Fertilization and development

Anatomy & PhysiologyMrs. Quezada

Aim: How does fertilization occur?

Key terms review: • Pregnancy: events that occur from time of

fertilization until birth• Gestation period: period of development

from last menstrual period until birth• Embryo: fertilized egg until 8 weeks• Fetus: 9 weeks until birth

• Fertilization occurs when the sperm’s chromosomes combine with those of an egg

• Fertilized egg= zygote• Sperm deposited in the vagina cannot fertilize

the oocyte immediately, they must be capacitated first

• Capacitated= sperm motility increases and membrane becomes weak to fuse with the egg (occurs in the female)

Structure of sperm

• Acrosome- contains enzymesto penetrate egg• Midpiece- contains mitochondria• Tail- propels the sperm• Nucleus- contains DNA

• Sperm use olfactory receptors to locate the egg

• The egg is surrounded by the corona radiata and the zona pellucida- transparent layers of extracellular matrix

• The sperm penetrates this layer using enzymes that digest the corona first, then binding to the zona pellucida

• The binding of sperm (releases Ca2+) to the zona pellucida causes the acrosomal reaction- breakdown of plasma membrane and acrosomal membrane, releasing enzymes that digest holes in the zona pellucida

• Binding of the sperm to receptors on the egg leads to the fusing of sperm and oocyte membrane and the contents of the sperm entering the oocyte cytoplasm

Blocks to Polyspermy

• Polyspermy- entry of several sperm into an egg

• This is prevented in human by the release Ca2+- causes the cortical reaction in the egg- zonal inhibiting proteins are released from granules and destroy the sperm receptors on the egg

Aim: What events occur during embryonic development?

• After fertilization of the egg, cleavage and implantation take place

• Cleavage- series of rapid mitotic divisions• ZygoteMorulaBlastocyst (hollow ball of

cells)

• Implantation- the outer cells of the blastocyst- trophoblast cells adhere to the endometrium and secrete digestive enzymes and growth factors

• The blastocyst burrows into the thick, velvety lining of the uterus

• After implantation, the corpus luteum is maintained by secretion of human chrorionic gonadotropin (HCG) by the trophobast cells- keeps progesterone levels high

• The embryo obtains nutrition from the endometrium for the first month, but by the second or third month the placenta is fully formed

• Placentation- formation of the placenta, a temporary organ that forms during pregnancy

• Cells from the inner blastocyst give rise to the chorionic villi- finger-like projections that absorb nutrients and oxygen from the mother’s blood

• Cells from the endometrium create the rest of the placental tissue

Functions of the placenta

• Provides nutrients and oxygen for the developing fetus, removes waste products, and maintains levels of progesterone and estrogen (corpus luteum breaks down after third month)

• Gastrulation- formation of three distinct layer in the embryo, from which all other tissues and organs will develop (third week)

• Ectoderm- nervous system and epidermis• Mesoderm- all other organs and tissues• Endoderm- digestive and respiratory systems

• Organogenesis- formation of body organs and organ systems- occurs by week 8 of pregnancy

• After week 8, growth and further development of organs take place until the end of pregnancy (week 40)