Embryology and Development

Somites and Limb Buds

• Somites are repeating patterns that develop into:
  • Bones (e.g., ribs)
  • Muscles (e.g., intercostal muscles)
  • Vertebrae
  • Blood vessels
• Limb buds develop into arms and legs (forelimbs and hindlimbs).
• These limb buds contain cells from the mesoderm.
  • Bones: humerus, radius, ulna, femur, fibula, tibia, etc.
  • Muscle tissues

Limb Development and Chemical Gradients

• The body knows how many bones to develop in each limb due to chemical gradients and induction.
• Signals from neighboring cells and varying chemical concentrations signal specific stem cells to differentiate.
• This allows for the formation of different:
  • Muscles
  • Bones
  • Connective tissues

Apoptosis in Limb Development

• Apoptosis: programmed cell death or cell suicide.
• It is crucial for proper development.
• Limb buds initially resemble paddles.
• Apoptosis sculpts the digits.
  • Apoptosis occurs between the developing fingers to separate them.
• Without apoptosis, webbing between digits can occur.
  • Example: Webbed feet (syndactyly)
  • Michael Phelps: Webbed feet, long limbs. Possible advantage in swimming

Limb Malformations

• Missing limbs can be caused by mutations or teratogens.
• Teratogens: Chemicals that disrupt development.
  • Example: Morning sickness pill caused limb malformations in babies during the critical stages of limb development.
• Pregnant individuals should be cautious about what they ingest.

Branchial Arches

• Branchial arches in fish develop into gills.
• In humans, branchial arches develop into:
  • Middle ear (auditory ossicles)
  • Hyoid bone
  • Mandible (lower jaw)
• Ontogeny recapitulates phylogeny: Embryonic structures are repurposed for different functions across species.
• Specific arch-to-structure relationships are not required knowledge.

Induction in Chicken Wing Development

• Normal chicken wing development:
  • Humerus, radius, ulna, digits
• Digits originate from stem cells in the posterior part of the limb.
• If mesoderm cells are placed on the anterior side, extra digits develop due to induction.
  • The neighboring cells signal the mesoderm cells to become digits.
• Normally, mesoderm doesn't grow on the anterior side, preventing extra digits.
• This concept applies to humans also; implanting mesoderm could cause extra digits.
• Naturally occurring extra digits may arise from accidental mesoderm cell transfer.

Endoderm Development

• The endoderm starts as a tube-like structure.
• The neural tube forms from the ectoderm through neurulation.
• The endoderm undergoes morphogenesis.
• The endoderm gives rise to:
  • Respiratory system
  • Digestive tract
  • Lungs, stomach, liver, intestines develop from this tube.
• Chemical gradients and induction drive the differentiation of the endoderm.

Chemical Gradients example

*Chemicals influence development.
*SOX2 is needed for the development of both lungs and stomach, but lungs also requir: FOXA8, FOXA38, FOXA4, and NKX2.1 while the stomach requires :PTX1, HOXA2, HOXB2, HOXA3, HOXB4

• Stomach cells can induce neighboring stem cells to become stomach cells.

Lung Development and Apoptosis

• Apoptosis is vital for lung development.
• Initial lung structure divides into two lungs through apoptosis.
• Further apoptosis creates lobes (three in the right lung, two in the left lung).
• Apoptosis continues to form primary and secondary bronchi, and alveoli.

Embryo and Fetus Development Timeline

• Embryo vs. Fetus Terminology
  • Embryo: General term for early development.
  • Fetus: Used when development clearly indicates a human.
• Four-week embryo, six-week fetus, nine-week fetus.
• By week nine, limbs are developing, and apoptosis is occurring in the hands.
• Somites and the neural tube are developing.
• The ten-week embryo is relatively small.
• Organogenesis (organ formation) is mostly complete by week four and done by week ten.

Risks During Organogenesis

• Many women don't realize they're pregnant until around week four, when organogenesis is already advanced.
• Exposure to alcohol or drugs during this period can disrupt organogenesis.
• This can retard development.

Facial Development and Alcohol

• Early embryo face:
  • Large mouth opening
  • Nostrils on the sides
  • Eyes on the sides
• Morphological movements are needed:
  • Mouth needs to get smaller and ends need to get closer
  • Nostrils must migrate anteriorly and medially.
  • Eyes must migrate medially.
• These processes can be slowed by alcohol exposure.
• Cleft Lip/Palate
  • Caused by incomplete development of the mouth.
  • Gap in the philtrum.
  • Failure to develop the hard and soft palates, creating a connection between the oral and nasal cavities.
  • Can be corrected with reconstructive surgery.
  • Can happen spontaneously even without alcohol or drug use.
• Fetal Alcohol Syndrome
  • Eyes are farther apart.
  • Nostrils are farther apart.
  • Mouth is wider.
  • Flat nasal bridge.
  • Mental issues and behavioral problems may occur (mental slowness can lead to behavioral issues).

Bipotential Stage of Genital Development

• Six-week-old embryo has undifferentiated genitalia.
• Masculinization (testosterone release) or feminization (no sex hormones) determines development.
• Male: testes release testosterone, leading to masculinization.
  • Scrotum formation
  • Male brain development
• Female: Ovaries remain quiet until puberty (no estrogen release during embryonic stages).
  • Labia majora formation.
  • Round ligament development.
• Men have nipples because the embryo develops as if it might become female.
• Labioscrotal folds:
  • Male: Scrotum.
  • Female: Labia majora.
• Urethral folds:
  • Male: Seam connecting scrotum and penis.
  • Female: Urethral opening.
• Genital tubercle:
  • Male: Glans penis.
  • Female: Glans clitoris.
• Homologous structures originate from similar structures in the bipotential stage.

Genital Development Issues

*Hypospadias
*Malformation of the urethra in males with the urethral opening found on the shaft of the penis and not at the tip of the glans penis
*This can be corrected through reconstructive surgery.
There are also some issues where the testes does not produce testosterone or very low levels of testosterone

Hormone Imbalances and Genital Development

• Genetic male (XY) with insufficient testosterone:
  • Scrotum resembles labia majora.
  • Urethral folds don't develop correctly.
  • Penis resembles an enlarged clitoris.
  • Can be corrected with reconstructive surgery.
• Genetic female (XX) with abnormally high testosterone:
  • Scrotum forms instead of labia majora.
  • The graph forms.
  • Enlarged glans clitoris resembles a penis.
• Hormones have a significant impact on development, not just on the reproductive system but on the entire body (e.g., brain development).

Intersex Conditions

• Intersex conditions can result from mutations.
  • Rare: about 1 in 750,000 to 2 million individuals.
• Sex chromosome misplacements:
  • XXY, XYY, XXX chromosomes.
• Broken hormone receptors:
  • Testicular feminization mutation (TFM): Receptors don't respond to testosterone.
  • Individuals with XY chromosomes may live as females due to non-functional testosterone receptors.
  • Identified during fertility issues.
• Macroscopic anatomical variations: Individuals with both male and female reproductive systems.

Childbirth and Labor

• At birth, the baby's head puts pressure on the cervix and uterus walls.
• Receptors detect the stretching.
• Signals are sent to the hypothalamus.
• Oxytocin is released from the brain.
• Oxytocin causes the myometrium (uterus muscle) to contract.
• Positive feedback loop: More stretching leads to more oxytocin and stronger contractions, until the baby is born.
• After birth, the umbilical cord pulls on the placenta, which detaches.
• The placenta has a thin epithelial tissue separating maternal and embryonic blood.
• Bleeding occurs after placental detachment due to open arterioles and venules.
• Historically, blood loss was a major cause of maternal death during labor.
• Oxytocin promotes faster blood clotting.
• Average blood loss during vaginal birth is one liter (10% of total blood volume).
• Women can still die from blood loss even in hospitals.

Childbirth Options

• Vaginal Birth:
  • Episiotomy is a surgical cut to prevent tearing of the urogenital diaphragm.
• Cesarean Section:
  • Incision through the rectus abdominis and uterus to remove the baby.

Arguments for Vaginal Birth

• Squeezing during vaginal birth:
  • Helps close heart shunts (bypassing the lungs during fetal development).
  • Helps expel fluids from the baby's lungs.
  • Transfers beneficial bacteria from mother to baby.
• Cesarean babies do not experience this physical trauma, so doctors may smack them to induce crying.

Medical Reasons for Cesarean Section

• Placenta previa: Placenta blocks the vaginal canal.
  • Risk of placental detachment and bleeding, endangering both mother and baby.
• Placenta abruptio: Placenta detaches from the uterus prematurely, causing bleeding.
• Transverse position: Baby is in an abnormal position.
• Umbilical cord prolapse: Umbilical cord protrudes through the cervix before the baby.
  • Can cause oxygen deprivation in the baby due to stopped blood flow.
• A large baby or a small maternal pelvis.
  Umbilical cord around the babys neck typically isn't a medical reason for a C section

Anatomical Variations

• Individuals have unique anatomies.
  *Renal Arteries and Veins:
  Textbook image shows a typical single renal artery and vein. Variations include:
  A single renal artery split into two or three.
  Anterior and posterior renal arteries.
  *Common Bile Ducts and Pancreatic Ducts
  Some cases. you have them coming in through different sphincters, and in other other cases, the pancreatic duct joins in with the common bile ducts before we even get close to the duodenum.
  *Aorta
  The organization of major arteries (brachiocephalic trunk, left common carotid artery, left subclavian artery) can vary.
  Variations include dual common carotid arteries.
  Surgeons discover a patient's specific anatomy during surgery.
  Anatomical variations aren't problematic; they're just differences.

Situs Inversus

• Situs inversus: Organs are mirrored (left and right are flipped).
• Functionally, it's not a problem.
• Diagnosis can be problematic.
  • Appendicitis symptoms would be on the left side, potentially leading to misdiagnosis.
• People may have three kidneys or other internal differences without knowing.

Final Remarks and Career Advice

• Anatomy knowledge is essential for healthcare professions.
• Nurses need a strong understanding of anatomy and physiology.
  *Nursing and other for-profit universities only care about how much money that the students are taking in
• Effort and effective study habits are crucial to success.