Embryology Overview Quiz

Embryology

• Embryological development occurs from fertilization to the first eight weeks.
• Fetal development occurs from week nine to birth.
• Major systems are established during the embryonic period (weeks 3-8), which is also called organogenesis.
• There are stages of development: fertilization, 1-week conceptus, 3-week embryo, 5-week embryo, 8-week embryo, 12-week fetus.
• The lengths of each development stage are provided in millimeters (mm).
• Gestation is 38-42 weeks (divided into three trimesters).

Learning Objectives

• Understanding the characteristics of gametes.
• Description of fertilization.
• Description of implantation.
• Listing the main functions of the placenta.

Gametes

• Egg (Oocyte):
  • Largest cell (0.1 mm diameter)
  • Non-motile
  • Active metabolically
  • Produced in the ovary before birth
  • Limited numbers
  • Arrested in metaphase II of meiosis II until fertilized.
  • Release of 4 viable sperms during fertilization
• Sperm:
  • Smallest cell (65 µm or 0.065 mm)
  • Highly motile
  • Little metabolic activity
  • Produced in the testes from puberty
  • Millions of sperm present
  • During maturation, there's a decrease in volume, followed by an increase with the gain of yolk granules.
  • Contains acrosome and nucleus
  • Contains mitochondria to produce ATP.
  • Contains flagellum with a 9+2 arrangement of microtubules to enable movement.

Structural features of the secondary oocyte

• Intracellular:
  • Large haploid nucleus
  • Dense cytoplasm
  • Surrounded by plasma membrane and vitelline membrane
• Extracellular:
  • Oocytes surrounded by granulosa cells forming corona radiata.
  • Corona radiata originates from the Graafian follicle in the ovary.
  • Granulosa cells sit in a matrix containing hyaluronan (hyaluronic acid).
  • Zona pellucida—a clear, transparent zone—lies below the corona radiata.
  • The zona pellucida is an extracellular matrix from oocyte and granulosa cells.
  • It includes glycoproteins like ZP3 receptors.

Structural features of spermatozoa

• Head:
  • Contains the nucleus and acrosome, each within its own membrane.
  • Acrosome contains hydrolytic enzymes (acrosin and hyaluronidase).
  • Acrosin digests proteins in the zona pellucida, facilitating fertilization.
  • Hyaluronidase breaks down hyaluronan in the corona radiata.
• Mid-piece:
  • Contains numerous mitochondria for ATP synthesis.
• Flagellum:
  • Enables movement, formed from long tubulin proteins surrounding the axoneme (central microtubules with 9 doublets).
  • 9+2 arrangement characteristic of eukaryotic flagella.
  • Enables movement.

Seminal Fluid

• Seminal fluid is a transport medium for sperm.
• A vital part of the fertilization procedure.
• Contains 60-70% seminal fluid, produced by seminal vesicles,
• Contains fructose (carbohydrate) for energy.
• Prostaglandins induce uterine contractions.
• Coagulase solidifies semen.
• Prostate gland contributes roughly 30% to seminal fluid
• Citrate is an energy source in the prostate gland fluid.

Quiz 1 (Reproductive System)

• Question 1: Male gametes are produced in the testes; female gametes in the ovaries.
• Question 2: Corona radiata - a layer of cells surrounding the oocyte, supplying nutrients and facilitating development to the oocyte.
• Question 3: Acrosome - a membrane-bound structure in the sperm containing hydrolytic enzymes to penetrate oocyte coverings for fertilization.
• Question 4: 9+2 microtubules are arranged radially and 2 centrally to form the flagellum in sperm.
• Question 5: Seminal vesicles and prostate gland produce seminal fluid that is vital as a transporting medium, and a source of nutrients and substances that assist in fertilization for the sperm, and also supplies energy for the sperm cells.
• Question 6: Fructose is preferred to glucose because the latter is used by bacteria; therefore, fructose reduces competition.

Implantation

• Implantation, typically 6-8 days post-fertilization , involves the blastocyst burrowing into the uterine wall (endometrium).
• The uterine wall envelops the blastocyst
• Provides nourishment as a part of the embryo’s nourishment.
• Human chorionic gonadotropin (hCG) is secreted for maintaining progesterone and to prevent menstruation.

Early Placental Development

• Trophoblast cells (outer cell mass) proliferate outside the blastocyst.
• They penetrate the uterine cells and form finger-like projections called chorionic villi.
• Cell membranes in multiple cells break down making regions of multinucleated cytoplasm and interact with maternal blood vessels.

Ectopic Pregnancy

• Ectopic pregnancy is an abnormal pregnancy that develops outside the uterine cavity.
• Fallopian tube ectopic pregnancies are the most common site.
• Prevalence is approximately 20 per 1000 pregnancies.
• Early symptoms can mimic typical early pregnancy, often with abdominal pain.
• Severity of complications correlates to the site of the ectopic pregnancy.

Week 3: Gastrulation

• Cells from the embryoblast (inner cell mass) proliferate, migrate, and form a trilaminar disc, which forms the ectoderm, mesoderm, and endoderm.
• Gastrulation is a vital part of shaping tissues and organs.

Week 3-8: Organogenesis

• The organs of the embryo start developing during the embryonic period (weeks 3–8).
• The organization process includes folding, splitting, and condensation.

Organogenesis

• Major organ systems begin to form during weeks 3–8.
• Key time-points include:
  • CNS and spinal cord (day 15)
  • Heart (day 18)
  • Eye and ear (day 22)
  • Digestive system (day 28)
  • Respiratory tract (day 35)
  • Skeletal system (day 35)
  • Muscular system (day 38)

Umbilical Cord Structure

• The umbilical cord contains embryonic blood vessels and carries blood to and from the placenta.
• One umbilical vein carries oxygenated blood to the embryo.
• Two umbilical arteries carry deoxygenated blood back to the placenta.

Early Cardiovascular System

• One umbilical vein carries oxygenated blood from the placenta to the embryo and into the embryonic circulation and through the inferior vena cava.
• Two umbilical arteries carry deoxygenated blood from the embryo back to the placenta.

Quiz 4

• Question 1: The mesoderm layer forms the muscles of the body.
• Question 2: hCG maintains the corpus luteum, which secretes progesterone to maintain the uterine lining in early pregnancy and prevents menstruation.
• Question 3: Trophoblast cells penetrate the endometrium and create chorionic villi.
• Question 4: The main processes for organ formation from the trilaminar disc are folding, splitting, and condensation.
• Question 5: The yolk sac provides nourishment before the placenta fully develops.
• Question 6: Placental functions include gas exchange, excretion, passive immunity, and anchorage.

Placenta Previa

• A placental abruption where the placenta forms over or near the internal os of the cervix, causes vaginal bleeding in second-half pregnancy.
• Requires close monitoring.
• Early delivery (C-section) may be necessary.

Foetal Alcohol Syndrome (FAS)

• Maternal alcohol use during pregnancy causes fetal abnormalities (FAS), with decreased growth, brain malformations, and specific facial features.
• This condition is considered the most serious fetal alcohol spectrum disorder.

Quiz 3 (Pregnancy)

• Question 1: Cleavage is the stage characterized by rapid mitotic cell division.
• Question 2: Oviduct.
• Question 3: Uterine secretions (uterine milk) nourish the embryo.
• Question 4: The cells' movement is facilitated by cilia and fimbriae, which waft them toward the uterus for implantation.
• Question 5: Blastulation transforms the ball of cells into a blastocyst.
• Question 6: Outer cell mass – trophoblast; inner cell mass – embryoblast.