Early Embryology 8/5

Where do PGCs originate?

In the wall of the yolk sac during weeks 4-6 of gestation.

Where do PGCs migrate to?

The genital ridge, which forms the future gonads.

When are PGCs called gonocytes?

After reaching the genital ridge.

What do gonocytes differentiate into?

Spermatogonia in males and oogonia in females.

What process leads to mature sperm and eggs?

Meiosis followed by cytoplasmic differentiation.

When does spermatogenesis begin and end?

Begins at puberty and continues throughout life.

When does oogenesis begin and resume?

Begins prenatally, pauses at prophase I, resumes at puberty, and completes upon fertilization.

How does the differences between spermatogenesis and oogenesis affect gamete production?

Men produce sperm continuously; women have a finite number of eggs.

What hormone regulates sperm production?

testosterone

How does testosterone act?

Through androgen receptors on Sertoli cells in the testes.

What is capacitation?

The physiological changes sperm undergo in the female tract to become capable of fertilization.

What is the acrosome reaction?

Release of enzymes that digest the zona pellucida and corona radiata to allow sperm entry.

When does the first meiotic division complete?

Just before ovulation, forming a secondary oocyte and polar body.

When is meiosis II completed?

Only after fertilization.

What causes chromosomal abnormalities?

Nondisjunction, translocations, duplications, deletions, and polyspermy.

How are chromosomal abnormalities linked to miscarriage?

They're a leading cause of early pregnancy loss, especially before week 2.

When is the pre-implantation period?

Week 1: fertilization to implantation.

When is the embryonic period?

Weeks 3-8: organ development.

When is the fetal period?

Week 9 to birth: growth and maturation.

What are teratogens?

Substances that cause birth defects.

When are major anomalies most likely?

During the embryonic period (weeks 3-8)

When are minor anomalies more likely?

During the fetal period (week 9 to birth).

Which systems continue development into childhood?

Brain, teeth, and skeletal system.

Where does fertilization usually occur?

In the ampulla of the uterine tube.

What layers surround the oocyte?

Corona radiata and zona pellucida.

Define zygote.

A single diploid cell formed by fusion of sperm and oocyte.

What is cleavage?

Series of mitotic divisions that increase cell number without increasing overall size.

What is the result of cleavage?

Formation of a morula (16-cell ball).

What is compaction?

Tight junction formation between outer cells of the morula, allowing separation of inner and outer cells.

What results from compaction?

A blastocyst with an inner cell mass and trophoblast.

What structure enters the uterus for implantation?

The blastocyst.

When does implantation begin and end?

Begins around day 6-7 and is completed by day 10

What are the components of the blastocyst?

Inner cell mass (embryoblast) and outer trophoblast.

How do fraternal (dizygotic) twins arise?

From two separate eggs and sperm.

What determines number of placentas and sacs?

Timing of the split — early = separate placentas/sacs; late = shared.

What are the basic IVF steps?

Hormonal stimulation → egg retrieval → fertilization in lab → culture to blastocyst → transfer to uterus.

Common sites for ectopic implantation?

Uterine tube (most common), abdominal cavity, ovary, cervix.

What does the embryoblast form?

Epiblast and hypoblast.

What does the trophoblast form?

Cytotrophoblast and syncytiotrophoblast.

What is the syncytiotrophoblast's function?

Invades the endometrium and secretes enzymes to promote implantation.

What produces hCG?

syncytiotrophoblast.

Clinical use of hCG?

Detected in pregnancy tests.

Cytotrophoblast role?

Structural support and proliferation.

What forms in the syncytiotrophoblast?

Lacunae — fluid-filled spaces that become maternal blood spaces.

What membranes form in week 2?

Amnion and yolk sac.

What membrane formed in week 2 is transient

yolk sac

what membrane formed in week 2 persists throughout pregnancy

amnion

Placenta function?

Nutrient/gas exchange and hormone production.

Placenta parts?

Fetal part (chorion) and maternal part (decidua basalis).

What are chorionic villi?

Finger-like projections of chorion that interface with maternal blood.

Amniotic fluid function?

Cushions the fetus, prevents desiccation, allows movement.

How does the fetus contribute to amniotic fluid?

Swallows and urinates into the fluid — important for lung and gut development.

What happens in week 3?

Gastrulation — formation of the trilaminar embryo.

Outcomes of gastrulation?

Formation of ectoderm, mesoderm, endoderm.

What is special about the oropharyngeal and cloacal membranes?

They are regions where ectoderm and endoderm meet directly (no mesoderm between).

What is the primitive streak?

A midline structure in the epiblast that initiates gastrulation.

What if the primitive streak persists?

May form a sacrococcygeal teratoma.

What is neurulation?

The process by which the neural plate folds to form the neural tube.

What does the neural tube become?

Brain and spinal cord.

Ectoderm derivatives?

Skin, nervous system.

Mesoderm derivatives?

Muscles, bones, blood, kidneys.

Endoderm derivatives?

GI and respiratory epithelium.

Neural crest derivatives?

PNS, melanocytes, adrenal medulla.

Ectomesenchyme derivatives?

Facial bones, connective tissue of head and neck.

What happens in week 4?

Embryo folds into cylindrical body shape.

How does folding affect the trilaminar embryo?

Converts it into a recognizable body form with gut tube and outer body wall.

What are the two types of folding?

Cephalocaudal and lateral folding.

What does lateral folding produce?

The gut tube and outer ectodermal body wall.

The gut tube and outer ectodermal body wall.

Brings the heart into the thoracic region and forms the oral cavity.