This light micrograph shows the cross section of seminiferous tubules, blood vessels and also the interstitial Leydig cells.
Leydig cells are responsible for the production of testosterone.
Each testis is composed of a tubular structure. It is from these seminiferous tubules that sperm are produced.
From puberty these tubules will produce sperm cells throughout the life of the man.
Note the basement membrane which surrounds each tubule.
Inside the basement membrane can be seen various cells which are the stages of the developing spermatozoa.
Between the seminiferous tubules are groups of cells called interstitial or Leydig cells that produce the male sex hormone, testosterone.
This image is of the wall of a seminiferous tubule.
a) Basement membrane
b) Germinal epithelium (2n) which divide by mitosis to produce
c) Spermatogonium (2n) which grow and enlarge
d) Primary spermatocytes (4n) go through Meioses I to Secondary Spermatocytes (2n).
Secondary Spermatocytes go through Meiosis II to produce spermatids (n)
e) Sertoli cells nourish and allow the spermatids to differentiate to spermatozoa. These are released into the lumen
The diagram carried information about the number of chromosomes per cell. At the stage of the primary spermatocyte there are 23 homologous pairs each represented by a pair of sister chromatids. Some authors choose to represent this as 4n = 46 chromosomes x 2 (each chromatid).
Spermatogonium (2n) are found at or near the basement membrane.
They have a high rate of cell division by mitosis to produce spermatogonia.
The spermatogonium grow to form Primary Spermatocytes which have completed S-phase.
The Primary spermatocytes separate the homologous pairs of chromosomes in meiosis I(reduction division) to form the haploid Secondary Spermatocytes.
The spermatids are formed from the separation of the sister chromatids in meiosis II.
The spermatids are found in association with the sertoli cells which nourish the spermatids as they differentiate into spermatozoa.
The rate of spermatozoa is high and continuous throughout the life on the sexually mature male.
The average number of spermatozoa in ejaculated semen is 32 x 106 ml-1
There are two hormones secreted from the anterior pituitary FSH and LH.
FSH stimulates the primary Spermatocytes which carry out meiosis I(reduction division) to separate homologous pairs of chromosomes and produce haploid secondary spermatocytes.
LH stimulates the interstitial cells to produce testosterone
Testosterone stimulates the maturation of secondary spermatocytes through meiosis and differentiation to spermatozoa.
a) Primary follicles in the medulla region (centre) each one contains an oogonia arrested at prophase I.
b) Sequence showing the development of the primary follicle (PI) into the secondary follicle (MII).
c) The mature secondary follicle is also known as a Graffian follicle. The size of this follicle will make the wall of the ovary bulge prior to ovulation. Note the exclusion of the 1st polar body that will degenerate.
d) Ovulation, with the rupture of the follicle wall the oocyte is released into the oviduct.
e) The Oocyte moves into the oviduct. This oocyte is at metaphase II and will complete meiosis only with fertilisation.
f) The Corpus Lute um forms from the now empty secondary follicle. This structure is responsible for the production of higher levels of progesterone
The light microscope image shows the mature secondary follicle.
At the centre of the follicle is the oocyte.
Note the Zona pellucida which surrounds the oocyte. This surrounds the oocyte and is composed glycoproteins that take part in the acrosome reaction.
Oogonium (2n) divide by mitosis to produce many oogonia
Each oogonia grows within the follicle of cells. Meiosis begins but stops in at prophase I. The oogonia are found within the primary follicles.
There are approx 400,000 primary follicles present in the ovary prior to puberty.
A Primary follicles (prophase I) may develop to secondary follicles (metaphase II) under the influence of FSH.
Note that the first polar body ( haploid set chromosomes) does not progress beyond metaphase II.
The Oocyte does not progress to the end of meiosis unless fertilisation takes place.
The structure of a mature sperm: remember this is a single cell ~50 um long and 3 um wide at the 'head'. The 40 um tail is omitted from this diagram.
The acrosome vesicle contains the enzymes required to digest its way though the ovum wall.
Haploid nuclei (n=23) containing the paternal chromosome set
The 'mid-section' of the sperm contains many mitochondria which synthesis ATP to provide the energy for the movement of the tails structure.
Protein fibres add longitudinal rigidity and provide a mechanism of propulsion.
The image shows the structure of the egg(diameter~ 100 um) at the point of ovulation.
The haploid nuclei (arrested at metaphase II ) sits inside a cell with a large volume of cytoplasm (yolk).
During follicle development unequal division of the cell during meiosis produces the 1st polar body that can be seen outside the plasma membrane. This will not develop.
The Zona pellucida surrounds the structure and is composed of glycoproteins. With the cortical granules they will be involved in the acrosome reaction at fertilisation.
Around the outside are the follicular cells.
testicle fluids are removed and the sperm concentrated
sperm mature here and develop the ability to swim
adds nutrients that include fructose sugar for respiration
mucus to protect sperm in the cell
adds fluids that neutralise the vaginal acids and minerals ions
In all species the egg cells release a specific signal usually a polypeptide that acts as a signal to the sperm cell to fertilise the egg and not other cells. The signals appear to be species specific and are one mechanism that prevents hybridisation although this is probably more important in aquatic species rather than humans. Other important events include the restoration of the diploid number (n+n=2n) and mechanisms that prevent polyspermy.
Fertilisation of the human egg
(a) The cumulus is a thick loose grouping of cells in a gelatinous matrix. The sperm cell must penetrate this mass to reach the zona pellucida, a glycoprotein matrix surrounding the egg plasma membrane.
(b) Contact between the zona pellucida and proteins in the sperm cells membrane trigger a the acrosome reaction.
(c) The acrosome vesicle fuses with the sperm plasma membrane and releases enzymes that digest a path through the zona pellucida.
(d) The membrane of the sperm cell and the ovum fuse together. This causes a prominent raising of the egg membrane. At the same time this results in a release of Ca2+ from the endoplasmic reticulum.
(e) The cortical vesicle fuse with the plasma membrane releasing enzymes that destroy the sperm binding proteins on the zona pellucida. This prevents polyspermy. The release of Ca2+ also activate meiosis and prepare the cell for completion of reduction division , MII and cell division.
a) The fertilised egg has developed into a blastocyte that will implant into the endometrium
b) Implantation of the blastocyst which begins to secrete human chorionic gonadotrophin (hCG)
c) hCG passes into the maternal blood. The concentration doubles every 2-3 days and reaches a peak at 8-10 wk's.
d) The hCG targets the ovary and the corpus luteum.
e) The corpus luteum secretes progesterone and oestrogen at high levels .
f) The oestrogen and progesterone continue to inhibit FSH and LH secretion from the pituitary.
g) The progesterone's prevent the breakdown of the endometrium and so the embryo can continue its development into a foetus.
Cell division in embryonic development is called cleavage.
Mammals show rotational cleavage in which the plane of division for successive divisions are at right angles.
At around the eight cell stage all the cells maximise there surface contact with each other and the ball of cells becomes tight.
The process outlined occurs as the cell mass moves down the oviduct.
Implantation occurs around day 6 in the uterus. Pre-implantation in the tubular walls is prevented by the zona pellucida (outer gray circle).
The blastocyst is a hollow ball of cells with a Inner cell mass (embryo) at the base.
Developmental biology studies the development of the inner cell mass as the cells differentiate into the tissues and organs of the organism. This area of biology has recently experienced significant growth particularly as knowledge of gene switches has developed. Interested students can do no better than read 'Endless forms most beautiful' by Sean Carroll.
a) Umbilical cord connects the foetus to the placenta
b) There are two umbilical arteries that carry the deoxygenated blood to the placenta.
c) The single umbilical vein returns the blood to the rest of the foetal circulation.
d) The placenta is normally about 190mm wide and 20 mm deep. The human placenta is more deeply integrated into the maternal tissue than any other animal.
e) The myometrium is composed on smooth muscle that produces the contraction in labor.
f) The endometrium which is maintained through out pregnancy by progesterone. Initially from the corpus luteum and later from the placenta itself.
g) The female blood supply which supplies the foetus with oxygen and nutrient. It will also remove waste from the foetal blood and excrete this through the maternal systems.
h) Open ended blood arterioles and capillaries that produce the inter-villous 'blood lakes'.
i) Inter-villus spaces filled with maternal blood. These surround the placental villi and allow for very efficient exchange.
j) Placental-Villi with large surface area for the exchange of nutrient and waste.As previously mentioned there are few membranes between the maternal blood and the foetal blood an association that is closer than an other mammal.
Placental Hormones and Pregnancy
During early pregnancy the endometrium is maintained by a secretion of hCG from the embryo.
Later the production of progesterone's is taken over by the placenta itself.
Note that the concentrations rise throughout the gestation.
This is an ultra sound image of a two month twin pregnancy.
In all the following images of unborn children the ultrasound scan shows both the foetus and the surrounding amniotic fluid along with the amniotic membrane.
The child develops within a fluid filled space, called the amniotic fluid. This fuid provides a supportive environment in which the foetus is suspended. The fluid, like most fluids, is largely incompressible and so is very good at absorbing pressure. So the fluid protects the child as a shock absorber of an impact to the uterus wall.
The fluid creates buoyancy for the foetus such that it does not have to support its own weight. The skeletal system of the child begins as flexible cartilage before hardening as bone. This process does not complete until after the child is born. The skeletal system therefore could not support the weight of the child.
The fluid prevents dehydration of the tissues by osmosis.
The image shows the position of the endometrium and two foetus
There is amniotic fluid surrounding each foetus to protect and support the foetus
The fluid is retained by an amniotic membrane
In this example there are two amniotic membranes one for each foetus.
This is a 3D Ultrasound scan of the twin pregnancy above.
The two 20 wk foetus are lying opposite each other but also upside down to each other.
Each of the children has its own set of placental membranes.
In the diagram there is a membrane section between which is the amniotic membranes.
Single foetus pregnancy.:
This 2D ultra sound scan shows the muscular myometrium and the uterine space.
The foetus is 14 wks and has its head to the left and feet to the right (un seen).
The amniotic membrane in this image is up against the wall of the uterus.
Click the image for labels.
The foetus develops and grows using materials obtained by exchange across the placental wall from mother to child. Excretory products are exchanged in the opposite direction from child to mother.
With the fall in progesterone the pituitary secretes this small polypeptide called oxytocin.
Stretching of the lower uterus walls by the foetus and its production of prostaglandin's add to the stimulus for the pituitary to secrete oxytocin.
The oxytocin causes the smooth muscle in the walls of the uterus to contract and labour has begun.
After nine months in the uterus the foetus is fully grown and takes up all the space available.
These cramped conditions push the baby down stretching the lower walls of the uterus. This sends impulses to the mother brain.
The foetus responds to the cramped conditions by producing hormones from the placenta (prostaglandin) which causes myometrial contraction
Progesterone is the hormone of pregnancy and at this stage the high levels of this hormone become less active.
All these changes stimulate the secretion of oxytocin from the pituitary and this causes the myometrial contractions of labour
In this system the stimuli to the brain increases the oxytocin production
In turn the oxytocin stimulate myometrial contraction
Myometrial contraction further stimulates the pituitary of the mother to release more oxytocin
The strength and frequency of the myometrial contractions is further increased.
In turn this further stimulates more oxytocin production
The process builds with stronger and stronger contractions
Final the child passes though the cervix and vagina to be born
Contractions continue for a further period until the placenta is delivered (after birth).