Chapter 5 Fetal Development

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

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

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)

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

Mitosis and Gametogenesis

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

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

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.

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

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 50-50 chance of either occurrence

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

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

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

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

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

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

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.

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

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

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

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

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

Fetal Circulation

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

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

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

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

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

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

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

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