Embryology Quiz: Fetal Development Milestones

Introduction to Embryology

• This is a presentation on embryology, covering fertilization, implantation, placental development, and fetal development. Sections include gastrulation, neurulation, and organogenesis.

Fertilization

• Fertilization is the fusion of a haploid sperm nucleus and a haploid secondary oocyte nucleus to create a single diploid zygote (containing 46 chromosomes).
• This occurs in the fallopian tubes within 24 hours of ovulation.
• Before fertilization, sperm and the oocyte must move through the fallopian tubes.
• Oocyte movement is aided by peristalsis and cilia.
• Sperm movement is assisted by prostaglandins within the seminal fluid.
• Sperm must undergo capacitation, where the acrosomal membrane of the sperm head becomes fragile.
• When zona pellucida protein 3 (ZP3) binds to the sperm head, it triggers the acrosomal reaction.
• Enzymes are released from the acrosome, allowing the sperm to penetrate the corona radiata and zona pellucida of the oocyte.
• The first sperm to fuse with the oocyte triggers a fast block to polyspermy, causing depolarization of the oocyte membrane, preventing further sperm binding.
• A slow block to polyspermy occurs when cortical granules release Ca2+, which blocks sperm receptors on the zona pellucida and hardens it.

Making a Zygote

• Sperm enters the oocyte, triggering completion of meiosis II, producing a second polar body.
• Inside the oocyte, the sperm loses its tail and becomes a male pronucleus.
• Male and female haploid pronuclei fuse, marking fertilization.
• The fertilized ovum is now called a zygote.

Cleavage

• After fertilization, the zygote undergoes mitotic cell divisions (cleavage), creating blastomeres.
• Blastomeres are identical daughter cells.
• Blastomeres are surrounded by a zona pellucida.
• Over a 10-12 hour period, cell divisions increase in size, forming 2, 4, 8, 16 cells.
• By day 3, there are 16 cells, which is called a morula.
• Once the blastomere has more than 16 cells, it's called an advanced morula.

Blastocyst Formation

• By day 5 or 6, uterine fluid enters the morula, creating a blastocoele.
• The cell mass is now called a blastocyst.
• The blastocyst has an outer layer of cells called the trophoblast.
• The trophoblast develops into the chorion and later becomes part of the placenta.
• The inner cell mass differentiates into the embryo.

Implantation

• At day 6, the blastocyst attaches to the endometrium.
• The trophoblast differentiates into two layers: syncytiotrophoblast and cytotrophoblast.
• Syncytiotrophoblast secretes enzymes to digest the endometrial cells.
• Cytotrophoblast forms a barrier that protects the embryo within the endometrial lining.
• A hormone called human chorionic gonadotropin (hCG) is secreted by the trophoblast to support corpus luteum and maintain uterine lining.
• hCG is detectable in urine and used in pregnancy tests.

Implantation & Placentation

• The placenta is not fully developed until the 3rd month of pregnancy.
• Chorionic villi originate from the trophoblast into the endometrium, creating maternal blood-filled lacunae.
• Maternal and fetal blood vessels do not intermingle.
• The placenta facilitates nutrient exchange, gas exchange, and waste removal.
• The umbilical cord connects the developing fetus to the placenta, carrying oxygenated blood to the fetus and deoxygenated blood back to the placenta.

Placentation - Day 21 - 56

• Images display the development of the placenta over time, showing initial formation to late stages.

Formation of the Amniotic Cavity & Yolk Sac

• The inner cell mass separates from the trophoblast to create a fluid-filled amniotic cavity.
• The remaining cells form the amnion.
• Two layers form: the epiblast (ectodermal) and hypoblast (endodermal) of the embryonic disc, with the amniotic cavity and yolk sac forming.
• A third germ layer, the mesoderm, forms between the endoderm and ectoderm. This triple-layer formation is called gastrulation.

Formation of Yolk Sac

• The yolk sac forms within the blastocoel.
• These cavities (amniotic cavity and yolk sac cavities) work as cushioning and nutrient providers.
• The outer surface of the yolk sac is lined with mesoderm.
• Mesoderm forms blood vessels to link the embryo to the placenta.

Embryogenesis Begins - Day 12-14

• The embryonic disc differentiates into the 3 germ layers (ectoderm, mesoderm, endoderm).
• Endoderm forms the gut and respiratory system.
• Mesoderm forms muscle, bone, connective tissue, blood vessels and the body stalk (umbilical cord).
• Ectoderm forms the skin, nervous system.

Formation of the Primitive Streak - Day 12-14

• Day 12: Cells within the ectoderm thicken creating the neural plate.
• Day 12-14: The primitive streak (or neural groove) forms at the end of the embryonic disc.
• Cells of these layers move (migrate).
• These cell movements are called a gastrulation process.

Neurulation - Day 12-18

• Notochord, a rod-like structure, stimulates cephalic end of neural plate thickening.
• Neural folds rise and fuse to form a neural tube, the precursor to the spinal cord. Neural crest cells, which migrate from the neural folds, are part of the developing nervous system peripheral structures.
• Somites, blocks of mesoderm, form beside the neural tube.

Organogenesis (including gut formation and folding) - Day 18

• Endoderm forms the gastrointestinal tract (gut), which forms the foregut, midgut, and hindgut.
• A tube is formed, initially connected to the yolk sac; this is the yolk stalk.
• The foregut and hindgut develop with oropharyngeal and cloacal membranes forming the mouth and anus, respectively.

Growth During Trimester 2 & 3

• Significant development and growth occur during trimesters 2 and 3, with the fetus increasing in size and weight.
• The fetus develops multiple systems and organs.

Brain Development - Day 33

• The brain, specifically the telencephalon region, begins to develop.

Limb Development

• Limbs emerge as limb buds, thickened ectodermal ridges, during the process of gestation.

Facial Development - Day 52

• The head takes on a more human shape and facial features begin to develop.

Gestation Period - 38 weeks

• This shows a timeline for the embryological gestation period.

Videos

• Links to videos related to various aspects of embryology are listed.

Quiz

• A QR code and instructions for an embryology quiz are provided.

Test your knowledge on key concepts of embryology relating to fetal development. This quiz covers various developmental milestones, processes following fertilization, and the structures involved during early pregnancy. Perfect for students in biology or medical fields.