There is an increase in the likelyhood of chromosomal abnormalities in the embryo after the mother reaches 35 years of age. The ideal maternal age for reproduction is considered to be from 18 to 35 years.

Older mothers have a high risk of Down’s syndrome (Or other trisomies).

There is also an increase in the incidence of fresh gene mutations with increase in age.

A higher risk of Achondroplasia is associated with the increasing age of the father.

Can Grossly abnormal (Morphologically abnormal) sperms cause birth defects?

It is beleived that most abnormal sperms are unable to pass through the mucus in the cervical canal, due to lack of normal motility.

Up to 10% of sperms may be grossly abnormal (eg with two tails or two heads etc).

Abnormal sperms are not believed to affect fertility, unless their number exceeds 20%.

Some of the causes of grossly abnormal sperms include X-Rays, severe allergic reactions and antispermatogenic agents.

Are there grossly abnormal oocytes (Human eggs)?

Some oocytes may have two or more nuclei. The incidence of grossly abnormal oocytes is much less than in sperms. It is believed that such abnormal oocytes never mature and get expelled during ovulation.

What is Nondisjunction?

During meiotic cell division, nondisjunction is the failure of homologous chromosomes to seperate and go to opposite poles of the germ cells.

This results in one gamete having 24 chromosomes while the other gamete from the same meiotic cell division has 22 chromosomes (The normal is 23 chromosomes per gamete).

If a gamete with 24 chromosomes unite with a normal gamete which has 23 chromosomes ,the zygote will have 47 chromosomes (They have three representatives of a particular chromosome – Therefore this condition is called trisomy).

References

Chapter 2, The Beginning of Human Development .The Developing Human. Clinically Oriented Embryology 6th edition by Keith L. Moore, Phd, FIAC, FRSM and TVN Persaud MD, PhD, DSc, FRCPPath (Lond).

]]> http://www.askdrshihaan.org/pregnancy/2008/11/gametogenesis-and-chromosomal-abnormalities/feed/ 0 http://www.askdrshihaan.org/pregnancy/2008/11/spermatogenesis/ http://www.askdrshihaan.org/pregnancy/2008/11/spermatogenesis/#comments Sat, 22 Nov 2008 11:21:06 +0000 Dr Shihaan http://www.askdrshihaan.org/pregnancy/?p=178 Spermatogenesis is defined as the sequence of events which leads to spermatatozoa (sperms) from primitive germ cells (spermatogonia).

Spermatogenesis begins at puberty (roughly 13 to 16 years) and continues well into old age.

The primitive germ cells( spermatogonia) remain dormant in the seminiferous tubules until the onset of puberty. With the onset of puberty the spermatogonia undergo several mitotic divisions which transforms them into primary spermatocytes, which are the largest germ cells in the seminiferous tubules.

Each primary spermatocyte undergoes a reduction division (meiotic division).  The first meiotic division gives rise to two haploid secondary spermatocytes (are about half the size of primary spermatocytes. The secondary spermatocytes then undergo a second mitotic division to form four haploid spermatids. The spermatids are about half the size of secondary spermatocytes.

The spermatids are then transformed into four mature sperms (by a differentiation process called spermiogenesis).

The entire process of spermatogenesis (including spermatogenesis) takes about two months.

The mature sperms (after spermiogenesis) enter the lumina of the seminiferous tubules.

Special cells (sertoli cells) lining the seminiferous tubules provide nutrition and support for the germ cells . They are also probably involved in the regulation of spermatogenesis. The sertoli cells secrete androgen binding protein (ABP), inhibin, and MIS. However they do not synthesize androgens. They contain the enzyme aromatase (CYP19) for converting androgens to estrogens. Inhibin inhibits follicle stimulating hormone secretion. MIS is responsible for the regression of the mullerian ducts in males during fetal life.

The sperms are then passively transported from the seminiferous tubules to the epididymis and stored until they become functionally mature.The epididymis is the elongated coil duct at the posterior border of the testis. It is continuous with the vas deferens (ductus deferens), which transports the sperms to the urethra.

Sperm Structure

The mature sperm consists of a head  and tail. It is free swimming.

The neck is the junction between the head and tail of the sperm.

The head of the sperm which contains the haploid nucleus is the largest part of the sperm.

The anterior(front end) of the nucleus is covered by a saccular organelle (sac like organelle) which contains several enzymes. The most important enzyme contained in the acrosome is acrosin.

The enzymes released from the acrosome facilitate the penetration of the zona pellucida and the corona radiata, while trying to fertilize the egg.

The tail of the sperm is made up of three pieces (three segments). The three segments are the middle piece (mid-piece), principal piece and end piece. The tail is responsible for the motility of the sperm. The middle piece is packed with large concentrations of mitochondria, which provide adequate the adenosine triphosphate (ATP) necessary for activity.

The mitochondrial (fibrous ) sheath is thought to be responsible for the lashing movements of the tail.

The membrane of late spermatids and spermatozoa contain a special type of germinal angiotensin II converting enzyme. Although the function of this enzyme in humans is not known, in mice desruption of the production of this enzyme reduces fertility.

Temperature and Spermatogenesis

For spermatogenesis to occur an optimal temperature which is considerably lower than that of the interior of the body must be maintained. The testis are normally maintained at a temperature of about 32 degrees celsius. The testis are kept cool by air circulating around the scrotum. It is also possible that a countercurrent heat exchange exists between the spermatic arteries and veins.

Hot baths (43- 45 degrees celsius for 30 minutes per day) and insulated clothing (eg atheletic supporters) reduce the sperm count , to as low as 90%.

It is also well established that if the testis are held intra-abdominally (in experimental animals or undescended due to embryological defect) can result in the degeneration of the tubular walls and sterility.

Hormones and Spermatogenesis

FSH (Follicle stimulating hormone) and androgens maintain spermatogenesis in the testis. LH (Leutenizing hormone) produces a high concentration of androgens in the testis. Maturation of spermatozoa (from spermatids) is also androgen dependent.

It is important to know that there is a high concentration of fluid in the rete testis. This helps in the concentration and reabsorption of the fluids. If this fails to occur, the sperms entering the epidedymis are diluted and infertility results.

References

Pages 22 to 23 Chapter 2, The Beginning of Human Development .The Developing Human. Clinically Oriented Embryology 6th edition by Keith L. Moore, Phd, FIAC, FRSM and TVN Persaud MD, PhD, DSc, FRCPPath (Lond).

Pages 412 to 414 Chapter 23, The Gonads; Development and Function of the Reproductive System. Review of Medical Physiology 20th Edition by William F Ganong.

The average volume of semen per ejaculate is 2.5 to 3.5 mL (after several days of continence).

There is a rapid decline in the volume of semen and the sperm count with repeated ejaculation.

The semen contains sperms and secretions from the prostate, seminal vesicles, urethral  and Cowper’s glands .

There are normally 100 million sperms per mL of semen.

It is important to note that almost all men with sperm counts under 20 million/mL are sterile. 50% of men with sperm counts of 20 -40 million are also sterile.

There is a very high concentration of prostaglandins in the semen and their functions in semen are not fully understood.

Motility of Sperms:

Human sperms move at the speed of about 3mm/ min through the female genital tract. Therefore sperms reach the uterine tubes about 30 to 60 minutes after copulation.

Summary of the Composition of Normal Human Semen

Colour: White, opalescent

Specific gravity: 1.028

pH: 7.35-7.50

Sperm count: Average about 100 million/mL, with fewer than 20% abnormal forms.

Other components:

From seminal vesicles (constitutes 60% of total volume):

Fructose (1.5-6.5 mg/ml)

Phosphorylcholine

Ergothioneine

Ascorbic acid

Flavins

Prostaglandins

From Prostate( Constitutes 20% of total volume):

Spermine

Citric acid

Cholesterol and phospholipids

Fibrinolysin and fibrinogenase

Zinc

Acid Phosphatase.

Buffers:

Phosphate

Bicarbonate

Component that cannot be classified above:

Hyaluronidase

References:

Pages 412 to 414 Chapter 23, The Gonads; Development and Function of the Reproductive System. Review of Medical Physiology 20th Edition by William F Ganong.

The male gamete is the sperm and the female gamete is the oocyte (egg). These are both highly specialized cells.

Both male and female gametes have half the number of chromosomes (haploid)than in the somatic (body) cells.

A special type of cell division called meiosis occurs during gametogenesis. This cell division reduces the chromosomes by half. The shape of the cells are also altered.

The maturation process of gametogenesis is called spermatogenesis in males and oogenesis in females.

Differences between male (sperms) and female (oocyte) gametes

1) The oocyte is a large cell while the sperm is a tiny cell.

2) The oocyte is immotile whiile the sperm is motile.

3) Sparse cytoplasm in the sperm cells compared to the oocyte. The oocyte has a large cytoplasm containing yolk granules, which provide nutrition to the zygote (fertilized egg) in the first week of development.

4) The oocyte (egg) is surrounded by the zona pellucida and a layer of follicular cells called the corona radiata.

5) Sperms have two types of sex chromosomal constitution: 23X and 23Y, while the oocyte has only one type 23X. Therefore it is the sperm which determine the sex of an embryo.

References:

Chapter 22 pages 18-23. The developing human- Clinically  oriented embryology 6th edition by Keith L. Moore, Phd, FIAC, FRSM and TVN Persaud MD, PhD, DSc, FRCPPath (Lond)