1. Sexual Reproduction
  2. Development of the reproductive system
    1. Formation of Testis and Ovaries
    2. Accessory Sex Organs
    3. External Genitalia
  3. Male Reproduction
    1. Testis
    2. Spermatogenesis
    3. Male Reproductive Tract
    4. Semen
  4. Female Reproduction
    1. Anatomy
    2. Oocyte and Follicle Development
    3. Menstrual Cycle
    4. Contraceptive Methods
    5. Fertilization
  1. Sexual Reproduction

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.

  1. Development of the Reproductive System
  1. Formation of Testis and Ovaries
  2. 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.

  3. Accessory Sex organs:

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

  1. External Genitalia

External genitalia of males and females are identical for the first 60 days.


  1. Male Reproductive System
  1. Testis:

The testis contain two compartments:

  1. Seminiferous tubules (90% of weight): Sertoli cells, spermatogenesis, stimulated by FSH
  2. Interstitial compartment: Leydig cells, stimulated by LH


  1. Spermatogenesis

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:

  1. oval-shaped head: contains the DNA
  2. midpiece or body: contains mitochondria for energy
  3. tail: for swimming
  1. Male Reproductive Tract

Testis® epididymis® Vas deferens® ejaculatory duct® urethra

  1. Semen
  1. seminal vesicles: 60% of semen volume comes from seminal vesicles — contains fructose
  2. prostate gland: citric acid, calcium, coagulation proteins
  1. Female Reproductive System
  1. Anatomy
  1. Oocyte and Follicle Development

Period in Life

Number of Oocytes

5-6 months

6-7 million (oogonia)


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:

  1. Menstrual Cycle

Changes in Ovary

Positive feedback loop: (LH surge): estradiol® ­ GnRH® ­ LH secretion® ­ estradiol by ovaries

Negative feedback loop: progesterone, estradiol ® ¯ FSH, LH secretion® ¯ development of follicles.

Changes in the endometrium

  1. Contraceptive Methods

Conception is most likely to occur when intercourse takes place 1-2 days prior to ovulation.

  1. Fertilization

Fertilization occurs in the uterine tubes.

  1. 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.
  2. Sperm fuses with ovulated oocyte in the uterine tube
  3. Fusion of one sperm prevents other sperm from fertilizing oocyte
  4. Zygote (diploid, 46 chromosomes) forms 12 hours after fertilization
  5. The zygote begins dividing (cleavage)
  6. Unfertilized oocyte will degenerate 12-24 hours after ovulation.

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.

Blastocyst Formation

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.