Reproduction in lower and Higher Animals (Part 1)

 Reproduction in lower and Higher Animals 

(part 1)

Reproduction:-

Reproduction is defined as the biological process of formation of new life forms from pre-existing similar life.

Asexual reproduction:-

• Asexual reproduction is a common method of reproduction among lower animals.
• It does not involve the fusion of two compatible gametes or sex cells.
• It is a process resulting in the production of genetically identical progeny from a single organism and inherits the genes of parents.
• Such morphologically and genetically identical individuals are called clones.
• Lower animals generally reproduce asexually either by gemmule formation, budding, or fragmentation.

Gemmule formation:-

• Sponges reproduce asexually via. gemmule formation. Gemmule is an internal bud formed only in sponges to overcome unfavorable conditions. 
• The structure of the gemmule includes the micropyle, spicule, inner layer, archaeocytes, and outer layer.
• It possesses an asexually produced mass or aggregation of dormant cells known as archaeocytes. 
• These cells are capable of developing into a new organism (totipotent). 
• The archaeocytes get coated by a thick resistant layer of secretion by amoebocytes. 
• Monaxon spicules (developed by growth along a single axis) are secreted by scleroblasts in between the inner and outer membrane. 
• In return for favorable conditions of water and temperature, the gemmules hatch and develop into a new individual. 
• There is a minute opening called micropyle through which the cells (new individuals) come out during favorable conditions.

Budding in Hydra:-

• Asexual reproduction in Hydra takes place through budding. 
• Budding normally occurs in favorable conditions. 
• This bud eventually develops (gets transformed) into a new individual.
• Later this bud developed as an independent individual.

Fragmentation in planaria:-

• The parental body of Planaria breaks up into two or more fragments. 
• The anterior end exerts a pull on the posterior end, resulting in a constriction in the middle part and splitting into two pieces. 
• Each piece grows into a new Planaria i.e. two clones of the original parent are formed.

Sexual reproduction:-

Sexual reproduction involves the fusion of two compatible gametes or sex cells.
It is also called amphimixis
It involves meiosis.

It involves mainly two phases:-

• Juvenile phase:- It is the earlier phase that mainly represents physical growth starting from birth. The animals cannot reproduce sexually in this phase.
• Vegetative phase:- It is the later phase in which physical growth is almost over. It involves the growth and activity of sex organs. The animals can reproduce sexually in this phase.

Human reproduction:-

• The process of human reproduction involves various steps such as gametogenesis, insemination, internal fertilization, zygote formation and embryogenesis, gestation, and parturition.

Male reproductive system :

• The male reproductive system consists of the following: 
• i.Primary sex organs (Gonads). Testes 
• ii. Accessory organs: Accessory ducts and accessory glands 
• iii. External genitalia: Scrotum and Penis 

Primary sex organs Testes:-

•  Testes are a pair of primary sex organs.
• which are mesodermal in origin.
•  They are located outside the abdomen in a pouch called the scrotum, .e. extra-abdominal in position. 
• Testes develop in the abdominal cavity early fetal life and later descend into the scrotal sac through a passage called the inguinal canal.
• They are suspended in the scrotal sac by the spermatic cord.
• Tested are ovule in shape.

Histology of Testis: 

• Externally, the testis is covered by three layers. These are: 

a. Tunica vaginalis: 

It is the outermost incomplete peritoneal covering made up of connective tissue and epithelium. 

b. Tunica albuginea: 

It is the middle layer formed by collagenous connective tissue. 

c. Tunica vasculosa/vascularis: 

It is the innermost layer. It is a thin and membranous layer. 

• Each testis is divided into about 200-300 testicular lobules by fibers from tunica albuginea. 
• Each lobule has 1 to 4 highly coiled seminiferous tubules. 
• Each seminiferous tubule is internally lined by a single layer of cuboidal germinal epithelial cells (spermatogonia) and few large pyramidal cells called Sertoli or sustentacular cells.  
• The germinal epithelial cells undergo gametogenesis to form spermatozoa. 
• Sertoli cells provide nutrition to the developing sperms. 
• Various stages of spermatogenesis can be seen in the seminiferous tubules. The innermost spermatogonial cell (2n), primary spermatocyte (2n), secondary spermatocyte (n), spermatids (n), and sperm (n). 
• Between seminiferous tubules, few groups of interstitial cells (Cells of Leydig) are present. 
• After puberty, interstitial cells produce a type of androgen i.e. testosterone.

Accessory ducts:-

•  Rete testis: The seminiferous tubules of the testis at the posterior surface form a network of tubules called rete testis.
• Testis open into vasa efferentia.
• Vasa efferentia: Vasa efferentia are 12-20 fine tubules arising from the rete testis and join to the epididymis.
• They carry sperms from the testis and open into the epididymis.
• Epididymis: It is a long and highly coiled tube that is differentiated into an upper caput, middle corpus- and lower cauda epididymis. 
• The sperm undergo maturation in the epididymis.
• Vas deferens:- Vas deferens travels up to the abdominal cavity and loop over the ureter to open into the urethra.
• Vas deferens join the seminal vesicle to form an ejaculatory duct.
• Ejaculatory ducts:- The ejaculatory duct passes through the prostate gland and opens into the urethra.
• Urethra:- The urethra provides a common passage for the urine and semen and hence is also called the urinogenital duct.
• In males, the urethra is long and extends through the penis.
•  It opens to the outside by an opening called the urethral meatus or urethral orifice.

 1. Seminal vesicles:-

•  These are a pair of small fibromuscular pouches present on the posterior side of the urinary bladder. 
• They secrete a seminal fluid (alkaline) containing citric acid, fructose, fibrinogen, and prostaglandins. 
• About 60% of the total volume of semen is made up of seminal fluid. 
• Fructose provides energy to sperms for swimming, while fibrinogen helps in the coagulation of semen after ejaculation for quick propulsion into the vagina.
• The prostaglandins stimulate reverse peristalsis in Vagina and uterus aiding the faster movement of sperms towards the egg in the female body.

 Prostate gland: 

• The prostate gland consists of 20 to 30 lobes and is located under the urinary bladder) It surrounds the urethra. 
• It releases a milky white, alkaline fluid called prostatic fluid into the urethra. 
• Prostatic fluid forms about 30% of the total volume of semen.
• lt contains citric acid, acid phosphatase protects, and various other enzymes.
• The acid phosphatase protects the sperm from the acidic environment of the vagina. 

Cowper's glands: 

• Cowper's glands are also known as bulbourethral glands.
• These are pea-sized and lie on either side of the membranous urethra. 
• They secrete a viscous, alkaline, mucous-like fluid that acts as a lubricant during copulation.

Semen:

•  It is the vicious, alkaline and milky fluid (pH 7:2 to 7.7) ejaculated by the male reproductive system. 
• Generally, 25 10 4.0 ml of semen is given out during a single ejaculation and it contains about 400 million sperms Semen contains secretion of the epididymis and the accessory glands for nourishing (fructose), neutralizing acidity (Ca, bicarbonates), activation for movement (prostaglandins).

 External genitalia: 

Penis: 

• The penis is the male copulatory organ. 
• It is cylindrical and muscular with three bundles of erectile tissue pairs of postero-lateral tissue called corpora cavernosa and a median corpus spongiosum.
• The swollen tip of the penis is called the glans penis.
•  It is covered by a loose fold of skin called the foreskin.

 Scrotum:-

• It is a loose pouch of pigmented skin lying behind the penis and is divided into a right and left scrotal by a septum of tunica dartos made of smooth muscle fibers. 
• The foetal testes are quick into and retained the scrotum by a short fibromuscular band called the gubernaculum. 
• The testes remain suspended in scronus by a spermatic chord. 

Female reproductive system:-

• The human female reproductive system consists of:-
• Internal genitalia.
• External genitalia.
• Accessory glands.

Internal genitalia:-

• Includes ovaries, oviducts, uterus, vagina.

Ovary:-

• It is the primary female sex organ.
• It is a solid, oval or almond-shaped organ, It is 3 cm in length, 1.5 cm in breadth, and I cm thick.
• It is located in the upper lateral part of the pelvis near the kidneys. 
• Each ovary is held in position by ligaments by attaching it to the uterus and the abdominal wall. 
• The largest of these is the broad ligament formed by a fold of the peritoneum. 
• It holds the ovary, oviduct, and uterus to the dorsal body wall, The ovarian ligament attaches the ovary to the uterus.

Structure and Development of the Ovary:-

• Each ovary is a compact structure differentiated into a central part called the medulla and the outer part called the cortex. 
• The cortex is covered externally by a layer of germinal epithelium. 
• The stroma of loose connective tissue of the medulla has blood vessels, lymph vessels, and nerve fibers. 
• The outer cortex is more compact and granular. 
• It shows a large number of tiny masses of cells called ovarian follicles. 
• These are collectively formed from the immature ova originating from cells of the dorsal endoderm of the yolk sac. 
• The cells migrate to the gonadal ridge during embryonic development and divide mitotically. 
• Now, these cells are called oogonia. 
• As the oogonia continue to grow in size they are surrounded by a layer of granulosa cells. 
• This assembly forms the rudiments of the ovarian follicles. 
• The process of oogenesis starts much before the birth of the female baby and by the end of twelve weeks, the ovary is fully formed. 
• The ovary has more than two million primordial follicles in it. 
• The cells of germinal epithelium give rise to groups of oogonia projecting into the cortex in the form of cords called egg tubes of Pfluger. 
• Each cord at its end has a round mass of oogonial cells called egg nests, from which the primordial ovarian follicles develop. 
• Each primordial follicle has, at its center, a large primary oocyte (2n) surrounded by a single layer of flat follicular cells. 
• The primary oocyte starts with its meiotic division but gets arrested at meiosis I. 
• Of the two million primordial follicles embedded in the foetal ovary only about one million remain at birth and only about 40,000 remain at the time of puberty.

 Functions: 

• Its main function is production.
• The ovary produces five hormones viz. estrogen, progesterone, relaxin, activin, and inhibin. 

Oviduct / Fallopian tube / Uterine tube: 

• These are a pair of muscular ducts lying horizontally over the peritoneal cavity. 
• The proximal part of the tube lies close to the ovary and distally it opens into the uterus. 
• Each tube is 10 to 12 cm in length. 
• It is internally lined by ciliated epithelium It can be divided into three regions:

1. Infundibulum: 

• The proximal funnel-like part with an opening called ostium is surrounded by many finger-like processes called fimbriae (of these at least one is long and connected to the ovary). 
• The cilia and the movement of fimbriae help in transporting the ovulated egg to the ostium. 

2. Ampulla: 

• It is the middle, long and straight part of the oviduct. 
• Fertilization of the ovum takes place in the egg or ovum and the female reproductive hormones. 

3. Isthmus Cornua: 

• The distal narrow part of the duct opening into the uterus. 

Functions: 

• Fallopian tubes carry the released egg from the ovary to the uterus. 
• Ampulla provides the site for fertilization of the ovum. 

Uterus: 

• It is commonly also called a womb. 
• It is a hollow, muscular, pear-shaped organ, located above and behind the urinary bladder.
• It is about 7.5 cm long, 5 cm broad and 2.5 cm thick. 
• Internally the uterine wall can be distinguished into three layers: Outermost perimetrium, middle thick muscular myometrium, made up of thick layers of smooth muscles. 
• Vigorous contractions of these muscles cause labor during parturition (childbirth). 
• The innermost layer called the endometrium or mucosal membrane is made up of stratified epithelium. 
• The thickness of this layer regularly undergoes changes during the menstrual cycle. 
• It is richly supplied with blood vessels and uterine glands. 
• These provide nourishment to the developing feotus. 
• The uterus can be divided into three regions: 

1. Fundus: 

• It is the upper dome-shaped part. Normally implantation of the embryo occurs in the fundus. 

2. Body: 

• It is the broad part of the uterus which gradually tapers downwards.

3. Cervix: 

• It is a narrow neck about 2.5 cm in length Ii extends into the vagina. 
• Its passage has two openings and an internal os' orifice towards the body, and an external os orifice towards the vagina.

Functions:-

• Uterus receives the ovum. 
• It provides a site for implantation, gestation, and parturition. 
• It forms the placenta for the development of foetus. 

Vagina: 

• It is a tubular, female copulatory organ, 7 to 9 cm in length. 
• It lies between the cervix and the vestibule. 
• The vaginal wall has an inner mucosal lining, the middle muscular layer, and an outer adventitia layer. 
• The mucosal epithelium is stratified and non-keratinized and stores glycogen. 
• There are no glands but the cervical secretion of mucus is received in the vagina. 
• The opening of the vagina into the vestibule is called vaginal orifice. 
• The vaginal orifice is partially covered by the hymen.

Functions:-

• The vagina acts as a passage for menstrual flow as well as a birth canal during parturition.

External genitalia (Vulva): 

• The external genital organs of females include parts external to the vagina collectively called vulva (covering ur wrapping), or pudendum.
•  They include the following parts: 

Vestibule:

•  It is a median vertical depression of the vulva enclosing the urethral and vaginal opening.

 Labia minora:

• These are another pair of thin folds inner to the labia majora with which they merge posteriorly to form the fourchette (frenulum), while towards the anterior end they converge into a hood-like covering around the clitoris.

Clitoris: 

• A small conical and sensitive projection lying at the anterior end of labia minora. 
• It has a pair of erectile tissue ie corpora cavernosal which is homologous to the penis 

Labia majora: 

• These are a pair of fleshy folds of skin forming the boundary of the vulva. 
• They are homologous to the scrotum. 
• They surround and protect the other parts of the external genitalia and enclose the urethral and vaginal openings in the vestibule.

Mons pubis:

• It is a fleshy elevation above the labia majora. 
• The Mons pubis and outer part of labia Majora show pubic hair.

Accessory glands: 

1 Vestibular glands / Bartholin's glands: 

• It is a pair of glands homologous to the Bulbourethral or Cowper's glands of the male.
• They open into the vestibule and release a lubricating fluid.

2. Mammary glands: 

• These are accessory organs of the female reproductive system for the production and release of milk after parturition. 
• Development of the mammary glands occurs at puberty under the influence of estrogen and progesterone.
• Lacto Tropic hormone (LTH) or prolactin helps in the development of lactiferous tubules during pregnancy.
• The mammary glands are a pair of rounded structures present in the subcutaneous tissue of the anterior.
• Each mammary gland contains fatty connective tissue and numerous lactiferous ducts.
• The glandular tissue of the cach breast is divided into 15-20 irregularly shaped mammary lobes, each with alveolar glands and lactiferous duct.
•  Alveolar glands secrete milk which is stored in the lumen of alveoli.
•  The alveoli open into mammary tubules.
• The tubules of each lobe join to form a mammary duct. Many mammary ducts.
• Many mammary ducts join to form a wider mammary ampulla, which is connected to the lactiferous duct.
• These converge towards the nipple located near the tip of the breast.
• It is surrounded by a dark brown colored and circular area of the skin called the areola.

 (Note: Prolactin secreted by lactotropic cells is also referred to as luteotropic hormone or luteotropin.)