Human Reproduction

Testis: held outside of body cavity in the scrotal sac to maintain a lower temperature for spermatogenesis

Seminiferous tubule: the site of spermatogenesis, between the tubules leydig cells produce testosterone

Epididymis: spermatozoa are stored and mature here prior to release at ***********

Vas deferens: the tubule down which sperm pass at ***********, seminal fluid is mixed with spermatozoa here

Urethra: the tube that leads to the outside for urine from the bladder and semen from the vas deferens

Prostate gland: produces and alkaline mucus rich secretion which is mixed with spermatozoa to neutralise the acidity of the ****** and aid sperm mobility

Seminal vesicles: produce a nutrient rich alkaline secretion to aid sperm mobility

Vagina: accommodates the penis during sexual intercourse

Cervix: prevents entry of foreign objects to the uterus

Uterus: region where the fetus develops

Endometrium: the lining of the uterus where the embryo is nourished after implantation

Myometrium: the muscular lining of the uterus which contracts to expel the fetus during childbirth

Fallopian tube: site of fertilisation

Ovary: produces ova in oogenesis and the female sex hormones oestrogen and progesterone

Fimbrae: 'catch' the oocyte released at ovulation and direct it into the fallopian tube

Spermatogenesis: the formation of spermatozoa in the seminiferous tubules

1. Germinal epithelium (2n) is located around the outside of the seminiferous tubules

2. It divides many time by mitosis to produce cells called spermatogonia (2n)

3. These then grow to form primary spermatocytes (2n)

4. Primary spermatocytes then undergo meiosis 1, to form secondary spermatocytes (n)

5. These then undergo meiosis 2 to form spermatids (n)

6. Spermatids then differentiate and mature into spermatozoa (n)

Sertoli cells: supply nutrients and O2, remove waste from the sperm cell, also protect the developing cells from the immune system response

Leydig cells: produce testosterone

The head: contains the nucleus which is the males genetic contribution to the zygote, and the acrosome - a vesicle containing hydrolytic enzymes used to gain access to the secondary oocyte

The midpiece: contain many mitochondria - to produce ATP for movement

The tail piece: a flagellum - for movement of the sperm

Oogenesis: the formation of secondary oocytes in the ovary. Oogenesis starts prior to the birth of a female fetus

1. Germinal epithelium (2n) divides by mitosis to form oogonia (2n)

2. These cells increase in size to form primary oocytes (2n)

3. Primary oocytes begin the process of meiosis 1 but stop at prophase 1

4. At puberty, each month, one of the primary oocytes completes meiosis 1, producing the secondary oocyte (n) and a polar body. Meiosis 2 begins but stops at metaphase 2

5. The polar body contains a set of chromosomes from the division, the secondary oocyte gains all the cytoplasm from the primary oocyte

6. The secondary oocyte is the cell which is release from the ovary and will be fertilised by a sperm

Follicle: the term given to the collection of follicle cells and the developing oocyte

The follicle cells surrounding the developing oocyte come from mitosis of the germinal epithelium. These cells are diploid (2n)

Primordial follice: contains the primary oocyte surrounded by a single layer of follicle cells

Primary follice: contains the primary oocyte and 2 layers of follicle cells and the zona pellucida is formed

Secondary follicle: contains the primary oocyte and larger layer of follicle cells, plus areas of fluid forming

Graafian follicle: contains the secondary oocyte and a large fluid filled space called the antrum

Ovulation: the Graafian follicle grows so big that the wall of the ovary bursts, allowing the secondary oocyte to be released

The layer of follicle cells surrounding the secondary oocyte is called the corona radiata, and the zona pellucida is a clear jelly-like layer of carbohydrates

The follicle cells remain in the ovary and form a structure called the corpus luteum which secretes progesterone

If the secondary oocyte is fertilised it completes meiosis 2 and releases a second polar body containing a set of chromosomes

During sexual intercourse an erect penis is inserted into the vagina of the female

The vagina widens and secretes mucus which lubricates it

Movements of the penis stimulate ***********

Semen containing sperm is deposited onto the cervix at the top of the vagina

The force of *********** is sufficient to propel some sperm through the cervix and into the uterus

Only a small proportion of sperm reach the fallopian tube

After *********** sperm can only fertilise the secondary oocyte after capacitation has taken place

Capacitation: the removal of glycoproteins from the cell membrane covering the acrosome. This membrane becomes more fluid and permeable to calcium ions Ca2+

Capacitated sperm swim more strongly

Once the sperm reaches the egg in the fallopian tube, it must fuse and penetrate it

The acrosome contains hydrolytic enzymes which can digest a pathway through the layer of follicle cells (corona radiata) and the zona pellucida of the secondary oocyte

As the sperm approaches the zona pellucida of the egg, the membranes surrounding the acrosome fuses with the plasma membrane of the oocyte, exposing the contents of the acrosome

The contents include numerous enzymes which are responsible for allowing the sperm to pass through the corona radiata and the zona pellucida to enable fertilisation to occur

The two main enzymes are: 

• Hyaluronidase - which digests hyaluronan (used to hold the corona radiata cells loosely together)
• Acrosin - which digests the zona pellucida

The cortical reaction prevents polyspermy (the entry of more than one sperm)

Within the secondary oocyte are cortical granules containing enzymes

When the sperm attaches to the secondary oocyte, it triggers the smooth endoplasmic reticulum of the oocyte to release Ca2+ into the cytoplasm

These ions cause the cortical granules to fuse with the membrane and release their enzymes

These enzymes react with the zona pellucida and cause it to harden - forming a fertilisation membrane, preventing other sperm from entering

1. Entry of the head of the spermatozoa restarts meiosis in the oocyte (from metaphase 2) and meiosis is completed

2. After 18-20 hours, the two nuclei combine to form a zygote (2n) - a zygote is a fertilised ovum, a single diploid cell

3. The zygote starts to divide by mitosis

4. Fertilisation takes place in the fallopian tube

1. The developing zygote slowly moves down the fallopian tube, taking approximately 3-4 days to reach the uterus. It is moved by cilia lining the oviduct

2. The zygote undergoes mitosis repeated times. The whole zygote does not increase in size, only in cell number, and this cell division is known as cleavage. It forms a solid ball of cells called the morula

3. The cells of the morula then rearrange themselves and the ball becomes hollow and is called the blastocyst. The cells continue to divide by cleavage. The cells on the outside layer of the blastocyst are called trophoblast cells, and some form the inner cell mass

4. The trophoblast cells will eventually form the placenta, and the inner cell mass will form the embryo

During all these evens the blastocyst is moving towards the uterus

5. After approximately 6-9 days, the blastocyst implants in the endometrium of the uterus. It does this by growing villi around its surface. These allow it to penetrate the endometrium layer and give it a larger surface area to absorb nutrients from the mother

Hormone

Where released from

Function

Other actions

FSH (Follicle stimulating hormone)

Anterior pituitary

Stimulates one follicle to develop

·        Stimulates oestrogen production early in cycle

·        Inhibits oestrogen late in cycle

LH (Lutenising hormone)

Anterior pituitary

Initiates ovulation

·        Causes the Graafian follicle to become the corpus luteum

·        Stimulates LH

·        Inhibits FSH

Oestrogen

Theca of the developing follicles; placenta

Rebuilding of endometrium

·        Inhibits FSH

·        Stimulates LH

Progesterone

Corpus luteum

Development of endometrium

·        Inhibits FSH

·        Inhibits LH

HCG (Human chorionic gonadotrophin)

Blastocyst and placenta

Maintain the corpus luteum

Oxytocin

Hypothalamus (released by posterior pituitary)

Stimulates smooth muscle contraction on the myometrium

Stimulates smooth muscle contraction in milk ducts

Prolactin

Anterior pituitary

Milk production

• FSH secreted by the anterior pituitary gland - triggers the maturation of a single follicle and secretion of oestrogen by the theca cells of this developing follicle
• Oestrogen levels increase as the follicle develops, triggering the repair of the uterus lining
• Increasing oestrogen levels inhibit FSH (to stop another follicle developing) and stimulates LH secretion from the anterior pituitary gland
• High LH levels trigger ovulation and cause the Graafian follicle to develop into the corpus luteum
• The corpus luteum then secretes progesterone which triggers the further thickening of the uterus lining ready for possible implantation if the oocyte is fertilised
• If there is no fertilisation/implantation, LH levels drop and as a result, progesterone levels fall
• Progesterone levels falling means that the endometrium lining is broken down and lost during menstruation
• As progesterone levels have dropped, there is no longer inhibition of FSH - so a new follicle can be triggered to start development. This starts the next menstrual cycle

• If there is fertilisation, the developing embryo begins to secrete Human Chorionic gonadotrophin (hCG). After implantation the hCG is secreted by the chorion layers of the placenta
• hCG causes the corpeus luteum to continue secreting progesterone, and maintains the endomedtrium during the first 16 weeks of the pregnancy
• hCG levels begin to drop as the placenta develops and begins to secrete progesterone and oestrogen
• The placenta develops and it secretes progesterone and oestrogen. These inhibit FSH and LH secretion
• Progesterone inhibits the contraction of the uterus
• Oestrogen stimulates the growth of the uterus, so it growns as the developing fetus grows. Plus, oestrogen stimulates the mammary glands to develop ready to provide milk to the young

• Oestrogen levels peak just before birth and are higher than the progesterone levels
• This removes the inhibition on the contractions of the uterus
• Oxytocin is released from the posterior pituitary gland and the fetus. This stimulates contraction of the myometrium (muscle layer of the uterus)
• There is a positive feedback loop - contractions of the myometrium cause more oxytocin release - then more contractions
• Prolactin is also secreted from the anterior pituitary gland of the mother during and after birth. Prolactin stimulates the production of milk by the mammary glands of the mother

The blastocyst arrives at the uterus after 3-4 days. It is moved by the action of cilia lining the fallopian tubes

The blastocyst secretes enzymes to help it 'burrow' into the endometrium of the uterus

The trophoblast cells divide by mitosis, producing villi which project into the endometrium

The villi develop blood vessels inside them - containing fetal blood

The endometrium develops spaces around these villi, called sinuses or intervillous spaces - which are filled with maternal blood

Oxygenated blood arrives at the sinuses from the maternal arteries

2 umbilical arteries bring deoxygenated blood to the placenta from the fetus. These branch to form capillary networks within each villus

Gas exchange takes place between the fetal blood and the maternal blood ( within the sinuses). The blood does not mix; it's just in very close contact. 

The placenta is a temporary endocrine gland and secretes hormones:

• hCG - maintains the corpus luteum, so keeps the endometrium in place; is detected by pregnancy tests
• Oestrogen - stimulates the development of mammary glands; stimulates the growth of the uterus
• Progesterone - inhibits the ability of the myometrium to contract, preventing early labour
• Human placental lactogen - promotes the growth of the mammary glands ready for milk secretion
• Relaxin - stimulates the relaxation of pelvic ligaments and softens the cervix, ready for labour and giving birth

There is counter current flow of blood between the villi and sinuses to allow efficient transfer of materials between fetus and maternal blood supplies

There is a large surface area as a result of many villi

The fetus is protected from the mother's immune system

The fetus can gain antibodies (IgG) from the mother so has early protection from infection

There is a barrier between the two blood supplies (mother and baby can have different blood groups)

What is transported between fetus and mother:

• Glucose - mother to fetus - faciliated diffusion
• Amino acids - mother to fetus - active transport
• Ions and vitamins - mother to fetus - diffusion/facilitated diffusion
• Immunoglobin (IgG) - mother to fetus - has specialised transporter on the placenta
• Water - mother to fetus - osmosis
• Urea - fetus to mother - diffusion

Drug

Effect

Effect on development

Nicotine

Increase heart rate and blood pressure in the mother

Passes directly across the placenta into the foetal blood

Increased risk of: spontaneous abortion, placenta previa, poor foetal growth, premature birth, abruption placentae, SIDS, heart defects, childhood asthma, growth deficiencies as children

Alcohol

Can affect fertility

Drinking during pregnancy can lead to foetal alcohol spectrum disorders

Abnormal appearance, short height, small head, hearing/sight problems, behaviour problems, low intelligence

Marijuana

AS used with tobacco

Slow foetal growth rates, lower birth weights

Cocaine

Restricts the flow of blood to the baby, and so could limit the nutrients that a baby receives

Possibly causing brain damage and even death, higher incidence of miscarriage in early pregnancy, to stillbirth in late pregnancy and to premature birth and low weight

Heroin

Readily crosses the placenta to developing baby and may seriously affect healthy development

Suppresses appetite in mother

Increased risk of various pregnancy complications such as poor foetal growth, low birth weight, prematurity and premature breaking of the waters and stillbirth

Babies born to heroin users have been observed to display withdrawal symptoms such as fever, trembling, irritability, excessive crying, diarrhoea and vomiting

The developing fetus is surrounded by a sac called the amnion which contains amniotic fluid

Amniotic fluid acts as a shock absorber to protect the developing fetus