Embryology and Human Development

Focus on embryology and human development, the last chapter of the semester.
Importance of cell differentiation: Initial similar cells become specialized, forming diverse cell types in the human body (e.g., muscle cells, neurons, epithelial cells).
Cell differentiation leads to unique structures and functions in various cell types.

Prenatal Development: From conception (fertilization) to childbirth (parturition).
  - Three key stages:
    - Pre-embryonic Period: First two weeks from fertilization to the implantation of the zygote in the uterine wall.
    - Embryonic Period: From implantation until the end of the eighth week, a critical time for major organ formation.
    - Fetal Period: From the ninth week until birth, characterized by organ growth and maturation.
Post-natal Development: Continues after birth until maturity, generally 15 to 20 years of age.

Definition: The union of two haploid gametes (sperm from male, oocyte from female) to form a diploid zygote.
Function of Gametes: Deliver chromosomes carrying genetic information.
- Sperm: Contributes paternal chromosomes;
- Oocyte: Provides maternal chromosomes, organelles (like mitochondria), and nutrients.
Maternal Lineage: Mitochondria have their own DNA, inherited maternally.

Location of Fertilization: Typically occurs in the ampulla of the uterine tube.
Process of Capacitation: Functional maturation of sperm within the female reproductive tract, increasing motility.
Statistics: Of approximately 200 million sperm deposited during intercourse:
  - ~10,000 reach uterine tubes (approx. 0.5%).
  - Fewer than 100 come into contact with the egg.
  - Typically, only one sperm fertilizes the egg, overcoming obstacles of oocyte membranes.

Penetration of Corona Radiata: Sperm must penetrate the cellular layer surrounding the oocyte, assisted by enzymes like hyaluronidase.
Oocyte Activation: Upon sperm entry, the oocyte membrane becomes impermeable to other sperm, preventing polyspermy (fertilization by more than one sperm).
Completion of Meiosis II: The oocyte completes meiosis II post-fertilization, resulting in mature ovum and second polar body.
Formation of Pronuclei: The nuclei from the sperm (male pronucleus) and oocyte (female pronucleus) migrate and fuse to form the zygote.

Lasts approximately 9 months, divided into trimesters.
First Trimester (weeks 1-13): Critical for organ formation (organogenesis). High rate of embryonic loss (40% do not survive).
Second Trimester: Continued organ development, refinement, significant fetal growth.
Third Trimester: Functional maturation of organ systems, preparation for birth.

Cleavage: Rapid mitotic divisions without significant growth, leading to formation of a multicellular structure (blastocyst).
Implantation: Blastocyst embeds into the endometrium of the uterus, with the trophoblast layer aiding attachment and invasion.
Placentation: Formation of the placenta for nutrient and waste exchange between mother and embryo.
Embryogenesis: Formation of the basic body plan and initial development of organ systems.

Cleavage results in smaller blastomeres; leads to morula formation (day 3-4) and then blastocyst formation (hollow ball of cells).
Components of Blastocyst:
- Inner Cell Mass: Pluripotent (can become all tissues of the embryo).
1. has 2 layers, Hypoblast
2. Epiblast
  - Trophoblast: Surrounds blastocoel, involved in nutrition provision and implantation.
  1. has 2 layers - Cytotrophoblast
  2. Syncytiotrophoblast: Multi-nucleated layer that invades the uterine lining and facilitates nutrient exchange between the mother and embryo.

Dramatic rearrangement of cells in the embryonic disc forming three germ layers: ectoderm, mesoderm, and endoderm, establishing the body's basic structure.
Ectoderm: Outer layer, contributes to skin and nervous system.
Mesoderm: Middle layer, forms muscles, bones, and cardiovascular system.
Endoderm: Inner layer, forms internal organs like the gut and liver.
Distinct rearrangement: epiblast cells migrate to form the primitive streak, and subsequent layers form from the epiblast and hypoblast.

Amnion: Derives from ectoderm/mesoderm, forms protective amniotic cavity filled with fluid.
Allantois: Derived from ectoderm/mesoderm, involved in urine bladder formation.
Yolk Sac: Source of early blood cell formation, formed from endoderm/mesoderm.
Chorion: Outermost membrane, involves mesoderm/trophoblast, contributes to blood vessel formation connecting mother to fetus.

Miscarriages often occur during implantation due to improper embryo implantation leading to immune responses targeting the embryo.

Interface between fetal and maternal circulation. Corionic villi invade and establish blood vessel networks for nutrient and waste exchange.
Umbilical Structures:
- Umbilical Arteries: Carry deoxygenated blood/wastes from fetus to placenta.
- Umbilical Veins: Carry oxygenated/nutrient-rich blood from placenta to fetus.

Embryogenesis: Around the fourth week, head and tail folds develop, forming a three-dimensional embryo.
Organogenesis: Development of organ systems begins within the first month—various organ systems share developmental mechanisms.

Uterus expands significantly during pregnancy (from ~7 cm to 30 cm).
Pressure from enlargement displaces surrounding organs, affecting maternal abdominal cavity structure.
Increased volume by ~5 liters and total weight of uterus and contents can reach approximately 22 pounds (just below 10 kg).