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Introduction & Gametogenesis
INTRODUCTION
Embryology :
Study of prenatal stages of development
from the fertilization of the ovum to the
birth of the new individual.
Why and How
Gross Form,
Correlation
Embryology
Errors in development
Structural
Molecular
(Clinical aspects)
Cellular and
Molecular
phenomena
Evolutionary
mirror
Sexual Reproduction
Vast potential for variation
 Basis for evolution

◦ Genes and gene pools
◦ Population genetics
However…
 Requires special cells : gametes

◦ Spermatozoa
◦ Ova (Oöcytes)

Overview :
◦ Reproductive system – male and female
◦ Two types of cell division
Descriptive terms of Position, Direction and Planes
Cell and Cell Division

Some familiarity with cell biology especially with reference to the nucleus is
presumed here. Some important concepts :

Chromatin : Coloured (when stained!) material in the nucleus, comprising nucleic
acids and proteins.

Chromosomes : supercoiled DNA with proteins, arranged as ‘coloured sticks’.
Chromosomes are visible as discrete structures only during cell division. Each
chromosome has two ‘arms’, short and long, with a small enlargement, the
centromere, between the two.

All nucleated cells in the human body, with the exception of the final stages of
gametogenesis, have 23 pairs of chromosomes in their nuclei. Two members of a pair
are known as homologous chromosomes. In each pair, one chromosome is paternal,
one maternal.

One pair of chromosomes (#23, X + Y) is designated sex chromosomes. Males have
X + Y, females have X + X.

A cell with 23 pairs of chromosomes is described as diploid; one with 23 single
chromosomes is termed haploid.

The genetic material in the nucleus of a cell undergoes ‘replication’ before cell
division. This phase in a cell’s life is designated the ‘S’ phase (‘s’ for synthesis). After
the S phase there is a quiescent period followed by cell division.
Cell and Cell Division




All body cells that can divide, with the exception of a stage of
gametogenesis, divide by ‘mitosis’. Mitotic cell division produces two
daughter cells which are genetically identical with the parent cell.
During gametogenesis, there is a stage when a two-stage division
produces four haploid daughter cells. This is meiosis or meiotic
division, also called a reduction division. However, there is more to
meiosis than mere reduction to the haploid state.
The following slides present the phenomena associated with
chromosomes during cell division. Details of the stages of cell
division do not feature in these. The focus is on nuclear events,
cytoplasmic details are not shown.
A few more terms and concepts will be introduced in the course of
the description.
Mitosis
The chromosomes have
replicated
prior
to
division, but not yet seen
as discrete structures.
The nuclear envelope is
intact.
A
B
Mitosis
In each pair, there is one
maternal
chromosome
and one paternal.
A chromosome, when fully
condensed
may
be
shown as in ‘A’. It has two
identical halves called
chromatids, shown at ‘B’.
The most important point
to be understood here is
that each chromosome
has replicated.
A chromosome in a nondividing
cell
is
a
chromatid, if at all we
could see it!
Only two chromosome pairs are shown in the nucleus
in these pictures. One pair is shown in pink, one in
blue. They are still being condensed.
Mitosis
During metaphase, the chromosomes are arranged
along a plane in the middle of the cell
(‘equatorial plane’).
A ‘spindle’ of microtubules (mitotic spindle) attaches
the chromosomes to the centrosomes.
Shortening of the microtubules separates the
chromatids. The chromatids (chromosomes of
the daughter cells) migrate towards the poles of
the dividing cell.
Mitosis
The cytoplasm divides to create two roughly equal
cells. Note how the chromosomes ‘disappear’.
Meiosis
The picture at top left shows the cell resting
after the replication of chromosomes.
Once again we see two representative
pairs of chromosomes (bottom). In each
pair,
paternal
and
maternal
chromosomes are shown in different
colours.
Meiosis is a two-stage division. The two
stages are often called meiosis I and
meiosis II. We are looking at meiosis I.
Meiosis
Homologous chromosomes are arranged next to
each other lengthwise. Their corresponding
arms cross at a number of points. These
points of crossing are called chiasmata.
At these points of crossing, homologous
chromosomes exchange segments.
When this exchange is complete, the two
homologous chromosomes cease to be
what they were. Each one has
segments
from
chromosomes.
Maternal :
Paternal :
both
parental
The possibilities of chiasmata formation are
virtually unlimited. Every cell undergoing
meiosis therefore, can give rise to a
unique
combination
of
resultant
chromosomes.
Meiosis
Separation of chromosomes now follows a
different pattern.
Chromatids do NOT separate. One entire
replicated chromosome from each pair
moves to one pole of the cell.
At the end of meiosis I, the two resultant cells
are thus haploid, containing one member
from each pair.
It is worth repeating that the chromosomes of
these cells are NOT identical with those of
any of the parents. Each of these cells has
genes coming from both parents.
Considering that the same process has been
followed in every meiotic division in the
parents of this individual, there has been a
great mixing of grandparental genes as well!
Meiosis
The cells begin the second division
soon, without an S phase.
Recall that each of these cells is haploid
and
the
chromosomes
are
replicated.
In meiosis II, the chromatids of these
chromosomes separate.
On the right, we see the beginning of the
formation of four cells.
Meiosis
At the end of meiosis therefore, four
unique, haploid cells are formed.
MITOSIS
MEIOSIS
GAMETOGENESIS
Oogenesis
Spermatogenesis
PRIMORDIAL GERM CELLS
Gametogenesis
• In both sexes, a pool of cells destined for gametogenesis is set
aside during embryonic life.
• During gametogenesis, these cells divide by mitosis first, to
retain a pool of cells.
• Essentially, some of the products of these mitotic divisions
must undergo meiosis.
• The details of the actual process differ in the male and the
female.
Male Reproductive System
•
•
The principal organ, testis, produces
male gametes or spermatozoa. A
spermatozoon is cell with very little
cytoplasm and nucleus forming its head,
a neck with a single, long, spiral
mitochondrion and a tail.
Spermatozoa produced by the testis are
carried by a tube, the ductus deferens to
the pelvis where glands add their
secretions, forming semen. Semen is
ejaculated through the penis and
deposited in the female reproductive
system.
Seminal
vesicle
Prostate
Testis
Ductus
deferens
Spermatogenesis
In the male, spermatogenic activity begins at puberty.
The spermatogenic cells, called spermatogonia undergo
mitotic division, whereby some cells remain as
spermatogonia and some take the path of meiosis.
Each meiotic division produces four haploid cells called
spermatids. Spermatids then undergo structural changes
– loss of some cytoplasm, formation of a tail.
Spermatogenesis is a continuous process.
SPERMATOGENESIS
SEMINIFEROUS TUBULE SHOWING
ARRANGEMENT OF SPERMATOCYTES
SPERMIOGENESIS
SPERMIOGENESIS
Spermatid to spermatozoon
Spermatid is circurlar cell containing nucleus,
golgi apparatus, centriole and mitochondria.
Nucleus – head
Golgi apparatus – acrosomic cap
Centriole-div 2 ,one lie in the neck and other
give rise to axial filament and the one in the
neck forms annulus
Mitochondria-sheath around middle piece
Female Reproductive System
•
•
•
The uterine tube has a funnel-like end facing the
ovary, called the infundibulum (meaning a
funnel!). The fronds forming its border are the
fimbriae.
The somewhat dilated portion (*) towards the
uterus is the ampulla, followed by the narrow
part leading to the uterus. Due to the thickness
of the uterine wall the last part of the tube is
actually within the uterine wall (intramural part).
The uterus has three parts – the fundus (A) at
the top, the large body (B) and the cervix
[‘neck’, (C)] at its junction with the vagina.
Uterine tube
*
*
A
B
Uterus
C
Ovary
Vagina
Oögenesis
• Process begins during intrauterine life
• Meiosis I incomplete, suspended until puberty
• During reproductive life :
– Each month several oöcytes begin maturation
– Normally only one is released (others degenerate)
• “Ovulation”
– Meiotic divisions highly unequal
– Oöcyte and ‘polar bodies’
– Meiosis completed after the entry of a sperm
Oögenesis : Implications
• Unequal meiotic divisions produce only one
gamete with a huge amount of cytoplasm. The
polar bodies are small cells, normally
unfertilised.
• Long suspension of division  probability of
abnormal separation of chromosomes – ‘nondisjunction’.
• Longer suspension  greater
chromosomal abnormalities.
chances
of
OOGENESIS
MATURATION OF OVUM
Primary Ovarian Follicle
Antral Follicle
SECONDARY OOCYTE AT
OVULATION
The Menstrual Cycle – 1
•
The release of one oöcyte every month has functional rationale.
•
The uterus must be prepared, in anticipation of fertilisation of the oöcyte, for
receiving it and sustaining development for the duration of pregnancy. This
preparation involves the lining of the uterus (‘endometrium’ = epithelium +
supporting tissues). If the oöcyte is not fertilised the prepared endometrium is
discarded. Thus there are coordinated cyclic changes in the ovary and the
uterus. All these are controlled by hormones.
•
The histological details of uterus, ovary and those of their hormonal control will
dealt with in the reproductive block. At this stage the salient features are
mentioned.
•
During each cycle, as the endometrium matures, it increases in thickness, blood
vessels proliferate and glands develop from the lining epithelium.
The Menstrual Cycle – 2
The average duration of the cycle of changes is 28 days, though cycles as short as
20 days or as long as 35 days are normal. In the average 28-day cycle, ovulation
takes place around day 14.
If the oöcyte is not fertilised, most of the endometrium is discarded. This is
accompanied by a significant amount of blood loss through the broken blood
vessels. The endometrial tissue with the blood constitutes menstrual flow which
lasts 3 to 5 days.
Menstrual flow is the only reliable external marker of the cyclic changes. The first
day of the flow is thus taken as “Day 1” of the cycle, though this in fact marks
the end of the previous cycle.
It is noteworthy that the interval between ovulation and the next menstrual period
is rather constant around 14 days, whereas that between day 1 and ovulation
varies according to the length of the cycle in a paticular individual.
COMPARISION
•
Fertilisation
Fertilisation is the union of the male and
female gametes. It is pertinent to note
that the female gamete has not completed
the second meiotic division at the time of
release from the ovary, and should be
correctly called an oöcyte. In the adjoining
picture note that its is surrounded by a
thick ‘wall’ called the zona pellucida (‘clear
zone’). This is in turn surounded by a layer
of supporting cells forming a ‘crown’, the
corona radiata. At this stage the zona
pellucida also includes the polar body
formed at the end of meiosis I (not shown
in this picture).
• Fertilisation most commonly takes place
in the ampulla of the uterine tube.
Zona
pellucida
Corona
radiata
Acrosome
Fertilisation
At the tip of the sperm head
is an enzyme-containing
structure called acrosome.
Acrosomal enzymes allow the
sperm to ‘bore’ through
the zona pellucida.
The entry of one sperm causes a molecular reaction in the zona pellucida which
prevents the entry of any other sperm.
The entry of the sperm is also followed by the completion of the second meiotic
division and a second polar body is formed.
The ovum now has two ‘pronuclei’ male and female. These soon lose their nuclear
membranes and a diploid cell is formed, called the zygote.
These events are shown in the next slide where the polar bodies are also shown.
Fertilisation thus has some ‘consequences’ :
•
•
•
•
Restoration of diploidy
Chromosomal sex determination
Completion of meiosis II in the oöcyte
Initiation of the first cell division (cleavage)
ABNORMAL GAMETES