Early Embryonic Development Summary

Clinical Age: Calculated from the date of the mother’s last menstrual period.
Postovulatory Age: Calculated from the direct time of fertilization; Clinical age week $3$ corresponds to Postovulatory week $1$ .
Germinal/Early Development Period (Weeks $0-2$ ): Encompasses fertilization, cleavage, and formation of primitive germ layers.
Embryonic Period (Weeks $2-8$ ): Period of organogenesis.
Fetal Period (Weeks $9-38$ ): Focusing on growth, differentiation, and tissue maturation.

Zygote: A single totipotent cell possessing a unique $2n$ genome.
Cleavage: Rapid mitotic divisions beginning within $24$ hours; blastomeres decrease in size with each division to pass through the ISTHMUS.
Morula: A solid ball of approximately $16$ blastomeres entering the uterus by day $4$ .
Compaction: Blastomeres flatten and reorganize to maximize cell-cell contact, creating inside-outside polarity.
Compaction Theories: * Inside-outside hypothesis: Position determines fate; internal cells express Oct4 and Nanog to become the Inner Cell Mass (ICM). * Polarization hypothesis: Differentiation depends on the plane of cleavage (parallel or perpendicular) relative to the zona pellucida.
Blastocyst: Formed by day $6$ ; consists of the trophoblast (outer epithelium) and the Inner Cell Mass (embryonic pole) surrounding a fluid-filled blastocyst cavity.

Hatching: The blastocyst utilizes enzymes to bore through the zona pellucida by day $5$ .
Adhesion: Occurs ~day $6$ on the posterior uterine wall; the uterus is primed by progesterone from the corpus luteum.
Trophoblast Differentiation: * Syncytiotrophoblast: Outer multinucleated layer that invades the endometrium and secretes hCG. * Cytotrophoblast: Inner mitotic layer lining the blastocyst wall.
Ectopic Pregnancy: Implantation occurring outside the uterine cavity; life-threatening due to potential blood vessel rupture.

Bilaminar Disc: The ICM reorganizes into the epiblast (dorsal/columnar) and the hypoblast (ventral/cuboidal).
Amnion: Forms by day $8$ between the epiblast and trophoblast; provides buoyancy and temperature regulation.
Yolk Sac: * Primary Yolk Sac: Formed by Heuser’s membrane (from the hypoblast) by day $8$ . * Secondary (Definitive) Yolk Sac: Replaces the primary sac by day $12$ ; vital for nutrient transfer in weeks $2-4$ .
Chorion: Composed of the trophoblast and extraembryonic mesoderm; encloses the chorionic cavity.
Connecting Stalk: A bridge of extraembryonic mesoderm that will become the umbilical cord.

Gastrulation: Formation of the three germ layers (ectoderm, mesoderm, endoderm) from the epiblast via the primitive streak.
Primitive Streak: Defines the major body axes (Cranial-Caudal, Medial-Lateral, Left-Right) and the primitive node (the organizer).
Germ Layer Derivatives: * Endoderm: Replaces the hypoblast; forms the GI tract, liver, and respiratory epithelium. * Mesoderm: Migrates between layers; forms muscles, skeleton, and circulatory system. * Ectoderm: Remaining epiblast; forms the epidermis and the nervous system.
Notochord: Midline mesodermal structure that induces the overlying ectoderm to form the neural plate (Neurulation).
Sirenomelia: A condition resulting from insufficient mesoderm formation in the caudal region.

Mesoderm Subdivisions: * Paraxial Mesoderm: Forms somites (paired blocks forming in a cranio-caudal direction at $3$ pairs/day). * Somite Derivatives: Dermomyotomes (muscle/connective tissue) and sclerotomes (bone/cartilage). * Intermediate Mesoderm: Forms the urogenital system. * Lateral Plate Mesoderm: Splits into parietal and visceral layers to form the intra-embryonic coelom (serous cavities).
Embryo Folding: * Longitudinal (Craniocaudal): Moves head and tail closer; caused by the rapid growth of the neural tube. * Lateral (Transverse): Fuses the left and right sides to create a cylindrical "tube within a tube" body form.
Ventral Body Wall Defects: * Ectopia cordis: Failure of thoracic fusion leaving the heart outside ( $1 \text{ in } 5 \text{ million}$ births). * Gastroschisis: Failure of abdominal fusion leaving intestines outside ( $3.5/10,000$ births).