pronucleus Formation Begins The sperm is absorbed into the cytoplasm, and the female pronucleus develops. Nucleus of fertilizing spermatozoon Female pronucleus

18. Figure 29-1b Fertilization Spindle Formation and Cleavage Preparation The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division. Male pronucleus Female pronucleus

19. Figure 29-1b Fertilization Amphimixis Occurs and Cleavage Begins Metaphase of first cleavage division

20. Figure 29-1b Fertilization Cleavage Begins The first cleavage division nears completion roughly 30 hours after fertilization. Blastomeres

21. 29-3 Gestation • Induction • Cells release chemical substances that affect differentiation of other embryonic cells • Can control highly complex processes • Gestation • Time spent in prenatal development • Consists of three integrated trimesters, each three months long

22. 29-3 Gestation • First Trimester • Period of embryological and early fetal development • Rudiments of all major organ systems appear • Second Trimester • Development of organs and organ systems • Body shape and proportions change • Third Trimester • Rapid fetal growth and deposition of adipose tissue • Most major organ systems are fully functional

23. 29-4 The First Trimester • First Trimester • Includes four major stages • Cleavage • Implantation • Placentation • Embryogenesis

24. 29-4 The First Trimester • Cleavage • Sequence of cell divisions begins immediately after fertilization • Zygote becomes a pre-embryo, which develops into multicellular blastocyst • Ends when blastocyst contacts uterine wall • Implantation • Begins with attachment of blastocyst to endometrium of uterus • Sets stage for formation of vital embryonic structures

25. 29-4 The First Trimester • Placentation • Occurs as blood vessels form around periphery of blastocyst and placenta develops • Embryogenesis • Formation of viable embryo • Establishes foundations for all major organ systems

26. 29-4 The First Trimester • The First Trimester • Most dangerous period in prenatal life • 40% of conceptions produce embryos that survive past first trimester

27. 29-4 The First Trimester • Cleavage and Blastocyst Formation • Blastomeres • Identical cells produced by cleavage divisions • Morula • Stage after three days of cleavage • Pre-embryo is solid ball of cells resembling mulberry • Reaches uterus on day 4

28. 29-4 The First Trimester • Cleavage and Blastocyst Formation • Blastocyst • Formed by blastomeres • Hollow ball with an inner cavity • Known as blastocoele

29. 29-4 The First Trimester • Cleavage and Blastocyst Formation • Trophoblast • Outer layer of cells separate outside world from blastocoele • Cells responsible for providing nutrients to developing embryo

30. 29-4 The First Trimester • Cleavage and Blastocyst Formation • Inner cell mass • Clustered at end of blastocyst • Exposed to blastocoele • Insulated from contact with outside environment by trophoblast • Will later form embryo

31. Figure 29-2 Cleavage and Blastocyst Formation Blastomeres Polar bodies 4-cell stage 2-cell stage DAY 2 DAY 1 First cleavage division DAY 0: Fertilization

32. Figure 29-2 Cleavage and Blastocyst Formation Early morula DAY 3 Advanced morula DAY 4 Hatching Inner cell mass DAY 6 Blastocoele Days 7–10: Trophoblast Implantation in uterine wall (See Figure 29–3) Blastocyst

33. 29-4 The First Trimester • Implantation • Occurs (begins) seven days after fertilization • Blastocyst adheres to uterine lining • Trophoblast cells divide rapidly, creating several layers

34. 29-4 The First Trimester • Implantation • Cellular trophoblast • Cells closest to interior of blastocyst • Syncytial trophoblast • Outer layer • Erodes path through uterine epithelium by secreting hyaluronidase

35. Figure 29-3 Stages in Implantation DAY 6 FUNCTIONAL ZONE OF ENDOMETRIUM UTERINE CAVITY Uterine glands Blastocyst DAY 7 Trophoblast Blastocoele Inner cell mass

36. Figure 29-3 Stages in Implantation DAY 8 Cellular trophoblast Syncytial trophoblast DAY 9 Developing villi Amniotic cavity Lacuna

37. 29-4 The First Trimester • Ectopic Pregnancy • Implantation occurs outside uterus • Does not produce viable embryo • Can be life threatening • Lacunae • Trophoblastic channels carrying maternal blood

38. 29-4 The First Trimester • Formation of the Amniotic Cavity • Villi extend away from trophoblast into endometrium • Increase in size and complexity until day 21 • Amniotic Cavity • A fluid-filled chamber • Inner cell mass is organized into an oval sheet two layers thick • Superficial layer faces amniotic cavity • Deeper layer is exposed to fluid contents of blastocoele

39. 29-4 The First Trimester • Gastrulation and Germ Layer Formation • Formation of third layer of cells • Cells in specific areas of surface move toward central line • Known as primitive streak

40. 29-4 The First Trimester • Primitive Streak • Migrating cells leave surface and move between two layers • Creates three distinct embryonic layers, or germ layers • Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass • Endoderm: consists of cells that face blastocoele • Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm

41. 29-4 The First Trimester Ectodermal Contributions Integumentary system: Epidermis, hair follicles and hairs, nails, and glands communicating with the skin (sweat glands, mammary glands, and sebaceous glands) Skeletal system: Pharyngeal cartilages and their derivatives in adults (portion of sphenoid, the auditory ossicles, the styloid processes of the temporal bones, the cornu and superior rim of the hyoid bone)* Nervous system: All neural tissue, including brain and spinal cord

42. 29-4 The First Trimester Ectodermal Contributions Endocrine system: Pituitary gland and adrenal medullae Respiratory system: Mucous epithelium of nasal passageways Digestive system: Mucous epithelium of mouth and anus, salivary glands

43. 29-4 The First Trimester Mesodermal Contributions Integumentary system: Dermis and hypodermis Skeletal system: All components except some pharyngeal derivatives Muscular system: All components

44. 29-4 The First Trimester Mesodermal Contributions Endocrine system: Adrenal cortex, endocrine tissues of heart, kidneys, and gonads Cardiovascular system: All components

45. 29-4 The First Trimester Mesodermal Contributions Lymphatic system: All components Urinary system: The kidneys, including the nephrons and the initial portions of the collecting system Reproductive system: The gonads and the adjacent portions of the duct systems Miscellaneous: The lining of the body cavities (pleural, pericardial, and peritoneal) and the connective tissues that support all organ systems

46. 29-4 The First Trimester Endodermal Contributions Endocrine system: Thymus, thyroid gland, and pancreas Respiratory system: Respiratory epithelium (except nasal passageways) and associated mucous glands Digestive system: Mucous epithelium (except mouth and anus), exocrine glands (except salivary glands), liver, and pancreas

47. 29-4 The First Trimester Endodermal Contributions Urinary system: Urinary bladder and distal portions of the duct system Reproductive system: Distal portions of the duct system, stem cells that produce gametes

48. 29-4 The First Trimester • Embryonic Disc • Oval, three-layered sheet • Produced by gastrulation • Will form body of embryo • Rest of blastocyst will be involved in forming extraembryonic membranes

49. Figure 29-4 The Inner Cell Mass and Gastrulation Day 10: Yolk Sac Formation Syncytial trophoblast Cellular trophoblast Amniotic cavity Yolk sac Blastocoele Lacunae Superficial layer Deep layer

50. Figure 29-4 The Inner Cell Mass and Gastrulation Day 12: Gastrulation Yolk sac Amnion Ectoderm Mesoderm Primitive streak Endoderm Blastodisc Embryonic disc