Page 472 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Male Reproduction / 457

androgen‐binding proteins and androgens, but each chromosome consists of two
chromatids. During the second meiotic
are higher than in typical extracellular fluid
VetBooks.ir because this functional barrier sequesters division of the two daughter cells, each of
them.
the resulting four cells receives one of the
chromatids. The overall result of meiosis is
the production of four daughter cells, each
Germ Cells and Spermatogenesis of which has a haploid number of chromo-
somes (Fig. 25‐3).
Spermatogenesis is the term for all In mammals, two of the daughter cells
processes involved in the formation of contain the Y chromosome that produce a
mature male gametes from the most undif- male offspring (XY) when combined with
ferentiated germ cells. It includes several an X chromosome from an ovum. The
mitotic cell divisions followed by two mei- other two contain the X chromosome to
otic cell divisions, during which the chro- produce female offspring (XX) when united
mosome number is reduced from diploid with the X chromosome‐containing ovum.
to haploid. This series of cell divisions is However, in avian species, all spermatozoa
termed spermatocytogenesis (Fig. 25‐2). contain the same sex chromosome, known
Some of the cells resulting from the in these species as the Z chromosome.
mitotic cell divisions of the most undiffer- Female birds carry one Z and a second sex
entiated germ cells remain at the base of chromosome, the W chromosome. An
the epithelium to maintain the supply of ovum may contain either a W or Z chromo-
stem cells. Others begin the sequence of some, so the ovum is the gamete that deter-
cell divisions (mitotic followed by meiotic mines the sex of the offspring in birds.
divisions) and developmental changes to When homologous chromosomes are
become spermatozoa. The mitotic cell paired in preparation for the first meiotic
divisions double the number of cells at division, crossing over may occur. During
each step, so a single spermatogonium this process, similar regions of chromo-
gives rise to many spermatozoa. somes may be exchanged between homol-
Meiosis entails two cell divisions and ogous chromosomes. Such exchanges
occurs only during the development of further increase genetic variability among
gametes in the testis and ovary. Prior to the the offspring, for they may now inherit
first division, the DNA is replicated in a chromosomes that are different from
manner similar to that of mitotic cell divi- either parent chromosome.
sion (see Chapter 2). This replication Spermatid is the term for the cells
results in chromosomes that consist of two resulting from the second meiotic division
identical chromatids. In preparation for in the seminiferous tubules. Spermatids
the first meiotic division, homologous undergo a series of functional and struc-
chromosomes pair up along the middle of tural changes to become spermatozoa, and
the cell. (Homologous chromosomes are this process is termed spermiogenesis
the similar chromosomes of a typical pair, (Fig. 25‐2). There is no further cellular
each of which was contributed by one of division after second meiotic division, so
the parent animals.) During the first mei- there is no further increase in cell number
otic division, one chromosome of each after that division.
homologous pair moves into each daugh-
ter cell. Which individual chromosome of
the homologous pairs moves to which Spermatozoa Morphology
daughter cell appears to be random. This and Spermatogenesis
mixing among homologous pairs provides
for genetic variation among the offspring. When first produced, spermatids are round
After the first division, each daughter cell immobile cells, while fully formed sperma-
has a haploid number of chromosomes, tozoa are mobile and consist of a head and