BIOLOGY Case study (4 Marks)

→ Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm.
→ A mesoderm soon appears between the ectoderm and the endoderm.
→ These three layers give rise to all tissues (organs) in adults.
→ The inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.
→ The human pregnancy lasts 9 months.
→ In human beings, after one month of pregnancy, the embryo's heart is formed. The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.
→ By the end of the second month of pregnancy, the foetus develops limbs and digits.
→ By the end of 12 weeks (first trimester), most of the major organ systems are formed, for example, the limbs and external genital organs are well-developed.
→ The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month.
→ By the end of about 24 weeks (end of second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed.
→ By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery.

→ The zygote is formed at the ampulla-isthmus junction of the fallopian tube when the union of secondary oocyte and spermatozoa occurs. The zygote undergoes various divisions and changes before it reaches the uterus for implantation.
→ The figure shows the various stages of development of a zygote as it passes through the oviduct and uterus:
(a) The zygote divides into a two-celled stage wide via division or 1st cleavage while the ozygote is still in the isthmus of the oviduct.
(b) Several more mitotic divisions or cleavages occur in the 2 celled stage to form a 2, 4, 8 and finally a 16-celled stage of the zygote.The 16 celled stage is called the morula and various cells formed after cleavage are called blastomeres.
(c) Morula then changes into a blastocyst after a few more divisions and this stage contains a fluid filled cavity in the embryo. The blastomeres become arranged and line up into an outer layer of cells called the trophoblast and an inner mass of cells. The fluid filled cavity is called blastocoel.
(d) Implantation of the embryo occurs at this blastocyst stage by the help of trophoblast layer which embeds itself into the uterine endometrium.

→ The process of formation of a mature female gamete is called oogenesis.
→ It is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth.
→ These cells (oogonia) start dividing and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes.
→ Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000- 80,000 primary follicles are left in each ovary.
→ At puberty the primary follicles get surrounded by more layers of granulosa cells and a new
theca and are called secondary follicles.
→ The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa.
→ It is at this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.
→ The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte.
→ The tertiary follicle further changes into the mature follicle or Graafian follicle.
→ The secondary oocyte forms a new membrane called zona pellucida surrounding it.
→ The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.