Chapter 32-6. Mammalian Embryology
Recommended Post: 【Biology】 Chapter 32. Embryology
2. Cleavage
3. Implantation
4. Extraembryonic Membrane Formation
6. Homeo Genes
7. Fetus
1. Fertilization
⑴ Mammalian Fertilization Process
2. Cleavage
⑴ Characteristics of Mammalian Cleavage
① Equal Cleavage
② Rotational Cleavage
③ Asynchrony in Early Division
④ Compaction
⑤ Mammalian cleavage is the slowest among animals
⑵ 2-cell stage: 30 hours after fertilization
⑶ 4-cell stage: 60 hours after fertilization
⑷ 8-cell stage: Formation of tight junctions between cells (compaction)
⑸ Morula: Reaches uterus between day 3 to 6 and forms a blastocyst (32 cells)
⑹ In early mammalian embryos, each blastomere is totipotent up to the 8-cell stage
① Totipotency: The ability of a single cell to develop into the entire organism including extraembryonic tissues (in species that form them)
3. Implantation
⑴ Implantation occurs only at the blastocyst stage
⑵ Blastocyst = Trophoblast + Inner Cell Mass (ICM) + Blastocoel
⑶ Trophoblast
① Initiates implantation and later forms chorion and villi
② Secretes enzymes to break down uterine wall
⑷ Trophoblast becomes placenta through chorion
4. Extraembryonic Membrane Formation
⑴ Formation after germ layers are established
⑵ Inner cell mass of the blastocyst forms not only the embryo but also extraembryonic membranes (allantois, yolk sac, amnion)
⑶ Types of Extraembryonic Membranes
① Chorion: Membrane that surrounds the entire embryo, made of ectoderm + mesoderm
○ Called serosa in birds
○ Involved in uterine implantation
○ Secretes hCG to prevent degeneration of corpus luteum
○ Since the corpus luteum lasts only until 3 months, hCG is secreted only until then: Placenta takes over hCG’s role afterward
② Amnion: Amniotic fluid, membrane that surrounds the embryo, ectoderm + mesoderm
○ Derived from inner cell mass
○ Protects the embryo
③ Allantois: Stores waste, mesoderm + endoderm
○ Related to umbilical cord formation in mammals
④ Yolk Sac: Provides nutrients to the embryo, mesoderm + endoderm
○ Has hematopoietic function
○ Unnecessary in mammals since nutrients are provided via the umbilical cord
⑷ Identical Twins: Classified by the timing of chorion and amnion formation
① Amnion forms later than chorion
② Separation before implantation (before day 5 post-fertilization): 2 chorions and 2 amnions
③ Separation between day 5–10 post-fertilization: 2 amnions within 1 chorion
④ Separation between day 10–14 post-fertilization: 1 chorion and 1 amnion, rare (4–6%), includes conjoined twins
Figure 1. Three types of identical twin development
5. Germ Layer Formation
⑴ Organizer
① Organizer in mammals: Hensen’s node
② Not an organizer in mammals: Dorsal lip of the blastopore
⑵ Neurulation
① During neurulation, the embryonic trunk mesoderm is subdivided into four parts (notochordal, paraxial, intermediate, lateral plate mesoderm)
② Paraxial mesoderm: Forms somites → which become bones and muscles
○ MyoD gene is expressed in somite cells that will form skeletal muscle
6. Homeo Genes
⑴ Terminology
① Homeo gene: Master gene that regulates other genes involved in body structure
○ Example: antennapedia gene, bithorax gene
② Homeobox (HOX)
○ A ~180 base pair sequence found within all homeo genes
○ Highly conserved evolutionarily
○ Contains each gene’s ORF and transcription factor binding site
③ Homeodomain
○ 60 base pair sequence encoded by the homeobox
○ Encodes helix-turn-helix DNA binding motif, transcription factor binding site
⑵ Homeo Genes in Drosophila: Drosophila has a single cluster
⑶ Vertebrate Homeobox
① All vertebrates have the same mechanism for vertebral development
② Vertebrates have 4 clusters on different chromosomes
○ Mammals have 4 sets of HOX complexes per genome
○ Homeobox genes are numbered 1 to 13 according to expression order from the anterior end of each cluster
○ HOX gene nearest to the 3’ end is expressed first, from the anterior part of the embryo
③ HOX genes with higher numbers are expressed later
7. Fetus
⑴ In humans,
① Embryo: Up to 8 weeks after fertilization
② Fetus: Week 9 to birth
Entered: 2019.03.17 14:54