Introduction to reproduction L14 Flashcards
Hypothalamus
Homeostatic regulator for reproduction, stress, body temp, hunger, thirst and sleep. Neuroendocrine organ which processes both neural and hormonal information.
Neurosecretory peptide hormones
Oxytocin and antidiuretic hormone (ADH or vasopressin)
Oxytocin and ADH
Synthesised in hypothalamus and transported to posterior pituitary, where they are stored and released.
Water soluble hormones (Peptides and proteins)
Gonadotrophin releasing hormone (GnRH) - Hypothalamus
Follicle-stimulating hormone - Anterior pituitary
Luteinising hormone - Anterior pituitary
Oxytocin - Posterior pituitary
Lipid soluble hormones (Steroid hormones)
Androgens - Testes
Oestrogens - Ovary
Progestagens - Ovary
Negative feedback
A mechanism which involves signalling between the hypothalamus, the pituitary and the target organ.
Regulation of pituitary secretion cells
Regulation of gonadotrophin secretion
Hormone FSH in females
Stimulates the growth and maturation of ovarian follicles, leading to estrogen production.
Hormone FSH in males
Stimulates Sertoli cells in the testes to support spermatogenesis (sperm production).
Hormone LH in females
Triggers ovulation by causing the release of a mature egg from the dominant follicle. Also supports the formation of the corpus luteum, which produces progesterone.
Hormone LH in males
Stimulates Leydig cells in the testes to produce testosterone, which is essential for sperm maturation and male secondary sexual characteristics.
Three major classes of sex steroid hormones
Androgens, estrogens and progestogens.
Tolestrone
Main secretory product of the testis, associated with the development and maintenance of male characteristics and fertility.
5 alpha Dihydrotestoterone (DHT)
More active form of testosterone which is involved in puberty and maintaining male structures throughout a lifetime.
Key properties of testosterone
Male sex development
Spermatogenesis
Sexual behaviour
Muscle development
Androgens
Group of reproductive hormones most abundant in males, produced from the adrenal cortex through adipose tissue. Important for hair growth, libido, bone health and muscle health.
Types of Oestrogens
Oestradiol, oestrone, oestriol
Main site of oestrogen production (and Oestradiol)
Granulosa cells of the growing follicle
Oestrogens
Development and maintenance of female characteristics and fertility
Oestrone
Weaker oestrogen produced in the adipose tissue for both males and females. Main source of oestrogen for males.
Oestriol
Weak oestrogen important in the maintenance of pregnancy, helps soften the cervix in childbirth.
Key properties of oestrogens
Female sex development
Regulation of the menstrual cycle
Growth of the endometrium (place where implantation of the embryo occurs)
Bone growth
Progesterone
Major steroidal hormone of the corpus luteum and the placenta only for females - produced after ovulation. Associated with the preparations for pregnancy and maintenance.
Type of progestagens
Progesterone
Sex determination
Commitment of the bipotential gonad to a testis or an ovary
Sex differentiation
Phenotypic development of genital structures due to the action of hormones produced by the gonad.
Y chromosome
Male gonads develop (testes) XY
Y chromosome absent
Female gonads develop (ovaries) XX
Sex Determination Factor
Sex-determining region (gene) on the Y chromosome provides the pathway for testes to develop. The presence of testis determines the sexual fate of the embryo, against a trend to become female. In the absence of SRY, the embryo develops into a female.
Sex determination
Female internal genitalia
Mullerian duct
Male internal genitalia
Wolffian duct
Difference between male and female sex systems
Two hormones driving the sex formation process in males, while oestrogens and progestagens are present and around but do not drive the development of the Mullerian duct system, it happens by itself, and the wolffian duct systems also regress by themselves. As the male system is being driven by hormones, it is happening faster and the female system lags behind slightly.
Male sex system
Female sex system
Development of the male internal reproductive system
Mullerian duct regression occurs under the control of Anti-mullerian hormone (AMH) secreted by sertoli cells.
Testosterone secreted by the testis (Leydig cells) actively maintain the Wollfian ducts.
The Wollfian ducts develop into epididymis, vas deferens, and seminal vesicles.
The testis descends from its internal position (high up in the pelvic cavity) to the scrotum, usually after the 7th month.
Development of the female internal reproductive system
The Wollfian ducts begin to regress slowly from about 10 weeks. The Mullerian ducts persist and develop to give rise to the uterine (fallopian) tubes, uterus, cervix, and upper vagina.
Female differentiation (development of the external genitals)
The urethral folds and labioscrotal swellings remain separate, thus forming the labia minor and majora.
The genital tubercle (glans area) forms the clitoris.
Development of the external genital (male and female external genitalia develop from a single bipotential precursor)
Male differentiation (development of the external genitals)
Fusion of the urethral folds enclosing the urethral tube forming the shaft of the penis.
Labioscrotal swellings fuse in the midline forming the scrotum.
The genital tubercle (glans area) expands forming the glans penis.
Androgen insensitivity syndome
The person has testes but the genital ducts and/or external genitals are female. The person has a mutation in their androgen receptor gene which prevents androgen function.
In the absence of androgen action, the external genitalia will appear female.
Puberty
The physical, emotional, sexual transition from childhood to adulthood. The transition is gradual, and it is punctuated by well-defined events and milestones.
Hormonal changes at puberty
Behavioural responses and physical changes
Plasma levels of gonadotrophins during childhood
Very low until the initiation of events leading to puberty.
Secondary sexual characteristics
Characteristics develop at different chronological ages in different individuals. The sequence in which the changes occur are quite characteristic for each sex. Staging criteria (Tanner stage) allows abnormalities to be detected, and comparisons made between individuals.
Females: sequence of events during puberty
Breast budding -> Pubic hair -> Height spurt -> Menarche
Breast development
First physical sign of secondary sexual maturation at age 10-11.
Oestrogen secretion leads to the appearance of a breast bud, followed by formation of a breast mound.
Ovulation, with subsequent progesterone secretion, leads to full breast development.
Sexual hair development
Usually within 6 months of the appearance of the breast bud at age 10-12.
Due to exposure of hair follicles to androgens.
Axillary (armpit) hair follows -1 year after pubic hair.
Growth spurt
Growth is stimulated by steroid hormones (oestrogen and androgen), with epiphyseal closure (bony ends) by oestrogen. Occurs in females 11 to 12, and males age 13-15.
Menarche (first menstrual period)
Occurs at an average age of 12-13 years. The first ovulation does not take place until 6-9 months after menarche, as the positive feedback mechanisms of oestrogen have not developed.
Regular ovulatory cycles are established 1-2 years after menarche.
Males: sequence of events during puberty
Testis -> Pubic hair -> Penis -> Height spurt
Testicular and penile enlargement
First signs of secondary sexual development is enlargement of the testicles.
Leydig cells enlarge and secrete testosterone, leading to increased testicular size.
Elongation and enlargement of the penis begins within year of testicular enlargement.
Physical changes in males and females
Body shape in males and females is determined by the differential affects of androgen and oestrogen
Physical changes in males
Sexual hair growth - pubic hair appears about 6 months after the beginning of testicular enlargement. Axillary hair begins 18 months later, and facial hair later.
Spermache - motile sperm is seen in urine at 13-14 years.
First ejaculation occurs soon after.
Timing of puberty
The age at which females first menstruate shows a clear trend towards earlier menarche.
In females, a critical weight must be attained before the activation of the hypothalamo-pituitary-gonadal axis can occur,
Mean weight of females at the beginning of menarche is ~47 kg.
Decreasing trend of menarche in females
Attainment of a critical weight due to improvements in nutrition, health care, and social living conditions.
Signifies that sufficient storage is required to sustain pregnancy and lactation.
Precocious puberty
The appearance of the physical & hormonal signs of puberty before:
7 years in females
9 years in males
Usually a GnRH dependent problem, often extreme cases are due to a hypothalamic tumour
Delayed puberty
The lack of appearance of the physical and hormonal signs of puberty
~13 years in females
~14 years in males
Occurs when the gonadotrophin signals from the pituitary are inadequate for sex steroid hormone secretion.
Menopause
The menopause is the consequence of the ovaries running out of follicles, and occurs between 50 and 52 years of age.
The last episode of natural menstrual bleeding signifies the end of her reproductive life and is referred to as the menopause. (Ovaries running out of follicles)
Phases associated with the end of reproductive life in females
By approximately one year after menopause, the ovary has essentially ceased producing hormones (ovarian senesence). Oestrogen production reduces to approximately _____. The oestrogen which is now produced is known as _____ which arises mainly from the stromal cells of ____ tissue.
By approximately one year after menopause, the ovary has essentially ceased producing hormones (ovarian senesence). Oestrogen production reduces to approximately 10% . The oestrogen which is now produced is known as oestrone which arises mainly from the stromal cells of adipose tissue.
