Lecture 2 Human Development and Biologic Sex

Indifferent stage

fertilization to 6 weeks, XX and XY fetuses have same body characteristics

Wolffian ducts

Ducts found in both male and female fetuses; in females they degenerate, and in males they develop into the epididymis, the vas deferens, and the ejaculatory duct.

Indifferent external structures at 6 weeks of development

-genital tubercle, labioscrotal swelling, urethral slit, and urethral folds
-see slide 11 of lecture 2

Chromesomes

-Humans have 23 pairs, total 46
-First 22 pairs are autosomes
-Pair number 23 are the sex chromosomes
-are packages of genes (instructions for our cells)
-half from sperm (haploid) , half from egg (haploid) = together form a zygote (diploid)

Sex Chromosomes

X and Y

X Chromesome

-the chromosome needed to live
-does not dictate sexual development

Y Chromesome

-the smallest chromosome, not necessary for life
-several important genes for male development
-SRY: Sex Determining Region, if present, indifferent gonads become testes
-other genes on Y chromosome are important for sperm production (spermatogenesis)
-genes that encode physical features (stature, tooth size, hand & foot size.)

Two main options for chromosomal sex

-XX=female (no SRY gene present)
-XY=male (SRY gene present)
-determined at fertilization
-Always get an X from the egg and can get either X or Y from the sperm

Embryogenesis

-The first 8 weeks of fetal development
-regardless of chromosomes, all embryos start out the exact same
-indifferent stage: up to 7 weeks of development, internal and external reproductive structures are the same in male and female embryos

Male development: Gonads

-if SRY genes is present, tells the gonads to become testes
-Leydig cells make testosterone (testosterone feeds Wolffian ducts)
-Sertoli cells make MIS (MIS inhibits Mullerian ducts)
-Germ cells become spermatogonia (baby sperm)

Male development: Internal

-Leydig cells make testosterone (testosterone feeds Wolffian ducts to help develop internal structures like epididymis, vas deferens, seminal vesicles, & ejaculatory duct)
Sertoli cells make MIS (MIS inhibits Mullerian ducts and causes them to degenerate)
-Prostate and bulbourethral glands develop from urogenital sinus as do the bladder and urethra
-see slide 36 from lecture 2

Male developement: Genitalia

-Testosterone causes the external genitalia to masculinize

-Genital tubercle becomes the glans and corpora cavernosa (top side) of penis

-Urogenital folds become corpus spongiosum (underside) of penis, through which the urethra will travel

-Urethra develops from urogenital sinus

-see slide 37-39 from lecture 2

Male development: testicular descent

-later in fetal life, or in the first few months after birth, testes descend out of the abdomen and down into the scrotum

Female development: gonads

-Occurs if no Y chromosome (in particular no SRY gene)

-Gonads become ovaries at about 12 weeks

-germ cells differentiate into oogonia (immature eggs), then into oocytes

Female development: ducts

-no testosterone- Wolffian ducts degenerate

-no MIS-Mullerian ducts differentiate into: fallopian tubes, uterus, & upper 1/3 of vagina

-Urogenital sunus (cloaca) forms: bladder, urethra, & lower 2/3 of vagina

-see slide 43 of lecture 2

Female development: genitalia

-genital tubercle becomes clitoris
-labioscrotal swelling becomes labia majora
-urogenital folds become labia minora
-urogenital sinus becomes lower 2/3 of vagina and urethra
-see slide 44 of lecture 2

Intersex conditions

disorders of sexual development

XY Intersex: Androgen Insensitivity Syndrome

-Normal XY and normal testes with normal testosterone levels
-the body, however, has no receptors for testosterone and cannot "see it"
-therefore, external genitalia develop as female and the testes never descend out of the abdomen
-they do make and see MIS, so no fallopian tubes, uterus, cervix, or upper 1/3 of vagina
-diagnosed when woman fails to menstruate

XX Males

-Have XX, but somehow the SRY gene is abnormally present

-Gonads develop into testes

-leydig cells make testosterone (development of male internal and external reproductive structures)

-sertoli cells make MIS (Mullerian ducts degenerate and do not develop)

-only missing some of the other genes found on Y chromosome os usually have small hands, feet and are shorter than other males

-may never come to medical attention and can be undiagnosed

True Gonadal Intersex

-may be XX or XY

-have both ovarian and testicular tissue

-varies anatomically depending on what hormones are produced and in what quantities

5 alpha-reductase deficiency / Guevedoces

-XY chromose, normal testes and normal testosterone levels
-don't see testosterone well so develop external female genitalia
-secrete MIS so no uterus, fallopian tubes or upper vagina
-at puberty develop male characteristics

congenital adrenal hyperplasia

-normal XX female characteristics

-adrenal gland is overdeveloped, resulting in a deficiency of certain hormones and an overproduction of others

-overproduce testosterone

-external genitalia become masculinized

Male anatomic disorders: Cryptorchidism

-undescended testicle

-can be normal finding in newborns

-can be treated with surgery

Male anatomic disorders: Hypospadias

-failure of fusion of the urogenital folds

-urethra opens onto underside of penis

Female anatomic disorders: Mullerian duct disorders

-partial or complete failure of fusion of the two ducts

-result in bicornuate uterus or uterus didelphys

-see slide 70 of lecture 2

Sex chromosome disorders: Turner's Syndrome

-XO female

-no second sex chromosome

-broad chest, webbed neck, short stature, and kidney problems

-infertile

Sex chromosome disorders: Klinefelter's Syndrome

-XXY Male

-develop breasts, have small male genitalia, undescended testes, and intellectual disability

-may also have hypospadias

Sex chromosome disorders: Supermales

-XYY Male
-tend to be tall and have acne
-low intelligence and social problems

Sex chromosome disorders: Fragile X syndrome

-X chromsomes has an abnormally long, fragile arm

-can occur in males and females, but symptoms more severe in males

-mental retardation, large ears, long face, behavioral and movement disorders