① Totipotency (organism-forming capacity): the ability of a single cell to form not only the embryo and the adult organism but, in species that possess them, also the extraembryonic membranes.

3. Implantation

⑴ Implantation occurs only at the blastocyst stage

⑵ Blastocyst = Trophoblast + Inner Cell Mass (ICM) + Blastocoel

⑶ Trophoblast cell layer (trophectoderm)

① Initiates implantation; after implantation, forms the chorion and chorionic villi.

② Secretes enzymes that degrade the uterine wall.

⑷ The trophoblast cell layer becomes the placenta via the chorion.

4. Extraembryonic Membrane Formation

⑴ Formation of extraembryonic membranes after germ layer formation

⑵ During the blastocyst stage, the inner cell mass not only forms the entire embryo but also gives rise to the extraembryonic membranes (allantois, yolk sac, amnion).

⑶ Types of extraembryonic membranes

① Chorion: the membrane that encloses the entire embryo; ectoderm + mesoderm

○ Called the serosa in birds.

○ Involved in uterine implantation.

○ Secretes hCG to prevent degeneration of the corpus luteum.

○ Because the corpus luteum is maintained only up to about 3 months, hCG is secreted only until then; thereafter, the placenta takes over this role.

② Amnion: the membrane surrounding the embryo and enclosing the amniotic fluid; ectoderm + mesoderm

○ The amnion is derived from the inner cell mass.

○ Protects the embryo.

③ Allantois: waste storage; mesoderm + endoderm

○ In mammals, associated with formation of the umbilical cord.

④ Yolk sac: provides nutrients to the embryo; mesoderm + endoderm

○ Hematopoietic function.

○ In mammals, it is unnecessary for nutrition because nutrients are supplied via the umbilical cord.

⑷ Monozygotic twins: classified according to the timing of chorion and amnion formation.

① The amnion is formed later than the chorion.

② Separation before implantation (before ~day 5 post-fertilization): two chorions and two amnions.

③ Separation on days 5–10 post-fertilization: two amnions within a single chorion.

④ Separation on days 10–14 post-fertilization: one chorion and one amnion; rare (about 4–6%); conjoined twins fall into this category.

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 formation

○ 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

○ In each complex, the HOX gene closest to the 3’ end is expressed first, and its expression occurs in the anterior region of the embryo.

③ HOX genes with higher numbers are expressed

7. Fetus

⑴ In humans,

① Embryo: Up to 8 weeks after fertilization

② Fetus: Week 9 to birth

⑵ Fetal Development

Entered: 2019.03.17 14:54

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Chapter 32-6. Mammalian Embryology

1. Fertilization

2. Cleavage

3. Implantation

4. Extraembryonic Membrane Formation

5. Germ Layer Formation

6. Homeo Genes

7. Fetus

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