The first week of human development begins with fertilization of the egg by sperm forming the zygote, followed by early cell division forming the blastocyst. These notes also cover events before fertilization formation of both the egg and sperm, gametogenesis. Cell division can occur in 2 forms, mitosis and meiosis. Mitosis can occur in all cells, while meiosis only occurs during the formation of the sex gametes.
Initially, there is a halving of choromosomal content in the gametes, which is restored by fertilization, allowing genetic recombination to occur. This is then followed by a series of cell divisions without cytoplasmic growth.
I have also included in the Week 1 notes
information about male sex determination and X
inactivation, which are not really specific to the
first week of development.
Zygote with 2 pronuclei
Dr S. J. DiMarzo
Cell division occurs at a specific phase of the cell cycle and occurs in 2 forms; mitosis and meiosis. Mitosis, is a process in which the nucleus divides to produce genetically identical copies of itself. Meiosis, is a type of cell division which leads to a halving of the chromosome number per nucleus and introduces genetic variation.
Cell division occurs at the stage of the cell cycle called mitosis. Progress through the cell cycle is regulated by a complex sequence proteins, the cyclins, discovered in the early 80's (Cell 33: 389-96, 1983). Each cell type has a variable G1 interphase period, though S (synthesis and DNA duplication) and G2 of interphase and mitosis (cell division) remains fairly constant. G0 is when cells become quiescent and withdraw from the cell cycle. Some cells withdraw permanently (neurons) while other cell types can reenter the cell cycle (muscle?).
Mitosis is a continous process but has been divided into 4 identifiable stages. These stages are shown over the page in a series during division in a living cell, figures on the right match the stages. Name and identify these stages.Note that the nucleus divides before the cytoplasm, which then furrows in the midline using an actin contractile ring to divide the cytoplasmic contents. What is the fate of cytoplasmic organelles, such as mitochondria?
Chromosomes, divide and are then separated on a specialized cytoskeleton called the mitotic apparatus. Two spindle polse form and between them extend the mitotic spindle fibres. Chromosomes have a specialized region, the kinetochore, which is attached to the spindle microtubules and the 2 daughter chromosomes are"pulled apart" to separate poles. The nuclear membrane reforms and the cytoplasm then divides.