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Primary: The chorionic villi are at first small and non-vascular. 13–15 days: trophoblast only [1] Secondary: The villi increase in size and ramify, while the mesoderm grows into them. 16–21 days: trophoblast and mesoderm [1] Tertiary: Branches of the umbilical artery and umbilical vein grow into the mesoderm, and in this way the chorionic ...
An undifferentiated cytotrophoblastic stem cell will differentiate into a villous cytotrophoblast, which is what constitutes primary chorionic villi, and will eventually coalesce into villous syncytiotrophoblast. The formation of syncytiotrophoblast from cytotrophoblast is a terminal differentiation step of trophoblastic cells.
In the placenta, the intervillous space is the space between chorionic villi, and contains maternal blood.. The trophoblast, which is a collection of cells that invades the maternal endometrium to gain access to nutrition for the fetus, proliferates rapidly and forms a network of branching processes which cover the entire embryo and invade and destroy the maternal tissues.
At the end of the second week of development, some cells of the trophoblast penetrate and form rounded columns into the syncytiotrophoblast. These columns are known as primary villi. At the same time, other migrating cells form into the exocoelomic cavity a new cavity named the secondary or definitive yolk sac, smaller than the primitive yolk sac.
A cytotrophoblast cap penetrates through the fetus' syncytiotrophoblasts and reaches the maternal decidua, forming the anchoring villus. [1] [3] [4] The cytotrophoblast layer spreads and contacts the cytotrophoblast layers of neighboring anchoring villi, creating a continuous layer called the cytotrophoblastic shell.
Primary yolk sac: it is the vesicle which develops in the second week, its floor is represented by Heuser's membrane and its ceiling by the hypoblast. It is also known as the exocoelomic cavity .
The first two in the sequence are by far the most common; 'tertiary' appears occasionally, and higher numbers are rare except in specialized contexts ('quaternary period'). The Greek series proto- , deutero- , trito- , ... is only found in prefixes, generally scholarly and technical coinages, e.g. protagonist, deuteragonist, tritagonist ...
Studies of monozygotic and dizygotic twins of the late 1990s support this idea, [16] particularly with regards to primary gyri and sulci, whereas there is more variability among secondary and tertiary gyri. [17] Therefore, one may hypothesize that secondary and tertiary folds could be more sensitive to genetic and environmental factors. [18]