Embryonic Development Stages

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.
• Understand that congenital anomalies result from disrupted embryonic development.

Fertilization

• Millions of sperm enter the female reproductive tract, but few reach the uterine tube.
• Sperm are "conditioned" (capacitated) in the uterine tube before fertilization.
• Fertilization is the fusion of gametes.

How Sperm Moves to the Tube?

• Contractions from smooth muscle cells in the uterine wall move sperm upwards into the fallopian tube.

What Happens After Sperm Enters the Tube

• Sperm survive in the fallopian tube for up to 5 days.
• Sperm are conditioned (capacitated) before fertilization.

Capacitation

• A period of time sperm need to reside in the female reproductive tract before acquiring the ability to fertilize oocytes.

Fertilization Process

• Sperm reaches the ovum.
• Chemicals are released, triggering the acrosome reaction.
• The acrosome swells and fuses with the sperm cell membrane.
• Digestive enzymes are released.
• Enzymes digest follicle cells and the zona pellucida.
• Sperm fuses with the ovum's membrane.
• Nuclei of ovum and sperm fuse.
• Enzymes thicken the ovum's outer layer, preventing other sperm entry.

Polyspermy Prevention

• The zona pellucida becomes impermeable, which prevents multiple sperm from fertilizing the ovum.

Clinical Relevance: Molar Pregnancy

• Two types:
  • Complete mole: Single sperm fertilizes an egg lacking genetic material. Abnormal trophoblast develops, but no embryo.
  • Partial mole: Normal egg fertilized by two sperms. Embryo develops but cannot survive.
• Molar pregnancy gives a positive test result.

The Zygote

• Fertilization produces a zygote that is genetically unique.

Cleavage

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

Morula

• Cleavage process produces a morula.

Blastomeres

• Cells formed from cleavage of the fertilized ovum.

Blastocyst Arrangement (Morula)

• Blastomeres are tightly compacted within the morula.

Blastocyst Structure

• Surrounded by the zona pellucida (glycoprotein coat).
• Cavity separates the cells into inner and outer groups.
• Inner cell mass (ICM) develops into the embryo.
• Outer cells (trophoblast) develop into the placenta.
• ICM cells differentiate into hypoblast and epiblast.

Blastocyst Hatching

• Blastocyst sheds its outer membrane and attaches to the uterine lining to prepare for implantation.

Implantation

• Three Stages:
  • Apposition: First physical contact between blastocyst and endometrium.
  • Adhesion: Blastocyst adheres to endometrium.
  • Invasion: Penetration of the endometrium by the trophoblast
• Complex signaling is necessary between blastocyst and endometrium for a successful implantation.
• Blastocyst must implant adequately but not too deeply.
• Blastocyst makeup is different from maternal tissue. This prevents rejection by the mother.

Implantation: Differentiation of Trophoblast

• Cytotrophoblast at the embryonic pole proliferates.
• Cytotrophoblast invades endometrium.
• Syncytiotrophoblast forms maternal blood filled spaces (lacunae).
• Trophoblast contacts maternal vessels, enabling gas exchange between maternal and fetal circulation.

Implantation: Changes in the Inner Cell Mass

• Inner cell mass flattens into two layers.
• Layers are epiblast and hypoblast.
• Blastocyst cavity is the primitive yolk sac.

Clinical Relevance: Implantation Problems

• Implantation may be unsuccessful if the blastocyst is abnormal or the endometrium is not receptive.
• Ectopic pregnancy occurs when implantation occurs outside the uterus (e.g., fallopian tubes, ovaries, abdomen, cervix).
• Suboptimal implantation can cause placental problems later in pregnancy.

Clinical Relevance: Ectopic Pregnancy

• Tubal (uterine tube) ectopic pregnancy.
• Pregnancy occurs outside the uterus.

Clinical Relevance: Invasive Placentation

• Placenta invades too far into the uterine muscle layers.
• Placenta may attach to adjacent tissues.
• Placenta does not detach easily after birth.
• Can cause severe hemorrhage.

Gastrulation (Week 3)

• Bilaminar disc develops into a trilaminar disc.
• Establishes three germ layers for development.
• Primitive streak: Marks the beginning of gastrulation.

Primitive Streak

• Appears as a groove (caudal end) on the epiblast.
• Cells migrate and invaginate through the epiblast.
• Cells form mesoderm and other layers.

Neurulation (Week 4)

• Process that forms the neural tube.
• Neural plate formation (ectoderm thickening) is induced by notochord.
• Neural plate bends to form a groove.
• Cells at the top of the folds become neural crest cells.
• Two sides of the groove fuse to form the neural tube.
• Neural tube detaches from the ectoderm.
  • Cranial end expands to form the brain.
  • Caudal end remains tubular, forming the spinal cord.

Clinical Relevance: Laterality Anomalies

• Congenital conditions involving abnormal "sidedness".
• Situs inversus: Thoracic and abdominal viscera are flipped.
• Dextrocardia: Heart is flipped.

Other Key Concepts

• Embryonic development is critical.
• Genetic abnormalities can cause spontaneous pregnancy loss.

This quiz covers the critical stages of embryonic development, from fertilization through the first four weeks. Understand the differentiation of the blastocyst and the formation of germ layers, as well as how congenital anomalies may arise from disrupted development. Test your knowledge on the processes that form the foundation of human development.