Reproduction (III) Animal Reproduction W.K. Leung P.1

Reproduction (III) Animal Reproduction

O

ne major advantage of sexual reproduction is the genetic variety it creates. Animals, with their capacity for locomotion, are able to move far and so produces a greater degree of out , greater mixing of genes within the gene p and hence greater v .

Gametes are usually differentiated into small m sperms which are produced in large number, and l , non-motile, f -storing, & much fewer ova (sing. ovum).

The organs that produce gametes are called go and are of two types: the ov____ which produce ova and the t______ which produce sperms.

I) Human Male Reproductive System

The testes are enclosed in the scr____. The opt temperature for sperm development ~ 2 °C below normal human body temperature.[1]
Inside the testes are convoluted sem tubules, each seminiferous tubule is lined by g______epithelium which gives rise to sperm by a series of divisions.[2]
The ep is where sperms are stored, and gain mot_____. The v__ d______carries sperms towards the u______. A Secretion known as seminal fluid comes from 3 sources:
·  Seminal vesicles produce a secretion which aids sperm mobility.
·  Prostate gland produces an alkaline secretion that neutralizes the acidity of any urine in the urethra as well as aiding sperm mobility
·  Cowper's glands secrete a sticky fluid into the urethra.
The combination of sperm and seminal fluid is called ‘s_____’ which passes along the urethra
The penis comprises three cylindrical masses of s______tissue covered by an elastic skin. /

II) Human Female Reproductive System

The female gonads, the ov , lie suspended in the abdominal cavity. The external coat is made up of germinal e which begins to d to form ova while the female is still a foetus.

At birth around 400,000 cells have reached prophase of the first meiotic division and are called primary oo . Each month after puberty, one of these completes its development into an ov .

The ovary consists of a number of oocytes at various stages of development. The most mature of the cells are called G follicles which are fluid-filled sacs each containing a sec______oocyte.

Close to the ovary is the funnel-shaped opening of the ovi or Fallopian tube. The oviducts have a muscular wall lined with a mucus-secreting layer of c epithelium The ovum is moved down the oviduct mostly by mu contractions of the oviduct wall, the beating of the c____ also assist..

The oviducts open into the u , or womb. It has walls of unstriated m and is lined internally by a mucus membrane called the endo .

The uterus opens into the va through a ring of muscle, the cer . The vagina opens to the outside through the vulva. Anterior to the vaginal opening is a small body of erectile tissue, the clitoris, which is homologous to the penis of the male.

III) Gametogenesis

The formation of gametes is called gamet______. The overall sequence of events is essentially the same in both sexes. Three stages are recognised: Mul , gr and diff______.

Sperm-formation is called spermatogenesis, egg-formation oogenesis

·  Phase of multiplication: diploid cells, divide repeatedly by mitosis.
·  Phase of growth: the daughter cells resulting from the mitotic divisions grow in size.
·  Phase of maturation: cells divide by meiosis, the resulting haploid cells differentiate into the gametes (eggs or sperm). /

In spermatogenesis the products of the initial phase of multiplication grow into spermatocytes (2n). The latter divide by m____ to form haploid sp (n) which differentiate into sp______(n). Sperms are formed by rep_____ cell divisions means that the huge number of sperms is made

Oo______ is basically similar to spermatogenesis, but differs from it in detail. Most of the cells formed by the initial phase of multiplication deg______.
The amount of gr that takes place at this stage is much greater than in spermatogenesis - this is what makes eggs so much l_____ than sperms.
The oocyte now undergoes meiosis, but the divisions are une_____ resulting in four daughter cells that di____ greatly in size.
One of the cells remains large and becomes the e___ cell. (the others are called polar bodies).
Q. Can you state three ways in which Oogenesis differs from spermatogenesis ? /

IV)  The Menstrual Cycle

In human females the onset of the first m_____ cycle represents the start of p_____. This takes place around the age of 12. The menstrual cycle, which lasts about 28 days, continues until the meno_____ at the age of 45-50. The events of the cycle are controlled by h______to ensure that the production of an o is synchronized with the readiness of the u to receive it, should it be fertilized.

·  The start of the cycle is taken to be the initial discharge of blood known as men______. This flow of blood, which lasts about five days, is due to the l of the uterus being shed, along with a little blood. (menstrual phase)
·  During the following days the lining re in readiness for a fertilized ovum. (follicular phase)
·  By day 14 it has thickened considerably and the Gr fo releases its ovum into the oviduct, the process being called ov . The journey to the uterus takes about 3 days during which time the ovum may be f______. If it is not, the ovum quickly dies and passes out via the vagina. (Ovulation phase)
·  The uterine lining is maintained for some time but finally b down again about 28 days after the start of the cycle. (Luteal Phase) /
Reference Reading : Sexual Cycles
Many animals have cycles of se activity in both males and females. These cycles often occur so that fertilization takes place at a t____ which gives the offspring the best chance of s , e.g. the offspring are produced at a time when the cl and f availability are most favourable. In mammals these cycles are of three main types:
·  The female undergoes a single period of sexual activity during the year, e.g. deer (monoestrus).
·  The female undergoes a number of periods of sexual activity during the year, each separated by a period of sexual inactivity, e.g. horses (polyoestrus).
·  The female has a more or less continuous cycle of activity where the end of one cycle is followed immediately by the start of the next, e.g. humans.

A)  Hormonal control of the Menstrual cycle

The control of the menstrual cycle is an excellent example of hormone inter . The production of hormone stimulate or inhibit the production of another. There are four hormones involved, two produced by the anterior lobe of the pituitary gland at the base of the brain, and two produced by the ovaries.

The release of hormones from the ovaries is controlled by the hormones from the a______p______. These hormones are thus referred to as the gonadotrophic hormones.

The two gonadotrophic hormones are f s hormone (FSH) and l______hormone (LH). These stimulate the ovaries to produce o and p respectively.

The functions of these hormones are as follows:

Follicle stimulating hormone

Þ  Causes Gr______follicles to develop in the ovary.

Þ  Stimulates the ovary to produce o .

Oestrogen

Þ  Causes repair of the u_____ lining following menstruation.

Þ  Stimulates the pituitary to produce l hormone.

Luteinizing hormone

Þ  Causes o______ to take place.

Þ  Stimulates the ovary to produce p from the c luteum.

Progesterone

Þ  Causes the uterus l______to be maintained in readiness for the young embryo.

Þ  Inhibits production of f by the pituitary.

The hormones are produced in the following sequence:

w  Progesterone at the end of the sequence inhibits the production of FSH. In turn, the production of the other hormones stops, including progesterone itself.

w  The absence of p______now means that the inhibition of FSH ceases and so the cycle starts again

w  The alternate switching o_ and o___ of the hormones produces a cycle of events - the menstrual cycle.

Interaction of pituitary and ovarian hormones in the control of menstrual cycle

/

B)  Artificial control of the menstrual cycle

The artificial control of the menstrual cycle has two main purposes: Firstly, as a contra ______device by preventing ovulation and secondly as a f device by stimulating ovulation.

a) Contraception

i) The Contraceptive Pill

The Pill contains both o and p and when taken daily it maintains high levels of these hormones in the blood. These high levels inhibit the production of the g______hormones from the pituitary, and the absence of LH in particular prevents o______. The Pill is normally taken for 21 consecutive days followed by a period of 7 days without it, during which the uterus lining b_____ down and a menstrual period occurs.

ii) The 'Morning-after' Pill

It contains a synthetic oestrogen, which is thought to prevent i __ of the fertilized ovum if it is present. Both types of pill are very effective forms of contraception.

b) Fertility drug

A fertility drug induces o______ in one of two ways:

·  It may provide g such as FSH which stimulate the development of Graafian follicles.

·  It may provide some chemical which inhibits the natural production of o . As oestrogen normally inhibits FSH production, the level of FSH increases and Graafian follicles develop.

·  Use of fertility drugs frequently results in multiple births.

·  Fertility drugs are employed to induce the production of o__ in IVF (in vitro fertilization).

Birth control methods

Method / How it works / Effect-iveness / Advantages / Disadvantages
Sterilization / Male (vasectomy) - The v__ d______are c__ and t___. Female (tubal ligation) - The ov______are cut and tied. / 100% / No artificial appliance is involved.
P______ / Irr______ in normal circumstances
Preventing ovulation
The contraceptive pill / Contains p______and o______
Prevents o______ / 100% / Totally reliable if taken regularly / Possible nausea, breast tenderness and water retention leading to an increase in weight
Preventing implantation
Morning-after Pill / Contains high level of o______. Probably works by preventing im______ / Probably 99-100%
Intra-Uterine Device (IUD) / A device usually made of plastic and/or copper which is inserted into the womb by a doctor and which prevents i______ / 99-100% / Once fitted, no further action is required except for annual check-ups / Possible menstrual discomfort. Must be inserted by a trained practitioner. Only really suitable for women who have had children
Barrier to fertilization
Male (condom) / a sheath of thin rubber unrolled onto the erect penis prior to intercourse. S____ is collected in teat at the tip. Best used with a sperm____ / Very reliable / Easily available, Easily used. Gives protection against S____ including AIDS / May reduce the sensitivity of the penis.
Female (diaphragm) / dome-shaped sheet of thin rubber with a thicker spring rim which is inserted into the vagina, over the cer___. Best used with sperm______ / Very reliable / Reliable.
Available for use by all women / Must be inserted prior to intercourse and should be removed 8-24 hours after intercourse. Initial fitting must be by a trained practitioner
Spermicide / Cream, jelly or foam inserted into vagina. Only effective with a mechanical barrier. K___ sperm / Not rel_____ alone / Easy to obtain and simple to use / Not effective on its own. May occasionally cause irritation
Natural Rhythm
Rhythm method / Refraining from intercourse during those times in the menstrual cycle when con______is most likely / Variable
N__ very reliable / No appliance required. Only acceptable method to some reli_____ groups / Not reliable. Restricts times when intercourse can take place. Unsuitable for women with ir______cycles
Reference Reading: Male sex hormones
Although male humans do not have a sexual cycle similar to that of females, they nonetheless produce some g______hormones from the anterior lobe of the pituitary gland.
Follicle stimulating hormone (FSH) stimulates s______development. Luteinizing hormone (LH) stimulates the i______cells between the seminiferous tubules of the testis to produce t . For this reason, in the male, LH is often called interstitial cell stimulating hormone (ICSH).
Testosterone is the most important of a group of male hormones or androgens. It is first produced in the foetus, where it controls the development of the m___ reproductive organs. An increase in production takes place at p______ and causes an enlargement of the reproductive organs and the development of the secondary s______characteristics. Removal of the testes (castration) prevents these changes taking place. It was used in the past as a rather drastic means of preventing choirboys' voices from breaking. C of animals is still practised in order to help fatten them and make the meat less tough.

V) Courtship and Secondary Sexual Characteristics

In many species it is necessary for both partners to follow a specific pattern of be______before m______can occur. C______behaviour is developed in sexually m_____ individuals. In this way mating between sexually immature individuals, that cannot produce offspring, are avoided.

On reaching sexual maturity many specie s develop easily recog______features which are sexually att______ to a potential partner. These are referred to as the s s characteristics.
They take a variety of forms, including bright plumage in many birds, the mane of a lion.
In humans, secondary sex characteristics include the growth of p____ hair in both sexes, increased mus___ growth, growth of facial h___ and deepening of the v___ in males and development of the br____ and broadening of the h__ in females. /

Apart from preparing the female for child-bearing the changes in many animals help to dist______males and females. In this way time and energy are not w on the fruitless courting of members of the same sex or sexually immature individuals of the opposite sex.

The females of many species undergo cycles of sexual activity during which they are only capable of conceiving for a very brief period. Courtship is used by the male to determine whether the female is rec or not.