1. Chapter 5 Fetal Development

2. Cellular Processes Soma cells
Make up the organs and tissues of the human body
Nucleus contains 23 pairs of chromosomes
22 pairs of autosomes
One pair of sex chromosomes
Chromosomes composed of genes

3. Cellular Processes (cont.)
Gametes (germ cells or sex cells)
Found in the reproductive glands only
Ovum is female gamete
Spermatozoon (sperm) is male gamete
Each gamete has 23 chromosomes

4. Types of Cellular Division
Mitosis
The process by which somatic (body) cells give birth to daughter cells
Each daughter cell contains the same number of chromosomes as the parent cell
Meiosis
The process by which gametes undergo two sequential cellular divisions of the nucleus (gametogenesis)

5. Types of Cellular Division (cont.)
Meiosis (cont.)
Gametogenesis = formation, development of germ cells
Spermatogenesis
Begins at puberty
Primary spermatocytes undergo the first meiotic division
Secondary spermatocytes undergo a second meiotic division
Spermatids undergo a change in form to become mature spermatozoa

6. Mitosis and Gametogenesis

7. Types of Cellular Division (cont.)
Meiosis (cont.)
Gametogenesis (cont.)
Oogenesis begins prebirth in ovaries; at birth, primary oocytes have completed prophase stage of first meiotic division
Completion of first meiotic division occurs before ovulation
Results in secondary oocyte and first polar body
First polar body disintegrates
Secondary oocyte begins second meiotic division at ovulation
Not completed unless fertilized by sperm

8. Question
In the development of a human being, a process occurs in which cells divide to produce a germ cell with 23 unpaired chromosomes. What is this process called?
a. Meiosis
b. Mitosis
c. Oogenesis
d. Gametogenesis

9. Answer
a. Meiosis
Rationale: Meiosis is the process by which gametes undergo two sequential cellular divisions of the nucleus. This process reduces the number of chromosomes in the gametes by half. Remember, each gamete has only 23 chromosomes, which is half, also known as the haploid number, of the total number of chromosomes required for human cells.

10. Stages of Fetal Development
Three stages
Pre-embryonic
Embryonic
Fetal
Begins at fertilization
Lasts through end of second week postfertilization

11. Stages of Fetal Development (cont.)
Pre-embryonic (cont.)
Conception usually occurs when the ovum is in the ampulla (the outermost half) of the fallopian tube
Zygote is formed and has diploid number of chromosomes
Gender determination
Occurs at time of fertilization
Dependent on whether sperm has X or Y chromosome
Research indicates that there is an approximately chance of either occurrence

12. Stages of Fetal Development (cont.)
Pre-embryonic (cont.)
Cellular reproduction
Cleavage begins
Three days after fertilization morula forms
Five days after fertilization blastocyst forms

13. Stages of Fetal Development (cont.)
Pre-embryonic (cont.)
Implantation
By 10th day after fertilization blastocyst is buried in uterine lining
Begins to produce human chorionic gonadotropin (hCG)
Endometrium is called the decidua from implantation through end of pregnancy

14. Stages of Fetal Development (cont.)
Embryonic stage
Lasts from the end of the second week postfertilization until end of eighth week
Developing conceptus becomes the embryo
Differentiation begins
Third week, three germ layers develop
Ectoderm
Mesoderm
Endoderm

15. Stages of Fetal Development (cont.)
Embryonic stage (cont.)
Three germ layers of the embryo
Ectoderm: Outer layer of cells
Forms skin, hair, nails, and the nervous system
Mesoderm: Middle layer
Forms skeletal, muscular, and circulatory systems
Endoderm: Inner layer
Forms glands, lungs, and urinary, digestive tracts

16. Stages of Fetal Development (cont.)
Fetal stage
From the beginning of the ninth week after fertilization and continues until birth
Additional growth and maturation of the organs and body systems

17. Question In the embryonic stage of fetal development what occurs?
a. Implantation begins
b. Gender differentiation takes place
c. Maturation of germ cells in ovaries
d. Differentiation begins

18. d. Differentiation begins
Answer
d. Differentiation begins
Rationale: During the embryonic period, the cells of the embryo multiply, and tissues begin to assume specific functions, a process known as differentiation.

19. Development of Supportive Structures
Fetal membranes
Amniotic cavity begins to develop around nine days postconception
Surrounded by amnion
Amnion is a thick fibrous lining, composed of several layers, that protects the fetus, and forms inner part of the sac in which the fetus grows
Chorion is a second layer of thick fibrous tissue
Surrounds amnion

20. Development of Supportive Structures (cont.)
Amniotic fluid
Fills the amniotic cavity
Serves four main functions
Physical protection
Temperature regulation
Provision of unrestricted movement
Symmetrical growth

21. Development of Supportive Structures (cont.)
Placenta
Organ that sustains, nourishes growing pregnancy
Three main functions
Provides for transfer, exchange of substances
Acts as barrier to certain substances
Functions as an endocrine gland, producing hormones

22. Development of Supportive Structures (cont.)
Umbilical cord
Extends from fetal umbilicus to fetal surface of the placenta
Two arteries bringing deoxygenated blood from fetus to placenta and one vein carrying oxygenated and nourished blood from placenta to fetus
Wharton jelly surrounds these three vessels

23. Fetal and Placental Circulation
Fetal circulation
Low oxygen tension
Three fetal shunts
Ductus venosus
Foramen ovale
Ductus arteriosus
Placental circulation
Exchange occurs in intervillous spaces

24. Fetal Circulation

25. Question Tell whether the following statement is true or false.
The ductus arteriosus carries oxygenated blood from the pulmonary artery to the aorta.

26. Answer
False
Rationale: The ductus arteriosus does connect the pulmonary artery and the aorta, but it carries deoxygenated blood.

27. Special Considerations of Fetal Development
Teratogens and the fetus
Substances that cause birth defects
Severity of the defect depends upon when during development the conceptus is exposed to the teratogen (i.e., which body systems are developing at the time of exposure) and the particular teratogenic agent to which the fetus is exposed

28. Special Considerations of Fetal Development (cont.)
Types of teratogenic agents
Ingested
Prescription and over-the-counter medications, illicit drugs, and alcohol
Infectious
Varicella, cytomegalovirus (CMV), and rubella
Environmental substance
Ionizing x-rays, radioactive substances, and certain chemicals

29. Special Considerations of Fetal Development (cont.)
Ectopic pregnancy
The zygote implants in places other than the uterus
95% occur in the fallopian tube
Usually caused by blockage or scarring of the fallopian tubes either from infection or trauma
Occurrence is 1% to 2% of all pregnancies

30. Special Considerations of Fetal Development (cont.)
Multifetal pregnancy
Monozygotic twins: Identical twins derived from one zygote
Share same genetic material; always the same sex
Dizygotic twins: Fraternal twins develop from separate egg and sperm fertilizations
Genetic material is not identical; may or may not be the same sex

31. Special Considerations of Fetal Development (cont.)
Multifetal pregnancy (cont.)
Classification of twins
Diamniotic-dichorionic twins
Each develop in their own amniotic sac
Placentas do not share any vessels
Diamniotic-monochorionic twins
Each have their own amniotic sac but share a common chorionic sac
Each have a separate placenta but the placentas share some vessels
Monoamniotic-monochorionic
Have one amniotic cavity that they both share

32. Special Considerations of Fetal Development (cont.)
Multifetal pregnancy (cont.)
Contributing factors for multifetal pregnancy
Family history of twins
Recent stoppage of oral contraceptive
Tall or large stature of mother
African American heritage
Fertility medication use