lecture 27 - human reproduction and development Flashcards
Describe the male reproductive system
Male gametes (sperm) are produced and mature in the testes that are located in the scrotum.
Sperm reaches urethra via vas deferens.
Each ejaculate contains several hundred million sperm (but only several hundred reach the egg)
Seminal vesicles: produce protein- and sugar-rich fluid, provides energy for sperm motility
Prostate gland: produces alkaline fluid, maintains sperm motility and counteracts acidity of female reproductive tract
Bulbourethral gland: fluid that lubricates urethra for sperm passage
Describe spermatogenesis
In the testes, sperm is produced in the seminiferous tubules. Sperm precursor cells undergo mitotic and meiotic divisions, and the sperm is released into the lumen of the seminiferous tubules. From the seminiferous tubules, sperm reaches the epididymis, where they become motile, and where they are stored prior to ejaculation. Sperm can be stored and nourished in the epididymis for up to five weeks.
A sexually mature man produces around 200-300 million spermatozoa each day.
Sertoli cells protect and nourish the developing sperm and regulate spermatogenesis. Leydig cells (are between the tubules) secrete testosterone. Myoid cells are contractile cells involved in the transport of sperm along the tubule.
In male humans, spermatogonia are formed in the embryo, but they only give rise to primary spermatocytes that undergo meiotic divisions to form sperm during puberty. Because spermatogonia continue to self-renew throughout the adult life, male humans never run out of sperm precursor cells.
Human spermatogenesis: 74 days
Describe sperm structure
Sperm are haploid cells with a unique shape and properties adapted for their function of delivering genetic material to the egg.
They have a small head containing minimal cytoplasm and a very densely packed nucleus.
The head is surrounded by a specialized organelle derived from the golgi, the acrosome, that contains enzymes that are used by sperm to transverse the outer coating of the egg.
Sperm have a long tail, or flagellum, which moves the cell by a whipping motion powered by the sperm’s mitochondria.
In adult men, sperm are continuously produced.
Describe the female reproductive system
Female gametes (oocytes) are produced in the ovaries and released monthly. After release from the ovary, the oocyte travels through the oviduct, where it is fertilised, to the uterus. If fertilisation has occurred, the embryo develops inside the uterus.
Describe egg production
Primary oocytes are formed during embryonic development, and arrest in prophase I of meiosis I. At the time of birth, a female has around 1-2 million primary oocytes arrested in meiosis I. Each menstrual cycle, one oocytes completes meiosis I, and the daughter cells arrest in meiosis II (metaphase). Meiosis II is completed after fertilisation. The meiotic divisions are asymmetric, resulting in three (or two, if the first polar body does not divide) polar bodies and one egg.
Before ovulation, both the oocyte and the surrounding follicular cells undergo a series of maturation processes. The oocyte loads up the cytoplasm with nutrient materials. The cells surrounding the oocyte divide and form several layers, and, together with the oocyte, secrete a glycoprotein that will form the zona pellucida surrounding the oocyte. These maturation processes already start around 2 month before the follicles are recruited for further maturation during the follicular phase of the menstrual cycle.
The oocyte completes meiosis I just before ovulation (triggered by high LH), and completes meiosis II after fertilisation.
Describe oocyte structure
The Oocyte is the largest cell (by volume) in the human body (around 100μm diameter). It contains large amounts of cytoplasm that provides nutrients for the embryo and contains molecules important for early cell divisions.
The oocyte is surrounded by the zona pellucida, a layer of extracellular matrix involved in sperm recognition, and by cells of the corona radiata that provide the oocyte with nutrients and are involved in sperm attraction
Describe hormonal control of the reproductive system
Testes and ovaries are part of the endocrine system; they respond to hormones and secrete hormones themselves. The hypothalamus secretes hormones that act on the pituitary gland, which in turn releases hormones that act on target organs. In the case of the reproductive system, the hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones act on the male and female gonads.
In males, these hormones are secreted almost continually, whereas in females, they are secreted cyclically.
Describe hormonal control of the male reproductive system
Leydig cells secrete testosterone in response to LH, and sertoli cells stimulate sperm production in response to testosterone and FSH. The negative feedback control by testosterone and inhibin maintains a relatively constant level of hormones.
Testosterone is also important for the development of male reproductive structures during embryogenesis, and male secondary sexual characteristics during puberty.
Describe the menstrual cycle
The menstrual cycle consists of cyclical and coordinated changes in the ovaries and uterus, timing the release of an oocyte from the ovary with the growth of the uterine lining so it can support the developing embryo if fertilisation occurs.
The menstrual cycle has two phases, the follicular phase and the luteal phase.
In the follicular phase, FSH and LH act on follicle cells (cells surrounding the oocyte). As a result, several oocytes begin to mature, but usually only one of these becomes completely mature and the others die off. The follicle cells secrete a form of ooestrogen known as estradiol. Estradiol in turn acts on the lining of the uterus, causing it to thicken.
As a result in part of rising ooestrogen levels, there is a rapid increase followed by a sharp decrease in the level of LH (and, to a lesser extend, FSH) produced by the anterior pituitary gland. This surge causes ovulation, and marks the beginning of the luteal phase. The follicle cells are converted to a structure known as the corpus luteum. The corpus luteum is a temporary endocrine structure that secretes the hormone progesterone, which maintains the thickened uterine lining.
If the oocyte is not fertilised, the corpus luteum degenerates, ooestrogen and progesterone levels drop, and the uterine lining is shed.
If it is fertilised (in the oviduct), the developing embryo travels to the uterus and implants into the uterine lining. The corpus luteum continues to secrete progesterone. It is maintained by the hormone human chorionic gonadotropin (hCG), which is released by the developing embryo. Eventually, the placenta takes over ooestrogen and progesterone production to maintain the uterine lining and stimulate growth of the uterus.
Describe hormonal control of the female reproductive system
Varying levels of hormone secretion by the anterior pituitary gland is due to feedback regulation. Low/medium levels of oestrogens (e.g. estradiol) inhibit LH and FSH secretion, whereas high levels stimulate it. High levels of progesterone have an inhibitory effect.
During the follicular phase, the follicular cells are secreting increasing amounts of oestrogens. When it has reached high levels, it stimulates the secretion of LH and FSH by the pituitary gland. LH causes ovulation and corpus luteum formation. The corpus luteum secretes progesterone, which inhibits LH and FSH production. After the degeneration of the corpus luteum, LH and FSH levels slowly increase again.
Describe how fertilisation occurs
- in mammals, fertilisation occurs internally
- to reach the egg, the sperm has to travel from the vagina via the uterus to the oviduct (only a small fraction of sperm deposited in the vagina reach the oviduct)
- the sperm is transported to the oviduct by muscular activity of the female genital tract; only in the oviduct, the flagellar movements are needed for sperm movement
- in the oviduct, the sperm is attracted to the egg by chemotaxis and thermotaxis (towards higher temperature)
In the female reproductive tract, the sperm undergoes a series of physiological changes (called capacitation) that allow it to fertilise the egg. These changes include changes in surface membrane proteins and increased motility.
Upon reaching the oocyte, the sperm passes through two layers—an outer layer of follicle cells called the corona radiata (or cumulus) and an inner matrix of glycoproteins called the zona pellucida—before fusing with the plasma membrane of the oocyte. Enzymes on the sperm surface allow passage through the cumulus cells, and release of acrosomal contents (contains enzymes that digests proteins and sugars) allow passage through the zona pellucida. The sperm and the zona pellucida and membrane proteins of the oocyte undergo species-specific interactions.
After passing through the layers surrounding the ooctye, the plasma membranes of the sperm and oocyte fuse. The oocyte then completes meiosis II and changes occur that prevent polyspermy, or fertilisation by more than one sperm. This includes the fusion of vesicles in the oocyte (cortical vesicles) with the plasma membrane, releasing the content of the vesicles. Enzymes released modify the zona pellucida and prevent further binding of sperm.
What is cleavage?
Cleavage is a series of mitotic divisions that occur while the embryo travels in the oviduct towards the uterus. The cell divisions are not accompanied with an overall increase of the size of the embryo.
Compaction:
at the 8 cell stage, the cells bind to each other more tightly (mediated by cell adhesion molecules such as E-Cadherin)
cells form tight junctions
cells become polarized
cells continue to divide, with some cells remaining at the surface and others forming the inner cell mass
Describe blastocyst formation
After compaction, blactocyst formation occurs. The outer cells of the blastocyst stage embryo are the trophoblast cells, whereas the inner cells are referred to as the inner cell mass.
Cavitation
- formation of the blastocoel
- outer cells pump sodium ions into the inside of the embryo -> osmotic effect -> water enters the embryo
The blastocyst hatches from the zona pellucida (the outer cells of the blastocyst secrete proteases that digest the zona pellucida)
Describe preimplantation development
After morula formation by cleavage, further cell divisions result in a fluid-filled ball of cells called blastula or blastocyst. The outer layer of the blastocyst will give rise to part of the placenta, whereas the inner cell mass will give rise to all the tissues of the developing embryo. The blastocyst hatches from the zona pellucida and implants into the uterine lining around five days after fertilisation. Implantation marks the beginning of pregnancy.
The placenta is formed both by maternal and embryonic cells and is the site of exchange of gas, nutrients and waste between maternal and fetal blood.
What is gastrulation?
Gastrulation is a highly coordinated set of cell movements that leads to a fundamental reorganization of the embryo.
After differentiation of the inner cell mass into epiblast and hypoblast, the cells of the epiblast divide and migrate inward. These migrating epiblast cells form three layers or sheets of cells called germ layers, which include the outer ectoderm, intermediate mesoderm, and inner endoderm. All of the adult tissues and organs are made from the three germ layers.
In humans, gastrulation starts in the third week after fertilisation.
