Embryology

• Primordial germ cells (PGCs) can be first identified within the wall of the yolk sac during the fourth to sixth weeks of gestation
• PGCs give rise to the germ line, which forms the sex cells (gametes) - egg and sperm
• Germ line is set aside for the next generation in the developing embryo
• PGCs migrate to the dorsal body wall of the embryo between the 6th and 12th weeks of gestation to populate the developing gonads
• Spermatogonia in males and oogonia in females differentiate from PGCs and are diploid, containing 23 pairs of chromosomes
• Gametogenesis (spermatogenesis in males, oogenesis in females) involves meiosis, halving the number of chromosomes to 23 (haploid)
• Spermatogenesis occurs in the seminiferous tubules of the testes after puberty, while oogenesis is initiated during fetal life
• Oogonia initiate meiosis during fetal life, but meiotic arrest occurs until after puberty
• Fertilization (uniting of egg and sperm) occurs in the oviduct, forming a zygote with 23 chromosomes
• Embryonic development begins at fertilization, with cleavage dividing the zygote into daughter cells without growth
• Morula differentiates into trophoblast (fetal placenta) and inner cell mass (embryoblast) by the 8- to 16-cell stage
• Blastocyst forms by the fifth to sixth day of development, implanting into the endometrial lining of the uterine wall
• During the second week, the embryoblast splits into epiblast and hypoblast, forming a bilaminar embryonic disc
• Amniotic cavity develops at the embryonic pole of the blastocyst, surrounded by cells from the epiblast, forming the amnion
• Hypoblast sends migratory endodermal cells to form the primary and secondary yolk sacs
• Extraembryonic mesoderm develops, forming the extraembryonic coelom and lining the trophoblast, amnion, and yolk sac
• Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast, contributing to the placenta
• Chorionic villi extend into maternal blood sinusoids, aiding in placental formation
• During the third week of development, the major event is gastrulation, which starts with the formation of the primitive streak in the epiblast near the caudal end of the bilaminar embryonic disc
• The primitive streak contains the primitive node at its cranial end, with a primitive pit and groove where cells leave the streak and move into the interior of the embryonic disc
• Cells leaving the primitive streak can invade the hypoblast, forming the definitive endoderm, or migrate bilaterally to form the intraembryonic mesoderm
• Gastrulation results in the formation of the three primary germ layers: ectoderm, mesoderm, and endoderm, which are the building blocks for organ rudiments
• Gastrulation also establishes the major body axes: cranial-caudal, dorsal-ventral, and left-right axes
• Neural induction occurs during gastrulation, where the primitive node induces the ectoderm to thicken and form the neural plate, the earliest rudiment of the central nervous system
• Formation of the tube-within-a-tube body plan and organogenesis are set by the end of the third week, where primitive organ rudiments are established and differentiated to form major organ systems
• During the fourth week, the tissue layers differentiate to form the primordia of most major organ systems, and the embryonic disc undergoes folding to create the tube-within-a-tube body plan
• Differential growth of the embryo drives folding, with the endodermal, mesodermal, and ectodermal layers fusing along the ventral midline to form a three-dimensional body form
• Neurulation converts the neural plate into a neural tube, which differentiates into the brain and spinal cord, with neural crest cells detaching and migrating to form various structures and cell types
• Somites continue to segregate from the paraxial mesoderm, subdividing into dermamyotomes and sclerotomes, which contribute to the musculature, dermis, and skeletal structures of the body
• Developmental events during embryonic development can sometimes lead to birth defects like gastroschisis, which is not caused by anything the mother did, but rather by developmental anomalies