BIOD27H3 Lecture : Textbook notes-Chapter 26
11 Aug 2010
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Chapter 26
Puberty: The period when a person makes the transition from being non-reproductive to
being reproductive
Pseudohermaphroditism: males having underdeveloped male external sex organs through a
gene causing deficiency in one of the male hormones (DHT); develop some but not all male
characteristics
x Humans are sexually dimorphic in that males and females are physically distinct
Sex Determination
The male and female sex organs consist of three sets of structures:
x gonads: organs that produce gametes
-male gonads are the testes that produce sperm
-female gonads are the ovaries that produce eggs
-male and female gonadal cells are undifferentiated as germ cells before differentiating
x internal genitalia: consists of accessory glands and ducts that connect gonads with
outside environment
x external genitalia : external reproductive structures
Other than the eggs and sperm, each nucleated cell contains 46 chromosomes in which 22 pairs
are autosomes (human body form) and one pair of sex chromosomes (sex organs)
The Sex Chromosomes Determine Genetic Sex
x Males inherit a Y chromosome from their father and an X chromosome from their
mother
x The presence or absence of a Y chromosome determines whether one is a female or
male
x Absence of an X chromosome causes death in a zygote because of essential genes
dµv[Çv}uWfemale has XO chromosome bu}v[µvP}v}uo(uo
reproductive functioning
x Once ovaries develop, one X chromosome inactivates and become nuclear chromatin
called Barr body
Sexual Differentiation Occur in the Second Month of Development (refer to Figure 26.3)
x Before the seventh week of development, the embryo tissues are considered
bipotential because they are neither male or female
x Bipotential internal genitalia consists of two pairs of accessory ducts called the Wolffian
duct and the Mullerian duct
x As development proceeds, one pair of ducts develops while the other degenerates
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x The bipotential external genitalia consist of a genital tubercle, urethral folds, urethral
groove, labioscrotal swellings (Figure 23.6b)
x Gender determination depends on the presence or absence of the SRY gene on the Y
chromosome (absence of the gene, gonads develop into ovaries)
Male Embryonic Development (refer to Figure 26.4)
x SRY gene activates additional genes that cause the gonadal medulla to develop into
testes ~}}v]v[t necessary; develops after testes are formed)
x The testes secrete three hormones that influence development of internal and external
genitalia
-Sertoli cells secrete anti-Müllerian hormone (AMH): Mullerian ducts regress
-Leydig cells secrtete testosterone and DHT: internal and external sex organ
development
x Testosterone also controls migration of the testes from the abdomen into the scrotum
Female Embryonic Development
x Female embryos have no SRY gene, so the gonadal cortex develops into ovarian tissue
x Absence of AMH causes the Mullerian ducts to develop into the vagina, uterus, and
Fallopian tubes
x Wolffian ducts degenerate and external genitalia take on female characteristics
Basic Patterns of Reproduction
Eggs are the largest cells in the body and are non-motile; move through reproductive tract by
smooth muscle contraction and cilia
Sperm are small and the only flagellated cells in the body, making them motile
Gametogenesis: gamete production (timing is different between males and females)
x Women are born will all the oocytes(eggs) they will ever have; eggs are released once
per month for 40 years until ceasing (menopause)
x Men manufacture sperm continuously from when they reach reproductive maturity;
sperm production diminishes with age but d}v[
Gametogenesis begins in Utero and Resume During Puberty (refer to Figure 26.5)
x tZvu]}]P]vUZoo[Eo]}µ]Àovðòµo]
chromosomes
x In the first meiotic division, the duplicated chromosomes remain paired as sister
chromatids so each secondary gamete contains 23 doubled chromosome
x After the second meiotic divison, secondary gametes divide and each gamete results in
two cells that have 23 single chromosomes
Male Gametogenesis
At birth, the testes have not progressed beyond the mitosis stage and contain immature germ
cells (spermatogonia)
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x At puberty, germ cell mitosis resumes and spermatogonia can still undergo mitosis or
become primary spermatocytes (meiosis)
- First meiotic division: primary spermatocytes divide and become secondary
- Secondary meiotic division: secondary spermatocytes divide and become spermatid
- Spermatids mature into sperm
Female Gametogenesis
x Oogonia (ovarian germ cells) complete mitotic replication and first stage of meiosis in
the fifth month of fetal development; at birth, primary oocytes are present
x dZ}}Ç[(]u]}]]À]]}vlo]vµÇ
- Primary oocyte divides into a secondary oocytes(large egg) and a polar body that
disintegrates
- Egg begins second meiotic division and meiosis pauses until egg is fertilized
- Ovulation occurs when the mature egg is released to be fertilized; if not fertilized,
u]}]]v[}uovPP]]vP
- Final step occurs when sperm fertilizes the egg; half the chromatids are sent to the
second polar body, which degenerates
The Brain Directs Reproduction
x Peptide trophic hormones from the hypothalamus and anterior pituitary control
secretion of sex hormones (androgens, estrogens, progesterone)
x Sex hormones are steroid hormones in which both males and females have androgens
and estrogens (more androgens in males, more estrogens in females)
x Testosterone is converted into DHT
x Both testes and ovaries contain the enzyme aromatase, which converts testosterone
into the female sex hormone estradiol
Control Pathways
x GnRH is released from the hypothalamus to control secretion of the gonadotropins LH
and FSH from the anterior pituitary
x FSH initiates gametogenesis while LH stimulates production of steroid sex hormones in
endocrine cells
x Inhibins and activins peptide hormones are secreted by ovaries and testes to activate
or inhibit FSH secretion
Feedback Pathways
x Gonadal steroids alter secretion of GnRH, FSH, and LH in a long loop response
x Pituitary gonadotropins inhibit GnRH release by a short loop response
x Androgens always maintain negative feedback on gonadotropin release when at high
concentrations while estrogen maintains negative feedback at low concentrations
Pulsatile GnRH Release
x At puberty, tonic GnRH release from the hypothalamus occurs in males and females
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Document Summary
The period when a person makes the transition from being non-reproductive to. Pseudohermaphroditism: males having underdeveloped male external sex organs through a gene causing deficiency in one of the male hormones (dht); develop some but not all male characteristics. N humans are sexually dimorphic in that males and females are physically distinct. The male and female sex organs consist of three sets of structures: Male gonads are the testes that produce sperm. Female gonads are the ovaries that produce eggs. Male and female gonadal cells are undifferentiated as germ cells before differentiating internal genitalia: consists of accessory glands and ducts that connect gonads with outside environment. Other than the eggs and sperm, each nucleated cell contains 46 chromosomes in which 22 pairs are autosomes (human body form) and one pair of sex chromosomes (sex organs) N males inherit a y chromosome from their father and an x chromosome from their mother.
