Rucete ✏ Biology In a Nutshell
1. Fertilization
- Begins when sperm recognizes and penetrates the egg membrane (zona pellucida)
- Triggers formation of the fertilization membrane to block other sperm
- Meiosis II in the secondary oocyte completes
- Nuclei fuse, and DNA replication begins
2. Cleavage
- Rapid mitotic cell division of the zygote
- Leads to formation of a solid ball of cells (morula)
- Establishes embryo polarity: animal pole and vegetal pole
- Types of cleavage: polar vs. equatorial, radial vs. spiral, indeterminate vs. determinate
3. Blastula Formation
- Forms a hollow ball of cells with a fluid-filled cavity (blastocoel)
4. Gastrulation
- Cells migrate to form three germ layers:
- Ectoderm (skin, nervous system)
- Mesoderm (muscles, bones, blood)
- Endoderm (digestive tract lining)
- Forms archenteron (primitive gut) and blastopore (becomes mouth in protostomes, anus in deuterostomes)
5. Extraembryonic Membranes (in amniotes)
- Chorion: forms part of the placenta
- Allantois: involved in gas exchange and waste storage
- Amnion: encloses the embryo in amniotic fluid
- Yolk sac: provides nutrients in early development
6. Organogenesis
- Development of organs from germ layers
- In chordates:
- Notochord forms from mesoderm
- Neural tube forms from ectoderm → becomes brain and spinal cord
- Neural crest forms various structures like skin pigment, skull muscles, bones, and teeth
7. Examples in Different Species
- Frog: Has a gray crescent, gastrulation starts at dorsal lip of the blastopore
- Bird: Blastodisc forms on top of yolk; primitive streak initiates gastrulation
- Humans: Form a blastocyst; trophoblast aids in implantation and forms HCG, maintaining the corpus luteum and forming the placenta
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Biology in a nutshell