Embryology 1

Gametogenesis

• During meiosis, diploid cells divide twice to produce four haploid cells (gametes)
• Humans have 46 chromosomes (2n) in a diploid cell, 23 chromosomes (n) in each gamete
• In spermatogenesis, each spermatogonium produces 4 active sperms
• In oogenesis, each primary oocyte produces one active ovum and four polar bodies

Fertilization

• Fertilization is the fusion of genetic material (pronuclei) from sperm and ovum to form a zygote
• Phases of fertilization: penetration of corona radiata, penetration of zona pellucida, fusion of oocyte and sperm cell membranes
• Fertilization typically occurs in the uterine tubes and the resulting zygote takes 4-5 days to implant in the uterine cavity

The Embryonic Period

• The embryonic period spans the first 8 weeks after fertilization
• During the first week, the oocyte finishes its second meiotic division, forming the female pronucleus
• The zona pellucida becomes impenetrable to other sperm
• The sperm’s head separates from the tail, swells, and forms the male pronucleus
• Cleavage begins in the first week, where the zygote undergoes rapid mitotic division
• Blastomeres form, which are progressively smaller cells produced by cleavage
• A morula (solid ball of cells) forms 3-4 days after fertilization, followed by the blastocyst (hollow ball of cells) a day later
• Implantation occurs 7-8 days after fertilization when the blastocyst attaches to the endometrium
• The decidua is the modified portion of the endometrium after implantation

Second Week of Development: Bilaminar Germ Disc

• The embryoblast differentiates into epiblast and hypoblast cells
• Epiblast layer forms dorsally and hypoblast layer forms ventrally, adjacent to the blastocyst cavity (blastocele)
• Anterior cells in the hypoblast layer differentiate into anterior visceral endoderm, defining the embryo's cranial side
• By 4 to 6 days of gestation, both dorso-ventral and cranio-caudal axes are established
• The trophoblast differentiates into two layers: the cytotrophoblast and syncytiotrophoblast
• The embryoblast forms two layers: the epiblast and hypoblast
• The extraembryonic mesoderm splits into two layers: the somatic and splanchnic layers
• Two cavities form: the amniotic and yolk sac cavities

Third Week of Development: Trilaminar Germ Disc

• The developing embryo continues to grow, forming a trilaminar embryonic disc destined to become the fetus
• Four extraembryonic membranes develop: amnion, yolk sac, allantois, and chorion
• Gastrulation occurs, establishing all three germ layers: ectoderm, mesoderm, and endoderm
• Ectoderm forms the superficial layer
• Mesoderm forms the middle layer
• Endoderm forms the inner layer

Trilaminar Germ Disc Development

• Ectoderm differentiates into the nervous system (brain and nerves) and the epidermis of the skin
• Mesoderm is loosely organized connective tissue (mesenchyme) that matures into blood, muscles, bones, etc.
• Endoderm gives rise to the epithelial lining of the digestive and respiratory tracts, as well as other organs. It also forms the endothelial lining of blood vessels.

Explore the processes of gametogenesis and fertilization in this quiz. Learn how meiosis leads to the formation of haploid gametes and understand the phases of fertilization. You'll also delve into the embryonic period and the early stages of human development.