Reproduction of lower and higher Animals (part 2)

 

 Reproduction in lower and Higher Animals 

(Part 2)

Menstrual Cycle :

• The menstrual cycle is a characteristic of primates. 
• It starts at the time of puberty and the period is called menarche.
• The menstrual cycle in humans lasts for 28 days.
• The days are numbered from the first day of blood flow in the menstrual period.
• A series of events occur regularly in females after every 26 to 30 years.
• The menstrual cycle includes four phases:-

I) Menstrual ovulation phase/Menstrual cycle phases :

• It is also called as bleeding phase this phase extends from the 1st to the 4th day of the menstrual cycle.
•  When the ovum is not fertilized, the high-level progesterone inhibits the secretion of luteinizing hormone, and the LH level decreases in the level of progesterone from the corpus leuteum.
•  After 14 days of ovulation, if the ovum is not fertilized, the lining of the uterus degenerates, and menstruation begins.
• This phase lasts about 4 days.
• The days when bleeding starts are considered the first days of the menstrual cycle.
• Menstrua flow consists of the secretion from the endometrial glands.
• Cell debris blood and unfertilized ovum.
• During menstrual flow, about 35-45 ml of blood is lost.
• It is also described as the weeping of the uterus for lost ovum or funeral of the unfertilized egg.

II) Proliferative phase/ follicular phase : 

• This phase extends from the 5th to 13th day of the menstrual cycle. 

Changes in the ovary :

• During this phase, the primordial follicle of the ovary develops into a Grafian follicle. 
• Many primordial follicles are already present in the ovary.
• The ovum becomes eccentric and it is connected by few follicular cells called germ hill/cumulus oophorus /discus proliferous the granulosa cells lining the antrum form membrane granulosa and follicular cells surrounding the ovum are called corona radiate. 
• The ovum increases in size. 
• A thick membrane is formed surrounding the outer surface of the ovum. 
• This is called Zona pellucida. 
• From the st roma of the ovary, the follicle is covered with two layers called Theca interna and theca externa.

Changes in Uterus :

• Estrogen secreted by follicular cells of the ovary stimulates endometrial glands. 
• This causes repair of the endometrium. 
• The endo trial cells proliferate and the thickness of the endometrium grows for abord Changes in the uterus 3mm to 5 mm. 

III) Ovulatory Phase: 

• Ovulation is the process in which there is a rupture of the Graffian follicle with the discharge of the ovum into the abdominal cavity. 
• It is under the influence of the luteinizing hormone. 
• A sudden rise in the level of LH stimulates ovulation which occurs usually on the 14th day of the menstrual cycle. 
• The rupture of Graffian follicles results in oozing out the follicular fluid. 
• The ovum along with radially arranged cells is releaséd into the abdominal cavity. 
• The ovum enters the fallopian tube through the ostium assisted by fimbriae. 
• The ovum is haploid since meiosis II is arrested at metaphase. 
• Ovum must be fertilized within 24 to 48 hours after ovulation. 
• Only during this time, it is viable for fertilization. 
• If fertilization takes the fallopian tube and reaches the uterus on the third day after ovulation. 

IV) Luteal Phase: 

• This phase extends from the 15th to 28th day of the menstrual cycle. 

Change in the ovary: 

• After ovulation, the ruptured follicle develops into a yellow body called the corpus luteum. 
• After the discharge and of ovum the corpus luteum. 
• It acts as a temporary endocrine gland that secretes progesterone, progesterone. 
• Secretion is under the influence of the luteinizing hormone, progesterone helps in maintaining the thickness of the endometrium. 
• So it is also called the pregnancy hormone. The 
• The Corpus luteum is active till the placenta takes up the function of secretion of the hormone human chorionic gonadotropin (HCG). 
• HCG is similar to LH. 
• If the ovum is not fertilized the corpus luteum degenerates and transforms into a whitish scar called corpus Albicans. 

Changes in Uterus: 

• Corpus luteum formed in ovary secretes progesterone. 
• It causes further growth of the endometrial glands. 
• uterine glands secret fluid which is rich in glycogen for nourishing the dividing embryo is implanted in thickness endometrium. 

2. Hormones secreted by Neurohypophysis and thyroid gland. 

Ans. : Neurohypophysis does not secrete any hormone but stores the hormones which are secreted by Hypothalamic neurons. I stores and releases the following hormones :

• a) Anti Diuretic Hormone (ADH) or Vasopressin: ADH increases the permeability of distal convoluted tubule or collecting tubules of uriniferous tubules of the kidney. 
• Absorption of water from the ultrafiltrate and regulation of water balance of body fluids ADH also controls constriction of arterioles to increase blood pressure in the kidney which facilitates ultrafiltration so-called vasopressin. 
• Its secretion is regulated by the increase or decrease of the osmotic pressure of blood in a feedback manner. 
• The osmotic pressure is detected by osmoreceptors in the hypothalamus. Deficiency of ADH causes diabetes insipidus i.e. loss of a large quantity of water through urine (polyuria or diuresis). 
• It results in polydipsia (increased thirst). 
• Hypersecretion causes antidiuresis (less urine formation) and stimulates water retention in body fluids. 

• b) Oxytocin (Birth hormone): It is a powerful stimulant for the contraction of uterine myometrium at the end of gestation to initiate labor pains for normal delivery it stimulates myoepithelial cells of mammary glands for milk ejection during breastfeeding. 
• It also helps in fertilization by powerful contraction of the uterine musculature to drive the sperms upward towards the fallopian tube. 

• c) Coherin: It is supposed to induce prolonged rhythmic integrated contractions of the jejunum. 

• Hormones of the Thyroid gland: The thyroid gland synthesize, store and discharge the thyroid hormones which are thyroxine (T4) also called tetraiodothyronine ( Ta) and thyrocalcitonin. 
• T3 and T are iodinated derivatives of amino acid tyrosine. 
• Both are stored and supposed to have the same function. The secretion of T3 and T is regulated by thyroid-stimulating hormone (TSH) or thyrotropin of the pituitary gland in a negative feedback manner. 
• T3 is more active and T is a more potent hormone.

Gametogenesis :

• Gametogenesis is the process of the formation of gametes in sexually reproducing animals.
• The male gamete is sperm and the female gamete is ovum or egg.
• The gametes are formed from primordial germ cells of gonads.

Spermatogenesis :

• Spermatogenesis is the process of formation of the male gamete (sperm) or spermatozoa from the germinal epithelium of the testis. 
• At the onset of puberty, the hypothalamus begins secretion of gonadotropin-releasing hormone (GNRH). 
• GnRH initiates a significant increase in the secretion of follicle-stimulating hormone (FSH) which in turn induces spermatogenesis. 
• Each seminiferous tubule is lined by a single layer of cuboidal epithelial cells called the germinal epithelium. 
• The cells of germinal epithelium undergo spermatogenesis to produce sperms.

The process of spermatogenesis involves three phases: 

• i. Multiplication phase: The primordial germ cells (2n) of the seminiferous tubules undergo repeated mitotic divisions to produce a large number of spermatogonia (2n). 
• Each spermatogonium is diploid and with 46 chromosomes. 
• ii. Growth phase: Some of the spermatogonia stop dividing and grow in size to develop into primary spermatocytes (2n) due to the accumulation of food. 
• Maturation phase: It involves meiotic ur reduction division. 
• The spermatocyte undergoes the first phase of meiotic division (meiosis I) leading to the formation of two haploid cells called secondary spermatocytes (n) having 23 chromosomes each. 
• The secondary spermatocyte undergoes the second phase of meiotic division (meiosis II) to produce four haploid spermatids. 
• The spermatid is non-motile and non-functional. It gets transformed into functional spermatozoa by the process called spermiogenesis. 

Changes during spermiogenesis: 

• During this process of change, the spermatids remain held to each other and to the Sertoli cells by cytoplasmic bridges. 
• The sperm heads remain attached to the Sertoli cells with their tails hanging in the lumen of the seminiferous tubule. 
• During spermiogenesis, the length of the spermatid increases. 
• Centrioles are rearranged as primary and distal centrioles. Mitochondria become spirally coiled and acrosome is formed from the Golgi complex.

Structure of the sperm (Spermatozoan) : 

(Sperm = seed; zoon = animal) 

• It is a microscopic, elongated haploid motile male gamete or parental gamete measuring about 0.055 mm (60u) in length. 
• Sperm remains viable for seventy-two hours but can fertilize the ovum in the first 12 to 14 hours only. 
• Head: It is a flat and oval region consisting of a large nucleus and like Acrosome secretes hydrolytic enzymes and acrosome. 
• Hyaluronidase helps in the penetration of the egg during fertilization. 
• The acrosome and anterior half of the nucleus are covered by a fibrillar sheath.
•  Neck: It is a very short region having two centrioles. 
• The proximal centriole plays a role in the first cleavage of the zygote. 
• The distal centriole gives rise to the axial filament of the sperm.
• Middle sperm: It serves as a powerhouse for sperm. 
• It has many mitochondria spirally coiled (Nebenkern) around the axial filament. 
• The mitochondria provide energy for the movement of the sperm in the female genital tract. 
• The posterior half of the nucleus, neck, middle piece of sperm are covered by a sheath.
•  Tail: The tail is a long, slender, and tapering structure formed of cytoplasm. 
• A fine thread, the axial filament arises from the distal centriole and transverses the middle piece of the tail.

  Oogenesis :

• Oogenesis is the process of formation of the haploid female gamete i.e. egg or ovum from the diploid germinal epithelium. 
• Oogenesis takes place in the ovaries and is initiated before the birth of the female baby. 
• It involves the process of mitosis and meiosis This process can be divided into three stages: 
• i. Multiplication phase ii. Growth phase iii. Maturation phase

• Multiplication phase: In this stage, the primary germinal cells PGCS (2n) of the ovary undergo repeated mitotic division to form millions of gamete mother cells or oogonial cells (2n). 
• This process is completed in the embryonic stage of human females. 
• Growth phase: Some of the oogonia stops division and begins to increase in size and form the primary 0ocytes (2n). 
• Cellular organelles like ER, Golgi apparatus, and mitochondria increase in number. 
• Maturation phase: The primary oocytes (2n) enter the maturation phase, which includes meiotic division Meiosis I and Meiosis II. 
• The diploid primary oocytes undergo meiosis I (reduction division) to form two haploid daughters ell having 23 chromosomes. 
• However, due to unequal division of the cytoplasm, of the two daughter cells, one is a large cell, called secondary oocyte (n) and another is a small cell, called I" polar body (n). 
• Generally, the 1" polar body does not enter meiosis I. The secondary oocyte (n) proceeds only up to metaphase II of meiosis II and its division is further stopped e arrested at this stage. 
• The secondary oocyte is shed from the Graafian follicle and ovary during ovulation. 
• Meiosis Il is completed only if fertilization occurs. 
• This last phase is usually completed in the ampulla of the fallopian tube at the time of fertilization. 
• In this division also, the two unequal daughter cells are formed i.e. the large cell is ovum (n) and the small cell is 2 polar body (n). 
• The ovum (n) so formed functions as the female gamete and is ready for fertilization. 
• The secondary oocyte is not fertilized by a sperm/spermatozoa, it is shed off along with menstruum.

The secondary oocyte :

• The secondary oocyte is the unfertilized egg which is released from the ovary (ovulated). 
• Size and structure: It is non-cleidoic (without shell) and microlecithal (yolk is present in very small quantity). 
• It is approximately 0.1mm (100 microns) in size., It is a rounded, nonmotile, haploid female gamete. 
• The egg shows polarity i.e. the side having a germinal vesicle and the first polar body is called an animal pole while the side opposite to it is called a vegetal pole. 
• Nucleus: The nucleus of the egg appears large and is called a germinal vesicle. 
• A typical nucleus or pronucleus is formed at the time of fertilization. 
• Cytoplasm: The cytoplasm of an egg is also called ooplasm. 
• It is devoid of centrioles.
• Membranes: The egg is surrounded by various coverings. The egg membrane is called the vitelline membrane.
• It secretes a non-cellular. 
• glycoproteins membrane with the zona pellucida on its outside. 
• Adhering to the outer surface of zona pellucida are several radially elongated cells forming the corona radiate. 
• These cells are derived from the innermost layer of granulosa cells. 
• They are firmly held to the zona pellucida and to each other by hyaluronic acid (mucopolysaccharide). Between the vitelline membrane and the zona pellucida is a fluid-filled perivitelline space. 
• The first polar body lies in this space.