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Samenvatting Human Development deel 1
- Instelling
- Vrije Universiteit Amsterdam (VU)
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Voorbeeld van de inhoud
Lecture 2a Male reproductionGametes have different reproductive system functions, such as production, storage, nourishment and
transport. Gametes are also necessary for fertilization. Fertilization is the fusion of male and female gametes
to form a zygote. The gametes and hormones are produced by organs called gonads. The ducts are the parts
that receive and transport gametes. The fluids in these ducts are secreted by accessory glands. The perineal
structures are collectively known as external genitalia.
Gender is about the social meaning that is attached to being male or female. Sexual/gender identity is one’s
identity as male or female. The expectations about how a male or female should behave has been
determined by the gender role. The gender develops in utero throughout childhood.
Females produce 1 gamete per month. Males, however, disseminate large quantities of gametes, approx. ½
billion sperm per day. The production of these sperm cells starts in the brain. The hypothalamus releases
GnRH to the anterior pituitary (hypofyse). This releases the hormones Gn and ACTH. The testes respond to
the Gn and the adrenal glands respond to the ACTH. Both testes and the adrenal glands produce
testosterone. The testosterone activates the reproductive glands.
The sperm cells are called spermatozoa. They travel a lot, because they are made in the testes, then go
through the epididymis, through the vas deferens (ductus deferens) and into the ejaculatory duct and penile
urethra. Fluids are secreted from the seminal vesicles, the prostate gland and the bulbourethral glands.
Spermatozoa development requires temperatures 1,1⁰C lower than the normal body temperature. The
cremaster and dartos muscles relax and contract to regulate temperature by pulling the testes closer to the
body or pushing them further away.
When taking a closer look to the structure of the testes, there can be seen that the testes are actually a
purple kind of spaghetti-like tissue. Those are divided by septa, so they are compartmentalized. In the hollow
part of the spaghetti threads are the spermatozoa made. Deep to the tunica vaginalis covering the testis is
the tunica albuginea, a dense layer of connective tissue rich in collagen fibers. These fibers are continuous
with those surrounding the adjacent epididymis and extend into the testis. There they form fibrous
partitions, or septa testis, that converge toward the region nearest the entrance to the epididymis. The
,connective tissues in this region support the blood vessels and lymphatic vessels that supply and drain the
testis, and the efferent ductules, which transport sperm to the epididymis.
The septa testis subdivides the testis into a series of lobules. Distributed among the lobules are
approximately 800 slender, tightly coiled seminiferous tubules. Each tubule averages about 80 cm in length. A
typical testis contains nearly one-half mile of seminiferous tubules. Sperm production takes place within
these tubules. Each lobule contains several seminiferous tubules that merge into straight tubules that enter
the mediastinum of the testis. Straight tubules are extensively interconnected, forming a maze of
passageways called the rete testis. Fifteen to 20 efferent ductules connect the rete testis to the epididymis.
The seminiferous tubules contain spermatogonia. These are stem cells involved in spermatogenesis. Because
the seminiferous tubules are tightly coiled, most tissue slides show them in transverse section. A delicate
connective tissue capsule surrounds each tubule, and areolar tissue fills the spaces between the tubules.
Within those spaces are numerous blood vessels and large interstitial endocrine cells (Leydig cells). Interstitial
endocrine cells produce androgens (androsterone and testosterone), the dominant sex hormones in males.
Testosterone is the most important androgen.
The body consists of two types of cells: somatic cells (non-sex) and germ cells (sex). The somatic cells are
diploid, meaning that they have 23 pairs of chromosomes – 23 from mom and 23 from dad. The germ cells
(gametes) are haploid, meaning that they have 23 different chromosomes (no pairs). The process by which
somatic cells reproduce, i.e., the process by which diploid cells produce identical diploid cells. This process
(mitosis) 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. Gamete production, i.e., haploid cells are produced from diploid cells. Male germ cells are
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). The products of meiosis are not
identical to the parent cell or to each other. For one generation of germ cells are two rounds of division
necessary. Recombination occurs in the first steps. This creates 4 different copies of each chromosome every
time meiosis occurs. Meiosis happens to increase the genetic variation in the zygote. Chromosomes are
randomly distributed and hence are segregated to daughter cells in meiosis 1. 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: 2n, where n is the number of homologous pairs. In a man’s testes, the number
of gamete types that can be produces based on independent assortment is 2²³, which equals 8,2 million
possibilities. Homologous chromosomes lie adjacent in meiosis 1. One chromosome segment exchanges
,position with its homologous counterpart. Genetic information is exchanged between the homologous
chromosomes. Two recombinant chromosomes are formed. On average, minimal one recombination event
happens per chromosome. A single egg is fertilized by a single sperm in a random manner. Considering
independent assortment (and random fertilization), an offspring represents one out of 72 x 10¹² (8,5 million x
8,5 million) zygote possibilities.
In the seminiferous tubules are different types of cells:
1. Sustentacular cells (Sertoli cell)
2. Interstitial cells (Leydig cell)
Spermatogenesis begins at the outermost layer of cells in the seminiferous tubules and proceeds toward the
lumen. At each step in this process, the daughter cells move closer to the lumen. First, spermatogonia divide
by mitosis to produce two daughter cells. One daughter cell from each division remains at that location as a
spermatogonium, while the other is pushed toward the lumen (space) of the tubule. The displaced cell
differentiates into a primary spermatocyte. Primary spermatocytes then begin meiosis, giving rise to
secondary spermatocytes that divide and differentiate into immature spermatids.
There are 5 steps in spermatogenesis:
1. Stem cells (spermatogonia) divide by mitosis to produce two diploid daughter cells. One remains as
spermatogonium and the second one differentiates into a primary spermatocyte.
2. Primary spermatocytes begin meiosis I and when meiosis I is finished, they form secondary
spermatocytes.
3. The secondary spermatocytes differentiate into spermatids. Meiosis II is finished, but the gametes
are still immature.
4. The spermatids differentiate into spermatozoa. This process is called spermiogenesis.
5. The spermatozoa lose contact with the wall of the seminiferous tubule and enter the fluid in the
lumen.
, Spermiogenesis is the last step of spermatogenesis. Each spermatid matures into one spermatozoon (sperm).
Sustentacular cells phagocytose shed cytoplasm and provides nutrients. The acrosomal vesicle is made of
fused saccules of spermatid’s golgi apparatus and peroxisome. It forms an acrosomal cap for the
spermatozoon, which contains degrading enzymes. A mature spermatozoon lacks an endoplasmic reticulum,
a golgi apparatus, lysosomes, peroxisomes, inclusions and other intracellular structures. It has mitochondria
and DNA. This reduces the sperm size and mass, which makes them move quicker. Sustentacular cells have 6
major
functions:
1. Maintain blood-testis barrier A blood testis barrier isolates the seminiferous tubules from the
general circulation. This barrier is comparable in function to the blood brain barrier. Nurse cells are
joined by tight junctions between basal cytoplasmic processes. They form a layer that divides the
seminiferous tubule into an outer basal compartment and an inner luminal compartment. The basal
compartment contains the spermatogonia. Meiosis and spermiogenesis occur in the luminal
compartment. Transport across the nurse cells is tightly regulated, so conditions in the luminal
compartment remain very stable. In addition, this barrier prevents immune system cells from
detecting and attacking the developing sperm.
2. Support mitosis and meiosis Circulating follicle-stimulating hormone (FSH) and testosterone
stimulate nurse cells. These stimulated nurse cells then promote the division of spermatogonia and
the meiotic divisions of spermatocytes.
3. Support spermiogenesis Spermiogenesis requires nurse cells. These cells surround and enfold the
spermatids, providing nutrients and chemical stimuli that promote their development. Nurse cells
also phagocytize cytoplasm that is shed by spermatids as they develop into sperm.
4. Secrete inhibin (Inh) Nurse cells secrete the peptide hormone inhibin (in-HIB-in) in response to
factors released by developing sperm. Inhibin depresses the pituitary production of FSH, and perhaps
the hypothalamic secretion of gonadotropin-releasing hormone (GnRH). The faster the rate of sperm
production, the more inhibin is secreted. By regulating FSH and GnRH secretion, nurse cells provide
negative feedback control of spermatogenesis.
5. Secrete androgen-binding protein (ABP) Androgen-binding protein (ABP) binds androgens
(primarily testosterone) in the luminal fluid of the seminiferous tubules. This protein is likely
important for increasing androgen concentration and stimulating spermiogenesis. FSH stimulates the
production of ABP.
6. Secrete Müllerian-inhibiting factor (MIF) Müllerian-inhibiting factor is secreted by sustentacular
cells in developing testes. It causes regression of fetal Müllerian ducts. In males, inadequate MIF
production during fetal development leads to retention of ducts and failure of testes to descend into
the scrotum.
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