Embryonic Development and Organogenesis

Physiology Sneak Peek

• The physiology course builds upon chemistry knowledge.
• Topics include:
  • Solubility
  • Pressure gradients
  • Blood pressure effects on the glomerulus and potential arteriosclerosis
• Essential chemistry concepts: Diffusion.
• Understanding pressure gradients.
• Basic chemistry knowledge is crucial for success in physiology.
• Physiology will cover calcium and hydrogen ions, their charges, concentration gradients, diffusion, and osmosis—topics typically learned in Chemistry 102 or 107.

Exam and Lab Practical Information

• The upcoming Monday is a regular lecture day.
• The Wednesday after that features your lecture final.
• The following Monday after the lecture final is Memorial Day (no class).
• Following lab practical is on that wednesday as well.
• Lab practical starts at 10:10 AM.

Fertilization

• Male ejaculation deposits sperm cells into the female reproductive system.
• Sperm cells release acrosomal digestive enzymes to penetrate the oocyte's jelly coat in the fallopian tube.
• One sperm cell injects its nucleus into the oocyte.
• Male infertility factors:
  • Temperature regulation.
  • Sperm cell production issues (e.g., two heads, no acrosome, jagged tail)
• Only about 5% of sperm cells are capable of fertilization.
• Husbands wanting children should "detox" (stop drinking/smoking, eat healthily).
• Female infertility factors:
  • STDs causing scar tissue.
  • Acidic uterus environment.
• Post-fertilization: formation of a fertilization membrane prevents additional sperm from entering.
• Sperm and oocytes are haploid, containing 23 chromosomes each.
• Human cells are typically diploid, with 46 chromosomes (two sets of 23).
• Triploid fertilized oocytes are inviable.
• The fertilization membrane prevents multiple sperm from fertilizing the oocyte, which would lead to a triploid cell.

Embryonic Cleavage

• After fertilization, the oocyte undergoes cleavage, becoming two cells, then four, eight, sixteen, thirty-two, and so on.
• This continues until the blastocyst stage, around ten days post-fertilization.
• The blastocyst implants in the uterus (specifically, the endometrium's stratum functionalis).
• Cleavage is different from mitosis.
  • Mitosis: One cell becomes two identical cells (same size).
  • Cleavage: One large cell divides into multiple smaller cells that fit within the original size.
• Cleavage technically is replication, but it's not the same as mitosis.
• Meiosis results in cells with different genetic variations, unlike mitosis, which makes identical copies.

Blastocyst and HCG

• The blastocyst releases human chorionic gonadotropin (HCG).
• HCG maintains the corpus luteum, preventing its degradation into corpus albicans.
• Corpus luteum is crucial for maintaining estrogen and progesterone levels.
• Drop in estrogen/progesterone leads to menstruation and loss of the stratum functionalis, resulting in loss of the blastocyst and prevents successful pregnancy.
• High levels of estrogen or progesterone prevent ovulation or implantation.
• The corpus luteum grows in size, releasing more estrogen and progesterone to support the stratum functionalis and the developing embryo.

Blastocyst Structure

• The blastocyst has two layers:
  • Trophoblast: Outer layer that becomes the placenta.
  • Inner cell mass: Becomes the embryo and contributes to extraembryonic membranes.
• The placenta is made by the embryo to obtain nutrients and oxygen from the mother.

Twins

• Fraternal twins: Two separate eggs are released and fertilized by two different sperm, resulting in different genetic makeups.
• Identical twins: The inner cell mass splits, resulting in two genetically identical inner cell masses derived from a single fertilized oocyte.
• Conjoined twins: Incomplete separation of the inner cell mass.

Ethical Considerations of Genetic Modification

• CRISPR technology allows for modification of a cell's DNA post-fertilization.
• Potential to alter traits like height, appearance, and other characteristics.
• Ethical question: Should we modify human DNA?

Placenta

• In fraternal twins, each embryo implants separately and has its own placenta.
• In identical twins, there is a single implantation, and the embryos share one placenta.
• Sharing a placenta can lead to complications due to nutrient competition.

Pregnancy Tests

• Pregnancy tests detect HCG, a hormone produced only when a blastocyst is present.
• ELISA tests detect the presence of HCG in a woman's urine.
• HCG levels spike after implantation, around week eight, which is when morning sickness usually begins.
• HCG levels eventually fall as the embryo/fetus no longer needs the corpus luteum.

Corpus Luteum and Corpus Albicans

• The corpus luteum is essential in early pregnancy and grows to be almost the same size as the ovary.
• Corpus albicans are remnants of previous pregnancies.

Implantation

• The blastocyst burrows into the stratum functionalis.
• Trophoblast cells develop and invade the endometrium, forming chorionic villi.

Chorionic Villi and Chorion

• Chorionic villi increase surface area for nutrient and waste exchange.
• The chorion is composed of chorionic villi and contains blood vessels.
• The chorion is the embryonic version of the placenta and eventually becomes the placenta.
• Both chorion and placenta supply nutrients/oxygen and remove wastes.
• The chorion surrounds the entire embryo, while the placenta is localized.

Chorionic Villi Structure

• Chorionic villi are made of simple squamous epithelium to facilitate diffusion between maternal and embryonic blood.
• Maternal blood fills spaces around the villi, allowing for exchange.
• The uterine artery supplies blood, and the uterine vein removes it.
• Umbilical arteries carry deoxygenated blood from the embryo's heart to the chorionic villi.
• Umbilical veins carry oxygenated blood back to the embryo's heart.
• Maternal and embryonic blood do not mix, but diffusion occurs.

Antibody and Virus Transfer

• Antibodies from the mother can pass to the child, providing immunity.
• Viruses (e.g., COVID-19), alcohol, and drugs can also cross the epithelium.

Placenta Previa

• Placenta previa occurs when the placenta covers the cervix, complicating delivery.

Fetal Circulation

• Umbilical arteries and veins facilitate fetal circulation.
• The umbilical vein passes through the liver, leaving a scar-tissue line.
• Shunts (arteriosus and foramen ovale) bypass the lungs since the fetus does not use them for oxygen exchange.

Gastrulation and Germ Layers

• After the blastocyst stage, gastrulation occurs, forming two layers (epiblast and hypoblast) that become three germ layers:
  • Ectoderm: Becomes epidermis, hair, nails, sweat glands, and nervous system.
  • Mesoderm: Becomes connective tissues (bone, cartilage, blood), muscles, and cardiovascular system.
  • Endoderm: Becomes gastrointestinal tract, liver, pancreas, lungs, and parts of the renal system.
• The germ layers undergo morphogenesis to change structure.

Germ Layer Orientation

• Ectoderm is outermost, appropriate for skin and nervous system components.
• Mesoderm is underneath, aligning with blood vessels and muscles.
• Endoderm is deepest, correlating with GI tract position.

Extraembryonic Membranes

• The endoderm forms the yolk sac, a vestigial structure in humans.
• Vestigial structures are evolutionary relics.
• Humans have evolved to not depend on the yolk sac for nutrients, unlike chickens.
• The ectoderm forms the amniotic sac, filled with amniotic fluid.

Amniotic Fluid

• Functions of amniotic fluid:
  • Suspends the embryo for three-dimensional development.
  • Cushions the embryo against trauma.

Ontogeny Recapitulates Phylogeny

• Ontogeny (embryonic development) recapitulates (repeats) phylogeny (evolutionary development).
• The yolk sac is evidence, as human embryos form it, but it's vestigial.
• Embryos developing in a watery environment is another piece of evidence.
• Early embryos of different species look very similar, suggesting a shared common ancestor.
• Cat bones are named similarly to human bones, indicating shared ancestry and evolutionary changes.

Organogenesis

• After two weeks of implantation, organogenesis occurs (production of organs).
• Involves morphogenesis (3D shape formation) and differentiation (cell specialization).

Differentiation Factors

• Chemical gradients: Specific chemicals in the stem cell's region influence its specialization.
• Induction: Physical contact with other cells influences differentiation (like peer pressure).

Chemical Gradients

• Chemical gradients influence stem cell differentiation.
  • Example: presence of activin A directs stem cells into mesoendoderm; minimal medium directs them into ectoderm.
• The amount of these chemicals also drives the type of differentiation.

Induction

• Peer pressure like induction in the cell world.
• Example: pluripotent stem cell next to cardiac cell will become cardiac muscle cell.
• Nearby cells signal to stem cell to direct the final product.

Embryonic Manipulation

• Mice mesoderm implanted into a chicken's beak can induce tooth growth.
• Chickens have teeth in their DNA, but they can not express it.
• During evolution, chickens stopped making mesoderm in the beak region.
• Meso implant to beak induces chicken to grow teeth.
• Chickens may be descendants of dinosaurs.

Neurulation

• Neurulation is the morphogenesis of the nervous system.
• Ectoderm folds to form the neural tube, running from anterior to posterior.
• If the neural tube doesn't close completely, disorders results:
  • Spina bifida: posterior failure to close leads to spinal cord and nerve issues, plus paralysis.
  • Anencephaly: anterior failure to close leads to no cerebrum and short life expectancy.
• Anxiety may be caused by many stimuli. Drug induced Phsychosis attacks thalamas, cerbreal cortex , and drugs effect nerurons.

Neural Crest

• The neural crest forms from the ectoderm and migrates to become the peripheral nervous system.
• Neural crest cells also form melanocytes (pigment cells).

Mongolian Spots

• Mongolian spots (bluish spots on the lower back) are common in individuals from Mongol regions due to overreacting melanocytes.
• Clinical curiosity; no functional purpose.

Somites

• Somites are cubes of mesoderm and determine segmentation patterns.
• Explain segmentation patterns like ribs, intercostal muscles, vertebrae, and dermatomes.

Four-Week-Old Embryo

• The heart is the largest organ and is already morphologically developed.
• The heart develops first due to the high pressure required to send blood to the chorion for nutrient and waste exchange.

Embryo vs. Fetus

• These terms are arbitrary.
• Embryo: Could be an embryo or other species.
• Fetus: Looks human.

Final Review

• Additional lecture material needs to be covered.
• Study to be prepared for the lecture.