Fetal Development Notes

Lesson 3.1 Objectives

  • Define key terms related to fetal development.

  • Describe gametogenesis in human reproduction, including spermatogenesis and oogenesis.

  • Explain human fertilization, including capacitation and the acrosomal reaction, and implantation processes.

  • Describe embryonic development, covering the major events in each week of the embryonic period.

  • Describe fetal development and maturation of body systems, detailing key milestones in each trimester.

Body Cell DNA

  • DNA and nucleus control cell function, housing genetic information that directs cellular activities.

  • Genes and chromosomes determine individual traits, with genes being segments of DNA that code for specific characteristics.

  • Each cell contains 46 chromosomes, organized into:

    • 22 pairs of autosomes (non-sex chromosomes)

    • 1 pair of sex chromosomes (XX for female, XY for male)

  • Biological development is influenced by:

    • External environment (teratogens), including exposure to harmful substances.

    • Drug use: Maternal drug use can lead to fetal alcohol syndrome or neonatal abstinence syndrome.

    • Undernutrition: Maternal undernutrition can impair fetal growth and development, leading to low birth weight.

    • Smoking: Maternal smoking can cause preterm birth and low birth weight.

Cell Division and Gametogenesis

  • Mitosis: Cell division resulting in two identical daughter cells; essential for growth and repair.

  • Meiosis: Cell division resulting in four genetically unique daughter cells; necessary for sexual reproduction (spermatogenesis and oogenesis).

Fertilization

  • Occurs when a sperm penetrates an ovum and their nuclei unite, forming a zygote.

  • Takes place in the outer third of the fallopian tube, near the ovary.

  • A chemical change in the membrane around the fertilized ovum (zona pellucida) prevents further sperm from penetrating, ensuring only one sperm fertilizes the egg.

Sex Determination

  • Sperm can carry either an X or a Y chromosome, determining the sex of the offspring.

  • The male determines the gender of the fetus.

  • pH of the female reproductive tract influences the survival rate of X- and Y-bearing sperm, including speed of motility; a more alkaline environment favors Y-bearing sperm.

  • XX results in a female.

  • XY results in a male.

Tubal Transport of the Zygote

  • Zygote is formed by the union of sperm and ovum, marking the beginning of pregnancy.

  • Transported through the fallopian tube into the uterus via peristaltic contractions and ciliary action.

  • During transport, the zygote undergoes rapid mitotic division (cleavage), increasing the number of cells without increasing the overall size.

  • The size of the zygote does not increase; individual cells become smaller as they divide and then form a solid ball (morula).

Implantation of the Zygote

  • Usually occurs in the upper section of the posterior uterine wall, where there is a rich blood supply for optimal development.

  • Cells burrow into the prepared lining—endometrium—via enzymatic action, embedding the blastocyst.

  • Endometrium is now called decidua, providing nourishment and protection for the implanted blastocyst.

  • The area under the blastocyst is the decidua basalis, contributing to the formation of the maternal part of the placenta.

  • Becomes the maternal part of the placenta, facilitating nutrient and waste exchange between mother and fetus.

Development

  • Cell differentiation: Process by which cells become specialized in structure and function.

  • Chorion: Outermost membrane surrounding the embryo, contributing to the formation of the placenta.

  • Amnion: Innermost membrane that encloses the embryo in amniotic fluid.

Functions of Amniotic Fluid

  • Maintains an even temperature, protecting the fetus from temperature fluctuations.

  • Prevents the amniotic sac from adhering to the fetal skin, allowing for free movement.

  • Allows symmetrical growth of the fetus, providing space for expansion.

  • Allows buoyancy and fetal movement, promoting musculoskeletal development.

  • Acts as a cushion to protect the fetus and umbilical cord from injury, absorbing impacts and pressure.

Yolk Sac

  • A cavity develops on the ninth day after fertilization, providing initial nourishment before the placenta is fully functional.

  • Functions only during embryonic life.

  • Initiates production of red blood cells, crucial for oxygen transport.

  • Continues until the fetal liver takes over around 6 weeks.

  • The umbilical cord encompasses the yolk sac, which then degenerates as its functions are taken over by other structures.

Germ Layers
Ectoderm

  • Outer layer of skin (epidermis)

  • Oil glands and hair follicles of skin

  • Nails and hair

  • External sense organs (e.g., eyes and ears)

  • Mucous membrane of mouth and anus

Mesoderm

  • True skin (dermis)

  • Skeleton (bones and cartilage)

  • Bone and cartilage

  • Connective tissue

  • Muscles (skeletal, smooth, and cardiac)

  • Blood and blood vessels

  • Kidneys and gonads

Endoderm

  • Lining of the trachea, pharynx, and bronchi

  • Lining of the digestive tract

  • Lining of the bladder and urethra

Three Stages of Prenatal Development

  • Zygote: cell formed by the union of sperm and ovum.

  • Embryo: second to eighth week of development, characterized by rapid differentiation and organogenesis.

  • Fetus: ninth week until birth, marked by growth and maturation of organ systems.

  • Age of viability: 20 weeks of gestation but requires NICU care to survive, referring to the earliest age at which a fetus has a chance of surviving outside the womb.

Lesson 3.2 Objectives

  • Describe the development and functions of the placenta, umbilical cord, and amniotic fluid, detailing their roles in fetal nourishment, gas exchange, and protection.

  • Compare fetal circulation to circulation after birth, explaining the changes that occur when the baby takes its first breath.

  • Explain the similarities and differences in the two types of twins, including their genetic origins and potential complications.

Accessory Structures of Pregnancy

  • Placenta: Supports fetus by providing oxygen, nutrients, and hormones, and removing waste products.

  • Umbilical cord: Connects the fetus to the placenta, facilitating the exchange of substances.

  • Fetal circulation: Unique circulatory system that bypasses the fetal lungs.

Placental Transfer

  • Fetal deoxygenated blood and waste products leave the fetus through two umbilical arteries, carrying them to the placenta for exchange.

  • Fetal and maternal blood do not normally mix, ensuring that there is no direct contact between the two bloodstreams.

  • Oxygenated, nutrient-rich blood from mother spurts into intervillous space from spiral arteries in the decidua, facilitating the transfer of essential substances to the fetal blood.

  • Fetal blood releases carbon dioxide and waste products into the intervillous space, which are then excreted via the maternal circulation.

Placental Hormones

  • Progesterone: Functions during pregnancy to maintain the endometrium and prevent uterine contractions.

  • Estrogen: Stimulates uterine growth and prepares the breasts for lactation.

  • Human chorionic gonadotropin (hCG): Supports the corpus luteum in early pregnancy, maintaining progesterone production.

  • Human placental lactogen (hPL): Affects glucose and protein metabolism, making glucose available to the fetus.

Umbilical Cord

  • Lifeline between mother and fetus, essential for fetal survival.

  • Two arteries carry blood away from the fetus, transporting deoxygenated blood and waste products to the placenta.

  • One vein returns blood to the fetus, carrying oxygenated blood and nutrients from the placenta.

  • Wharton jelly covers and cushions cord vessels, protecting them from compression and ensuring uninterrupted blood flow.

  • Normal length is 55 cm (22 inches), allowing the fetus to move freely without tangling the cord.

  • Usually protrudes near the center of the placenta, but variations can occur without necessarily causing complications.

Fetal Circulatory Shunts

  • Foramen ovale: An opening between the right and left atria of the fetal heart.

  • Ductus arteriosus: A blood vessel connecting the pulmonary artery to the aorta, bypassing the fetal lungs.

  • Ductus venosus: A shunt that allows oxygenated blood from the umbilical vein to bypass the liver and enter the inferior vena cava.

Circulation Before Birth

  • Blood enters the fetal body through the umbilical vein, carrying oxygen and nutrients from the placenta.

  • About half goes to the liver; the remainder enters the inferior vena cava through the ductus venosus, goes through the foramen ovale, and then the ductus arteriosus.

  • Blood containing waste products is returned to the placenta through umbilical arteries.

Circulation After Birth

  • Foramen ovale closes within 2 hours after birth (permanently by age 3 months) due to increased pressure in the left atrium.

  • Ductus arteriosus closes within 15 hours (permanently in about 3 weeks) as a result of increased oxygen levels and decreased prostaglandins.

  • Ductus venosus closes functionally when the cord is cut (permanently in about 1 week) as blood flow through the umbilical vein ceases.

  • After permanent closure, the ductus arteriosus and ductus venosus become ligaments.

Lung Preparation for Birth

  • Lung fluid maintains lung expansion in utero, keeping the alveoli open and ready for gas exchange.

  • Fluid decreases during labor due to hormonal changes and pressure from uterine contractions.

  • Several hormones increase during labor to decrease lung fluid production and increase resorption in preparation for breathing air, facilitating the transition to extrauterine life.

Impaired Prenatal Development

  • Undernutrition: Inadequate intake of essential nutrients by the mother can impair fetal growth and development.

  • Intrauterine growth restriction (IUGR): A condition in which the fetus does not grow to its expected size in utero.

Impaired Prenatal Development and Subsequent Illness

  • Undernutrition in utero can result in:

    • Permanent changes in fetal structure, physiology, metabolism, predisposing the individual to chronic diseases.

    • Development of chronic conditions later in life, such as cardiovascular disease, diabetes, and obesity.

  • To prevent illness of the next generation, this generation must focus on their own health practices, including nutrition, exercise, and avoiding harmful substances.

Multifetal Pregnancy

  • Twins occur once in every 90 pregnancies, but the rate is increasing due to assisted reproductive technologies.

  • When hormones are used to assist with ovulation, twinning and other multifetal pregnancies occur more frequently.

  • Monozygotic twins are from a single fertilized ovum (identical), sharing the same genetic material.

  • Dizygotic twins are from two separate fertilized ova (fraternal), having different genetic compositions similar to siblings born at different times.