Download Gametogenesis, Fertilization and Blastula Formation

Gametogenesis, Fertilization and Blastula Formation
Spermatogenesis
The production of mature sperm cells in adult males
Continuous process in adult males; can make 100’s of millions of sperm per day
For each cell that enters meiosis, 4 sperm cells are produced
It is continuous for the reproductive life of a male and occurs in an uninterrupted
sequence
Sperm structure
Head: contains the haploid nucleus and an acrosome—vesicle containing enzymes
Middle piece: contains a long, spiral shaped mitochondria which uses fructose as an
energy source
Tail: flagellum for movement
Oogenesis
The development of ova (mature, unfertilized egg cells)
Begins in the embryo when primordial germ cells undergo mitosis to produce diploid
oogonia
Each oogonium develops into a primary oocyte by the time of birth resulting in all
potential ova being present in the ovaries at birth
Between
birth and puberty they enlarge and the surrounding follicles grow; they
replicate their DNA and enter prophase 1 where they stay until activated by hormones
After puberty, during each cycle, hormones stimulate the oocyte to complete meiosis 1
which is then released from the follicle; if a sperm cell penetrates the membrane meiosis
2 occurs to complete oogenesis
Is under the control of many hormones
Egg structure
Egg cell: contains a nucleus and lots of cytoplasm and organelles; has a heterogeneous
distribution of mRNA, proteins, and other chemicals; surrounded by the plasma
membrane and 2 other layers
Vitelline layer: contains species specific receptor sites
Jelly coat: outermost layer
Fertilization
1. Sperm encounters the jelly coat and the acrosome releases enzymes that digest a
cavity into the jelly coat (acrosomal reaction)
2. Recognition proteins on the sperm bind with the receptor site on the egg
3. The plasma membranes of the sperm and egg fuse
4. The cell becomes activated which prevents polyspermy (fast block)
5. The sperm nucleus enters the egg cell
6. The fertilization membrane forms (slow block) through the cortical reaction which
causes the vitelline layer to swell, separate from the plasma membrane and harden
7. Sperm and egg nuclei fuse creating a zygote
Phase 1: cleavage—rapid succession of cell divisions that produce a ball of cells (the
blastula) from the zygote
Cells replicate DNA and divide rapidly but no protein synthesis takes place
Different regions of the zygote can divide at different rates
Nutrients
from the egg cytoplasm (yolk) nourish the zygote
The cell division organizes the developing embryo into developmental regions
The morula (solid ball of cells) gets rearranged into the blastula (hollow ball of cells)
with a fluid filled center (blastocoel)
The yolk gradient is a key factor in determining the polarity of the blastula: the most
yolk forms the vegetal pole, the least forms the animal pole (forms the anterior of most
animals)
Phase 2: Gastrulation: sorts the cells of the blastula into distinct germ layers
Phase 3: organogenesis: formation of organs from the germ layers