Gastrulation and Germ Cell Formation

Gastrulation: Formation of Germ Cells

• Germ cells are crucial for species continuation
• Germ cells include sperm (males) and eggs (females)
• Germ cells undergo meiosis to pass genetic material
• Germ cell formation is called gametogenesis
• PGCs (Primordial Germ Cells) are precursors to sperm and egg
• PGCs are specified in the epiblast during the third week of development
• PGCs are initially undifferentiated
• PGC induction is influenced by BMPs and Wnt signaling molecules
• PGCs migrate from the epiblast to the gonadal ridges
• Migration is guided by chemotactic signals
• PGCs differentiate into spermatogonia (males) or oogonia (females)
• Gametogenesis (sperm and egg formation) follows
• Spermatogonia develop into sperm
• Oogonia mature into primary oocytes
• Primary oocytes arrest in prophase I until puberty

Gonadal differentiation and transition to gametogenesis

• PGCs colonize gonads
• Gonads differentiate into testes (males) or ovaries (females)
• SRY gene triggers testes development
• Absence of SRY gene leads to ovaries development
• Sex-specific differentiation of PGCs into spermatogonia or oogonia
• Spermatogonia mature into sperm
• Oogonia mature into primary oocytes

Key Mechanisms in germ cell development

• Germ cell development depends on transcription factors (Prdm1, Prdm14, BLIMP1, Sox2, and Oct4)
• These factors are important in establishing PGCs identity and pluripotency
• DNA demethylation and histone modification play a crucial role in epigenetic reprogramming of PGCs

Sexual Determination and Differentiation

• The genetic sex of the embryo determines germ cell differentiation
• Testes develop in males due to SRY gene expression
• Ovaries develop in females due to the lack of SRY gene

Neurulation: Formation of Neural tube

• Neural plate transforms into neural tube
• Neural tube becomes central nervous system
• Neurulation occurs in the fourth week of human development, typically
• The process involves induction, shape and patterning of the neural plate, formation of neural folds and neural groove, and the closure of the neural tube
• Neural tube defects can occur due to closure failure
• Anencephaly and spina bifida are examples of neural tube defects

Fate of Somite Derivatives

• Somites differentiate into sclerotome, dermomyotome
• Sclerotome forms vertebrae and ribs
• Dermomyotome forms muscles and dermis
• These processes are crucial for body segmentation and organization
• Somites are blocks of mesoderm that form along the neural tube
• They are involved in musculoskeletal system development

Formation and Development of Somites

• Somites arise from paraxial mesoderm
• Somites appear sequentially from cranial to caudal (head to tail)
• Each somite differentiates into sclerotome (vertebrae/ribs) and dermomyotome (muscles/skin dermis)
• The timing and number of somites can vary, typically around 42–44 pairs in human embryos

Development of Intraembryonic Coelom

• Intraembryonic coelom forms as small cavities that merge
• It subdivides into Pericardial, Pleural and Peritoneal cavities
• It's essential for organ development and positioning
• Mesoderm divides into somatic and splanchnic layers during development
• The mesoderm plays a vital role in forming mesenteries

Early Development of the Cardiovascular System

• Blood islands form in the mesoderm are the first structures in development
• Blood island cells differentiate into angioblasts (precursors to endothelial cells of blood vessels) and hemocytoblasts (precursors to blood cells)
• Vasculogenesis forms a rudimentary circulatory system from blood islands
• The heart tube begins to beat around day 22
• Development of major blood vessels precedes the fully developed heart

Development of Chorionic Villi

• Chorionic villi are microscopic finger-like projections that form the placenta
• They are essential for nutrient and gas exchange between the mother and fetus during pregnancy
• Villi are composed of trophoblast cells and mesoderm, increasing in complexity with development from primary, secondary, to tertiary
• Chorionic villi form a barrier, but not entirely impermeable, to infection from the mother's blood supply

Molar Pregnancy

• Abnormal growth of the chorionic villi can result in a molar pregnancy
• Molar pregnancy is a condition marked by abnormal growth of chorionic villi, often forming cyst-like structures