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