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HUMAN REPRODUCTION AND EMBRYONIC DEVELOPMENT
Human beings reproduce sexually and are viviparous i.e. they give birth to
young ones. The reproductive units in sexual reproduction are specialized cells
called gametes.
PRIMARY SEX ORGANS
Primary sex organs are those sex organs which produce gametes. They are
also called gonads.
They also produce steroid hormones essential for reproduction as well as the
growth and development of the entire body.
SECONDARY SEX ORGANS
Sex organs, glands and ducts which do not produce gametes and hormones
but are otherwise essential for sexual reproduction are known as secondary
sex organs.
Secondary sex organs of human male reproductive system are vasa efferentia,
epididymes, vas deferentia, ejaculatory ducts seminal vesicles, cowper’s gland,
urethra, prostate gland and penis.
Secondary sex organs of human female reproductive system are fallopian
tubes, uterus, vagina, external genitalia and mammary glands.
ACCESSORY OR EXTERNAL OR SECONDARY SEX CHARACTERS
They are those external features which provide distinctiveness to the two
sexes.
Major accessory or external or secondary sex characters of human male are
facial hair, body hair, more height, more muscles, broadening of shoulder, low
pitched voice, narrow but strong pelvis
Major secondary sex characters of human female are development of breasts,
broader pelvis, rounded body contours, fat deposition in thighs, buttocks and
face, high pitched voice, pubic hair and sterna breathing
PUBERTY
Beginning of sexual maturity or ability to reproduce is known as puberty. It is
the period when primary sex organs become functional and start secreting sex
hormones which bring about developments of secondary sex organs and
appearance of secondary sex characters.
Puberty is characterized by rapid growth. The average age of onset for girls is
10 -14 years and for boys, 12 -15 years old
In boys pubertal changes occur in response to testosterone, whereas in
female child, estrogen is produced by ovarian follicles.
MALE REPRODUCTIVE SYSTEM
It is made of a pair of testes, scrotum, vasa efferentia, a pair of epididymes, a pair of
vasa differentia, a pair of seminal vesicles, a pair of ejaculatory ducts, urethra,
prostate gland, a pair of cowper’s glands and penis
TESTES
They are a pair of oval, pinkish primary sex organs of male reproductive
system, each of with a size of 5cm ( length), 3cm ( thickness) and 2.5cm
( width), weight about 12gm and lying obliquely in scrotum.
During early foetal life, the testes develop in the abdominal cavity just below
the kidney then they descend into the scrotum.
The testis is surrounded by three layers
(i) Tunica vaginalis, a bilayer of peritoneum with narrow coelomic cavity
filled with coelomic fluid. It helps the testis in frictionless sliding
(ii) Tunic albuginea, is the actual covering of the testes. The covering is
made up of dense bluish with fibrous or collangenous connective
tissue
(iii) Tunica vasculosa, is delicate loose connective tissue which lines the
testicular lobules inner to tunica albuginea. It has rich supply of blood
vessels.
Tunica albuginea also projects inside testes to form a thick incomplete vertical
column called mediastimum and a number of transverse septa. The septa
produces 200 – 300 conicular testicular lobules and each lobules is filled with
connective tissue and 1-3 yellow convoluted seminiferous tubules. A total of
900 – 1000 yelloish seminiferous tubule occur in each testis. Each tubule is
about 70-80 cm long. Seminiferous tubule join to form 20-30 short straight
ducts called tubulic recti. Tubuli recti enter a network of channels called rete
testes.
The connective tissue lying in between seminiferous tubules are called
Leydig’s cell and it contain yellow pigment granules. Leydig cells are large
polyhedral cells which have eccentric nucleus and small lipid containg
vacuoles. They secrete testosterone and other androgens. The lining of seminiferous tubules is formed by a single layered germinal
epithelium. The epithelium has two types of cells, spermatogenic ( primary
germ cells) and sertoli cells ( supporting cells), Spermatogenic cells form 4-8
layers. The cells are destined to undergo spermatogenesis and form
supermatozoa.
Sertoli cells are large, elongated and pyramidal cells with bases resting on
basal lamina ( basement membrane) and apics projecting into the lumen of
the seminiferous tubules. They secrete spermatogonic substances for
nourishing and differentiation of cells undergoing spermatogenesis. They
also secrete hormone inhibin for controlling FSH secretion.
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| fig 1.1 : Cross section of human testes |
SCROTUM
It is a pouch of deeply pigmented skin arising from lower abdominal wall
below the pubic symphysis and hanging between and infront of groin part of
the thighs. Scrotal skin bears sebaceous glands that produce a characteristic
odour, sweat glands and nerve endings. Wall of scrotum has three layers – outer wrinkled skin, connective tissue and
smooth muscles. An internal septum scroti divides the scrotum into two sacs,
each for one testis. Left testis lies a bit lower than the right one. Scrotum
possesses a smooth involuntary muscle called dartos muscle. A testis rests in
its chamber over pad called gubernaculum.
The scrotum remains connected with the abdomen or pelvic cavity by the
inguinal canals. The spermaticord, formed from the spermatic artery, vein and
nerve, bound together with connective tissue passes into the testis through
inguinal canal.
The testes develop in the abdominal cavity during the 7th month of gestation
descend permanently into the respective scrotal sacs
The scrotum acts as a thermoregulator, maintaining the testes at a
temperature 2 degree C lower than that of the body. Movement of dartos muscle
help in changing position of testes to keep them at proper temperature. When
the body is chilled the smooth muscle contracts and brings the testes closer to
the pelvic cavity to get warmth
In some person testes fail to descend in scrotum. The condition is called
cryptorchidism. It results in sterility
VAS EFFERENTIA
Rete testies is connected to caput epididymis by 12-20 fine tubules called vasa
efferentia
Their lining epithelium is pseudostratified. It has large columnar ciliated cells
and small nonciliated cells with endocytic activity, ciliated cells help in
conducting sperms. Tubuli recti, rete testis and vas efferentia constitute an
intratesticular genital duct system
EPIDIDYMES
The epididymes is a mass of long narrow closely coiled tubule which lies along
the inner side of each testis. coiling forms three parts – upper caput
epididymes or head middle corpus epididymes or body and lower cauda
epididymes or tail
In the head of the epididymis, the sperms undergo physiological maturation,
acquiring increased motility and fertilising capacity. In the tail of the
epididymis sperms are stored before entering the vas deferens Epididymis is lined by pseudostratified epithelium that secretes nutrients
required for maturation of spermatozoa. Non-ejaculated sperms are broken
down after an interval.
VAS DEFERENS
The vas deferens is a continuation of cauda epididymis which leaves the
scrotal sac and enters the abdominal cavity. After entering abdomen it loops
over the urinary bladder and dilates to spindle like ampulla for temporary
storage of spermatozoa. It also conducts sperms.
EJACULATORY DUCTS
They are shorter ducts of about 2cm length where male ejaculate is produced.
Each duct is formed by joining of vas deferens and duct of seminal vesicle
Ejaculatory ducts enter the prostate gland and join the prostatic urethra to
produce a single urinogenital duct. In ejaculatiory duct, the sperm mix up with
secretation of seminal vesicles. The walls of ejaculatory ducts are muscular to
quickly conduct the ejaculate through urinogenital duct.
URETHRA
It arises from urinary bladder and is about 20cm long, differentiated into three
regions
(i) A short proximal prostatic urethra which is surrounded by prostate
glands.
(ii) A very short middle membranous urethra without any covering
(iii) A long distal penile urethra that passes through a penis
The urethra has internal sphincter of smooth muscle fibres at its beginning
and external sphincter of straited muscle fibres around its membranous part
PENIS
It is male erectile copulatory organ which transfers semen into reproductive
tract of female during sexual intercourse
The penis contains three cyclindrical masses of erectile tissue – two dorsal
corpora cavernosa and one ventral corpus spongiosum
The corpus spongiosum, which contains the penile urethra, is enlarged at the
distal end of the penis to form glans penis. It is covered with smooth skin,
foreskin.
SEMINAL VESICLES
They are a pair of lobulated contorted musculoglandular sacs of 5cm length
between urinary bladder and rectum. Their ducts join the vasa deferens to
form ejaculatory ducts.
They produce an alkaline secretion which forms 60% of the volume of semen.
The secretion of seminal vesicles contains fructose, citrate, prostaglandins,
inositol and clotting protein.
Alkaline nature of the acidic environmental of the male urethra which
otherwise would inactivate and kill sperms
The prostaglandins stimulate uterine contractions and thus may help the
sperm to be moved towards female’s oviducts, where fertilization takes place.
The clotting protein help semen coagulate after ejaculate.
PROSTATE GLAND
It is a large grayish to red pyramidal gland of 4cm with and 3cm height that
encloses a part of urethra including its junction with ejaculatory ducts
The produces a milky slightly alkaline secretion which forms 25% of the
volume of semen. It possesses calcium, phosphate, bicarbonates, enzymes,
clotting enzymes, prefibrolysin and prostaglandins. Secretion of the prostate
gland nourish and activates the spermatozoa to swim. Prostaglandins helps in
liquefying cervical mucus and helps in liquefying cervical mucus and
stimulating movement in the female tract.
BULBOURETHRAL GLANDS
A pair of small yellow pea seed sized lobulated tubualveolar glands, 4-5cm
below prostate and opening into membraneous urethra by separate ducts
The secretion has abundant mucus for lubrication of reproductive tract. It
neutralizes the urethra from remains of urine. Secretion of cowper’s gland is
produced before the ejaculation of semen
SEMEN
The secretion of accessory sex glands and mucus are added to sperms to form
seminal fluid, or semen or seminal plasma
It rich in fructose, calcium and certain enzymes. It has a pH of 7.35 to 7.5. It is ejected from the penis during ejaculation. A single ejaculation may
contain 300 million sperms. It has many functions
(i) It provides a fluid medium for transmission of sperms into vagina of the
female.
(ii) It nourishes and activate the sperms to keep them viable and motile
(iii) It neutralizes the acidity of the urine in the urethra of male and vagina
of female to protect the sperms
(iv) It facilitates the sexual act by lubricating the reproductive tract of the
female.
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HORMONAL CONTROL OF MALE REPRODUCTIVE SYSTEM
The growth, maintenance and functions of the male reproductive organs are
under the hormonal control. GnRH ( Gonadotropin releasing hormone) is
secreted by hypothalamus. It stimulates the anterior lobe of pituitary gland to
secrete and release LH and FSH. In male, LH is called interstitial cell stimulating
hormone (ICSH) because it stimulates Leydig’s cell of the testes to secrete
androgens Testosterone is the principle androgen. The growth, maintenance and
functions of secondary sex organs and functions of secondary sex organs and
accessory glands are under the control of testosterone.
FSH stimulates sertoli cells of testis to secrete an androgen binding protein
(ABP) that concentrates testosterone in seminiferous tubule and also secrete
a peptide inhibin which suppresses FSH synthesis. FSH acts directly on
spermatogonia to stimulate sperm production.
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| Fig1.3. Hormonal Control on human male reproductive system |
COMMON DISORDERS OF MALE REPRODUCTIVE SYSTEM
(i) Prostatitis – It is inflammation of prostate generally caused by infection
(ii) Benign prostatic hypertrophy ( BPH) – This is the enlargement of prostate
gland. It often causes
in old age causing frequent night urination.
Untreated BHP may lead to kidney damage.
(iii) Prostate carcinoma – It is cancer of prostate which may grow unnoticed up
to stage of metastatis
(iv) Impotence – It is the inability of the adult male to achieve penile erection.
(v) Sterility – Sperms are unable to fertilize the ovum due to low count or less
motility.
(vi) Cryptochidism – It is a failure of one or both of the testicles to descend
into scrotum. It is often results in pushing of an intestinal loop into
scrotum, resulting in its abnormal size and discomfort.
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