BIOLOGY OF SEX

Sex is defined as the sum of morphological, physiological and psychological differences that
distinguish male and female.
Gonads
Sexual differentiation encompasses Rep tract
Somatic features
Male & Female sex behaviour
(1) Differentiation of gonads.
(2) Differentiation of the reproductive tract
(3) Differentiation of the somatic features
(4) Development of male and female sexual behavioural characteristics.
These processes start at birth and are completed at puberty and they are influenced by hormones.
Sexual differentiation was well elucidated by Jost Paradigm. He observed that:

Chromosomal sex leads to gonadal sex

Gonadal sex leads to phenotypic sex Chromosomes......>Gonads.....>Phenotype

CLASSIFICATION OF SEX

Sex can be classified into:

• Genetic

• gonadal

• hormonal

• phenotypic

• brain

• Behavioural

Genetic/ Chromosomal Sex

Genetic sex deals with the organisation of sex chromosomes. In mammals, males determine sex
by the XY chromosomes. The Y chromosome will always cause testis formation no matter how
many X chromosomes that is present. XX chromosome without a Y chromosome is necessary
 for ovarian development. In 1990, SRY gene was discovered. SRY (Sex Related to Y
chromosome) gene is the sex determining gene located on the Y chromosome. It has 2 functions

• a) It encodes the testes determining factor

Homogametic (female)
• b) It acts as switch for testis differentiation
Heterogametic (male)

The female sex chromosomes pair is identical and has the designate XX. In male the sex
chromosome consist of one X and a shorter Y chromosome. During gamete production,
chromosome numbers are halved to form haploid sperm and ova; upon fertilization these
recombine to give the normal diploid compliment. Thus each ova will have X chromosome
(homogametic) but 50% of the sperm will be X and 50% Y chromosome (heterogametic). In
recombination any combination involving a Y chromosome will give genetic male and XX will
be female, thus there is 50% chance of fertilization resulting in male and 50% chance of
fertilization resulting in a female offspring.

Gonadal Sex

Male determining genes by way of the SRY gene on the Y chromosome influences the genital
ridge. A bilateral condensation of tissue near the adrenal and convert it into testis. In the
absence of male determining genes (SRY gene) due to the absence of Y chromosomes then the
indifferent gonad develops into ovary. Gonadogenesis is the development of the gonads. They
develop from the genital ridge located on the medial aspect of the ‘primitive’ kidney
(mesonephros). The indifferent gonad is composed of an inner medulla and an outer cortex.
Primordial germ cells which have been multiplying by mitosis migrate to genital ridges from the

GONADOGENESIS

= Development of gonads from genital ridge in the medial aspect of the primitive kidney
= Indifferent gonad has an inner medulla and an outer cortex
=Primordial germ cells multiplied by mitosis migrate to genital ridges from the yolk sac endoderm
=Primordial germ cells invade medulla to form primary sex cords leading to gonadal differentiation while bipotentiality ceases (MALE)

MALE FEMALE

=SRY gene makes medulla develop into testes AND cells in the = SRY gene is absent
primary sex cord into precursor of spermatozoa =Premordial germ cells in the epithelium invade the cortex as
= Cortex regresses secondary sex cords
=Leydig cells producing testosterone appear = Cortex develops into an ovary
=Mullerian Regression Factor secreted by Sertoli cells appear = Medulla regresses
= Embryonic ovaries do not secrete hormones
yolk sac endoderm.
Later they invade the medulla to form primary sex cords. At this stage, gonadal differentiation
occurs and bipotentiality ceases. The presence of the SRY gene leads to the development of the
medulla into a testis and the cells in the primary sex cords into precursors of spermatozoa. The
cortex regresses. Leydig cells appear producing testosterone, also Mullerian Regression Factor
(MRF) secreted by Sertoli cells. In genetic females where SRY gene is absent persisting
primordial germ cells in the epithelium invade the cortex as secondary sex cords. The cortex
develops into an ovary and the medulla regresses. The embryonic ovary does not secret
hormones.
 HORMONAL SEX
The sex hormone produced depend on the nature of the gonad and not on the genetic makeup.
Male
The testis produce male sex hormone whether it is genetically XY (normal), XXY, XYY or XO.
XY
Alfred Jost performed 2 experimented to prove Hormonal Sex. XXY
XYY
XO
PHENOTYPIC SEX

This refers to the external sexual appearance. In most cases genetic, gonadal and hormonal sex
culminates in phenotypic appearance appropriate for the sex. Males have testis and penis
females have vagina, vulva, mammary gland etc.

BRAIN SEX

If newborn female mice are injected with testosterone, they will not show oestrous cycles after
puberty- the brain has been masculinised. Anatomical evidence for brain sex is found in the
medial preoptic nucleus of rodents which is eight times larger in male rats than female rats.
Castration of newborn males reduces the nucleus, testosterone injection of newborn females
expand it. The nucleus is concerned with sexual behaviour. Medial preoptic nucleus of rodents is an
anatomical evidence for brain sex

SEXUAL BEHAVIOUR (BEHAVIOURAL SEX OR PSYCHIC SEX):

Sexual behaviour encompasses:

Social mannerism
• social mannerism Maternal behaviour
Mating behaviour (sex drive & phases of copulation)

• maternal behaviour

• mating behaviour Sex drive

Phases of copulation

Mating behaviour has two components

• a) sex drive (libido) and

• b) phases of copulation such as postural adjustment, intromission, ejaculation, orgasm

and post-copulatory behaviour.

Sexual behaviour is influenced by:

 • internal factors.

• external factors.

Internal factors

• Ovarian hormones

• Gonadotropins Secreted by gonadotrope cells of PITUITARY GLAND

• Other endocrine gland secretions such as thyroid and adrenal gland secretions

The External factors

• Season

• Nutrition

• Domestication and confinement

• Social behaviour

• Pheromones

Pheromones

Is a chemical that is produced by an animal that acts at a distance to trigger a natural behavioural
response or other physiologic change in another member of same species. There are alarm
pheromone, food trail pheromone and sex pheromone.

Sex pheromone

They are chemical messengers that facilitate sexual communication between mammals. They
elicit attraction and mating behaviour. Males are attracted to female by odours emanating from
vaginal secretions. One compound Methyl-P-hydroxybenzoate has been isolated from vaginal
secretion of dogs in proestrus and estrus and shown to elicit intense anogenital interest by male
when applied to females. Females are also influence by odours of the male e.g sow on estrus
will assume a breeding stance (hind legs braced i.e rigidity reflex) if exposed to the urine of a
boar.
 The Phenomena on Pheromones

• The Whitten Effect When pheromones contained in the urine of male mice stimulate
synchronous estrus in a population of female mice

• The ram effect Non-cycling ewes are stimulated to ovulate by the sudden
introduction of a novel ram

• McClintock effect Menstrual Synchrony / Wellesley effect

• The Bruce Effect Female rodents abort pregnancy when

exposed to new males

Puberty

This is regarded as ‘great awakening’. It is when the gonad becomes fully functional. In the
male, it is when fertile spermatozoa appear in the ejaculate. It can be divided into: 1.
Physiological; and 2. Behavioural aspects. In the female it is the time in which female comes
with estrus and ovulate. In the male spermatozoa is seen while ova is seen in the female. In
humans, puberty is regarded as series of events occurring over a number of years.

There are two types of mechanisms that have been put forward to explain how puberty comes
about:

1. Gonadostat hypothesis
Luteinizing hormone Follicle Stimulating Hormone
2. Hypothalamic maturation hypothesis.
In the gonadostat hypothesis, the concept begins with the idea that negative feedback of FSH and
LH of the pituitary on the hypothalamus is very sensitive. This implies that a very sensitive
response to the concentration of steroids i.e there is a low set point. At puberty the set point
increases and there is less sensitivity to steroids. The steroids in the plasma now rises and as a
result of that the level of steroids in d plasma now rises. However, this observation is now
thought to be secondary to the onset of puberty. The primary cause of puberty is now thought to
be hypothalamic maturation. In this hypothesis, there is maturation of the CNS and increase
output of the GnRH which leads to the production of gonadotropins.

Silent ovulation: this is seen in the 1st or 2nd cycle at the breeding season.

Precocious puberty: this is an abnormal early onset of puberty.

 Factors affecting attainment of puberty
1. Age: breed dependent
2. Environmental factor: esp. phtoperiodism in seasonal breeders.
3. Social factors: presence of other females and introduction of strange males.
4. Nutrition

MALE REPRODUCTIVE PHYSIOLOGY

• Reproductive tracts in domestic species of animal are basically similar; differing only in
position of testis in relation to the body, penis structure, presence/absence and shape of
accessory glands.

• The male genital system consists of the penis, scrotum, testis (male gonad), rete
tubules, efferent tubules, epididymides, vas deferens and several accessory glands e.
g. ampulla, prostate, vesicular glands (seminal vesicles) and Bulbourethral glands
(Cowper’s gland).
 Unique features of reproductive organs:

• Ram and Billy goat: prostate diffuse, urethral process present.

• Boar: relatively large vesicular and bulbourethral glands which contribute to the large
volume of semen produced. The boar has the largest testes per unit body weight. A
preputial diverticulum is present.

• Stallion: penis very vascular, no sigmoid flexure as in bull, ram, buck and boar.
• Dog: Os penis present, all accessory glands are absent except prostate. Cat penis has
spines (so has a lion’s penis).

• Gonad = testis in male, ovary in female.

• Accessory sex organs = all reproductive organs except the gonad, ie scrotum, penis,
prepuce, preputial glands, urethra, efferent ducts, epididymis, vas deferentia (pl of vas
deferens), plus the accessory glands. These include the ampullae, vesicular glands, and
prostate glands

Accessory glands secretion

• Known collectively as seminal plasma, this fluid from accessory gland is released during
sexual stimulation and ejaculation. It dilutes, nourishes and protects sperm within the
female reproductive tract.

• The prostate gland produces a secretion that buffers the vaginal pH, which is normally
3.5 to 4.

• The seminal vesicles add fructose to nourish the sperm and prostaglandins to promote
contractions in the female.

• The bulbourethral gland (Cowper’s gland) produces mucus which lubricates the penis.

• Fructose is the major source of energy for bull, ram, goat and rabbit spermatozoa.
Derived mainly from vesicular glands, it is metabolized by sperm to lactic acid.

• Stallion semen contains sorbitol which is reduced to fructose, an energy source.

• Boar vesicular glands produce inositol which is probably converted to fructose.

• Ergothioneine from boar vesicular glands and stallion ampulla protects sperm from
toxic effects of oxidizing agents. This is important because of high volume, low sperm
concentration and low levels of glycolysable sugars in semen.

• Ascorbic acid is the antioxidant in other species.

• Prostaglandins (E and F) produced by seminal vesicles (ram prostate also produces them)
by causing contraction (PGF) or relaxation (PGE) if smooth muscle probably aid sperm
passage through female reproductive tract

• The excurrent tract (or ducts)

• Included are rete testis, efferent ducts, the epididymis, the vas deferens (pl = vasa
deferentia = deferent ducts in English) and the urethra. Rete testis is network of straight
tubules which connect efferent ducts to the sperm-producing convoluted seminiferous
tubules.

Epididymides

• The epididymis consists of head (caput), body (corpus) and tail (cauda): It is attached to
both pole and one side of the testis. From the cauda eqididymis, the vas deferens courses
back into the abdominals cavity via the spermatic cord and inguinal canal.
 Testes

• Involve in male germ cells (spermatozoa) production and the production of male
hormones notably testosterone.

The testes lie outside the abdomen in the scrotum (derived from abdominal skin and
fascia). Each testis lies in the vaginal process, an extension of the peritoneum. This
process goes via the inguinal rings along with its contents, which include the spermatic
cord (blood) vessels.
• Each testis contains 250 to 300 lobules, and each of these contains one to three tightly
coiled seminiferous tubules. Sperm are produced within the seminiferous tubules.

Descent of testes

• Involves abdominal, inguinal and scrotal migration. Testes enter the scrotum in mid-
pregnancy in domestic ruminants, the third trimester in the boar and just before or after
birth in the stallion.

• CRYPTORCHIDISM, results when testes fail to descend. Causes are believed to be

hormonal.

Functions of testes

• The functional unit of the testes is the seminiferous tubule where sperm develop.

• A cross section of testis shows seminiferous tubules with developing germ cells at
different stages, sertoli cells. Outside the basement membrane of the tubule are the
interstitial cells of Leydig’, blood and lymph vessels.

Sertoli Cell function

 Sertoli cells or sustentacular or nurse cells are found within the seminiferous tubule.
Have long processes which surround germ cells throughout their development. Perform
these functions:

1. During development, the Sertoli cells, under the influence of SRY, produce Mullerian

inhibiting substance (MIS). The latter inhibits the development of the embryonic

mullerian duct derivatives (oviduct, uterus and cervix).

2. They form a physical support for the developing sperm cells.

3. They help to nourish the developing sperm cells. There are gap junctions between

Sertoli cells as well as between Sertoli and developing sperm cells. Nutrients and

other metabolites can pass from the Sertoli cells to the developing sperm cells.

4. They protect the developing sperm cells. The Sertoli cells form the blood-testis

barrier (details later).

5. They secrete factors that are important in sperm development:

 Androgen binding protein secreted by the Sertoli cells maintains a high

concentration of the male hormone testosterone in the tubular lumens. This is critical for
sperm development.

 Potasium and bicarbonate secreted by the Sertoli cells into the testicular fluid help to
propel the nonmotile sperms out of the testis.
 Inhibin is a hormone that inhibits the secretion of FSH (follicle stimulating
hormone) from the pituitary as well as gonadotropin releasing factor from the

hypothalamus.

 Activin is a hormone that exerts a positive feedback on the secretion of

FSH.
7. The Sertoli cells phagocytize defective sperms and residual bodies.
8. Secrete androgen binding protein (ABP) under influence of FSH.

9. They undergo cyclic changes with the seminiferous epithelium and this way coordinate
spermatogenesis, they are involved in spermiation (release of sperm, analogous with
ovulation)

Other functions of testes include:

(a) Sex differentiation – before birth, the foetus could develop in either the male or
female direction. Presence of testes ensures that development proceeds in the male
direction.

(b) Puberty – At the onset of puberty, the hypothalamus/pituitary axis becomes less
responsive to negative feedback by testicular androgens (testosterone, androstandedione
etc). This leads into excess LH and testosterone production which promotes development
of reproductive tract and organs during puberty.

The testes and thermoregulation

The cells of the seminal lineage are susceptible to toxic chemicals, heavy metals, ionizing
radiation and especially, high temperature. For sperm to develop properly, they need to
be in the scrotal sacs, which are 1.5 to 2.5, °C lower in temperature than that of the
abdominal cavity. The scrotal sac and the spermatic cord play an important role in the
thermal regulation of the testis. Mechanisms to maintain ideal temperatures for
spermatogenesis include:

(1) Countercurrent exchange transfer of heat from the arterial to the venous side in
the pampiniform plexus just before blood enters the testis.

(2) The cremaster and dartos muscles contract and draw testes towards the body in
cold weather, they relax and move them away as the temperature increases.

3) Scrotal skin has no subcutaneous fat, it is rich in sweat glands.

4) Testicular arteries and veins are close to skin surface, this promotes direct heat loss.
5) Temperature receptors in the scrotal skin (of sheep) initiate responses that lower body
temperature.

Spermatogenesis

Spermatogenesis. Germ cells near the outer wall of the seminiferous tubules of the testis
differentiate into stem cells called spermatogonia. Spermatogonia divide by mitosis, and
mature into primary spermatocytes. Each primary spermatocyte undergoes meiosis:
Meiosis I yields 2 haploid secondary spermatocytes. meiosis II yields 4 equal-sized
spermatids. The spermatids migrate toward the lumen (central opening). Sertoli cells
supply nutrients for the spermatids, which mature into motile sperm in the epididymis.

During formation of the embryo, the primordial germ cells migrate into the testes and
become immature germ cells called spermatogonia which lie in two or three layers of the
inner surfaces of the seminiferous tubules.

The spermatogonia begin to undergo mitotic division, beginning at puberty, and

continually proliferate and differentiate through definite stages of development to form
sperm. Spermatogenesis occurs in the seminiferous tubules during active sexual life as
the result of stimulation by anterior pituitary gonadotropic hormones, beginning at an
average age of 13 years and continuing throughout most of the remainder of life but
decreasing markedly in old age. In the first stage of spermatogenesis, the spermatogonia
migrate among Sertoli cells toward the central lumen of the seminiferous tubule.

The Sertoli cells are very large, with overflowing cytoplasmic envelopes that surround
the developing spermatogonia all the way to the central lumen of the tubule.

Meiosis. Spermatogonia that cross the barrier into the Sertoli cell layer become
progressively modified and enlarged to form large primary spermatocytes. Each of these,
in turn, undergoes meiotic division to form two secondary spermatocytes. After another
few days, these too divide to form spermatids that are eventually modified to become
spermatozoa (sperm).

Sex Chromosomes
 In each spermatogonium, one of the 23 pairs of chromosomes carries the genetic
information that determines the sex of each eventual offspring.

This pair is composed of one X chromosome, which is called the female chromosome,
and one Y chromosome, the male chromosome. During meiotic division, the male Y
chromosome goes to one spermatid that then becomes a male sperm, and the female X
chromosome goes to another spermatid that becomes a female sperm.

The sex of the eventual offspring is determined by which of these two types of sperm
fertilizes the ovum.

HORMONAL CONTROL OF SPERMATOGENESIS

Several hormones play essential roles in spermatogenesis. Some of these are as follows:

• Testosterone, secreted by the Leydig cells located in the interstitium of the testis, is
essential for growth and division of the testicular germinal cells, which is the first stage in
forming sperm.

• Luteinizing hormone, secreted by the anterior pituitary gland, stimulates the Leydig cells
to secrete testosterone.

• Follicle-stimulating hormone, also secreted by the anterior pituitary gland, stimulates the
Sertoli cells; without this stimulation, the conversion of the spermatids to sperm (the
process of spermiogenesis) will not occur.

• Estrogens, formed from testosterone by the Sertoli cells when they are stimulated by
follicle-stimulating hormone, are probably also essential for spermiogenesis.

• Growth hormone (as well as most of the other body hormones) is necessary for
controlling background metabolic functions of the testes. Growth hormone specifically
promotes early division of the spermatogonia themselves; in its absence, as in pituitary
dwarfs, spermatogenesis is severely deficient or absent, thus causing infertility
THE BLOOD TESTIS BARRIER

During sperm development new genetic combinations appeared a result of crossing over.
Since sexual maturity occurs long after the development of immunocompetence, the
differentiating sperm cells can be recognized as ‘foreign’ and provoke an immune
response that will destroy the sperms. This does not happen because a blood-testis
barrier protects the developing sperms. Very elaborate occluding or tight junctions make
up the blood-testis barrier between Sertoli cells. The junctions divide the seminiferous
epithelium into a basal containing the spermatogonia and early-stage primary
spermatocytes, and an adluminal compartment, containing later-stage primary
spermatocytes and all subsequent stages of sperm cells. As early primary spermatocytes
move apically from the basal compartment to the adluminal one, the tight junctions need
to be disassembled and reassembled. These tight junctions form the physical basis for the
blood-testis barrier, which creates a specialized, immunologically safe microenvironment
for developing sperm. By blocking paracellular diffusion, the tight junctions restrict
movement of substances between the blood and the developing germ cells through a
trans–Sertoli cell transport pathway and in this manner allow the Sertoli cell to control
nutrient availability to germ cells. Large molecules such as immunoglobulins cannot
penetrate the tight junctions and thus are unable to reach the developing sperms. Note
that the spermatogonia are located outside the barrier (they are genetically identical to the
somatic cells) and can respond to various factors. The spermatocytes are inside the barrier
and they may express novel proteins on their surfaces.