12.4 Embryo development

Three parts of the prenatal period and their length

• The prenatal period is divided into three stages based on developmental milestones:

• Most congenital anomalies develop during the embryonic period, particularly between weeks 3-8

Define clinical age and developmental age

• Clinical age (gestation age):

  • Measured from the first day of the last menstrual period (LMP)

  • Used routinely in obstetrics

  • Overestimates the developmental age by approximately 2 weeks

• Developmental age (fertilisation age):

  • Begins at fertilisation (conception)

  • More accurate from a biological standpoint

  • Used in embryology and developmental biology

Pre-implantation development and implantation process

• Pre-implantation development

  • Day 0: Fertilisation occurs in the fallopian tube

  • Day 1-3: Cleavage divisions produce a 2-cell, then 4-cell, then 8-cell embro

  • Day 3: Formation of the morula (16-cell stage); cells begin to differentiate into “inside” and “outside” groups

  • Day 5: Formation of the blastocyst, which contains:

    • Trophoblast: outer layer, gives rise to the placenta

    • Inner cell mass (ICM): gives rise to the embryo

    • Blastocyst has polarity: dorsal side contains the ICM, ventral side faces the cavity

• Implantation

  • Begins ~Day 6-7 post-fertilisation

  • Involves:

    • Apposition and adhesion: Blastocyst loosely aligns with endometrial epithelium

    • Attachment: Mediated by molecular interactions between trophoblast and uterine lining

  • Trophoblast differentiates:

    • Cytotrophoblast: progenitor cells

    • Syncytiotrophoblast: Invasive multinucleated layer; secretes enzymes and human chorionic gonadotropin (hCG)

  • Blastocyst is fully embedded by maternal endometrium by Day 11

Role of hCG in maintaining pregnancy

• Produced by: Syncytiotrophoblast from day 6

• Key functions:

  • Maintains the corpus luteum, which secretes progesterone in early pregnancy

  • Prevents menstruation by maintaining endometrial lining

  • Basis for pregnancy tests (detectable in urine and serum

• Peak levels occur around week 8-10, then decline as the placenta takes over progesterone production

Post-implantation development: Gastrulation and neurulation

Week 2: Bilaminar disc formation

• After implantation, the inner cell mass (ICM) of the blastocyst undergoes further differentiation into two distinct layers, forming the bilaminar embryonic disc

  1. Epiblast

     • Located on the dorsal (upper) side

     • Composed of tall, columnar cells

     • This layer is pluripotent and gives rise to all three germ layers (ectoderm, mesoderm, endoderm) during gastrulation in week 3

     • Forms the embryo proper as well as contributing to amniotic membrane development

  2. Hypoblast:

     • Located on the ventral (lower) side

     • Composed of cuboidal cells

     • Does not contribute to the embryo itself but forms extra-embryonic structures, including the primary yolk sac and parts of the extra-embryonic mesoderm

     • Helps pattern the embryo by influencing epiblast development through cell signalling

• A fluid-filled cavity forms above the epiblast, called the amniotic cavity. it will later surround and cushion the developing embryo

Week 3: Gastrulation — formation of three germ layers

• Primitive streak appears on the epiblast

• Epiblast cells migrate through the primitive streak in a process called epithelial-to-mesenchymal transition (EMT) to form:

  • Endoderm, which replaces the hypoblast and forms the gut lining, liver, and lungs

  • Mesoderm, which becomes muscle, skeleton, and the cardiovascular system

  • Ectoderm, which includes the non-migrates epiblast cells and forms skin and the nervous system

• Gastrulation establishes the anterior-posterior, left-right, and dorsal-ventral body axes

Neurulation — formation of the neural tube (week 3-4)

• The notochord (derived from mesoderm) induces the overlying ectoderm to thicken and form the neural plate

• The neural plate folds into the neural groove (around Day 18) and fuses to form the neural tube (between days 21-28)

• Neural crest cells develop at the edges of the neural plate and migrate to form:

  • Peripheral nervous system

  • Autonomic ganglia

  • Adrenal medulla

  • Pigment cells

• The neural canal inside the neural tube becomes:

  • The ventricular system of the brain

  • The central canal of the spinal cord

• Failure of neural tube closure results in neural tube defects (NTDs) such as:

  • Spina bifida

  • Anencephaly

Heart development

• Timing and origin

  • The heart is the first functional organ in human development

  • Begins forming from the mesoderm (specifically, the cardiac crescent) around Day 18-20

  • Heart tube formation begins during week 3

• Developmental process

  1. Day 20: Two endocardial tubes form and fuse into a single primitive heart tube

  2. Day 22: The heart begins to beat rhythmically

  3. Day 24-25: The heart tube starts to pump blood through the developing circulatory system

  4. The primitive heart undergoes folding, looping, and septation to form the mature chambers and outflow tracts

• Key structures

  • Cardiac jelly surrounds the heart tube and helps shape valves

  • Neural crest cells contribute to the outflow tract (conotruncal region)

  • Defects in this region can lead to congenital heart anomalies like Tetralogy of Fallot

Musculoskeletal system development

• Somite formation

  • Derived from paraxial mesoderm, somites are blocks of mesoderms the form along the neural tube

  • 1st pair of somites appear around Day 20

  • By day 30, ~40-44 pairs of somites have developed

• Somite differentiation

  • Each somite differentiates into:

    1. Sclerotome:

       • Forms vertebrae and ribs

    2. Myotome:

       • Forms skeletal muscles of the body and limbs

    3. Dermatome:

       • Forms the dermis of the skin in the back and neck

• Muscle innervation

  • Motor axons from the spinal cord grow into the myotome regions, helping establish the future pattern of muscle innervation

Gametogenesis

• Primordial germ cells (PGCs)

  • Arise from the endoderm near the yolk sac (allantois region) during week 3-4.

  • Migrate to the genital ridge (next to the developing mesonephros)

• Key events

  • PGCs are the precursors of sperm (spermatogonia) and oocytes (oogonia)

  • Once in the genital ridge, these cells undergo mitosis and prepare for meiosis (which begins later depending on sex)

  • Gametogenesis begins prenatally in females (meiosis I arrest at birth), and postnatally in males (puberty onset)

Birth defects

• Prevalence and causes

  • Major structural birth defects occur in ~3% of live births

  • Minor anomalies are found in up to 15%

  • Historically believed to be genetic, now known that environmental factors (teratogens) contribute significantly to

• Common teratogens and effects

  • Thalidomide: Limb malformations (failure of long bone development)

  • Alcohol: Fetal alcohol spectrum disorder (affects CNS and facial development)

  • Infections:

    • Rubella, varicella, cytomegalovirus, parvovirus B19 → miscarriage, anemia, neurodevelopmental issues

  • Drugs, chemicals, and radiation can also disrupt development

Environmental exposures and critical windows

• Timing matters

  • Weeks 3-8 (embryonic period): Highest vulnerability to teratogens; most organs are forming.

  • Earlier exposures (pre-week 3) often result in pregnancy loss rather than defects

  • Each organ system has its own window of susceptibility depending on its development timeline

• Examples of Non-chemical teratogens

  • Maternal fever in early pregnancy increases the risk of:

    • Neural tube defects

    • Heart defects

    • Oral clefts

• Developmental programming

  • Environmental factors during critical developmental periods can result in long-term changes in physiology or disease susceptibility:

    • Low birthweight associated with:

      • Type 2 diabetes, cardiovascular disease, hypertension, PCOS, osteoporosis, schizophrenia, cognitive impairment

    • High birthweight associated with:

      • Allergy, asthma, certain cancers (breast, ovarian, prostate)