11.4.1 Micrograph of testis.

testis structure

leydig

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.

 

 

 

 

 

 

 

spermatogenesis diagram

 

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.

 

 

 

 

 

 

 

 

 

spermatogenesisThis 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

 

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11.4.2 Spermatogenesis.

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).

spermatogenesis

 

 

 

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11.4.3 The role of some reproductive hormones.

LH FSH spermatogenesis

 

 

 

 

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11.4.4 Ovary structure.

ovary

 

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

 

 

 

follicles.

 

 

 

 

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11.4.5 Oogenesis.

oogenesis

 

 

 

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11.4.6 Diagram of a mature sperm and egg.

sperm cell

 

 

 

 

 

 

 

oocyte

 

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11.4.7 Production of semen.

semen

Epididymis:

Seminal vesicles:

Prostate:

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11.4.8 Comparison of oogenesis and spermatogenesis.

oogenesis spermatogenesis comparison

 

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11.4.9 Fertilisation, acrosome reaction and cortical reaction.

Fertilisation:

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.

 

acrosome

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.

 

 

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11.4.10 HCG in early pregnancy.

HCG

 

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.

 

 

 

 

 

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11.4.11 Early embryonic development.

blastocystCell 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.

 

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11.4.12 The placenta.

placenta

 

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

placental hormones

 

 

 

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11.4.13. Protection of the foetus by amniotic sac and fluid.

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.

amniotic membranes

 

 

 

 

 

 

 

 

amniotic twins

 

 

 

 

 

 

 

 

 

 

 

 

Single foetus pregnancy.:

amniotic membrane

 

Click the image for labels.

 

 

 

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11.4.14 Maternal, foetal exchanges across placenta.

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.

placental exchange

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11.4.15 Outline of birth.

 

Parturition:

  • At the end of gestation it is believed that there is a drop in the high levels of progesterone.

  • In recent years it has been shown that the progesterone receptor are also blocked. Therefore the progesterone is not effective.

oxytocin

 

 

 

 

 

birth

After nine months in the uterus the foetus is fully grown and takes up all the space available.

All these changes stimulate the secretion of oxytocin from the pituitary and this causes the myometrial contractions of labour

 

 

Positive feedback:

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Click4Biology: Topic 11.4 Reproduction

 

Reproduction

11.4.1 Micrograph of testis.

11.4.2 Spermatogenesis.

11.4.3 The role of some reproductive hormones.

11.4.4 Ovary structure.

11.4.5 Oogenesis.

11.4.6 Diagram of a mature sperm and egg.

11.4.7 Production of semen.

11.4.8 Comparison of oogenesis and spermatogenesis.

11.4.9 Fertilisation, acrosome reaction and cortical reaction.

11.4.10 HCG in early pregnancy.

11.4.11 Early embryonic development.

11.4.12 The placenta.

11.4.13. Protection of the foetus by amniotic sac and fluid.

11.4.14 Maternal, foetal exchanges across placenta.

11.4.15 Outline of birth