34.Animal development. Mechanisms in mammals and humans.

Embryonic Development in Animals

• Overview: Sexual reproduction ensures genetic variation and adaptability in populations. Two haploid gametes (n) produced by parents fuse to form a new, unique diploid (2n) individual. Meiosis halves the genetic material during gamete production.
• Meiosis:
• Meiosis I separates homologous chromosomes.
• Meiosis II separates sister chromatids.
• Gametes:
• Spermatogenesis: Produces male haploid gametes (sperm).
• Oogenesis: Produces female haploid gametes (ova/eggs).
• Sperm and ova are specialized cells adapted to their functions.
• Fertilization: brings haploid gametes together and activates developmental processes in the egg.
• Chromosomal Sex Determination:
• Males (XY):
• Y chromosome: ~59 million base pairs, mostly non-functional repeats, 48 protein-coding genes including SRY.
• SRY gene encodes Testis Determining Factor (TDF), a transcription factor regulating other genes.
• Females (XX):
• X chromosome: ~155 million base pairs, ~5% of haploid genome.
• Encodes housekeeping and specialized genes like Wnt-4 and DAX-1 for ovary development.
• X inactivation: One X chromosome randomly inactivated, creating a mosaic of maternal and paternal X activity.
• Sexual Determination (Mammalian):
• SRY gene: Genetic switch for male development; expressed in indifferent gonad → becomes testis. Absence → ovary.
• SOX9: Transcription factor expressed in males after SRY; absence in XY → female development or fatal issues.
• SF1 (Steroidogenic Factor 1):
• Activated by SRY; cofactor for steroid synthesis genes.
• Absence → adrenal and hypothalamus failure.
• Note: SRY presence is necessary but not sufficient for male development.
• Ovarian Development:
• DAX1: Double dose disrupts testicular development.
• Wnt4a: Expressed in genital ridge, shuts off in XY, stays on in XX. Deletion → malformed ovary with testis-specific markers (AMH, testosterone enzymes).
• Animal Embryogenesis (General Patterns):
• Cleavage: Rapid mitotic divisions; cytoplasm divided into blastomeres forming a blastula.
• Gastrulation: Blastomeres rearrange, forming ectoderm, endoderm, mesoderm.
• Organogenesis: Cells interact and migrate to form tissues/organs; some organs formed from multiple germ layers.
• Migrating cells: precursors of blood, lymph, pigment cells, gametes.
• Germ Cells vs Somatic Cells:
• Germ cells → gametes; migrate to gonads.
• Somatic cells → form the individual body.
• Gametogenesis completes at physical maturity.
• Larval Stage:
• Organism may hatch immature (larva), adapted for feeding/dispersal.
• Adult stage may be brief and mainly for reproduction.
• Example: Silkworm moths, adults cannot feed; reproduction occurs immediately after metamorphosis.