Reproductive Anatomy and Physiology

Female Reproductive Anatomy

Ovary: produce female eggs/ovum and female hormones (estrogen, progesterone)

Ovum: female reproductive cells that contribute X chromosomes.

Fallopian tubes: tubes that transport ovum to the uterus; passageway where fertilization occurs; site of ectopic pregnancy; site where tubal ligation (sterilization) occurs.

Uterus: also known as the “womb” when an egg is fertilized and implants in the lining of the uterus; when no fertilized egg is present the lining of the uterus sheds for the menstrual cycle.

Cervix: lower end of the uterus; an opening between the uterus and vagina that passes sperm, menstrual fluid, and a fetus.

Vagina: pathway for menstrual flow, birth canal, sperm, and STIs. Opening for sexual intercourse and contains vaginal secretions/lubrication.

Urethra: opening for urine.

Clitoris: sensitive tissue with sole purpose for sexual stimulation.

Passageway of Ovum:
Ovary – Fallopian tube – Uterus – Cervical Opening – Vagina

 

 

Menstruation

At the time the ovaries are formed in the fetus, there are approximately 6000000 primordial follicles, which decrease to about 600000 at birth, to 300000 at the first menstrual cycle, to about 10000 at the time of menopause.

The Cycle

– The average cycle is 28 days and has two distinct phases

The Follicular Phase

– The follicular phase starts on day one of the menstrual cycle (the first full day of bleeding)
– The hypothalamus in the brain releases gonadotropin-releasing hormone (GnRH)
– GnRH signals the pituitary gland to release follicle stimulating hormone (FSH)
– FSH stimulates the eggs inside the ovaries to grow
– About 20 immature eggs response and begin to develop within sacs known as follicles
– Follicles provide nourishment to the eggs
– As the eggs develop, the ovaries release estrogen.
– Estrogen signals the pituitary gland to reduce FSH production
– Only enough RSH is now released to stimulate one egg to continue developing, the rest of the eggs shrivel away.
– Estrogen stimulates the lining of the uterus to thicken
– The primary follicle contains the contains the egg that has grown the most rapidly.
– Estrogen continues to rise until it triggers a surge of luteinizing hormone (LH) from the pituitary gland
– LH stimulates ovualtion
– The follicle ruptures and the egg is released along with the follicular fluid onto the surface of the ovary.

The Luteal Phase

– The ruptured follicle continues to receive LH
– The LH enables the follicle to turn into a small cyst known as the corpus luteum
– The corpus luteum produces progesterone
– Progesterone

1) builds and thickens the endometrium, developing glandular structures and blood vessels that supply nutrients to the developing embryo
2) it switches off FSH an LH
3) it raises the basal body temperature (BBT) by half a degree, warming the uterus and fertilized egg.

The Journey of the Egg

– The egg is surrounded by the zona pellucida, a protective shell
– The shell is surrounded by a mass of sticky cells called the cumulus oophorus
– These sticky cells allow the finger like projections at the end of each fallopian tube, to pick up the egg and sweep it into the tube
– The channel from the fimbriae to the uterus is lined with cilia, which together with muscular contractions move the ovum along the tube to the uterus.
– The journey from the ovary to the uterus take about 6 days
– If the egg is not fertilized it will disintegrate and is absorbed

 

 

 

Fertilization & Implantation

– the genetic material from a sperm cell (spermatozoon) and secondary oocyte merge into a single nucleus
– fertilization normally occurs in the Fallopian tube 12-24 hours after ovulation
– ejaculated sperm can remain viable for about 48 hours and a secondary oocyte is viable for about 24 hours after ovulation
– peristaltic contractions and the action from the cilia transport the oocyte through the tube
– sperm swim to the oocyte by whip-like movements from its tail
– sperm produce an enzyme (acrosin) that stimulates sperm motility and migration
– uterine contractions stimulated by the prostaglandins in semen aids sperm movement
– sperm undergo functional changes (capacitation) in the female reproductive tract: the membrane around the acrosome becomes fragile so that several destructive enzymes are released
– the enzymes help penetrate the ring of cells (corona radiate) that surround the oocyte
– one sperm penetrates and enters a secondary oocyte, this is called syngamy
– syngamy causes depolarization, which triggers the release of calcium ions into the cell
– calcium ions stimulate the release of granules, that in turn, promote change changes in the zona pelllucida to block entry of the other sperm
– oocyte completes equatorial division (meiosis ll)
– the nucleus from the sperms head and the nucleus from the ovum fuse to produce segmentation nucleus
– the segmentation nucleus contains 23 chromosomes from the male pronucleus and 23 chromosomes from the female pronucleus
– the fertilized ovum is now called a zygote
– rapid mitotic cell divisions of the zygote are called cleavage
– cleavage is completed 30 hours after fertilization
– successive cleavages produce a solid sphere of cells called morula
– the morula is the same size as the original zygote
– at 4 1/2 – 5 days, the dense cluster of cells has developed into a hollow ball of cells and enters the uterine cavity, it is now called a blastocyst

Implantation

– blastocyst remains free within the uterine cavity for a short period of time
– blastocyst enlarges and receives nourishment from glycogen rich secretions from the uterine glands
– 6 days after fertilization the blastocyst attaches to the endometrium
– usually implants on the posterior wall of the fundus
– develops two layers in the region of contact between the blastocyst and endometrium, synctiotrophoblast and cytotrophobast
– syncytiotrophoblast secretes enzymes that enables the blastocyst to penetrate the uterine lining, the enzymes digest and liquefy the endometrial cells
– the trophoblast secretes hCG that rescues the corpus luteum from degeneration and sustains its secrestions of progesterone and estrogens, thus menstruation does not begin

Must Read: Reactive oxygen species: potential cause for DNA fragmentation in human spermatozoa

Embryonic Development

– inner cell mass of the blastocyst begins to differentiate into three primary germ layers: ectoderm, endoderm, and mesoderm
– within 8 days after fertilization the cells of the cytotrophoblast proliferate and form a fetal membrane ( amnion)
– about the 12th day after fertilization a yolk sac is forming
– about the 14th day cells differentiates into three distinct layers: ectoderm, endoderm, and mesoderm
– as the embryo develops, the endoderm becomes the epithelial lining of the GI tract, respiratory tract and several other organs. The mesoderm forms muscle, bone, and other connective tissue, the ectoderm develops into the skin and nervous system
– formation of the embryonic membranes, the membranes lie outside the embryo and protect and nourish the embryo and later the fetus
– the membranes are the yolk sac ( the primary source of nourishment for the embryo), amnion (protective membrane) chorion (the structure for exchange of materials between mother and fetus, produces hCG) , and allantois ( serves as an early site of blood formation)
– placenta is developed, it is formed by the chorion of the embryo and a portion of the endometrium of the mother
– the placenta allows oxygen and nutrients to diffuse into fetal blood from maternal blood: carbon dioxide and wastes diffuse from fetal blood into maternal blood
– the umbilical cord is a vascular connection between mother and fetus
– the cord consist of 2 umbilical arteries , and surrounded by a layer of amnion