BIOM 3200 Lecture 6: 6 Reproduction
1 May 2018
School
Department
Course
Professor
BIOM3200 – Reproduction
Pages: 701-748
Sexual Reproduction
• A particular gene on the Y chromosome induces the embryonic gonads to become testes
o Embryonic testes secrete testosterone, which induces the development of male
accessory organs and external genitalia
o The absence of testes in the female embryo causes the development of female
accessory organs
• Germ cells (gametes) are formed within the gonads by a process of reduction division
=meiosis
o Normal number of chromosomes (46) is divided into two so each gamete receives
23 chromosomes
o Fusion of the sperm and egg cell = fertilization restoration of 46 chromosomes
in the zygote (fertilized egg)
o Growth of the zygote occurs via mitotic divisions
o When the individual reached puberty, mature sperm or ova will be formed by
meiosis within the gonads
• Sex Determination:
o 46 chromosomes (diploid) 23 pairs of homologous chromosomes
▪ 1st 22 pairs = autosomes
▪ 23rd pair = sex chromosomes
• female: XX (one is inactive via X chromosomal inactivation=barr
body)
• male: XY
o X - ~1090 genes
o Y - ~80 genes (only 54 are homologous genes outside
are restricted to X chromosome in recombination X-
linked genes)
o Since all maternal eggs with have X, and paternal sperm with have either X or Y,
the chromosomal sex of the zygote is determined by the fertilizing sperm cell
• Formation of Testes and Ovaries:
o Following conception, the gonads of males and females are similar in appearance
for the first 40 days
▪ During this time, cells that give rise to the sperm or ova migrate from the
yolk sac to developing embryonic gonads
• Sperm spermatogonia cells
• Egg oogonia cells
▪ At this stage the embryonic structures have potential to become either
testes or ovaries
find more resources at oneclass.com
find more resources at oneclass.com
• The hypothetical substance that promotes their conversion to testes
=testis-determining factor (TDF)
o XY zygote + TDF in indifferent gonads testes
▪ seminiferous tubules - appear early; two different
cell types:
• germinal cells – eventually become sperm
through meiosis and subsequent
specialization
• nongerminal cells = Sertoli cells
▪ interstitial cells = Leydig cells
• appear in embryonic testes, clustered in the
interstitial tissue that surrounds the
seminiferous tubules
• constitute the endocrine tissue of the testes
o secrete large amounts of androgens
(testosterone)
o *as the testes develop they move
within abdominal cavity and
gradually descend into the scrotum
o *temperature of the scrotum is
maintained for spermatogenesis
o XX zygote with no TDF in indifferent gonads ovaries
▪ Functional units (ovarian follicles) do not appear
until second trimester
• The gene (SRY – sex-determining-region) for TDF is located on
the short Y chromosome
Development of Accessory Organs and External Genitalia:
• Most accessory organs are derived from two systems of embryonic ducts:
o Male organs are derived from the wolfflan (mesonephric) ducts
o Females organs are derived from the mullerlian (paramesonephric) ducts
o *two duct systems are both present in male and female embryos between day 25-
50 have potential to form accessory organs of either sex
▪ removal of the testes results in regression of the wolffian ducts and
development of mullerian ducts into female accessory organs (the uterus
and fallopian tubes) female organs develop as a result of testes absence
rather than presence of ovaries
• In males, the Sertoli cells of the seminiferous tubules secrete a mullerian inhibition factor
(MIF) that causes regression of the mullerian ducts ~60 days after fertilization
o Secretion of testosterone by Leydig cells subsequently causes the growth and
development of the wolffian duct into male accessory organs (epididymis, ductus
vas deferns, seminal vesicles and ejaculatory duct)
• The external genitalia of the male and females are essentially identical during first 6
weeks of development, sharing: urogenital sinus, genital tubercle, urethra fold and a pair
of labioscrotal swellings
find more resources at oneclass.com
find more resources at oneclass.com
o The secretion of the testes masculinize these structures penis + spongy urethra,
prostate and scrotum
▪ In absence of testosterone, genital tubercle (that would form penis)
clitoris
• Clitoris + penis = homologous structures
▪ Labioscrotal swellings scrotum in male or labia majora in female
*homologous
o Masculinization of the embryonic structures occurs as a result of testosterone
secreted by the testes:
▪ Once inside particular target cells, testosterone is converted by the
5(alpha)-reductase into active dihydrotestosterone (DHT)
▪ DHT is needed for the development and maintenance of the penism
spongy urethra, scrotum and prostate
Disorders of Embryonic Sexual Development
• Hermaphroditism – both ovarian and testicular tissue is present
o Results when some embryonic cells receive the short arm of the Y chromosome
(with SRY genes) and some do not
o Pseudohermaphrodites – accessory organs and external genitalia are incompletely
development or do not match chromosomal ssex
▪ * most common cause of female pseudohermaphroditism =congenital
adrenal hyperplasia (excessive secretions of androgens from adrenal
cortex – no MIF)
▪ * most common cause of male pseudohermaphroditism =testicular
feminizing syndrome (normally functioning testes but lack receptor for
testosterone OR lack ability to produce enzyme ; MIF is still secreted so
uterus and fallopian tubes do not develop)
Developmental Timetable:
Approx. Time After Fertilization
Developmental Changes
Days
Trimester
Indifferent
Male
Female
19
First
Germ cells migrate from yolk
sac
25-30
Wolffian ducts begin
development
44-48
Mullerian ducts begin
development
50-52
Urogenital sinus and tubercle
develop
53-60
Tubules and Sertoli
cells appear; Mullerian
ducts behin to regress
find more resources at oneclass.com
find more resources at oneclass.com
