Anatomy and Physiology

Similarities:

• Both are synovial joints
• Both are involved in the movement of the leg

Differences:

• Many nuclei (fibres are long and were formed from many muscle cells fusing together, hence the fibres are multinucleated)
• Large number of mitochondria (muscle contraction requires a lot of ATP)
• Tubular myofibrils, divided into sections called sarcomeres, and made of two different myofilaments (proteins responsible for contraction)The membrane surrounding a muscle fibre is called the sarcolemma
  • Where thin (actin) and thick (myosin) filaments overlap, a dark band occurs, and this is flanked by light regions containing thin filament only
• The internal membranous network is called the sacroplasmic reticulum, it is analogous to endoplasmic reticulum but is specialised for muscle contraction (it contains high levels of Ca²⁺ ions)

• Action potential from a motor neuron triggers release of Ca²⁺ from the sarcoplasmic reticulum
• Calcium ions expose the myosin heads by binding to a blocking molecule and causing it to move
• The myosin heads form a cross-bridge with actin binding sites
• ATP binds to the myosin heads and breaks the cross-bridge
• The hydrolysis of ATP causes the myosin heads to change shape and swivel - this moves them towards the next actin binding site
• The movement of the myosin heads cause the actin filaments to slide over the myosin filaments, shortening the length of the sarcomere

Ultrafiltration occurs when hydrostatic pressure forces blood through a semi-permeable membrane, separating blood cells and large proteins from the remainder of the serum. Ultrafiltration occurs between the glomerulus and the Bowman's capsule and requires two things to form the filtrate:

Hydrostatic Pressure

• The glomerulus increases BP by forming narrow branches (also increases surface area)
• This pressure is maintained by a narrow efferent arteriole, which restricts the outflow of blood 
• The net pressure gradient in the glomerulus forces blood into the capsule space

Basement Membrane (filtration barrier)

• The basement membrane is a fine mesh that restricts the passage of blood cells and proteins
• It lies between the glomerulus and Bowman's capsule
• Blood can exit the glomerulus directly through pores as the capillaries are fenestrated, and enter Bowman'c capsule.

Proteins: Present in blood plasma, but not present in glomerular filtrate or urine. This is because proteins cannot pass across the basement membrane during ultrafiltration and thus cannot form part of the filtrate

Glucose: Present in blood plasma and glomerular filtrate, but not present in urine (normally). This is because the glucose is selectively reabsorbed in the proximal convoluted tubule. It is reabsorbed from the filtrate into the blood by active transport (symport with Na⁺ ions)

Urea: Present in blood plasma, glomerular filtrate and urine. Only about 50% of urea is reabsorbed (some urea is reabsorbed to help regulate the medullary osmolarity gradient). Because water is reabsorbed from the filtrate (by osmosis, due to the hypertonicity of the medulla), urea becomes more concentrated in urine. The concentration of urea in the urine will depend on the amount of water in the urine.

• The urine of non-diabetic patients should contain no glucose as it is selectively reabsorbed from the filtrate in the proximal convoluted tubule
• Diabetics have higher levels of blood glucose due to either a lack of insulin secretion (type I) or insensitivity to insulin secretions (type II)
• Because of this, not all of the glucose in diabetics is reabsorbed into the blood (protein pumps in tubule wall become saturated)
• This results in the presence of glucose in the urine of untreated diabetics, which can be detected using test strips

• Spermatogenesis describes the production of spermatozoa (sperm) in the seminiferous tubules of the testes
• The first stage of sperm production is the division of germline epithelium by mitosis
• These cells (spermatogonia) then undergo a period of growth
• This is followed by two meiotic divisions that result in four haploid daughter cells
• These haploid cells then differentiate to form sperm cells
• The developing sperm cells are nourished throughout by the Sertoli cells

• The process begins during foetal development, when a large number of cells (oogonia) are formed by mitosis before undergoing a period of growth
• These cells begin meiosis but are arrested in prophase I until puberty
• At puberty, some follicles continue to develop each month is response to FSH secretion
• These follicles complete the first meiotic division to form two cells of unequal size
• The cell with less cytoplasm is a polar body (which degenerates), while the larger cell forms a secondary oocyte
• The secondary oocyte begins the second meiotic division but is arrested in prophase II (until fertilisation)
• It is released from the ovary (ruptured follicle develops into corpus luteum) and, if fertilisation occurs, will complete meiosis
• The second meiotic division will produce an ovum and a second polar body

Similarities:

• Both processes result in the formation of haploid gametes
• Both processes involve mitosis, growth and meiosis

Differences:

• The sperm is attracted to the egg due to the release of chemical signals from the secondary oocyte (chemotaxis)
• Fertilisation generally occurs in the oviduct (fallopian tube)
• To enter the egg membrane, the sperm must penetrate the protective jelly coat (zona pellucida) surrounding the egg via the acrosome reaction
• The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
• The membrane of the egg and sperm fuse and the sperm nucleus enters the egg
• Now fertilised, the nucleus of the secondary oocyte completes meiosis II and then the egg and sperm nuclei fuse to form a diploid zygote
• The cortical granules release enzymes that destroy the sperm-binding proteins on the jelly coat

• The endometrium is a blood-rich environment in which an implanted zygote can grow and it is sustained by the hormone progesterone
• If progesterone levels aren't maintained (i.e. the corpus luteum degenerates), then the endometrium will be sloughed away (menstruation)
• A fertilised zygote develops into a blastocyst that secretes human chorionic gonadotrophin (hCG)
• hCG maintains the corpus luteum post-ovulation so that the blastocyst can remain embedded in the endometrium and continue to develop
• Gradually the placenta develops and produces progesterone (at around 8 - 10 weeks), at which point the corpus luteum is no longer needed

• After fertilisation, the zygote undergoes several mitotic divisions to create a solid ball of cells called a morula (at around 4 days)
• Unequal divisions beyond this stage cause a fluid-filled cavity to form in the middle - this makes a blastocyst (at around 5 days)
• The blastocyst consists of:
  • An inner mass of cells (this will develop into the embryo)
  • An outer layer called the trophoblast (this will develop into the placenta)
  • A fluid filled cavity (called the blastocoele)
• These developments all occur as the developing embryo is moving from the oviduct to the uterus
• When the blastocyst reaches the uterus, it will embed in the endometrium (implantation)

Structure and Function: The placenta is a disc-shaped structure that nourishes the developing embryo. It is formed from the development of the trophoblast upon implantation and invades the uterine wall. The umbilical cord connects the fetus to the placenta and maternal blood pools via open ended arterioles into intervillous spaces (lacunae). Chorionic villi extend into these spaces and facilitate the exchange of materials between the maternal blood and fetal capillaries. Nutrients, oxygen and antibodies will be taken up by the fetus, while carbon dioxide and waste products will be removed. The placenta is expelled from the uterus after childbirth.

Hormonal Role: The placenta also takes over the hormonal role of the ovary. Estrogen stimulates growth of the muscles of the uterus (myometrium) and the development of the mammary glands. Progesterone maintains the endometrium, as well as reduces uterine contractions and maternal immune response (no antibodies against fetus). Both estrogen and progesterone levels drop near time of birth.

• The process of childbirth is called parturition and is controlled by  oxytocin
• After nine months, the fetus is fully grown and takes up all available space in the uterus, stretching the walls of the uterus
• This sends a signal to the brain, releasing oxytocin from the posterior pituitary
• Oxytocin inhibits progesterone, which was inhibiting uterine contractions
• Oxytocin also directly stimulates the smooth muscle of the uterine wall to contract 
• The contraction of the uterine wall causes further stretching, which triggers more oxytocin to be released (causing even more contraction)
• Additionally, the fetus responds to the cramped conditions by releasing prostaglandins which cause further myometrial contractions
• As the stimulus causing oxytocin release is increased by the effects of oxytocin, this creates a positive feedback pathway
• Contractions will stop when labour is complete and the uterus is no longer stretched