Spermiogenesis Oogenesis-III Lectures – Anatomy Lectures – MBBS Lectures

Specific learning objectives:

By the end of this session you should be able to:

• Explain the process of SPERMIOGENESIS             
• Enlist the differences between spermatogenesis and spermiogenesis
• Justify the relationship of sub-fertility with production of abnormal sperms
• Define terms like azospermia, oligospermia
• Describe the maturation of oocytes before birth

Spermiogenesis refers to the differentiative process by which spermatids are transformed into spermatozoa.
The main features of this transformation include:

Sperm

• Important organelles are present in spermatid; there is a prominent Golgi apparatus, a pair of centrioles and numerous mitochondria.
• The formation of a spermatozoon involves elaborate changes in all these cellular elements.
• The Golgi apparatus shifts its position to come to lie close to the nucleus. Eventually, it gives rise to a large vesicle, called acrosome, which becomes applied to one pole of the nucleus in the form of a cap. The acrosomal material is rich in carbohydrates and contains hydrolytic enzymes.
• The nucleus itself gradually moves toward the cell membrane, becomes progressively condensed and assumes a slightly flattened and elongated shape. The nucleus and the overlying acrosomal cap migrate to a position adjacent to the cell membrane.

• The area occupied by the nucleus and acrosomal cap becomes the head region of the developing spermatozoon.
• As the acrosome is forming, the two centrioles move to the caudal pole of the nucleus. In this location one of them, called distal centriole, gives rise to a flagellum which forms the axial filament of the developing sperm tail.
• The cell gradually elongates; this elongation occurs due to the displacement of the cytoplasm caudally.
• The mitochondria gradually move toward the caudal region of the cell, partially fuse to one another and finally form a sheath around the proximal part of the flagellum.
• The area containing the mitochondrial sheath comes to be known as middle piece of the sperm .

• In the final stages of the maturation of the spermatid, the surplus (residual) cytoplasm is partitioned off from the remainder of the spermatid.
• When the sperm is released from the seminiferous epithelium, the excess cytoplasm becomes detached from the sperm as a membrane-bound structure called residual body which is phagocytized by the Sertoli cells.
• The end piece of the sperm tail finally consist mainly of the axial filament (flagellum) surrounded by the plasma membrane.
• The net result of these changes is that a fully developed spermatozoon:
  • (i)  attains a very small size,
  • (ii)  it is capable of achieving motility, and
  • (iii)  it retains only those cellular structures which are essential to make it capable of fertilizing an ovum.

                                Morphological Abnormalities

• It is extremely rare to find an abnormality of shape in the female gametes (ova).
• However, morphological abnormalities of spermatozoa are quite common and presence of 10% abnormal spermatozoa in the semen is considered to be normal.
• Radiography, severe allergic reactions, & certain antispermatogenic agents increase the %age of abnormal shaped sperms (affect fertility if no. exceeds 20%)
• Several types of morphological abnormalities of spermatozoa are known; the commonly found abnormalities include:
  • two tails,
  • two heads,
  • very small head (called pinhead),
  • very large head, and
  • abnormal alignment of head and tail, etc.
  • Relationship of sub-fertility with abnormal sperms?
• Abnormal spermatozoa are incapable of fertilizing an ovum.

Sperm counts

• Sperms accounts for less than 10% of the semen
• Remainder consists of secretions of the  seminal glands, prostate & bulbourethral glands.
• Usually more than 100 million sperms per ml of semen
• A man with 20 million sperms/ml  or 50 million/ejaculate is fertile
• A man with less than 10 million sperms/ml is likely to be sterile, especially when the sperms are abnormal & immotile
• For potential fertility, 50% of sperms should be motile after 2hrs & some should be motile after 24 hrs.
• Male infertility may result from low sperm count (oligospermia), poor sperm motility, medications, endocrine disorders, exposure to environmental pollutants, cigarette smoking, abnormal sperms or obstruction of genital tract & accounts for 15% to 30% of infertility in couples.
• Oligospermia: Presence of very few live sperms in the ejaculate
• Azospermia: Absence of live sperms in the semen

Oogenesis

The process whereby oogonia differentiate into mature oocytes.

OOGENESIS Site?/

Ovaries

                                                OOGENESIS

• The process whereby oogonia differentiate into mature oocytes.
• In the female, the primordial germ cells differentiate into oogonia.
• PGCs originate in the endoderm of yolk sac during the 4th week of development.
• From here, they migrate to and settle down in the developing ovaries.
• In the ovaries they pass through a phase of proliferation in which they increase in number by a series of mitotic divisions.

PRENATAL MATURATION OF OOCYTE

4th wk i.u.l.: PRIMORDIAL GERM CELLS, 20-30 in number, 12-20 um,

5th-12th wk i.u.l.: OOGONIA, Mitosis, arranged in clusters surrounded by a layer of flat epithelial cells which originate from surface epithelium. All of the oogonia in one cluster are probably derived from a single cell.

12^th wk to 5^th month i.u.l.: Mitosis continues,

 5^th Month i.u.l.: Max No. 7000,000/ovary, then cell death begins, some oogonia enlarge to form PRIMARY OOCYTES 

All surviving PRIMARY OOCYTES (approx. 100um) are surrounded by a single layer of flat follicular cells forming primordial follicle (cytophysiological unit, and have entered prophase of meiosis-I but arrested at diplotene (dictyotene starts)

Arrested state is produced by oocyte maturation inhibitor (OMI), a small peptide secreted by follicular cells

Prenatal maturation of Oocyte

Postnatal maturation of Oocyte

Differences between Spermatogenesis & Spermiogenesis