31 Years NEET Previous Year Questions: Human Reproduction - 1

Sperm plasma membrane receptors bind to ZP3 on the zona pellucida. Binding to ZP3 allows sperm to adhere to the zona pellucida, which is an important step in fertilisation.

The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.

Fig: Sperm binding receptors

Relaxin is produced first by the corpus luteum of the ovary and later by the placenta. It helps in parturition as it increases the flexibility of the pubic symphysis and helps dilate the uterine cervix.

LH (Luteinizing hormone) is produced by the pituitary gland in the brain. It triggers ovulation & promotes the development of the corpus luteum by rupture of the Graafian follicle and high concentration of estrogen is released by mature Graafian  follicle. The egg ruptures the ovary wall and heads into the fallopian tubes where it waits for the sperm.

• Amnion, in reptiles, birds and mammals is a membrane forming a fluid-filled cavity (the amniotic sac) that encloses the embryo.
• During development the amnion arises by a folding of a mass of extra- embryonic tissue called the somatopleure.
• Lined with ectoderm and covered with mesoderm, the amnion contains a thin transparent fluid in which the embryo is suspended, thus providing a cushion against mechanical injury.

• The sperms in the female’s genital tract are made capable of fertilising the egg by secretions of the female genital tract.
• These secretions remove coating substances deposited on the surface of the sperms particularly those on the acrosome
• Thus the receptor sites on the acrosome are exposed and sperm becomes active to penetrate the egg. 
• This phenomenon of sperm activation in mammals is known as capacitation.

• The prostate gland lies in the pelvic cavity in front of the rectum and behind the symphysis pubis, surrounding the first part of the urethra.
• The prostate gland secretes a milky fluid that makes up about 30% of semen with the milky appearance. It is slightly alkaline, which provides a protective local environment for sperm arriving in the acidic vagina.
• It also contains a clotting enzyme, which thickens the semen in the vagina, increasing the likelihood of semen being retained in the vicinity of the cervix. Secretions of this gland, along with secretions of seminal vesicles, also make the seminal plasma rich in fructose, calcium and certain other enzymes too. 

It is made almost exclusively in the placenta. HCG hormone levels found in the mother's blood and urine rise a lot during the first trimester. They may play a part in the nausea and vomiting often linked to pregnancy.

In follicular phase of menstrual cycle, LH and FSH increase gradually and stimulate follicular development as well as secretion of estrogens by the growing follicles.

• The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube.
• The ovum released by the ovary is also transported to the ampullary isthmic junction where fertilization lakes place.
• Fertilization can only occur if the ovum and sperms are transported simultaneously to the ampullaryisthmic junction. This is the reason why not all copulations lead to fertilization and pregnancy.

• Capacitation and acrosomal reaction occur before fertilization.
• The process of capacitation happens when sperms enter vagina.
• The secretions of the female genital tract remove coating substances deposited on the surface of the sperms particularly those on the acrosome.
• Thus, the receptor sites on the acrosome are exposed and sperm becomes active to penetrate the egg.
• This phenomenon of sperm activation in mammals is known as capacitation.

• Prolactin is the hormone which induces lactation (enables females to produce milk) and has no role in parturition.
• Parturition is a phenomenon of giving birth. It is a process of delivering a child from the womb to the outside. It is usually initiated by the release of oxytocin hormone. 
• This hormone increases the contraction of uterine muscles. Prostaglandins are synthesized and released in response to the oxytocin hormone. Also, an increase in estrogen and progesterone ratio indicates the initiation of the parturition process.

During gametogenesis, spermatogonia are diploid because of undergoing the process of meiosis.

Spermatogenesis:

• Estradiol is a female sex hormone and plays a role in the development of female secondary sexual characteristics.
• It is an estrogen steroid hormone. It has a role in maintaining the lining of the vagina, endometrium and fallopian tubes. However, there is no direct co-relation with the event of ovulation specifically.
• All the other events mentioned, are involved in the ovulation phase of the menstrual cycle in the human female.

• GIFT (Gamete Intra Fallopian Transfer) is a technique helping to have a child.
• Gamete Intra Fallopian Transfer (GIFT) is an assisted reproductive technology in which both the sperm and unfertilised oocytes are transferred into the Fallopian tubes. Fertilisation takes place in vivo (inside the body of the female).

Gamete intra fallopian transfer:

• Follicles that form an antrum during maturation are called Antral Follicle or Graafian follicle.
• During the development of the follicle, a glycoprotein polymer capsule called the Zona pellucida which is acellular, forms around the oocyte, separating it from the surrounding granulosa cells.
• This layer remains with the oocyte after ovulation and contains enzymes that catalyze with sperm to allow penetration.

Urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids from the body. The urethra travels through the penis and carries semen as well as urine.

The corpus luteum is a temporary endocrine structure in female mammals that is involved in the production of relatively high levels of progesterone.

Corpus Luteum:

Synthesis of estrogen and progesterone due to high level of hCG is a normal gonadotropic activity in a normal pregnant female. 

• Human chorionic gonadotropin is the embryonic hormone, which ensures the corpus luteum continues to produce progesterone throughout the first trimester of pregnancy.  
• As well as maintaining progesterone production from the ovary, human chorionic gonadotropin may also play a role in making sure the lining of the uterus (endometrium) is ready to receive the implanting embryo. hCG may also promote the production of other steroids (e.g., estrogen) and protein (e.g., relaxin, inhibin A) hormones whose actions in the uterus may facilitate gestation.
• So, the correct answer is 'High level of hCG stimulates the synthesis of estrogen and progesterone.'

• Atretic follicles are found in the ovary.
• The ovary of an adult female has about 120,000-160,000 follicles.
• Out of which, only one ovarian follicle matures every 28 days alternatively by the two ovaries.
• The rest of them degenerate at different times. These are known as atretic follicles and the degeneration process is called follicular atresia.

• Both corpus luteum and macula lutea are characterised by yellow colour pigment, called lutein.  
• After an egg is released from the ovary, the cells that line the ovarian follicle change shape and colour to become a corpus luteum (yellow body).
• Macula lutea occurs as a yellow area of retina as yellow spot.
• Lutein, zeaxanthin, α-carotene, and β-cryptoxanthin are responsible for its yellow colouration.