Embryology Quiz on Developmental Processes

Learning Objectives

• Describe the stages of development from fertilization to week 4
• Understand how a blastocyst differentiates into an embryoblast and trophoblast
• Understand how an embryoblast develops into a bilaminar disc
• Understand gastrulation and the formation of three germ layers
• Understand neurulation
• Appreciate that congenital anomalies can result from disruptions in embryonic development

Fertilization

• Millions of sperm enter the female reproductive tract; few reach the uterine tube
• Sperm are "conditioned" (capacitated) in the uterine tube before fertilization

Fertilization Process

• Sperm reaches the ovum
• Chemicals are released, triggering the acrosome reaction
• The acrosome swells, fusing with the sperm's cell surface membrane
• Digestive enzymes are released, digesting the follicle cells and zona pellucida
• The sperm nucleus enters the ovum
• Nuclei of the ovum and sperm fuse
• Enzymes thicken the jelly-like layer, preventing other sperm entry

Polyspermy Prevention

• The zona pellucida becomes impermeable

Clinical Relevance: Molar Pregnancy

• Two types: complete mole and partial mole
• Complete mole: Sperm fertilizes an egg with no genetic material; abnormal trophoblast develops, not an embryo
• Partial mole: Normal egg fertilized by two sperm; embryo starts developing but cannot survive

The Zygote

• Fertilization produces a zygote that is genetically unique

Cleavage

• Zygote undergoes cell division immediately after fertilization
• No increase in overall size during early cell division

The Morula

• Cleavage produces a morula
• Blastomeres are cells formed from cleavage of the fertilized ovum
• Blastomeres are tightly compacted in the morula

The Blastocyst

• The morula forms a fluid-filled cavity
• The cavity separates the cells into inner and outer groups
• The inner cell mass (ICM) develops into the embryo
• The outer cells become the trophoblast, which develops into the placenta
• Cells in the ICM differentiate into the hypoblast and epiblast

Blastocyst Hatching

• The blastocyst sheds its outer membrane and attaches to the uterine lining to prepare for implantation

Implantation

• Begins around day 6
• Three main stages: apposition, adhesion, invasion
• The endometrium is the inner-most lining of the uterus

Implantation: Apposition

• First physical contact between the blastocyst and the endometrium

Implantation: Adhesion

• Blastocyst adheres to the endometrium

Implantation: Invasion

• Trophoblast invades the endometrium
• Complex signalling between endometrium and trophoblast
• Blastocyst must implant adequately but not too deeply to avoid rejection by the mother

Implantation: Trophoblast Differentiation

• Cytotrophoblast proliferates at the embryonic pole
• Cytotrophoblast invades the endometrium
• Syncytiotrophoblast forms blood-filled spaces called lacunae
• Trophoblast contacts maternal vessels, allowing gas exchange between maternal and fetal circulation

Implantation: Changes in the Inner Cell Mass

• The ICM flattens into two layers: epiblast and hypoblast
• Amniotic cavity develops between the epiblast and trophoblast
• The blastocyst cavity forms the primitive yolk sac

Clinical Relevance: Implantation Problems

• Implantation may not be successful due to: abnormal blastocyst, blastocyst not reaching the uterus, non-receptive endometrium
• Ectopic implantation: outside the body of the uterus (fallopian tubes, ovary, abdomen, or cervix)
• Sub-optimal implantation can lead to placental problems later in pregnancy
• Clinical Relevance: Ectopic pregnancy: Implantation outside the uterus (often in the fallopian tubes)
• Clinical Relevance: Invasive placentation: Placenta invades the myometrium (muscle wall of the uterus). Severe hemorrhage can result if the placenta does not detach easily after birth

Gastrulation (Week 3)

• A critical event
• Bilaminar disc develops into a trilaminar disc
• Establishes the three germ layers (ectoderm, mesoderm, endoderm)
• Further specification of body axes: Head-to-tail (AP), Front-to-back (DV), Left-to-right (L/R)

The Primitive Streak

• Appears as a groove at the caudal end of the epiblast

Epiblast and Mesoderm Formation

• Epiblast cells migrate towards and through the primitive streak
• Epiblast cells settle between epiblast and hypoblast forming the mesoderm

Clinical Relevance: Laterality Anomalies

• Laterality anomalies, like Situs Inversus and Dextrocardia, are conditions of abnormal sidedness
• Situs Inversus: Thoracic and abdominal viscera are reversed (mirror image)
• Dextrocardia: Heart position is reversed

Neurulation (Week 4)

• The process of forming the neural tube
• Neural plate forms from thickening of the ectoderm (induced by notochord (mesoderm structure))
• Plate bends to form a groove; cells at the top of folds are neural crest cells
• Neural tube forms from the closing of the neural groove
• Neural tube detaches from the ectoderm
• Cranial end forms the brain
• Caudal end remains as the spinal cord (spinal cord and tube)

Test your knowledge on key embryological concepts such as blastocyst differentiation, gastrulation, and neurulation. Understand the significance of these processes in embryonic development. This quiz also covers topics like capacitation and polyspermy prevention in fertilization.