Fertilization and Embryo Development

• A sperm cell is motile, much smaller than an egg, and is divided into a head, middle piece, and a tail.
• The sperm head contains the haploid nucleus with the cell's genetic material and the acrosome, a vesicle with enzymes.
• Enzymes from the acrosome dissolve the jelly layer around the egg for fertilization.
• The sperm middle piece contains many mitochondria, producing energy for movement.
• A flagellum powers the sperm's movement.
• Sperm last for 2-3 days before disintegrating, while ova survive 24 hours.
• Typically, one egg is released from an ovary each month.
• Fertilization by a sperm results in a zygote, which divides and becomes an embryo.
• The embryo travels down the oviduct to the uterus.
• Implantation occurs when the embryo embeds in the uterine lining and attaches to the uterine wall.
• Before ovulation, the uterine wall prepares to receive an embryo, becoming thick, spongy, and vascularized to nourish the embryo with nutrients and oxygen.
• Unfertilized eggs die and exit the body through the vagina.
• If fertilization doesn't occur, hormone levels change, causing the endometrium to disintegrate and exit through the vagina (menstruation/period); unfertilized eggs do not attach to the uterus wall.
• Menstruation, the discharge of endometrial fragments and blood, occurs about once per month.
• The first menstruation happens at puberty shortly after first ovulation.
• Menstruation lasts about five days, after which the endometrium prepares for a potential embryo.
• The menstrual cycle is defined as the sequence of events from the first day of menstruation to the first day of the following menstruation; ovulation occurs in each cycle.
• The average menstrual cycle lasts 28 days but varies.
• Menstrual cycles occur regularly until menopause or pregnancy.
• Menopause is when the ovaries stop ovulating, menstruation stops, and the menstrual cycle ends, generally occurring between ages 45-55, after which a female can no longer have children.
• Events during the menstrual cycle are controlled by ovarian hormones (estrogen, progesterone) and pituitary hormones (FSH, LH).
• During menstruation, estrogen and progesterone levels are low.
• The pituitary gland then secretes FSH, which stimulates the development of several follicles, sacs within the ovaries where the eggs mature, containing supporting cells.
• Typically, only one follicle matures per cycle, while others disintegrate.
• Follicles release estrogen as they develop.
• Estrogen causes the uterine lining to grow and the endometrium to thicken and become spongy.
• When estrogen reaches a certain level, the pituitary gland releases a surge of LH and FSH.
• The LH surge triggers ovulation, where the mature follicle bursts, releasing the egg to the oviduct.

Sperm cells, smaller than eggs, have a head with a nucleus and acrosome, a middle piece with mitochondria, and a flagellum for movement. Fertilization results in a zygote, which develops into an embryo. The embryo implants in the uterine lining, which thickens to provide nourishment.