Page 48 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

30 Veterinary Histology of Domestic Mammals and Birds

Endomitosis ation exists in the events that are ascribed to each stage, the
VetBooks.ir Endomitosis is an incomplete internal form of division that stages are consistently referred to as:
bears only partial similarity with mitosis. After replicating
· leptotene (= thin strand),
during the S phase, the chromatids appear and separate,
but subsequent nuclear and cell division does not occur. · zygotene (= yoke-like strand),
In contrast to mitosis the nuclear envelope does not · pachytene (= thick strand),
disintegrate, and a mitotic spindle is not formed. · diplotene (= double strand) and
As the chromatids are not divided among two nuclei, · diakinesis.
the nucleus retains double the usual number of chromo-
somes. Repeated endomitoses may occur, resulting in a In the leptotene stage, chromatin fibrils undergo con-
high degree of polyploidy. This is usually accompanied by densation to form elongated, thin chromatin strands
the formation of multiple nucleoli, an increase in nuclear (containing DNA that was duplicated in the previous S
volume, elevated metabolic activity and growth in cell phase). Homologous pairs of sister chromatids draw
volume. The nuclei of highly differentiated cells, such as near to one another (homologous pairing) and become
megakaryocytes of the bone marrow or particular liver connected by point-like synaptonemal complexes
cells, often undergo maturation through endomitosis. (zygotene). The chromatin strands become shorter
and thicker. In the pachytene stage the homologous
Amitosis chromatid pairs are very closely associated. There is
In contrast to all of the previously described processes of exchange of genetic material between maternal and
cell division, amitosis is characterised by direct division of paternal chromosomes (crossing over). In the final stage
the nucleus and cytoplasm without the formation of vis- of prophase I (diplotene), homologous chromosomes
ible chromosomes or breakdown of the nuclear envelope. begin to separate. Junctions between the chromosomes
Division of the nucleus is brought about by microtubules (chiasmata) represent sites at which crossing over has
that form a ‘drawstring’ around the nucleus, separating occurred.
it into two portions of equivalent volume. The original Towards the end of prophase, the nuclear envelope
organelle population is retained. disintegrates and chromosomes become recognisable
Amitosis frequently results only in nuclear division, giv- as separate entities. During this stage (diakinesis), kine-
ing rise to multinucleate cells. tochore and spindle microtubules are formed.
The subsequent phases (metaphase, anaphase and telo-
Meiosis phase) proceed in a similar manner to their counterparts in
Mammalian fertilisation (syngamy, karyogamy) occurs mitosis. In contrast to mitosis, however, the centromeres
through union of the nuclei of male and female gametes that connect sister chromatids do not split. Consequently,
(oocytes and spermatozoa). In order to prevent doubling of it is homologous chromatid pairs (rather than sister chro-
the number of chromosomes with each generation, the dip- matids) that become separated. This results in halving of
loid set of chromosomes in the germ cells must be reduced the number of chromosomes (Figure 1.39).
by half to become haploid. This is achieved by meiosis.
Meiosis involves two nuclear divisions. In the diploid Meiosis II (equatorial division)
germ cell (2n), homologous chromosomes (equivalent The sequence of events during meiosis II is similar to that
chromosomes of maternal and paternal origin) each occurring during mitosis, with the exception that only a
form genetically identical chromatid pairs (4d). These haploid set of chromosomes undergoes separation (Figure
exchange genetic material (‘crossing over’) before becom- 1.39). In contrast to meiosis I, the centromeres of each
ing separated in an initial division (meiosis I, reductional chromatid pair become separated, allowing sister chro-
division). This gives rise to a haploid gamete in which matids to be detached from one another. Division of the
the number of chromosomes and the amount of DNA is two nuclei thus results in four haploid gametes (1n, 1d).
halved (1n, 2d). In a subsequent division, meiosis II (equa- When the male and female gametes fuse at fertilisation,
torial division), the amount of DNA (but not the number the two haploid nuclei combine to form a diploid somatic
of chromosomes) is halved again. This is achieved, as in cell with a full complement of DNA (2n, 2d).
mitosis, by the separation of sister chromatids. In contrast
to mitosis, however, this division is not preceded by (fur- Cell death
ther) duplication of DNA (Figure 1.39). A physiological equilibrium exists in multicellular organs
and tissues between the number of proliferating and
Meiosis I (reductional division) degenerating cells. Abnormalities can result in an increase

Meiosis I begins with a prolonged prophase (prophase I) in cell number (e.g. hyperplasia, neoplasia, autoimmune
that is subdivided into five stages (Figure 1.39). While vari- disease) or a reduction in the cell population (e.g. atrophy,

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