4.1 Sexual Reproduction in Humans

Scrotum

External sac of skin containing the testes

Testes

Produce gametes and testosterone

Epididymis

Sperm are stored here and mature to become fully mobile

Vas deferens

Carries sperm towards the penis during ejaculation

Seminal vesicle

Secretes seminal fluid into the vas deferens

What does seminal fluid provide

Nutrients for sperm such as amino acids

Why is seminal fluid alkaline

Helps to neutralise the acidity of any urine remaining in the urethra and the acidity of the vaginal tract

What does the prostate gland do

Secretes prostate fluid into the vas deferens

Why is prostate fluid alkaline

Helps to neutralise the acidity of any urine remaining in the urethra and the acidity of the vaginal tract

Urethra

Carries semen and urine out of the body

Penis

Specialised organ adapted to transfer semen to the vagina during sexual intercourse

What is produced in the ovaries

gametes

oestrogen and progesterone

What do fallopian tubes have a lining of and why?

Ciliated epithelial cells which move the secondary oocyte to the uterus

Uterus

Holds the developing foetus until birth

Endometrium

The inner most layer of the uterus wall. It has a good blood supply and builds up every month during the menstrual cycle. If implantation of an embryo does not happen then the endometrium is shed during menstruation

Cervix

A narrow ring of connective tissue and muscle

It acts as a barrier between the uterus and the outside environment during pregnancy

During pregnancy, a mucous plug forms in the cervix which helps prevent entry of pathogens

Vagina

It has muscular walls and opens at the vulva
Semen is deposited in the vagina during sexual intercourse and the foetus is able to pass out from the uterus through the vagina during birth

Where does spermatogenesis take place

Seminiferous tubules

Process of spermatogenesis

Germinal epithelium cells divide by mitosis to produce spermatogonia

Spermatagonia divide many times by mitosis and increase in size to form primary spermatocytes

Primary spermatocytes undergo meiosis to form secondary spermatocytes

Secondary spermatocytes undergo meiosis to form spermatids

Spermatids mature and differentiate into spermatozoa

what do Sertoli cells provide

nourishment for spermatids

protection against the male's immune system

What surrounds the seminiferous tubule

Basement membrane of connective tissue

Interstitial cells

Secrete testosterone which stimulates spermatogenesis

What does the acrosome contain

Protease enzymes

Where is the mitochondria in sperm cells

Spiraled around microtubules

What does the mid section of sperm contain

Microtubules that are responsible for movement of tail.

Large numbers of mitochondria to provide ATP

Where is the axial filament

In the tail

Tail

Whiplash movement of tail propel the spermatozoa

Oogenesis

Production of ovum

Process of oogeneis

Germinal epithelium divide by mitosis to produce oogonia

Oogonia divide many times and enlarge to produce large number of primary oocytes

Primary oocytes present at birth, but development delayed at prophase I

Primary oocytes undergoes meiosis I to produce secondary oocytes just before ovulation

Meiosis II stops at metaphase II

Meiosis II completed in fertiliation occurs to produce an ovum

Follicle

A ball of diploid cells surrounding Graafian follicle and oocyte.

The cells of the follicle develop and increase in numbers as the oocyte develops

Graafian follicle

Contains secondary oocyte ready for ovulation

Primary follicle

Contains primary oocyte

Corpus luteum

If fertilisation occurs, it starts secreting progesterone and oestrogen

Ovulation

Secondary oocyte being released

Process of fertilisation

Following sexual intercourse, spermatozoa move into the fallopian tubes

Capacitation increases the permeability of the cell membrane in the head of the sperm

Acrosome reaction releases hydrolase enzymes which digest the zona pellucida

Fusion of sperm and secondary oocyte membranes

Genetic material of sperm cell enters the secondary oocyte triggering completion of meiosis II and formation of ovum and second polar body

Cortical reaction in which cortical granules fuse with the cell membrane and modify the zona pellucida to form the fertilisation membrane; this prevents polyspermy

Nuclei of the sperm and ovum fuse to form a zygotic nucleus

Cleavage

The zygote undergoes repeated mitotic divisions as it moves down the oviduct to form a ball of cells called the blastocyst

Implantation

The blastocyst moves into the uterus where it attaches and sinks into the endometrium. Cells on the outside of the blastocyst, the trophoblast cells, form trophoblastic villi that penetrate the endometrium. The villi increase the surface area for the absorption of nutrients from the endometrium

Formation of the placenta

The placenta begins to develop from the trophoblast cells

What does the placenta exchange with the foetus

nutrients

waste products

oxygen and carbon dioxide

Role of the placenta in providing a barrier between maternal and foetal blood

Protects foetal capillaries from higher blood pressure and changes in blood pressure of the mother

Cells of the chorionic villi fuse together preventing the mother's phagocytes from passing into the foetus.

The mother's antibodies are small enough to cross into the foetal blood and provide passive immunity to the foetus

Role of the placenta in secretion of hormones

Following implantation the placenta takes over secretions of HCG from the blastocyst

This maintains the corpus luteum and its secretions of progesterone and oestrogen

As the placenta develops it takes over secretions of progesterone and oestrogen from the corpus luteum

Amniotic fluid

Acts as shock absorber protecting the

foetus from injury during development

Adaptations of chorionic villi

Microvilli that increase surface area

Thin walls therefore distance for diffusion is short

A counter-current flow of blood between foetal and maternal blood that maintains the concentration gradient

Role of umbilical arteries and veins

Transport materials between the foetus and mother

Intervillious spaces

Contain mother's blood surrounding chorionic villi

Maternal and foetal blood do not mix

Menstrual Cycle

1. FSH secreted by the anterior pituitary gland stimulates the maturation of a follicle and stimulates the production of oestrogen

2. Following menstruation, the level of oestrogen secreted by the developing follicle increases, which triggers the repair of the endometrium - this inhibits FSH production and stimulates LH production

3. High level of LH initiates ovulation - Graafian follicle develops into corpus luteum

4. Progesterone causes the development of the endometrium

5. If implantation does not occur, falling FSH and LH levels cause the corpus luteum to degenerate, progesterone levels fall, the endometrium breaks down and is lost in menstruation

Pregnancy

1. The developing embryo secretes HCG, which maintains the corpus luteum for 16 weeks

2. The placenta then secretes progesterone and oestrogen, which rise to high levels

3. FSH and LH are inhibited

4. Progesterone suppresses the uterine wall’s ability to contract

5. Oestrogen stimulates the growth of the uterus and mammary glands

Birth

1. Oestrogen levels increase and progesterone decreases - the uterine wall can now contract

2. Oxytocin, secreted by the posterior pituitary gland, stimulates contraction which stimulates the secretion of more oxytocin

3. Prolactin is released during and after birth to stimulate the production of milk