THE REPRODUCTIVE SYSTEM
- Sexual Reproduction
- Development of the reproductive system
- Male Reproduction
- Female Reproduction
Medicine Human reproductive system
I. Sexual Reproduction
- In sexual reproduction, genes from two individuals are combined in random and novel ways. This generates diversity within the species.
- Normally each cell in the adult has 23 pairs of chromosomes or 46 total chromosomes.
- 22 pairs are called autosomal chromosomes
- 1 pair is called the sex chromosomes
- XX sex chromosome is female
- XY sex chromosome is male.
- XX sex chromosome is female
Formation of the zygote
At puberty cells in the gonads (testis or ovaries) undergo meiosis. 23 pairs of homologous chromosomes become 23 chromosomes. The germ cell from the male (sperm) will then fuse with the germ cell of the female (ovum) during reproduction to reform a cell with 23 pairs of homologous chromosomes.
The sex of the zygote is determined by the sex chromosome of the fertilizing sperm.
II. Development of the Reproductive System
- Formation of Testis and Ovaries
- Accessory Sex organs:
- Sertoli cells: Mullerian inhibitory factor (MIF): regression of the Mullerian ducts
- Leydig cells: Testosterone: epididymis, ductus (vas) deferens, seminal vesicles, ejaculatory duct.
- External Genitalia
- Male: penis, urethra, prostate, and scrotum
- Female: clitoris, labia majora
After conception the embryonic gonads of males and females are similar (for about the first 40 days). Therefore the embryo can form either testes or ovaries. The presence or absence of the Y chromosome determines what happens. SRY (sex determining region of the Y chromosome) on the Y chromosome ® male. SRY gene encodes the testi-determining factor.
For the first 40 days the reproductive system of the embryo is undifferentiated and has accessory organs characteristic of either sex.
Male: Wolffian ducts ® epididymis, ductus (vas deferens), seminal vesicles, ejaculatory duct.
Female: Mullerian ducts ® uterus, fallopian tubes
External genitalia of males and females are identical for the first 60 days.
III. Male Reproductive System
- Seminiferous tubules (90% of weight): Sertoli cells, spermatogenesis, stimulated by FSH
- Interstitial compartment: Leydig cells, stimulated by LH
- Testosterone required for spermiogenesis in adult.
- Later stages of spermatogenesis require FSH. Testosterone and FSH act on Sertoli cells which probably release paracrine substances that stimulate spermatogenesis
- spermatozoa are non-motile in testes. They become motile and undergo other changes outside of the testes.
- oval-shaped head: contains the DNA
- midpiece or body: contains mitochondria for energy
- tail: for swimming
- Male Reproductive Tract
- Testes: formation of sperm
- Epididymis: sperm maturation and sperm storage
- Vas deferens: duct to transport spermatozoa towards the seminal vesicles
- Seminal vesicles: add secretions to form semen
- Prostate gland: add secretions to semen
- seminal vesicles: 60% of semen volume comes from seminal vesicles — contains fructose
- prostate gland: citric acid, calcium, coagulation proteins
The testis contain two compartments:
From spermatogonia (original stem cell in gonad) to spermatozoa. Cells migrate from the embryonic yolk sac to the testes. In the seminiferous tubules they become spermatogonia and then through a process called spermatogenesis the spermatogonia become spermatids and then mature spermatozoa. The process from spermatids to spermatozoa is called spermiogenesis.
Spermatozoa have 3 parts:
Testis® epididymis® Vas deferens® ejaculatory duct® urethra
IV. Female Reproductive System
- Ovaries: contain follicles which contain ova.
- Accessory Sex Organs
- Uterine or Fallopian tubes: ducts directly connected to uterus
- Uterus: 3 layers perimetrium (connective tissue), myometrium (smooth muscle), endometrium (epithelium)
- Oocyte and Follicle Development
- Oocyte Development: oogonia (46 chromosomes)® primary oocyte (46 chromosomes) ® secondary oocyte (23 chromosomes) ® zygote (if fertilized)
- Follicular Development: primary follicles ® secondary follicles ® graafian follicle ® corpus luteum (after ovulation)
- Primary follicle: immature: primary oocyte + a single layer of follicular cells mature: primary oocyte + a number of layers of follicular cells.
- Secondary follicle: primary oocyte or secondary oocyte plus numerous layers of granulosa cells and fluid filled vesicular cavities
- Graafian follicle: secondary oocyte, arrested before second meiotic division, plus layers of granulosa cells and a single large fluid filled cavity (antrum).
- Menstrual Cycle
- Menstruation (Day 1 to Day 4or 5): steroid hormones lowest, ovaries contain primary follicles
- Follicular Phase (Day 1 to Day 13, highly variable): FSH ® growth of follicles, one becomes mature graafian follicle, granulosa cells secrete estradiol
- Positive feedback loop: (LH surge): estradiol® GnRH® LH secretion® estradiol by ovaries
- Ovulation: FSH, followed by LH surge causes rupture of graafian follicle, expulsion of secondary oocyte into uterine tubes. Occurs about 24 hours after beginning of LH surge
- Luteal Phase: empty follicle becomes corpus luteum, secretes estradiol and progesterone.
- Proliferative Phase: proliferation of endometrium and increase in blood vessels (spiral arteries)
- Secretory Phase: development of endometrium — thick, vascular, spongy — in preparation for the embryo.
- Menstrual Phase: constriction of spiral arteries ® cell death ® sloughing of layer of endometrium. Bleeding phase.
- Contraceptive Methods
- Contraceptive pill: contains estrogen and progesterone, maintains negative feedback throughout cycle, so no LH surge and therefore no ovulation.
- Rhythm method: measure slight variations in temperature that normally occur just prior to ovulation. Often hard to detect temperature changes.
- Sperm capacitation, a series of changes which makes sperm fertile, occurs in the female tract. Sperm can last up to 3 days in female reproductive tract.
- Sperm fuses with ovulated oocyte in the uterine tube
- Fusion of one sperm prevents other sperm from fertilizing oocyte
- Zygote (diploid, 46 chromosomes) forms 12 hours after fertilization
- The zygote begins dividing (cleavage)
- Unfertilized oocyte will degenerate 12-24 hours after ovulation.
- 3 days after ovulation the embryo (8 cells at this point) enters the uterus
- 2 days later blastocyst forms
- 7 days after fertilization embryo is implanted in the uterine wall (endometrium)
|Period in Life||Number of Oocytes|
|5-6 months||6-7 million (oogonia)|
|Birth||1-2 million (primary oocytes)|
At 5 months of gestation there is a peak of about 6-7 million oogonia. After 5-6 months production of oogonia stops and never resumes. These oogonia become primary oocytes by the end of gestation. At puberty there are only about 400,000 primary oocytes. Only about 400 of these oocytes will actually ovulate during a woman's lifetime.
Types of Follicles:
Changes in Ovary
Negative feedback loop: progesterone, estradiol ® ¯ FSH, LH secretion® ¯ development of follicles.
Changes in the endometrium
Conception is most likely to occur when intercourse takes place 1-2 days prior to ovulation.
Fertilization occurs in the uterine tubes.
Fertilization cannot take place more than 1 day after ovulation. Since sperm live for 3 days in the female reproductive tract, fertilization can occur up to 3 days prior to ovulation.
Human chorionic gonadotropin
The embryo secretes chorionic gonadotropin (hCG). This maintains the corpus luteum and thus estradiol and progesterone secretion remain elevated. Since estradiol and progesterone levels remain elevated the endometrium is maintained, menstruation is prevented, and the embryo continues to grow.
- The Reproductive System, University of California, Berkeley (Dept. of Molecular & Cell Biology).