(a) Briefly explain the events of fertilisation and implantation in an adult human female.
(b) Comment on the role of placenta as an endocrine gland.


(a) During coitus, the semen having sperms is released into the vagina.

i. It passes through the cervix of the uterus and reaches the ampullary-isthmic junction of the fallopian tube.

ii. The ovum is also released into the junction for fertilisation to occur.

iii. The process of fusion of the sperm and the ovum is known as fertilisation.

iv. During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.

v. The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum.

vi. This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body.

vi. Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote.

vii. Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8 and 16 daughter cells called blastomeres.

viii. The 8−16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus.

ix. The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.

x. The trophoblast gets attached to the uterine endometrium and the process is called implantation. This leads to pregnancy.

xi. The inner cell mass gets differentiated to form the embryo.

(b) Placenta acts as an endocrine gland and produces the following hormones
i. Human chorionic gonadotropins (hCG), 
ii. human placental lactogen (hPL), 
iii. Estrogen and progesterone
iv. Relaxin (in the later stages of pregnancy).

These hormones support foetal growth and help in the maintenance of pregnancy. Hormones like oestrogen, progestogen, cortisol, thyroxine and prolactin are increased several folds in the maternal blood.

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(a) Explain the menstrual phase in a human female. State the levels of ovarian and pituitary hormones during this phase.
(b) Why is follicular phase in the menstrual cycle also referred as a proliferative phase ? Explain.
(c) Explain the events that occur in a Graafian follicle at the time of ovulation and thereafter.
(d) Draw a graafian follicle and label antrum and secondary oocyte


(a) The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy.

At the beginning of menstrual cycle, the brain signals the pituitary gland to produce a hormone called the follicle-stimulating hormone (FSH), which in turn signals the ovaries to begin forming and maturing eggs for this cycle. As these follicles mature, the level of estrogen in the body rises, indicating that an egg is ready. This first half of the menstrual cycle is called the follicular phase.

*When high estrogen levels signal an egg is ready, the pituitary gland then produces a luteinizing hormone (LH), triggering the ovary to release the mature egg by the process of ovulation. The day ovulation happens is the first day of the second half of the cycle, the luteal phase.

During the luteal phase of the menstrual cycle the ovaries (the corpus luteum) begin to increase the levels of progesterone in the body. Progesterone, secreted by ovary regulates and maintains the inner lining of the uterus both during menstrual cycle. Progesterone is a hormone that prepares the lining of the uterus for pregnancy. If the egg is fertilized and implanted, the body continues to produce progesterone and if not, then progesterone levels fall, and that month's menstrual period begins.

(b) The follicular phase is known as the Proliferative phase because the primary follicle grows into the mature Graafian follicle and the endometrium of the uterus regenerates through proliferation simultaneously. The hormones LH and FSH stimulate follicular development. 

(c) The primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of

pituitary and ovarian hormones. The secretion of LH and FSH increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles. The LH surge induces rupture
of Graafian follicle and thereby the release of ovum (ovulation). The
ovulation (ovulatory phase) is followed by the luteal phase during which
the remaining parts of the Graafian follicle transform as the corpus
luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium.



(d) 

 

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(a) Arrange the following hormones in the sequence of their secretion in a pregnant woman.
(b) Mention their source and the function they perform.


(a) The sequence of secretion of the given hormones in a pregnant woman is:
FSH-LH-hCG-Relaxin

(b)

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Changes in GnRH pulse frequency in females is controlled by circulating levels of

  • estrogen and inhibin

  • progesterone only

  • progesterone and inhibin

  • progesterone and inhibin


D.

progesterone and inhibin

High levels of estrogen and progesterone give negative feedback to the hypothalamus for the release of GnRH. Thus, inhibiting the gonadotropin release. 

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Identify the correct statement on 'inhibin'

  • is produced by granulosa cells in ovary and inhibits the secretion of FSH

  • is produced by granulosa cells in ovary and inhibits the secretion of LH

  • is produced by nurse cells in testes and inhibits the secretion of LH

  • is produced by nurse cells in testes and inhibits the secretion of LH


A.

is produced by granulosa cells in ovary and inhibits the secretion of FSH

Inhibin is produced by the granulosa cells of the ovarian follicles and has a negative feedback effect on the secretion of FSH.

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