1. Gametogenesis : Conversion of Germ Cells Into Male and Female Gametes

4. Gametes Development begins with fertilization
The male gamete, the sperm + female gamete the oocyte = Zygote.
Gametes derived from germ cells in the wall of the yolk sac - fourth week of development.
They migrate to the developing gonads - end of the fifth week.
Mitotic divisions increase their number during their migration and also when they arrive in the gonad.
In preparation for fertilization, germ cells undergo gametogenesis, which includes meiosis to reduce the number of chrosomes and cytodifferentiation to complete their maturation

5. Transverse sections showing development of the mesodermal layer. A
Transverse sections showing development of the mesodermal layer. A. day 17 B. Day 21. The thin mesodermal sheet gives rises to paraxial mesoderm (future somites), intermediate mesoderm (future excretory units) and lateral plate, which is split into parietal and visceral mesoderm layers lining the intraembryonic coelomic cavity

6. Oogenesis Maturation of oocytes begins before birth
Primordial germ cells arrive in the gonad
Differentiate into oogonia
Meiosis is the cell division that takes place in the germ cells
Meiosis requires two cell divisions
Meiosis I and meiosis II,
cells

7. As a result of meiosis the number of chromosomes in the cells is reduced to half. That is instead of 23 pairs 23 singles – this number is called the haploid number 23 pairs - is called diploid number. It generates female gametes Some of the oogonia differentiate into primary oocytes A primary oocyte together with its surrounding flat epithelial cells, is known as a primordial follicle. Mitotic divisions clusters surrounded by a layer of flat epithelial cells called follicular

8. Maturation continues at puberty.
At puberty 5 to 15 primordial follicles begin to mature with each ovarian cycle
Zona pellucida is formed by the secretion of a glycoprotein
Fluid filled spaces appear between the oocyte and the surrounding granulosa cells  coalesce  follicular antrum  maturation  graafian follicle

9. Only one reaches full maturity
With each ovarian cycle many follicles begin to develop.
Only one reaches full maturity
Others degenerate and become atretic
On maturing the primary oocyte resumes the temporarily stopped meiosis i.e. the first meiotic division  secondary oocyte and first polar body. The polar body lies between the zona pellucida and the cell membrane of the secondary oocyte
The second oocyte completes the second meiotic division only if fertilized.
If not fertilized the cell degenerates approximately 24 hours after ovulation.
Whether or not the first polar body undergoes a second division is uncertain, but fertilized ova accompanied by three polar bodies have been observed.

10. Certain important considerations
During meiosis one primary oocyte gives rise to four daughter cells
Each with (x) chromosomes.
Only one of these develops into a mature gamete, the oocyte. The other three, the polar bodies , receive little cytoplasm and degenerate during subsequent development.

11. Similarly one primary spermatocyte gives rise to four daughter(
Similarly one primary spermatocyte gives rise to four daughter(? Son) cells
Two with (x) chromosomes and two with (y) cromosomes
In contrast to oocyte formation, all four develop into mature gametes.

14. Clinical Correlates
Chromosomal abnormalities can be in number or in structure
They may result in Birth Defects and Spontaneous Abortions:
Abortions may be due to major or minor chromosome abnormalities.
Most common chromosome abnormalities are (Turner Syndrome),
gene mutations account for about 8% of abortions.
Trisomy 21 (Down Syndrome)
trisomy - xxy - Klinefelter’s syndrome – xxxy may also occur.
In trisomy an extra chrosome is present  total 47

15. Maturation of the gametes begins before birth.
Spermatogenesis
Begins at puberty
Primordial germ cells in the seminiferous tubules give rise to spermatogonia
spermatogonia  mitosis  primay spermatocytes  meiosis I  secondary spermatocytes  meioss II  spermatids
Spermatids are haploid because of meiotic division – i.e. reduction division.

16. Spermiogenesis
Process of transformaion of spermatids into spermatozoa –
acrosome is formed
condensation of the nuleus - formation of neck, middle piece, and tail -shedding of most of the cytoplasm
Fully formed spermatozoa enter the lumen of seminiferous tubules. From there they are pushed toward the epididymis by contractile elements in the wall of the seminiferous tubules
In the beginning spermatozoa are only slightly motile – full motility is gained while in the epididymis.
Spermatogonia and spermatids and spermatozoa remain embedded in deep recesses of sertoli cells throughout their development.
Sertoli cells support and protect the germ cells, participate in their nutrition, and assist in the release of mature spermatozoa

23. Turner’s Syndrome
What is Turner syndrome? Chromosomal condition that exclusively affects girls and women.
One of the two X chromosomes normally found in females is missing or incomplete How is Turner syndrome diagnosed? A blood test, called a karyotype, analyzes the chromosomal composition of the individual. This is the most commonly used blood test to diagnose Turner syndrome.

24. What are the most common characteristics of Turner syndrome
What are the most common characteristics of Turner syndrome? short stature
lack of ovarian development.
webbed neck,
arms that turn out slightly at the elbow,
a low hairline in the back of the head
cardiovascular problems,
kidney and thyroid problems,
skeletal disorders such as scoliosis
dislocated hips
hearing and ear disturbances What can be done? no "cure" for Turner syndrome.
minimize the symptoms
Growth hormone, either alone or with a low dose of androgen, will improve growth velocity and probably final adult height. Growth hormone is approved by the Food and Drug Administration for treatment of Turner syndrome and is covered by many insurance plans.

25. Estrogen replacement therapy to promote development of
secondary sexual characteristics. Estrogens are also important for
maintaining good tissue and bone integrity.
Modern reproductive technologies have also been used to helpwomen with Turner syndrome become pregnant if they desire. A donor egg can be used to create an embryo, which is carried by the Turner syndrome woman.

27. Klinefelter Syndrome normal phallus enlarged breasts 47 chromosomes, XXY

28. Down’s Syndrome
Flat broad face, oblique palpebral fissures; epicanthus; furrowed lower libs; broad hand with single transvers or simian creases. Mental retardation, congenital heart anomalies