BIOM 3200 Lecture 6: 6 Reproduction

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BIOM3200 Reproduction
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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
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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
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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
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
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