Summary human development
The reproduction system
The human reproduction system ensures the continued existence of the human species- by
producing, storing, nourishing, and transporting functional male and female reproductive
cells. These cells are called gametes.
The reproductive system includes the following basic structures:
- Gonads are reproductive organs that produce gametes and hormones
- Ducts that receive and transport gametes
- Accessory glands and organs that secrete fluids into the reproductive system ducts or
into other excretory ducts
- Perineal structures are collectively known as the external genitalia.
In both males and females, the ducts are connected to chambers and passageways that open
to the outside. The structures involved make up the reproductive tract.
Reproductive system functions in gamete
- Production
- Storage
- Nourishment
- Transport
Fertilization: Fusion of male and female gametes to form a zygote
Female produces 1 gamete per month (retains and nurtures zygote)
Male disseminates large quantities of gametes (produces 1/2 billion sperm per day)
,How many cells does your body consist of? These cells can be either:
Somatic cells (non-sex cells)
- Example: lung, skin, heart, penis
- These cells are diploid, meaning that they have 23 pairs of chromosomes – 23 from
mom and 23 from dad.
Germ cells (sex cells)
- Gametes
- These cells are haploid, meaning that they have 23 different chromosomes (no pairs
as in a diploid cell)
The process by which somatic cells reproduce, i.e., the process by which diploid cells
produce identical diploid cells. In a nutshell, it consists of:
- Replication of chromosomes (which contain the DNA)
- Separation of the chromosomes
- Division of the nucleus and division of the cytoplasm
- The products of mitosis are identical to one another and to the parent cell.
Generation of germ cells: two rounds of division necessary. The products of meiosis are not
identical to the parent cell or to each other. This is because recombination occurs in the first
steps, this creates 4 different copies of each chromosome every time meiosis occurs. This is
to increase genetic variation in the zygote.
Homologous chromosomes lie adjacent in meiosis I
• One chromosome segment exchanges position with
its homologous counterpart
• Genetic information is exchanged between
homologous chromosomes
• Two recombinant chromosomes are formed
• One average minimal 1 recombination event per
chromosome
Fusion of maternal and paternal DNA
- Each gamete contains 23 chromosomes
Segregation & Independent assortment
- Random segregation of Maternal & Paternal DNA
- Zygote: 23 pairs of chromosomes / 46 chromosomes
Recombination
- Crossing over during late prophase
Who wins the race
- Random fertilization
Chromosomes are randomly distributed and hence are segregated to daughter cells in
meiosis 1 (always a complete set)
Alleles on different pairs of homologous chromosomes are again distributed independently
in meiosis 2
The number of different types of gametes can be calculated by this formula: 2N, where n is
the number of homologous pairs
, Male reproduction
In adult males, the testes are male gonads that secrete sex hormones called androgens. The
main androgen is testosterone. The testes also produce the male gametes, called sperm.
During emission, mature sperm travel along a lengthy duct system, where they are mixed
with the secretions of accessory glands. The mixture created is known as semen. During
ejaculation, semen is expelled from the body.
figure 1: The adrenal glands are on top of the kidney and the testes
produce testoron. This is why females also produce testosteron.
Pathway of spermatozoa
• Testis
• Epididymis
• Ductus deferens (Vas deferens)
• Ejaculatory duct
Accessory organs
• Seminal vesicles
• Prostate gland
• Bulbourethral glands (Cowper)
• Scrotal sac encloses testes
• Penis
External genitalia in the male: figure 2: Spermatozoa travel from testis within epididymis; to ductus
• Scrotal sac (encloses testes) deferens; to ejaculatory duct and penile urethra. Fluids are secreted from
the seminal vesicles, the prostate gland, and the bulbourethral glands
• Penis
Spermatozoa development requires
temperatures ~1.1°C lower than the normal
body temperature. Cremaster and dartos
muscles relax/contract to regulate
temperature (they move the scrotal sac).
Inguinal canals are passageways through
abdominal musculature for testes descent (the
testes are made inside the body and need to
, move to their place); closed later in life
Males: risk of Inguinal Hernia
Connective tissue:
Supports blood and
lymphatic vessels that
supply and drain the
testis, and the efferent
ductules, which transport
spermatozoa to the
epididymis (ependymes).
Tubule: ~80 cm
~500.000 tubules / testis
Role: Spermatozoa
production
The septa testis subdivides the testis into a series of lobules. Distributed among the lobules
are tightly coiled seminiferous tubules. Sperm production takes place in these tubules.
Spermatozoa go from the seminiferous tubules via rete testis (a maze of passageways) to the
epididymis. Interstitial endocrine cells (Leydig cell) found in the connective tissue around
the tubules (within the septa) produce androgens (androsterone and testosterone), the
dominant sex hormones in males.
Seminiferous tubules
- Contain spermatogonia
• Stem cells involved in spermatogenesis
• Production of spermatozoa
Contain sustentacular cells
- Sustain and promote the development of sperm
Male germ cells: known as spermatogonia (stem cells)
• Some of them continually duplicate themselves (via mitosis; type A) throughout a male’s
reproductive life (maintain population) = self-renewal; symmetric division
• Other spermatogonia (type B) are destined to develop into sperm (via meiosis) => when
different fates; asymmetric division
• Males produce spermatozoa
The reproduction system
The human reproduction system ensures the continued existence of the human species- by
producing, storing, nourishing, and transporting functional male and female reproductive
cells. These cells are called gametes.
The reproductive system includes the following basic structures:
- Gonads are reproductive organs that produce gametes and hormones
- Ducts that receive and transport gametes
- Accessory glands and organs that secrete fluids into the reproductive system ducts or
into other excretory ducts
- Perineal structures are collectively known as the external genitalia.
In both males and females, the ducts are connected to chambers and passageways that open
to the outside. The structures involved make up the reproductive tract.
Reproductive system functions in gamete
- Production
- Storage
- Nourishment
- Transport
Fertilization: Fusion of male and female gametes to form a zygote
Female produces 1 gamete per month (retains and nurtures zygote)
Male disseminates large quantities of gametes (produces 1/2 billion sperm per day)
,How many cells does your body consist of? These cells can be either:
Somatic cells (non-sex cells)
- Example: lung, skin, heart, penis
- These cells are diploid, meaning that they have 23 pairs of chromosomes – 23 from
mom and 23 from dad.
Germ cells (sex cells)
- Gametes
- These cells are haploid, meaning that they have 23 different chromosomes (no pairs
as in a diploid cell)
The process by which somatic cells reproduce, i.e., the process by which diploid cells
produce identical diploid cells. In a nutshell, it consists of:
- Replication of chromosomes (which contain the DNA)
- Separation of the chromosomes
- Division of the nucleus and division of the cytoplasm
- The products of mitosis are identical to one another and to the parent cell.
Generation of germ cells: two rounds of division necessary. The products of meiosis are not
identical to the parent cell or to each other. This is because recombination occurs in the first
steps, this creates 4 different copies of each chromosome every time meiosis occurs. This is
to increase genetic variation in the zygote.
Homologous chromosomes lie adjacent in meiosis I
• One chromosome segment exchanges position with
its homologous counterpart
• Genetic information is exchanged between
homologous chromosomes
• Two recombinant chromosomes are formed
• One average minimal 1 recombination event per
chromosome
Fusion of maternal and paternal DNA
- Each gamete contains 23 chromosomes
Segregation & Independent assortment
- Random segregation of Maternal & Paternal DNA
- Zygote: 23 pairs of chromosomes / 46 chromosomes
Recombination
- Crossing over during late prophase
Who wins the race
- Random fertilization
Chromosomes are randomly distributed and hence are segregated to daughter cells in
meiosis 1 (always a complete set)
Alleles on different pairs of homologous chromosomes are again distributed independently
in meiosis 2
The number of different types of gametes can be calculated by this formula: 2N, where n is
the number of homologous pairs
, Male reproduction
In adult males, the testes are male gonads that secrete sex hormones called androgens. The
main androgen is testosterone. The testes also produce the male gametes, called sperm.
During emission, mature sperm travel along a lengthy duct system, where they are mixed
with the secretions of accessory glands. The mixture created is known as semen. During
ejaculation, semen is expelled from the body.
figure 1: The adrenal glands are on top of the kidney and the testes
produce testoron. This is why females also produce testosteron.
Pathway of spermatozoa
• Testis
• Epididymis
• Ductus deferens (Vas deferens)
• Ejaculatory duct
Accessory organs
• Seminal vesicles
• Prostate gland
• Bulbourethral glands (Cowper)
• Scrotal sac encloses testes
• Penis
External genitalia in the male: figure 2: Spermatozoa travel from testis within epididymis; to ductus
• Scrotal sac (encloses testes) deferens; to ejaculatory duct and penile urethra. Fluids are secreted from
the seminal vesicles, the prostate gland, and the bulbourethral glands
• Penis
Spermatozoa development requires
temperatures ~1.1°C lower than the normal
body temperature. Cremaster and dartos
muscles relax/contract to regulate
temperature (they move the scrotal sac).
Inguinal canals are passageways through
abdominal musculature for testes descent (the
testes are made inside the body and need to
, move to their place); closed later in life
Males: risk of Inguinal Hernia
Connective tissue:
Supports blood and
lymphatic vessels that
supply and drain the
testis, and the efferent
ductules, which transport
spermatozoa to the
epididymis (ependymes).
Tubule: ~80 cm
~500.000 tubules / testis
Role: Spermatozoa
production
The septa testis subdivides the testis into a series of lobules. Distributed among the lobules
are tightly coiled seminiferous tubules. Sperm production takes place in these tubules.
Spermatozoa go from the seminiferous tubules via rete testis (a maze of passageways) to the
epididymis. Interstitial endocrine cells (Leydig cell) found in the connective tissue around
the tubules (within the septa) produce androgens (androsterone and testosterone), the
dominant sex hormones in males.
Seminiferous tubules
- Contain spermatogonia
• Stem cells involved in spermatogenesis
• Production of spermatozoa
Contain sustentacular cells
- Sustain and promote the development of sperm
Male germ cells: known as spermatogonia (stem cells)
• Some of them continually duplicate themselves (via mitosis; type A) throughout a male’s
reproductive life (maintain population) = self-renewal; symmetric division
• Other spermatogonia (type B) are destined to develop into sperm (via meiosis) => when
different fates; asymmetric division
• Males produce spermatozoa
