Early Human Development and Genetics Study Guide

Early Embryonic Cleavage and the Morula Stage- lecture

Cells undergo division through mitosis during the early stages of development.
The term "Morula" refers to a solid ball of cells phase, typically characterized by having at least $16$ cells, but commonly ranging from $16$ to $32$ cells under normal conditions.
A critical observation in early development is that the overall size of the embryo does not change during these initial divisions. Although the cell number increases, the diameter remains the same because the embryo is still contained within the fallopian tube.
It is only when the embryo reaches the blastocyst stage and enters the uterus that it begins to grow significantly larger. Many diagrams incorrectly depict the embryo growing in size during the cleavage stages.

The Blastocyst and Implantation

The blastocyst consists of a fluid-filled cavity and the inner cell mass ( $\text{ICM}$ ).
The inner cell mass is the specific cluster of cells that will eventually differentiate into the embryo and then the fetus.
Implantation typically occurs between day $5$ and day $7$ after fertilization ( $5$ to $7$ days is the general range, though some stages are noted around day $6$ ).
The blastocyst travels through the fallopian tube toward the uterus. It normally implants on the posterior side of the uterine wall, which is considered the safest location.
During implantation, the trophoblast cells (the outer layer of the blastocyst) secrete enzymes. These enzymes dissolve maternal cells to allow the blastocyst to burrow into the endometrium.
Eventually, the endometrium grows over the blastocyst, securing it within the uterine wall.
Prior to implantation, the embryo sheds the $Zona \, Pellucida$ , which is an outer protective covering located just outside the cells and the $Corona \, Radiata$ .
During the orientation of implantation, the inner cell mass is always positioned toward the uterine wall.

Abnormal Implantation: Ectopic Pregnancy and Placenta Previa

An ectopic pregnancy occurs when the embryo implants in a location other than the uterus.
The most common type of ectopic pregnancy is a tubal pregnancy, where the embryo implants in the wall of the fallopian tube. This is a dangerous condition as it starts growing in a confined space.
Another abnormal condition involves implantation too close to the internal os (the opening of the cervix), often referred to erroneously in common parlance as a "placental hernia" in this context, but specifically described as the embryo implanting near the opening.
This occurs in approximately $1$ out of every $200$ pregnant women (a ratio of $0.5\%$ ).
This condition can cause vaginal bleeding, characterized by dark red blood, during the second and third trimesters as the placenta forms over the opening.

Timeline of Fertilization and Early Development

Fertilization occurs specifically in the $Ampulla$ area of the fallopian tube.
For fertilization to be successful, sperm must already be in the female reproductive system because the secondary oocyte only lives for approximately $24$ hours after ovulation.
Within the first day after fertilization, cleavage begins.
Three to four days after fertilization, the embryo reaches the morula stage ( $16$ to $32$ cells).
Five to seven days after fertilization, the blastocyst reaches the uterus and undergoes implantation.
At approximately day $7$ , the embryo "hatches" by shedding the $Zona \, Pellucida$ , rotates, and implants into the endometrium.

Hormonal Regulation: The Role of hCG

The most prominent hormone right after fertilization is $hCG$ ( $Human \, Chorionic \, Gonadotropin$ ).
$hCG$ secretion begins around day $8$ to day $10$ post-fertilization.
This hormone serves as the primary signal to the maternal reproductive system that implantation has occurred, triggering preparations for pregnancy.
$hCG$ levels are extremely high during the first $3$ months of pregnancy.
The function of $hCG$ is to maintain the $Corpus \, Luteum$ . Normally, the $Corpus \, Luteum$ degenerates if fertilization does not occur.
However, if fertilization and implantation occur, $hCG$ keeps the $Corpus \, Luteum$ active for approximately $3$ months to secrete estrogen and progesterone, which are essential for maintaining the uterine wall (endometrium).
After the third month, the placenta is fully formed and takes over the production of estrogen and progesterone, at which point $hCG$ levels drop drastically and the $Corpus \, Luteum$ degenerates.

Pregnancy Testing Mechanisms

Pregnancy tests detect the presence of the hormone $hCG$ .
In these tests, the hormone acts as an antigen. The test strips are soaked with specific antibodies.
When urine containing the antigen ( $hCG$ ) is applied to the strip, an antigen-antibody reaction occurs, producing a visible line.
Tests can be performed as early as $2$ weeks after fertilization, but testing too early (e.g., at $7$ days) may result in a false negative because $hCG$ levels are not yet high enough for detection.
Other factors like diluents can cause false results.

Week Two of Development: The Bilaminar Embryonic Disc

In the second week, rapid divisions occur, and the inner cell mass (also called the embryoblast) forms the embryonic disc.
The embryonic disc consists of two layers (bilaminar):
1. $Hypoblast$ : The outer layer that eventually becomes the primary yolk sac and the $Chorion$ (the outermost extraembryonic membrane).
2. $Epiblast$ : The inner layer that develops into the actual embryo and the three primary germ layers.
The trophoblast also divides into two parts:
1. $Cytotrophoblast$
2. $Syncytiotrophoblast$ : This part burrows into the endometrium, secures the embryo, and is responsible for secreting $hCG$ .

Week Three of Development: Gastrulation and Germ Layers

During the third week, the bilaminar disc undergoes a process called $Gastrulation$ .
Gastrulation converts the two-layered disc into a three-layered structure known as a $Gastrula$ .
The three primary germ layers are:
    1. $Ectoderm$ : Becomes the nervous system and the integumentary system (skin).
    2. $Mesoderm$ : Becomes the middle structures (muscles, bones, circulatory system).
    3. $Endoderm$ : Becomes the internal linings of organs.
During this week, the foundation for all major organ systems is established, and the heartbeat begins.

Extraembryonic Membranes and Their Functions

There are four major extraembryonic membranes:
    1. $Yolk \, Sac$ : Provides nutrients during the first $2$ weeks of development. It is the first site of $Hematopoiesis$ (blood cell formation) before the liver and spleen take over. it also houses the primitive gonadal stem cells ( $Spermatogonia$ and $Oogonia$ ) before they migrate to the developing ovaries or testes.
    2. $Amnion$ : A single layer that covers the amniotic cavity and secretes amniotic fluid. This "bag of water" protects the fetus from drying out and cushions it. The fetus eventually releases nitrogenous waste (urine) into this fluid.
    3. $Chorion$ : The outermost layer. Its primary function is gas exchange. The part of the chorion that attaches to the endometrium becomes the fetal portion of the placenta.
    4. $Allantois$ : Located within the umbilical cord. it assists in gas exchange and the removal of waste products from the fetus.

Prenatal Diagnostic Testing: Amniocentesis

Amniocentesis is a genetic test recommended especially for mothers over the age of $35$ or $40$ due to increased risks of genetic abnormalities during mitosis and meiosis.
The procedure involves using ultrasound to locate the baby and placenta, then inserting a needle to take a sample of amniotic fluid.
The amniotic fluid contains fetal cells that have sloughed off the body. By analyzing the $DNA$ in these cells, doctors can detect genetic disorders such as Down syndrome or other degenerative disorders.
Risk Factor: The test can lead to complications such as leaking amniotic fluid or miscarriages.

Physiology of the Placenta and Umbilical Cord

The placenta and umbilical cord are fully formed by the third month ( $3$ months).
$Chorionic \, Villi$ : These are branching structures on the maternal side of the placenta that increase the surface area to facilitate efficient gas, nutrient, and waste exchange.
Interaction of Blood: Maternal blood (from arteries) fills spaces around the chorionic villi, and deoxygenated blood returns to the mother via veins.
CRITICAL RULE: Maternal and fetal blood do NOT mix. They remain separated by the walls of the fetal capillaries. Mixing typically only occurs during childbirth, which can introduce fetal antigens into the mother's bloodstream (relevant for the $Rh$ factor).
Protective Transfer: Certain antibodies, such as $IgG$ , can cross the placenta to provide immunity to the fetus. However, harmful microorganisms and substances can also cross.
Endocrine Function: The placenta secretes $hCG$ (minimal amounts later on), estrogen, and progesterone to maintain the pregnancy.

Teratogens and Fetal Damage

Teratogens are substances that cause developmental malformations (fetotoxic substances).
$Alcohol$ : causes Fetal Alcohol Syndrome ( $FAS$ ), leading to irreversible brain damage and growth problems.
$Smoking \, and \, Vaping$ : Can cause low infant birth weight, cardiac abnormalities, and $Anencephaly$ (where parts of the brain and skull bones are missing at birth, leading to certain death).
$Radiation$ : Ionizing radiation (e.g., $X-rays$ ) can cause $Microcephaly$ (abnormally small head).
$Other \, Substances$ : Viruses, certain bacteria, chemicals, hormones, antibiotics, cocaine, and drugs can all cross the placenta and cause harm.

Twinning and Multiple Births

$Dizygotic \, Twins$ : Formed from two separate eggs fertilized by two separate sperm. They possess unique genetic structures and typically have separate placentas and amniotic cavities.
$Monozygotic \, Twins$ : Formed from a single zygote that splits into two embryos. They are genetically identical.
The degree of sharing depends on when the split occurs:
    - Day $1$ to $3$ : Dichorionic and diamniotic (separate everything).
    - Day $4$ to $8$ : Monochorionic but diamniotic (share a placenta, separate amniotic sacs).
    - Day $8$ to $13$ : Monochorionic and monoamniotic (share placenta and amniotic sac).
    - After Day $13$ : If the split is incomplete, it results in conjoined twins because the germ layers ( $Mesoderm$ , $Endoderm$ ) and organs have already begun to form.
$Superfertility$ : In rare cases, humans can have eggs ovulated and fertilized weeks apart, though this is more common in animals.

Principles of Genetics and Inheritance

$Mendel's \, Second \, Law \, (Segregation)$ : Describes how alleles for a trait separate during the formation of gametes (sex cells).
$Punnett \, Squares$ : Used to determine the potential genotypes (genetic makeup) and phenotypes (physical appearance) of offspring based on parental alleles.
$Sex \, Determination$ : The father determines the sex of the baby because he carries both $X$ and $Y$ chromosomes, whereas the mother only contributes $X$ chromosomes.
$Simple \, Inheritance$ : One pair of alleles for a single gene (dominant and recessive).
$Incomplete \, Dominance$ : Example: Sickle Cell Anemia.
    - Normal Hemoglobin: $HBA$
    - Mutated Hemoglobin: $HBS$
    - Heterozygous ( $HBA/HBS$ ): Known as the sickle cell trait. These individuals are usually asymptomatic but may have issues during strenuous exercise. Their blood contains a mixture ( $50/50$ ) of normal and sickle-shaped cells. Sickle cells live only $10$ to $20$ days compared to the normal $120$ days, leading to anemia.
$Codominance$ : Multiple alleles are equally dominant.
- Example: $ABO$ blood types. The alleles $IA$ and $IB$ are codominant, while lowercase $i$ is recessive. This results in the $AB$ blood type where both proteins appear on the cell surface.
$Polygenic \, Inheritance$ : Traits determined by more than one gene, resulting in a wide continuous range of phenotypes (e.g., skin color, hair color). In a population, these traits often follow a bell curve distribution.
$X-Linked \, Inheritance$ : Traits located on the $X$ chromosome. Since males only have one $X$ chromosome, recessive traits (like color blindness) are expressed more frequently in males. Females must have two recessive alleles to show the trait.

Questions & Discussion

Student Observation: "Is it really rare? Like, it's called super fertile for humans."
Professor Response: It is very rare in humans but common in animals. There are cases where eggs are fertilized at different times.
Student Comment on Pregnancy Symptoms: Students discussed how pregnant women "look like a watermelon," experiencing swollen ankles and weight gains of up to $80$ pounds. They noted how "scary" the physical toll of pregnancy can be.
Dialogue on Genetic Testing: Regarding amniocentesis, the discussion touched upon the risks of the wound leaking amniotic fluid after the needle is inserted.
Student Question: "Can you go back to the previous question where you asked about the conjoined split?"
Professor Response: Re-explaining that conjoined twins occur when the embryo does not completely split after day $13$ , at which point organs have already started to form.

Cells undergo division through mitosis during the early stages of development. These initial divisions are crucial, as they set the stage for subsequent cell differentiation and growth.
The term "Morula" refers to a solid ball of cells phase, typically characterized by having at least $16$ cells, but commonly ranging from $16$ to $32$ cells under normal conditions. This stage represents a critical transition from a single cell to multiple cells, establishing the foundation for further embryonic development.
A critical observation in early development is that the overall size of the embryo does not change during these initial divisions. Although the cell number increases, the diameter remains the same because the embryo is still contained within the fallopian tube. This is significant as it helps to understand the mechanics of early development.
It is only when the embryo reaches the blastocyst stage and enters the uterus that it begins to grow significantly larger, preparing for implantation and subsequent stages of development. Many diagrams incorrectly depict the embryo growing in size during the cleavage stages, leading to misunderstandings about embryological growth processes.

The Blastocyst and Implantation

The blastocyst consists of a fluid-filled cavity and the inner cell mass ( $ext{ICM}$ ), which is crucial for future development as it differentiates into various tissues.
The inner cell mass (ICM) is the specific cluster of cells that will eventually differentiate into the embryo and then the fetus, highlighting the importance of proper cell lineage in developmental biology.
Implantation typically occurs between day $5$ and day $7$ after fertilization, with variations noted around day $6$ . This timing is essential for the synchronization of embryonic and maternal development.
The blastocyst travels through the fallopian tube toward the uterus, where it normally implants on the posterior side of the uterine wall, which is considered the safest location for successful pregnancy.
During implantation, the trophoblast cells (the outer layer of the blastocyst) secrete enzymes that dissolve maternal cells, facilitating the blastocyst's burrowing into the endometrium, which is critical for establishing a maternal-fetal connection.
Eventually, the endometrium grows over the blastocyst, securing it within the uterine wall, an essential step for maintaining pregnancy.
Prior to implantation, the embryo sheds the $Zona ext{Pellucida}$ , which is an outer protective covering located just outside the cells, alongside the $Corona ext{Radiata}$ , marking the transition from a protected to an invasive growth stage.
During the orientation of implantation, the inner cell mass is always positioned toward the uterine wall, demonstrating the specificity of embryonic placement in uterine tissue.