Embryology: Gametogenesis and Fertilization

• Embryology studies the prenatal development of gametes (sex cells), fertilization, as well as the development of embryos and fetuses.
• The study of the formation and early development of living organisms is encompassed within embryology.
• Congenital disorders that occur before birth are also a concern of embryology.

Gametogenesis

• Gametogenesis creates gametes (sperm and egg cells) through meiosis in the gonads (ovaries and testes).
• Spermatogenesis in males produces sperm.
• Oogenesis in females produces eggs (ova).
• Spermatogenesis occurs continuously after puberty, while oogenesis is a cyclic process, starting before birth and continuing until menopause.
• Meiosis in gametogenesis ensures gametes have half the number of chromosomes as somatic cells, maintaining the species' chromosome number after fertilization.

Fertilization

• Fertilization describes the fusion of male and female gametes (sperm and egg) to form a zygote.
• Sperm capacitation, acrosome reaction, penetration of the zona pellucida, and fusion of the sperm and egg cell membranes are all involved.
• After fertilization, the zygote has a diploid set of chromosomes, with half from each parent.
• Fertilization activates the egg, starting embryonic development.

Cleavage

• Cleavage is rapid mitotic cell division of the zygote immediately after fertilization.
• During cleavage, the embryo's size does not increase, but the number of cells (blastomeres) does.
• Cleavage leads to the formation of a morula, a solid ball of cells.
• The morula then develops into a blastocyst, a hollow ball of cells with an inner cell mass (ICM) and a trophoblast.

Gastrulation

• Gastrulation is a crucial stage where the three primary germ layers are established; ectoderm, mesoderm, and endoderm.
• Complex cell movements and rearrangements take place during gastrulation.
• The ectoderm leads to the epidermis, nervous system, and neural crest cells.
• The mesoderm gives rise to muscles, bones, blood, heart, and urogenital system.
• The endoderm leads to the lining of the digestive tract, respiratory system, and associated organs.

Neurulation

• Neurulation forms of the neural tube, which becomes the central nervous system (brain and spinal cord).
• It starts with the formation of the neural plate, a thickened region of the ectoderm.
• The neural plate folds inward to form the neural groove, which then closes to form the neural tube.
• Neural crest cells arise from the edges of the neural plate, migrate, and differentiate into peripheral neurons, pigment cells, and skeletal/connective tissue of the head.

Organogenesis

• Organogenesis refers to organ formation.
• The three germ layers differentiate into specific tissues and organs.
• Interactions between different cell types and germ layers are crucial for organ development.
• Cell proliferation, cell migration, cell differentiation, and programmed cell death (apoptosis) are all components.

Fetal Development

• After the embryonic period (first 8 weeks), the developing organism becomes a fetus.
• Organs and tissues grow and mature during fetal development.
• This period involves rapid growth and increased complexity.
• By the end of fetal development, the fetus can survive outside the uterus.

Congenital Disorders

• Congenital disorders (birth defects) are structural or functional abnormalities during prenatal development.
• They can be caused by genetic and environmental factors (teratogens), or a combination of both.
• Drugs, alcohol, radiation, and infections are examples of teratogens.
• Some congenital disorders are minor, while others are severe and life-threatening.
• Ultrasound, amniocentesis, and chorionic villus sampling help detect these disorders before birth.