Biology Test 327

Interstitial cells or Leydig cells are the cells interspersed between the seminiferous tubules of the testis. They secrete androgens (e.g., testosterone) in response to stimulation by luteinizing hormone from the anterior pituitary gland.

After ovulation which occurs in the middle of menstrual cycle, empty Graafian follicle continues growth under the influence of LH. The follicular cells are converted into lutein cells by deposition of yellowish lipid inclusions. The phenomenon is called luteinization. The ruptured Graafian follicle is now called corpus luteum. It secretes hormones, mainly progesterone and small quantity of estrogen. Both LH and progesterone help in further growth and thickening of endometrium. The major change is that the endometrial glands become secretory. The uterine wall becomes ready for nourishing and anchoring blastocyst if fertilization takes place. Hence, this phase of menstrual cycle is called luteal or secretory phase. The phase lasts for about 13 days i.e.,15−28 days of 28 days menstrual cycle.

The testes are situated outside the abdominal cavity within a pouch called scrotum. The scrotum helps in maintaining the low temperature of the testes (2−2.5^∘C right. lower than the normal internal body temperature) necessary for spermatogenesis.

The process of fusion of a sperm with an ovum is called fertilization. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilize an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. In contact with the surface of egg covering, the acrosome releases its contained hydrolytic enzymes, also called sperm lysins. It is known as acrosomal reaction. Acrosome reaction results in dissolving of corona cells and degeneration of zona pellucida which helps in sperm penetration.

Sertoli cells (named after Italian histologist Enrico Sertoli) are found in the walls of the seminiferous tubules of the testis. Compared with the germ cells they appear large and pale. They anchor and probably nourish the developing germ cells, especially the spermatids, which become partly embedded within them.

Semen or seminal fluid or seminal plasma is the fluid ejaculated from the penis at sexual climax. Each ejaculate may contain 300−500 million spermatozoa suspended in a fluid secreted by the prostate gland and seminal vesicles with a small contribution from Cowper's glands. It is rich in fructose, calcium and certain enzymes. It provides a fluid medium for transport of sperms, nourishes and activates sperms, lubricates the reproductive tract of female and neutralizes the acidity of the vagina of female to protect the sperms.

Oogenesis starts with division of oogonia (gamete mother cells) giving rise to primary oocyte which enters into prophase I of the meiotic divison and get temporarily arrested at this stage. These primary oocyte gets surrounded by primary, secondary and tertiary follicles respectively. The tertiary follicle grow in size and completes its first meiotic divison to give rise to haploid secondary oocyte. This secondary oocyte forms a new membrane called zona pellucida surrounding it. During fertilisation the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division (2nd right. divison) of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid).

A morula is an embryo at an early stage of embryonic development, consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida. The morula is produced by embryonic cleavage, the rapid division of the zygote. The increase in number of cells does not change the size of the original mass. The divisions are rapid because there is no net growth of the embryo-the cell cycle alternates between DNA replication and mitosis. In the absence of growth, the cell number in the embryo increases while the cell size decreases. Thus, it has almost equal quantity of cytoplasm as an uncleaved zygote but much more DNA.

Assisted reproductive technologies (ART) helps couple who cannot conceive naturally.

Transfer of an ovum collected from a healthy donor is done into the Fallopian tube of another healthy female who cannot produce ovum naturally but can provide a suitable environment for fertilisation and further development is another method attempted. This is called GIFT or Gametic Intra Fallopian Transfer.

Amniocentesis is a prenatal diagnostic test that involves the extraction of a small amount of amniotic fluid from the amniotic sac surrounding the fetus. This fluid contains fetal cells and other substances that can be analyzed for various purposes.

1. Biochemical abnormalities in the fetus: Amniotic fluid can be tested for specific substances, such as alpha-fetoprotein (AFP), which can indicate certain fetal abnormalities like neural tube defects or kidney problems.

2. Errors of metabolism in the fetus: Amniotic fluid can also be used to test for specific metabolic disorders, such as phenylketonuria (PKU) or cystic fibrosis, by evaluating enzyme levels or other metabolic markers.

3. Chromosomal abnormalities in the fetus: The fetal cells present in the amniotic fluid can be used to perform chromosomal analysis, such as karyotyping or fluorescence in situ hybridization (FISH), to detect conditions like Down syndrome, trisomy 18, or other chromosomal abnormalities.

In summary, amniocentesis is a valuable prenatal diagnostic tool that can provide information about a variety of potential abnormalities in a developing fetus, including biochemical, metabolic, and chromosomal issues.