Online Course: Biosex Variations
Courtesy of Prof. M. Italiano & Genderhealth, Inc. who are the holders of the copyright
Prof. M. Italiano is the Advisor on Biosex Variations for the Organisation Intersex International, a Canadian intersex organisation with coalition members worldwide. OII offers this course and other resources in its effort to educate the public and health professionals about sex variations.
Module 1
TYPICAL SEX BIOLOGY
Part 1 Genetics and sex determination
Part 2 Embryology and gonadal differentiation
Part 3 Anatomy & Microanatomy of internal genital systems
Part 4 Anatomy & Microanatomy of external genital systems
Part 5 Endocrinology
Part 1: Genetics and sex determination
Introduction
The determination of an individual's genetic sex is the result of a complex series of events beginning with the merging of egg and sperm cells. These mature GERM CELLS are known as GAMETES, having become such, as the result of another complex series of events known as GAMETOGENESIS.
Cells in the body, other than germ cells, are referred to as SOMATIC CELLS, and typically, in humans, possess 46 CHROMOSOMES. (1) Chromosomes are strands of DNA (deoxyribonucleic acid), which are hereditary material that is chemically composed of four bases interwoven around themselves in the form of a double helix. (2) GENES located on the chromosomes (being made up of DNA) encode for the synthesis of a protein by means of RNA (ribonucleic acid). Genes are usually responsible for the development of many physical traits. DNA which does not encode for the synthesis of a protein and is not responsible for the transmission of hereditary traits is referred to as JUNK DNA and may comprise 98% of the HUMAN GENOME - the sum total of all of the genes in a typical somatic cell. (3) The sum total of chromosomes in a somatic cell is referred to as a KARYOTYPE.
Since the usual number of chromosomes in a human somatic cell is 46, the first important purpose of gametogenesis is to reduce the number in half, so that half are derived from the egg and the other half are derived from the sperm. This is accomplished through a series of cell divisions known as MEIOSIS 1 or REDUCTION DIVISION and MEIOSIS 2 or EQUATION DIVISION.
An equally important purpose of gametogenesis is to prepare the egg to provide nutrients for the developing embryo, and to prepare the sperm for its mobility and fertilizing ability.
Thus the preparation of germ cells for typical egg development is referred to as OOGENESIS and for typical sperm development is referred to as SPERMATOGENESIS. Once mature, the sperm and egg are prepared to merge.
In order to merge, the sperm must recognize and penetrate the egg (which at this stage is a loose term for its technically correct label as an OOCYTE.)
The preparation for the penetrating of the egg is a process known as CAPACITATION.
The sperm then releases enzymes which digest the ZONA PELLUCIDA (region surrounding the oocyte), allowing the sperm to be dragged into the egg.
These enzymatic processes are referred to as the ACROSOME REACTION.
The egg then becomes refractory to further fertilization, and the acrosome reaction stimulates the egg (oocyte) nucleus to resume equation division.
This finishes meiosis 2 and the oocyte may now be properly referred to as an OVUM - the technical definition for an egg. Thus, technically, a woman who has never had sex has never produced an egg, in the strict sense of that term.
Although during penetration, the sperm has penetrated the egg, the nucleus of each, which contains the HAPLOID, or half of the total number of chromosomes, have not intermingled as of yet. Each nucleus is still referred to as the MALE PRONUCLEUS and the FEMALE PRONUCLEUS.
Together, they are still referred to as PRONUCLEI.
When the pronuclei walls disintegrate, the union of the sperm and egg nuclei, containing the chromosomes from each, occurs. This process is known as SYNGAMY. This restores the chromosome complement of the new cell, called a ZYGOTE, to its DIPLOID (possessing the complete number of chromosomes) state, with the usual 46 chromosomes.
Of the 23 chromosomes in the sperm and egg, 22 are called AUTOSOMES (or non-sex chromosomes). The other chromosome in the sperm and the egg are termed the SEX CHROMOSOMES.
The egg typically has a sex chromosome known as an X chromosome and the sperm typically has one sex chromosome, which may also be an X chromosome or a chromosome known as a Y chromosome.
If an X chromosome from the female has joined with a Y chromosome from the male, the resulting individual will have an XY chromosome complement, which usually results in a male.
If the X chromosome from the female has joined with an X chromosome from the male, the resulting individual will have an XX complement, which usually results in a female. Thus, the CYTOGENETIC (“cyto” meaning cellular) basis of sex determination at the level of the chromosomes as visualized genetically (thus cytogenetic) is roughly determined.
The Y chromosome is very small and does not contain nearly the amount of genes as does the X chromosome which is necessary for survival of the embryo.
Since the X chromosome contains many genes and is necessary for survival, typical females who have two X chromosomes (XX), have extra genes that XY embryos do not have. Thus, some of the genes on the extra X chromosome are inactivated (turned off) by a process known as DOSAGE COMPENSATION (4) (which compensates for the extra X chromosome in XX individuals) or Lyonization, named after scientist Mary F. Lyon who discovered the process in humans. (5)
The inactivated X chromosome (the one which is inactive is usually random and not necessarily the one that is inactive in another cell) forms what is known as SEX CHROMATIN or a BARR BODY, named after physician Murray Barr who discovered it. (6)
Some genes on the inactivated chromosome are not inactivated (escape inactivation).
It is thought that some of these do have their counterpart on the Y chromosome in an area called the PSEUDOAUTOSOMAL REGION. (7)
Although a person's cytogenetic sex is roughly determined as defined by the chromosome complement, genetic sex gets far more complex than that, because what actually determines and develops a person's GONADAL (sex glands) sex are genes on the X and Y chromosomes, and perhaps even more importantly, genes on the autosomes. One primary gene, known as SRY (for sex determining region of the Y chromosome), is a main sex determining gene for maleness (8), usually causing a person to become a male. However, importantly, it does so only indirectly, since it is a regulator, i.e., it turns other genes on other chromosomes on or off. (9) These other genes actually then produce chemicals which will cause gonadal maleness or femaleness. (10)
If these other genes on other chromosomes (often even on autosomes) work in concert with SRY, chemicals will be produced which will lead to the differentiation of the indifferent GONADS (embryonic sex glands) into testes.
If there is no Y chromosome, but instead there is a second X chromosome, then there will be no SRY gene to interact with other chromosomal genes to produce testes. Instead, the formula for such an individual with an XX chromosome complement will usually be to lead to chemicals which cause the differentiation of the gonads into ovaries, thus initiating femaleness.
Thus, genes on the X, Y, and autosomes all contribute to producing chemicals which lead to male (testicular) or female (ovarian) differentiation. This is the genetic basis of initial (gonadal) sex differentiation.
Gonadal differentiation is referred to as being primary since (1) it occurs first and (2) it is where both the germ cells locate and where the sex hormones are produced.
Part 2: Embryology and gonadal differentiation
The PRIMORDIAL GERM CELLS, that is, sexually undifferentiated germ cells (except with regard to their chromosomal complement) develop embryologically in a region known as the EPIBLAST. (11)
They move to the junction of the gut tube and the umbilical yolk stalk and eventually become embedded in the gut and in tissue that lines the abdominal cavity.
They are chemically distinct as germ cells (12), although anatomically they are sexually undifferentiated.
They migrate along a chemical gradient from the hindgut to a place called the GENITAL (GONADAL) RIDGE, whose chemical signal attracted them. (13) The gonads themselves arise from MESODERM (a middle embryonic germ layer) and are located bilaterally in the lower pelvic cavity.
Thus, the gonads are not initially the source of the germ cells, but receive them after they migrate there. The gonads at this point in development are not sexually differentiated, but are referred to as NEUTRAL gonads.
They are also known as BIPOTENTIAL gonads, since they can develop into either ovaries or testes.
If the chromosomes are XY, the SRY gene will typically regulate genes on the chromosomes to produce a protein which will develop the MEDULLA (the inner part of the gonad) into testicular tissue. The CORTEX (the outer part of the gonad) will degenerate. In the absence of the male protein, the cortex will develop into ovarian tissue, and the medulla will degenerate.
Among the first signs of testicular specific differentiation is the formation of SEMINIFEROUS TUBULES, where sperm will be manufactured. The appearance of two cell types unique to testicular tissue occurs.
The first is the development of SERTOLI CELLS (14) in the tubules. These Sertoli cells have two functions:
1) To nurse the primordial germ cells (they will actually surround them) and
2) To produce a substance known as MIS, for MULLERIAN INHIBITING SUBSTANCE. (15)
The second cell type is called LEYDIG CELLS (16) or INTERSTITIAL CELLS which will produce the hormone TESTOSTERONE, which is a potent ANDROGEN (a general name for a male hormone, although females also produce androgens). In the absence of the SRY gene, and in the presence of two normal X chromosomes, normal ovaries usually will be actively developed.
The cortex will develop into ovarian tissue whereas the medulla will degenerate. Cell types unique to the ovary will include ovarian FOLLICLES, which will participate in the development of the germ cells through stages of oogenesis, and THECA CELLS, which will produce ESTROGENS (a general name for female hormones, although males also produce estrogens).
Thus, the gonads are truly bipotential. They have the ability to develop into testes or ovaries depending on chemicals produced by the X chromosome, Y chromosome, and autosomes.
Part 3: Anatomy & Microanatomy of internal genital systems
There exist two drainage tubes along the neutral gonads. These are known as the WOLFFIAN DUCTS (17) and the MULLERIAN DUCTS. (18) There are a pair of each of these ducts on each side (left and right) in the pelvic cavity. The Wolffian ducts develop into the SPERM DRAINAGE SYSTEM (Seminal Vesicles, Vas Deferens) and the Mullerian ducts develop into the EGG TRANSPORT SYSTEM (Uterus, Fallopian Tubes, Cervix, and upper portion of the Vagina.)
In typical male development, the Sertoli cells of the testes produce MIS (Mullerian Inhibiting Substance). MIS inhibits the Mullerian Ducts from developing. The remnants of the Mullerian ducts in typical males are called the PROSTATIC UTRICLE or UTERUS MASCULINIS.
It is also believed that MIS may assist in the descent of the testes from the pelvic cavity to the outer part of the body into the SCROTUM (a sac). Whereas, MIS inhibits the development of these Mullerian ducts, testosterone, which is produced by the Leydig cells, causes the Wolffian ducts to develop.
The sperm drainage system modifies the sperm as it is transported from the testes and eventually out of the penis.
Thus, in the absence of the action of MIS and Testosterone, the Mullerian Ducts lead to the development of the egg delivery system.
This provides a place for fertilization (Fallopian Tube) and a path for the embryo to implant into the uterus. The degenerated Wolffian Ducts in females is known as the EPOOPHERON.
Although internal female genital development is referred to as a passive or default process (since it occurs in the absence of MIS and testosterone), it is undeniably a biologically active process, which is just overridden in typical males by MIS and testosterone action.
Significant fetal estrogen production from the fetal ovaries does not contribute significantly to female development (19), which occurs naturally, and is termed as occurring by default, in the absence of the influence of male substances.
The active process for female internal genital development remains poorly understood.
Part 4: Anatomy & Microanatomy of external genital systems
Typical embryos have a common opening for the genital and urinary systems which leads to the outside. This is known as the UROGENITAL SINUS. Typical embryos also have in common a set of structures in their sexually "neutral" anatomical stage.
They are known as the GENITAL TUBERCLE, GENITAL FOLDS, and GENITAL SWELLINGS.
A modified form of testosterone, known as dihydrotestosterone (DHT), produced by an enzyme called 5 ALPHA REDUCTASE 2, modifies these genital structures, changing them into male external genitalia.
Thus, whereas testosterone is responsible or the development of the Wolffian ducts, it is only DHT which converts the external genitalia to male in typical males. (20)
The genital tubercle forms the PENILE GLANS or head of the penis. The genital folds form the penile shaft, which includes part of the urethra known as the PENILE URETHRA, for the passageway of urine and semen.
This part of the urethra which is surrounded by erectile tissue, spongiosum, leads to the BULB OF THE PENIS, which also contains vascular tissue.
Two additional erectile bodies contained within the shaft of the penis are called the CORPORA CAVERNOSA, which spread out to form the CRURA (like legs), which are attached to the PUBIC BONE.
There is a dense network surrounding the corpora and crura which is made of a complex neural and vascular communication system, which is referred to as the NEUROVASCULAR BUNDLES.
These originate from the PUDENDAL NERVE and form the neurovascular bundles of the PROSTATE, which extend out along the crura before fanning out along the DORSAL (top) part of the penile corpora. (21)
Also, these nerves surround and innervate the VENTRAL (bottom) surface, and perforate the corpus spongiosum. The corpora cavernosa is enveloped and bound by connective tissue known as the TUNICA ALBUGINEA.
In addition, there is another separate nerve pathway which innervates the bottom part of the glans penis and the CORONA (ridged crown area around the base of the glans penis).
The glans penis serves as a cap for the corpora and is also made of spongy tissue. It contains few sensory receptors, except for the area around the corona and the RIDGED BAND (which is the area of the fusion of the glans with its covering before it separates naturally). Its covering, known as the PREPUCE or FORESKIN, is rich in receptors.
The most noted of these are VATER-PACINIAN CORPUSCLES, which are specialized sensory nerve endings, especially sensitive to deep pressure sensation. (22)
However, the foreskin is often removed in male infants during a procedure known as CIRCUMCISION, which removes the most erogenous and sensitive parts of the male genitalia.
The shaft of the penis, by the base, is suspended by the SUSPENSORY LIGAMENT of the penis, and a thickening of fibrous tissue from the lower abdominal wall called SCARPA'S FASCIA (23), known as the FUNDIFORM LIGAMENT. DHT is also responsible for the development of the COWPER'S GLANDS (24), which secrete droplets into the urethra to keep it moist and the development of the male PROSTATE, which surrounds the neck of the bladder and whose function is to form part of what makes up SEMEN, a fluid which delivers sperm.
The area of the urethra surrounded by the prostate is known as the PROSTATIC URETHRA and the prostate opens into the urethra in the form of EJACULATORY DUCTS on a raised part of the prostatic urethra known as the VERUMONTANUM. In typical males, the genital swellings enlarge and swing around to fuse into a sac, known as the SCROTUM. The area of the fusion is known as the SCROTAL RAPHE and extends up to the bottom of the fused genital folds, which forms the PENILE RAPHE. Loose muscular tissue investing the raphe is known as the TUNICA DARTOS. Another connective tissue layer which surrounds the testes after they've descended into the scrotum and elevates them for the maintenance of temperature requirements necessary for the development of healthy sperm is known as the CREMASTERIC MUSCLE.
This also covers the EPIDIDYMIS, which is technically part of the testis (gonad) proper.
In the absence of the action of DHT, as in typical females, the external genitalia become female.
The genital tubercle becomes the CLITORAL GLANS, which as in the male glans, is made up of spongiosa tissue.
On average, the female glans has far more Vater-Pacinian corpuscles than either the male glans or the male prepuce. (23) The clitoral glans is also covered by a CLITORAL PREPUCE or foreskin also known as the CLITORAL HOOD. The corpora cavernosa also exist in females and are an extension of the female crura, which is also attached to the pubic bone.
The corpora are smaller than in typical males, but are also encased in a tunica albuginea and become fused as they get nearer to the glans clitoris, which forms a cap over the fused corpora cavernosa.
The female also has neurovascular bundles with an analogous innervation of the clitoris as is found in the male with its innervation of the penis.
The corpus spongiosum also exists in the female and extends from beneath the VESTIBULE which is the area of entrance for the vaginal, urethral and other ductal openings. This continues to the PARS INTERMEDIA, which includes the union of the two CLITORAL BULBS, which are paired and are homologous to the bulb of the penis. However, the female urethra, which is shorter than the male urethra, does not traverse the tip of the clitoris.
However, there is a dimple or groove on the underside near the tip of the clitoris, which represents its counterpart in females.
There is also a suspensory ligament and a fundiform ligament of the clitoris, although smaller than in the typical male.
The clitoral bulbs run parallel with the crura. They contain a rich vascular tissue network, which become engorged during arousal. They have previously been known as the VESTIBULAR BULBS, although their location relates them more accurately as the clitoral bulbs, being located more anterior than previously believed. (25)
Thus, their role in providing "support" or rigidity for the penis during sexual intercourse remains doubtful.
The GENITAL FOLDS, (also called URETHRAL FOLDS), in the female remain unfused, and become the outer portion of the LABIA MINORA, which is usually darker in skin tone and measure about .5 of a centimeter in thickness (26), but vary greatly in their width and length. (27) Thus, they are very thin and sometimes fold back on themselves. They contain sweat and sebaceous follicles, but usually no hair follicles. (28) The inner part of the labia minora is referred to as mucosa and is continuous with the mucosa of the vestibule. (28) The rear part of the vestibule represents a depressed area known as the FOSSA NAVICULARIS, which may disappear after childbirth. The EPITHELIUM (a surface layer of cells) of the inner part of the labia minora and the vestibule however doesn't have mucous secreting glands and is actually very poorly KERATANIZED (keratin is a protein which makes up skin) skin. (28) This tissue is also called STRATIFIED SQUAMOUS EPITHELIUM (stratified meaning more than one layer; squamous meaning a type of flat cells in appearance; and epithelium, meaning the surface layer of cells which lines the epidermis or outer skin layer or which forms the surface layer of mucous membranes). Thus, they are referred to as the SEX SKIN (29) or NYMPHAE. They surround the vestibule, below which lies the female corpus spongiosum (30) (containing erectile tissue and lined by ENDOTHELIUM), and in the inner part of the labia minora which present a dramatic color change, signifying impending orgasm. (31)
The demarcation between the outer part of the labia minora, with its deep color and keratinization, and the inner part of the labia minora, with its pink, poorly keratanized area is known as HART'S LINE, named after Scottish gynecologist, D. Barry Hart. (32) In some individuals this represents an immediate change, and in others there is about a millimeter or so of a gradual decrease in keratinization as the tissue gets nearer to the vestibule. At the rear part of the labia minora there exists a ridged elevated area known as the POSTERIOR FOURCHETTE.
This may become FRIABLE, or disintegrate upon sexual activity, or with the occurrence of adhesions or agglutination of the labia minora, which occurs predominantly in younger girls before this area becomes thicker due to the effects of pubertal estrogens.(33) Pubertal estrogens also change the color of the inner labia minora and the vestibule from a red to a pink color, as the epithelium responds to estrogens.(33) There are small gland-like structures in the vestibule in some, but not in all typical females, known as the MINOR VESTIBULAR GLANDS.(34)
Inflammation of these may result in a condition known as VESTIBULITIS, which may need to be treated medically or surgically. Other glands, which open into the vestibule, and are homologous with the male's Cowper's glands are known as BARTHOLIN'S GLANDS or MAJOR VESTIBULAR GLANDS. (35)
There is usually one opening for each of these glands on both sides of the vestibule.
They may contribute a small amount of lubrication during sexual arousal. (36)
Likewise, they may also get infected and may require medical or surgical treatment. At the entrance to the vagina, but within the vestibule, there exists in most typical females, membranous tissue, known as a hymen. This is very variable in structure, and is no indication of a state of virginity, as it may tear from horseback riding, the insertion of a tampon, etc. It will usually leave remnants known as HYMENAL TAGS.
The labia minora, also usually (but not always) (27), joins together at the bottom of the clitoral glans (sometimes with the clitoral dimple mentioned above), and forms a FRENULUM OF THE CLITORIS. Sometimes, it also attaches to the clitoral hood (prepuce).
The female urethra enters the vestibule just anterior to the opening of the vagina.
Surrounding the female urethra is the FEMALE PROSTATE (37), a more accurate description than its former name of SKENE'S GLANDS (38) or PARAURETHRAL GLANDS, which probably represent two other ducts, which open into the urethra. However, the female prostate may have up to 32 ducts entering the urethra (39) and may ejaculate considerably more fluid than seen in males. (40)
This female ejaculate contains a PROSTATE SPECIFIC ANTIGEN (41) and thus must not be dismissed as urine. It is thought that maybe with its acidity that it wards off infections in females who ejaculate. (42)
The GENITAL SWELLINGS also remain unfused in females and are called the LABIA MAJORA. They are skin folds and contain adipose tissue. The outer surface is typically hair bearing. The inner surface is usually hairless and may develop from part of the genital folds.
They are located from the pubic mound anteriorly (which is known as the MONS PUBIS or MONS VENERIS. The mons has its upper hair growth called the ESCUTCHEON, sharply defined in its upper border in a horizontal direction) all the way posteriorly to the POSTERIOR COMMISSURE, where they usually are tapered and are usually not joined, but blend with the surrounding skin.
Just behind the posterior labia majora lies the PERINEUM, which overlies the PERINEAL BODY (a type of crisscross between several separate muscles). This is usually more tense than the male perineal body, as it needs to support childbirth.
With the urogenital sinus left open in embryological development, the vaginal opening leads to the VAGINA. This passageway for intercourse, menstrual flow, and for childbirth is the birth canal, is made of an OUTER MUSCULAR LAYER, AN INNER LAMINA PROPRIA (a layer containing vascular and connective tissue), and a lining of STRATIFIED SQUAMOUS EPITHELIUM (28), which is not mucosa (43), although it is often termed mucosa, simply because it lines a body passageway and is not keratinized. The vagina also contains an adventitia and fornices. It is simply NON-KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM. (43) Any vaginal lubrication is the result of a TRANSUDATE (a fluid that has passed through a membrane as a result of certain imbalanced forces) from the VENOUS PLEXUS (a vascular network) surrounding the vaginal wall, which is squeezed through the spaces in the vaginal wall cell tissue during arousal by the engorgement of this plexus (44) or from cervical secretion.
There is also a muscular layer at the area of the vagina which surrounds it near its entrance. This may grip the penis and facilitate intercourse, although it does not trap the penis. Involuntary spasms of this muscle may be painful and result in DYSPAREUNEA (painful intercourse), a condition known as VAGINISMUS.
The lower part of the vagina is usually presented as having folds, and the lower part of the vagina is the only part which is rich in nerve endings.
Part 5: Endocrinology
Endocrinology is the study of the endocrine glands and their interaction with other parts of the body. “Endo” means that these endocrine glands secrete their substances called HORMONES directly into the bloodstream.
The hormones may then be changed by components of target (receiving) cells, which receive their messages to fit the purposes or requirements of the cell.
The ability to express oneself sexually is largely the influence of sex hormones. As we have seen, they have their role in developing the genitalia.
However, they also have an important role in the development and maintenance in overall body shape, texture, and even on behavior, thinking and mood.
They are essential for the control of gametogenesis. There are four main sources of sex hormone production.
First, the gonads mainly produce hormones known as STEROID HORMONES.
The ADRENAL GLANDS in their CORTICAL region also produce steroid hormones.
The third source is the PITUITARY GLAND (a regulatory gland in the brain) which produces hormones that in turn regulate the gonads and the adrenals.
STEROIDS are complex molecules which are initially synthesized from CHOLESTROL. (45)
Many of the effects of hormones begin at PUBERTY. The resumption of the maturation of germ cells begins at puberty.
The HYPOTHALAMUS, an endocrine gland in the brain, begins to secrete a hormone called GONADOTROPIN RELEASING HORMONE (GnRH) as the child matures and enters puberty.
This causes another gland in the brain, known as the PITUITARY GLAND, to secrete LUTEINIZING HORMONE (LH), which will cause the typical pubescent male to resume the production of testosterone by the testes.
In typical females, LH will also cause a structure from a ruptured follicle known as the CORPUS LUTEUM to produce a hormone known as PROGESTERONE. A second hormone produced by the pituitary is called FOLLICLE STIMULATING HORMONE (FSH).
This causes the Sertoli cells in the testes to produce ANDROGEN BINDING PROTEIN (ABP), which will, when combined with testosterone, assist in spermatogenesis.
In the typical female, FSH will induce the ovaries to release estrogens and it will cause the ripening of the egg (oocyte) during oogenesis. The feedback system between ovarian and pituitary hormones in typical females is responsible for the regulation of the MENSTRUAL CYCLE, a process beginning at puberty, with a first period known as MENARCHE. The menstrual cycle begins with day 1 of the cycle, which averages 5 days, and lasts until about day 28.
It is here that an oocyte will be ovulated and the lining of the uterus matured to receive the fertilized egg.
If fertilization does not occur, the uterine lining, known as the endometrium, is not maintained in this matured state, but is shed along with menstrual blood. If fertilization does occur, the LH function is taken over by early embryonic tissue, which releases HUMAN CHORIONIC GONADOTROPIN (HCG).This will cause the PLACENTA (an organ which provides metabolic interchange with the fetus) to produce necessary progesterone for the duration of the pregnancy.
SECONDARY SEX CHARACTERISTICS are those which are believed not to relate directly to reproduction. The sex hormones are largely responsible for secondary sex characteristics. In females this includes fatty development which enlarges the breasts. This is not seen in many other primates and is believed not to be related to the function of the breasts in producing milk, a phenomenon known as LACTATION. This requires development of the ALVEOLI, ACINI, A DUCTAL SYSTEM and GLANDULAR DEVELOPMENT (which are functional parts of the breasts for lactation). Although it was previously believed that this is present in humans before birth as appears in some lower mammals (47), this is not the case and some typical males have been able to get a totally normal breast development histologically and functionally.(48)
Hormones that are responsible for the functional development of lactation include PROLACTIN, produced by the anterior pituitary gland. Other secondary sex characteristics in typical pubertal girls include the development of soft skin and the deposition of subcutaneous fat in the area of the hips. The skeleton also has features unique to males and females, especially with regard to the width of the pelvis, facilitating childbirth. (49) Most pelvic features develop their sexual differences after puberty. (49) However, there are two pelvic differences which appear before birth. One is called the GREATER SCIATIC NOTCH and the other the SUBPUBIC ANGLE. (49) However, there is enough overlap that in 10% of the population the sex would not be able to be determined, even if one had the entire pelvis. (50)
There are also skeletal differences sexually in the jaw. However, again, if one has the entire skeleton, one could only accurately predict the sex in 98% of the population due to the amount of overlap. (50)
Secondary sex characteristics in pubertal males include the development of facial hair, increased muscle mass, and a deepening voice.
The measurement of sex development during puberty is called the TANNER SCALE. (51) It measures especially the development of the breasts and of the pubic hair.
Hormones are believed to be related to sexual desire (libido) (52) and as we will discuss in an upcoming module, also of sex differentiation of the brain, gender identity, and gender role behavior.
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51) Tanner, JM (1962) Growth and Adolescence (2nd edition). Oxford, England.
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