Fertilization, Development and Birth G.Burgess Accessed Dec.4, 2001.

Fertilization completion of meiosis and the development of a viable organism Ova and sperm on their own are not able to live. Successful fertilization requires that no more than one sperm fuses with an egg Fertilization by more than one sperm is polyspermy and almost always leads to early death of the embryo.

Spermatozoa

Structure of Spermatozoa Acrosomal cap: encloses and protects DNA, also contains hydrolytic enzymes to break through zona pelludica of ovum. Mitochondria: used to generate ATP to move flagellum Flagellum: propel spermatozoa through fluids Seman: composed of sperm and protective/ nutrient fluids. –Nutrient fluid from seminal vesicle –Protective fluids –prostate adds base fluid to protect sperm from acids in vaginal canal –Cowper’s gland adds base fluid to protect sperm from acids in urethra

Release of Spermatozoa Path: epididymis, vas deferens, seminal vesicle, prostate, Cowper’s gland, urethra, vaginal canal, fallopian tube, ovum Fluids in vaginal canal flow out, preventing deformed/weak sperm from entering fallopian tube

Capacitation The changing and activation of the sperm Makes sperm swim when released into the female reproductive track destabilizes the sperm’s membrane to prepare for the acrosomal reaction

Factors for Successful Fertilization mobility (constant propulsive force from the sperm’s tail) zona-digesting enzymes

Stopping Polyspermy Polyspermy is the entrance of more than one sperm into the ova during fertilization. –Cortical Reaction

Conception

Fertilization Fertilized Ovum: Zygote (2n) Single cell Ovulated ova are inactive (they have stopped maturing) Entry of sperm causes; –Completion of meiosis Acrosomal reaction: The entering of a sperm activates the ova by releasing Nitric Oxide into the egg. Cortical reaction: causes a thickening of the membrane around the ovum so that no other sperm may enter the cell. Egg Activation: Activate DNA (combining of male and female DNA) and beginning of metabolism Development activation: beginning of cell division (cleavage)

Development: Cell division: mitosis (no growth in beginning/ only duplication of DNA/ cellular organelles/ and cleavage) –Type cleavage is determined by quantity of yolk: ie. Humans & sea urchins have little yolk; do symmetrical radial cleavage (form cells of equal size) Chickens; cleavage is asymmetrical. (cells are of different sizes) –Human Zygote divides to 64 blastomeres(cells): called a morula (single mass of cells) All cells are undifferentiated Twinning may occur during this time

Blastulation: Cells are cleaving, no growth Formation of Blastula –a hollow ball (hollow area is called a blastocoel).

Gastrulation (Gastrula) Formation of the primitive gut Develop; endo/ ecto and meso -derm –Endoderm: inside layer Forms digestive tract Makes tissues of back of mouth, pharynx, esophagus, stomach, and intestine –Ectoderm: outside layer Forms skin Nails/hair/epidermis/ears, sweat glands, sensory cells Spinal cord/ nervous system –Mesoderm: middle layer Forms: muscle/fat/ligaments/ bone Cells are beginning to differentiate –Identified for individual tasks.

Neurulation: Cells of animal pole fold and pinch inward forming a tube of cells that stretches the length of the embryo forming a neurula –Development of spine and nervous system –Determines head / thoracic cavity/ abdomen –Determines segmentation

Pregnancy 280 days from time of first day of last menstrual cycle. Blastocyst implants in endometrium and begins formation of placental tissues –Amnion: –Chorion: –Allantois: –Placenta: Endometrium: developed by mother’s tissue Chorionic villi: developed by embryonic tissue