reproductive physiology. what will be covered: gametogenesis how gametes form sexual differentiation...
DESCRIPTION
Gametogenesis This is the formation of gamete cells via meiosis Gametes are specialised as they contain only half of the genetic material required to reproduce; this is so they can bind with another gamete via sexual reproduction In meiosis 4 haploid gametes are formed from a diploid oogonium (female) or spematogonium (male) The process differs slightly between oocytes and spermatocytesTRANSCRIPT
Reproductive Physiology
What will be covered:• Gametogenesis• How gametes form• Sexual differentiation
• Pituitary-gonadal axis• Female reproductive physiology• Ovarian cycle• Uterine cycle• Hormonal control and changes
• Male reproductive physiology
Gametogenesis• This is the formation of gamete cells via meiosis
• Gametes are specialised as they contain only half of the genetic material required to reproduce; this is so they can bind with another gamete via sexual reproduction
• In meiosis 4 haploid gametes are formed from a diploid oogonium (female) or spematogonium (male)
• The process differs slightly between oocytes and spermatocytes
Spermatogenesis• During development germ cells are produced,
however these remain inactive until puberty• Hormones act on germ cells, causing them to
undergo mitosis and meiosis to form sperm cells
Stages of sperm production:1. Spermatocytogenesis2. Spermatidogenesis3. Spermiogenesis
SpermatogenesisSpermatogonium undergo mitosis to ‘double up’ DNA, ready for meiosis. The spermatocytes undergo meiosis twice to produce four spermatids each with one set of DNA, ready to mature into sperm and
eventually bind with an ovum in sexual reproduction
Mitosis
Meiosis I
Meiosis II
Spermatogonium
Primary spermatocyte
Secondary spermatocyte
Spermatid
Sperm
Oogenesis• Oogenesis produces a secondary oocyte which is
released during the ovulatory stage of the menstrual cycle
• If no fertilisation occurs it will not undergo further meiotic division (i.e. will not undergo meiosis II on diagram)
Stages of oocyte production:1. Oocytogenesis – finished by birth2. Ootidogenesis3. Fertilisation of ovum
OogenesisThe primary oocyte divides by meiosis to produce a secondary oocyte and a polar body
(these are non-functioning waste products). Meiosis II occurs after fertalisation.
Mitosis
Meiosis I
Meiosis II
Oogonium
Primary oocyte
Secondary oocyte
Ootid
Ovum
Primary polar body
Secondary polar body
Sperm
Further differences in male vs female gamete productionMale Female• Continuous production of
sperm from puberty indefinitely
• Stem cells are retained• Sperm cells are mobile with
little cytoplasm
• All oocytes are produced before birth
• Stem cells are used up• Sperm cells are large, and
are immobile• Oocytes are surrounded by
follicular cells, and together form a follicle
Gender Determination• Chromosomes determine gender• Humans have 23 chromosomes donated by the
egg, and 23 donated by the sperm• Two gametes fuse to form a zygote. This requires
fusion of both the cytoplasm and the genetic material
X X X Y
XX XY XX XY Zygotes
Non-disjunction• Occasionally, zygotes form without ‘normal’
separation of X/Y chromosomes. This is non-disjunction in meiosis I or II
• This can result in monosomy (only one sex chromosome) or polyploidy (more than two sex chromosomes)
• Monosomy: XO = Turner Syndrome
• Polyploidy: XXY = Kleinfelter SyndromeXYY = Jacob Syndrome
XXX = Trisomy X
Turner Syndrome (XO)Females have typical phenotype of short stature, neck skin folds, primitive sexual characteristics, no menstruation and typical facial features
Kleinfelter Syndrome (XXY)Males with small testes, infertility, feminine sexual characteristics, normal IQ
Jacob Syndrome (XYY)Males with above-average height and reduced IQ
Trisomy X (XXX)Females who are normal, and are only identifiable by karyotype
Gender Determination• At week 7 of embryonic development the SRY gene is activated
(SRY = sex-determining region of Y chromosome)
• As the name suggests, it is only present in males
• The SRY gene determines the development of sexual characteristics• Absence of SRY causes gonads to become ovaries• Presence of SRY causes gonads to descend down the Wolffian duct and
become testes.
• The testes produce anti-mullerian hormone which causes the mullerian duct to degenerate. The Wolffian duct becomes the vas deferens
• The ovaries then produce female hormones and there is an absence of testosterone, so the Wolffian duct degenerates. An absence of anti-mullerian hormone allows the Mullerian duct continue developoing to become fallopian tubes
Pituitary-Gonadal Axis• The hypothalamus produces gonadotrophin-
releasing hormone (GnRH)
• GnRH stimulates the release of follicle-stimulating hormone (FSH) and lutenising hormone (LH) from the anterior pituitary gland
• FSH acts on ovaries/testes to stimulate them to produce gametes
• LH acts on endocrine cells to stimulate them to secrete steroid and peptide hormones
• This is a negative feedback system
Female Reproduction• The pituitary-gonadal axis controls menstruation
in females
What the hormones do
Oestrogen: produced by developing follicle. Stimulates LH surge.
Progesterone: maintains endometrium for implantation.
LH: oocyte maturation and subsequent release from follicle.
FSH: stimulates a few follicles to complete meiosis.
Menstrual Cycle• Lasts 28days ( range is 24-35days)
• Starts with shedding of endometrium (period) and release of FSH
• Involves a ovarian cycle and a uterine cycle• Ovarian cycle
• Development of ovarian follicle• Production of hormones• Ovum release
• Uterine cycle• Shedding of endometrium prior to ovum release• Preparation for implantation of potential embryo via hormones
• These cycles are linked by hormone production/release
The CyclesOvarian cycle: follicular phase (follicle develops), ovulation phase (follicle released),
luteal phase (unfertalised follicle degenerates)Uterine cycle: menses (endometrium sheds), proliferative phase (endometrium builds
up), secretory phase (endometrium secretes progesterone for potential zygote)
If Fertalisation Occurs…• The endometrium lining is maintained by:• Progesterone from the corpus luteum • Human chorionic gonadotrophin (hCG) until w7• Progesterone from the placenta from w7
• The placenta also produces oestrogen (breast development) and human placental lactogen (milk production)
• Birth is stimulated by:• Increased corticotrophin-releasing hormone from the
placenta for 2weeks prior to birth• Reduced progesterone, increased oxytocin (uterine
contraction), increased inhibin (cervical relaxation)
Male Reproduction• The aim of the male reproductive system is to
produce, maintain and transport viable sperm
• It also produces hormones which:• Develop secondary sexual characteristics• Offer feedback for spermatogenesis
• Parts of the male reproductive system:• Testes• Accessory glands
• Prostate• Seminal vesicles• Bulbourethral glands
Testes• Site of sperm production
• Divided into lobules, each with one seminferous tubule
• Seminferous tubule maintains a suitable environment for spermto develop by…• Blood-testis barrier• Compartments,
therefore low glucose/high hormone
• Hormone production
Testes cellsSertoli cells
Anti-mullerian hormone
Secreted in embryology, causes Mullerian duct degeneration
Inhibin Regulates FSH releaseAndrogen-binding hormone
Binds to testosterone/DHT and reduces their loses, increasing testosterone activity
Estradiols/Aromatase Support spermatogenesisLeydig cells
Testosterone Spermatogenesis and secondary sexual characteristics
(stimulated by LH to produce testosterone)
FSH increases response to LH
Spermatogenesis Hormonal Control
GnRHhypothalamus
FSHant. Pituitary
Androgen-binding peptide release
Sertoli cells
Inhibin releaseSertoli cells
-ve feedback loop
LHant. pituitary
TestosteroneLeydig cells
Spermatogenesis
Secondary characteristics
Males and Reproduction• The role of the male in reproduction is to deliver
sperm to the vagina• This requires an erection reflex• Erotic stimuli stimulate the autonomic nervous
system, which vasodilates penile arterioles• This causes blood to flood the penile tissue, causing
an erection• Following the erection reflex an ejaculation
reflex is required• Emission (movement of sperm from vas deferens
into urethra, along with seminal fluids) is stimulated by the sympathetic nervous system
• Strong muscle contractions (bulbospongiosus) due to a spinal reflex cause expulsion of semen - ejaculation
ProblemsSometimes problems occur with this:
• Erectile dysfunction = unable to initiate an ejection or maintain it until ejaculation
• Premature ejaculation = male reaches orgasm and ejaculates too quickly for partner to achieve enjoyment
• Prolonged ejaculation = inability for male to orgasm and ejaculate