Spermatogenesis
In the last
chapter we talked about the importance of meiosis in sexual reproduction and
diversity, and saw how haploid cells are formed. In males, meiosis occurs
during spermatogenesis, in which spermatogonia in the testes become spermatozoa.
The germ
cells that will form the male gametes (spermatozoa) are derived from germ cells
that migrate from the yolk sac into the site of early gonad formation (see
Chapter 38).
Spermatogonia
are diploid germ cells in the testes that maintain their numbers by mitosis,
thus maintaining spermatozoa numbers through life. Spermatogonia contain both
Xand Ysex chromosomes. At a certain point a spermatogonium will stop its other
duties and begin meiosis. The cells that result will then pass through more
stages of maturation and development and will become mature spermatozoa capable
of travelling to and fertilising an ovum.
The testis
is made up of very long, tightly coiled tubes called the seminiferous
tubules that are surrounded by layers of connective tissue, blood vessels
and nerves (Figure 8.1). The seminiferous tubules are linked to straight
tubules and a network of tubes called the rete testis which lead to
the epididymis. The epididymis is another collection of tubes on the
posterior edge of the testis that passes inferiorly and is continuous with the ductus
deferens (also known as the vas deferens). The ductus deferens carries
mature spermatozoa from the testis to the urethra.
Spermatogonia
are found in the walls of the seminiferous tubules, and as they progress
through spermatogenesis they pass towards the lumina of those tubules. Leydig
cells within the testes produce testosterone. Sertoli cells are also found in
the seminifer- ous tubules, and produce a number of hormones.
The
spermatogonia that we begin the process with are called spermatogonia A cells
(Figure 8.2). These are the stem cells that proliferate and replenish the root
source of all spermatozoa. The cells that are about to begin meiosis are called
spermatogonia B cells, and can be recognised partly because they are
connected to one another by cytoplasmic bridges. They continue to divide by
mitosis until they become primary spermatocytes. The cytoplasmic bridges
will maintain connections between a group of cells during spermatogenesis, synchronising the
process and batch producing groups of spermatozoa.
The primary
spermatocytes enter meiosis I. Homologous recombination of chromosomes
occurs in this stage. One primary spermatocyte becomes two secondary
spermatocytes. These cells are now haploid. Each secondary spermatocyte may
contain an X or a Y sex chromosome.
Secondary
spermatocytes enter meiosis II and again divide, forming spermatids.
As the DNA was not replicated in meiosis II these cells have half their
original DNA. During fertilisation this DNA will be combined with the DNA of
the maternal ovum. This is the end of the first stage of spermatogenesis, known
as spermatocytogenesis.
During
spermiogenesis the rounded spermatid cell changes shape, becoming elongated and
developing the familiar head and tail. The cell loses cytoplasm, the nucleus is
packed into the head, mito- chondria become concentrated in the first part of
the tail and an acrosome forms around the tip of the head. The acrosome
contains enzymes that will help the sperm penetrate the outer layers of the
ovum during fertilisation.
At the end
of spermiogenesis the spermatids have become spermatozoa (Figure 8.3).
Spermatogenesis
takes around 64 days to produce spermatozoa from germ cells in the above
processes. The spermatozoa are then passed in an inactive state to the
epididymis, where they continue to mature. During the next week they descend
within the epididymis and become motile and ready to be passed into the ductus
deferens during ejaculation.
Clinical relevance
Abnormalities
in spermatogenesis are common, and during fertility investigations the number
and concentration of spermatozoa, and the proportion of abnormal sperm, are
counted in a semen sample. A number of biological and environmental factors
will affect the sperm count and fertility, such as smoking, sexuall smitted diseases,
toxins, testicular overheating and radiation.
