Embryology, Fetal Development, and Genetics

Flashcards reviewing embryology, gametogenesis, fertilization, fetal organ development, placental structures, chromosomal abnormalities, and prenatal genetic testing.

What is the definition of embryology?

The study of the science concerned with the origin and development of a human being from a zygote to birth of an infant.

How is a zygote formed?

A zygote is formed by the union of an oocyte (egg) and a sperm (spermatozoon).

What is the difference between an embryo and a fetus?

An embryo is a developing human during early stages of development (until 8 weeks gestation), while a fetus is a developing human after the embryonic period (from 9 weeks until birth).

How do mitosis and meiosis differ in terms of cell division type and outcome?

Mitosis replicates somatic cells where a single cell divides into two identical cells with the same genetic material to facilitate growth, development, or replacement. Meiosis divides sex (germ) cells into haploid cells, with Meiosis II resulting in 4 gametes (sperm or oocytes).

When does oogenesis begin, and what happens to female eggs prior to puberty?

Oogenesis begins during embryological and fetal development of the female. All potential eggs start to undergo the first meiotic division to replicate DNA but stop before it is finished until puberty.

What are the products of the first meiotic division in oogenesis after puberty?

One secondary oocyte and a small polar body, each containing 22 autosomes and 1 X chromosome.

When does the second meiotic division in oogenesis complete?

The second meiotic division begins after ovulation and completes only if the oocyte is fertilized.

What is a spermatogonium and what are the steps of spermatogenesis?

A spermatogonium is a primitive sperm cell produced in the testes at the onset of puberty. It undergoes a first meiotic division to form 2 haploid secondary spermatocytes (each with 22 autosomes and 1 sex chromosome), which then divide to result in 4 sperm.

Where does fertilization typically occur?

In the ampulla (outer third) of the uterine tube.

What is the zona reaction?

A chemical reaction that occurs during fertilization to prevent more than one sperm from entering the ovum.

What are the viability timeframes for an ovum and a sperm cell?

An ovum is viable for approximately $24\text{ hours}$ after ovulation, while sperm are viable for up to $5\text{ days}$.

What are the two protective layers surrounding the ovum?

The zona pellucida and the corona radiata.

What development occurs on Day 3 and Day 4 after fertilization?

On Day 3, the conceptus becomes a morula (an undifferentiated cell mass). On Day 4, it becomes a blastocyst.

What structures develop from the inner and outer layers of a blastocyst?

The inner layer (Embryoblast) forms the embryo, umbilical cord, and amnion. The outer layer (Trophoblast) forms the chorion and placenta, and produces hCG.

Where does implantation usually occur in the uterus?

The blastocyst embeds in the endometrium, usually in the fundus.

What are chorionic villi and when can they be tested for genetic abnormalities?

Chorionic villi are finger-like projections from the trophoblast that link to the maternal blood supply and excrete waste. They can be tested for genetic abnormalities between $10-12\text{ weeks}$ gestation.

What body systems derive from the three embryonic germ layers?

1. Endoderm layer becomes: Digestive system, liver, pancreas, and inner layers of lungs.
2. Mesoderm layer becomes: Circulatory system, epithelial layers of lungs, skeletal system, and muscular system.
3. Ectoderm layer becomes: Hair, nails, skin, and nervous system.

What are the key structural features and weight of the placenta at term?

The maternal side is rough, convoluted, and dull red. The fetal side is shiny and smooth. The placenta weighs about $1\text{ lb}$ at term and contains approximately $500\text{ mL}$ of blood.

What are the two fetal membranes and their origins?

The chorion develops from the trophoblast, and the amnion develops from the blastocyst.

What are the main functions of amniotic fluid?

Maintains constant temperature, cushions the fetus, allows movement and development of fetal limbs, provides vitamins, proteins, IgG, and electrolytes, exhibits antibacterial properties, and is vital for fetal lung development.

What is the vessel structure and length of the umbilical cord?

The umbilical cord contains 3 vessels (AVA: 2 arteries and 1 vein), measures $30-90\text{ cm}$ in length (average $55\text{ cm}$), is covered with Wharton's jelly, and develops from the connecting stalk.

What defines fetal viability and at what gestational age is it reached?

Viability is the ability to survive outside the uterus based on organ development (lungs, neuro, GI, renal), typically reached between $22-25\text{ weeks}$.

When does the fetal heartbeat begin and what is the typical fetal heart rate range?

The heartbeat starts in the 3rd week after conception and runs at a rate of $110-160\text{ BPM}$.

What physical clinical signs are associated with Down Syndrome (Trisomy 21)?

Low set ears, Brushfield spots in eyes, a single palmar crease, and hypotonia.

Based on the image below, identify the types of structural chromosomal changes shown.

The structural chromosomal changes shown are Deletion, Duplication, Inversion, Translocation, and Isochromosome.

What are the differences between autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive inheritance patterns?

1. Autosomal dominant: Only one mutated allele is needed for expression (e.g., Marfan's Syndrome).
2. Autosomal recessive: Two copies of the mutant gene are needed for expression (e.g., Sickle cell disease).
3. X-linked dominant: Dominant gene carried on an X-chromosome (e.g., Fragile X).
4. X-linked recessive: Recessive gene carried on an X-chromosome (e.g., Hemophilia).

What is Alpha-Fetoprotein (AFP) screening and when is it performed?

AFP is a maternal serum test performed between $15-18\text{ weeks}$ gestation. Increased levels are associated with neural tube defects (NTDs).

What is Fetal Nuchal Translucency (FNT) screening and what does it indicate?

FNT is a vaginal probe ultrasound performed between weeks $10-14$ to measure fluid collection at the base of the fetal neck. It is associated with genetic disorders, trisomy 13, 18, 21, and Turner syndrome.

What components are measured in Triple & Quad Screening and what is its purpose?

It measures maternal serum levels of AFP, estriol, beta-HCG, and inhibin A between weeks $15-18$ to identify risk for Down syndrome, neural tube defects (NTDs), and other trisomy conditions.

What are the indications for recommending prenatal genetic testing?

Advanced maternal age (AMA), known or suspected chromosomal rearrangement, previous pregnancy with a congenital disorder, history of perinatal loss, abnormal quad screening or ultrasound, and family history of congenital anomalies or genetic disorders.