Immunogenesis Lecture Notes

Gametogenesis is the process of producing reproductive cells (gametes).
Reproduction in complex organisms requires the fusion of chromosomes from two different members of the population.
Typically, this involves a large, immobile, nutrient-rich gamete (egg) and a small, mobile gamete (sperm).
Internal fertilization, where male gametes are introduced into the female reproductive tract, is common, even in insects like dragonflies.

Oogenesis aims to produce a single oocyte at a time, which must be fertilized or it will die.
Female bodies do not store viable, fertile gametes.
In placental mammals, development occurs within the female reproductive tract.
While the oocyte is produced, the female reproductive tract prepares for the potential development of a human.

Spermatogenesis involves producing a large number of viable, motile reproductive cells.
To achieve a one in four chance of fertilizing an egg on the day of ovulation, a male needs to introduce approximately 250,000,000 sperm into the female reproductive tract.
A male producing less than 60,000,000 sperm in his ejaculate is considered sterile.
The immune system attacks sperm as foreign invaders, necessitating a high number to overwhelm this defense.

Gametogenesis begins with progenitor cells, which have the potential to become different types of cells.
Progenitor cells can either proliferate through mitosis, creating clones, or differentiate to acquire new characteristics.
Differentiation leads the cells on a path to becoming gametes.
Before meiosis, a body cell contains four copies of DNA from two sources.
After differentiation, this remains unchanged until meiosis begins.

During the S phase of cell division, DNA is duplicated before division.
After the first meiotic division (meiosis I), primary cells contain either two maternal or two paternal copies of chromosomes, making them haploid.
These primary cells then undergo meiosis II to produce gametes.
Gametes contain one copy of genetic material from one source (maternal or paternal).

In females, all phases before the secondary cell stage occur while the female is an embryo.
Every woman has all her eggs at birth, with meiosis I already initiated.
After reaching reproductive maturity, primary oocytes finish meiosis I and begin meiosis II during each menstrual cycle.
The cell ovulated is a secondary oocyte, not an egg.
The secondary oocyte only becomes an egg if fertilization occurs.
Without fertilization, the secondary oocyte remains arrested in meiosis II and eventually dies due to lacking support from the follicle.
Once females reach their adult reproductive state, primary cells can only be converted into secondary cells; oogenesis completes only upon fertilization.

In males, the gonad (testis) is divided into lobules, which contain seminiferous tubules.
Interstitial cells, located between the tubules, produce testosterone.
Spermatogenesis occurs within the walls of the seminiferous tubules.
The youngest, least differentiated cells are located near the outer edge, with development progressing towards the center.
Sperm are eventually released into the tubule.
Sertoli cells within the tubule walls support developing spermatocytes, providing a unique microenvironment.

Progenitor cells in spermatogenesis are called spermatogonia (2n).
Spermatogonia proliferate to create more spermatogonia.
Spermatogonia differentiate into primary spermatocytes.
Primary spermatocytes undergo mitosis for amplification (e.g., one spermatogonium can produce 64 primary spermatocytes).
Primary spermatocytes duplicate their DNA during the S phase, resulting in four copies.
These then undergo meiosis I to become secondary spermatocytes.
Secondary spermatocytes go through meiosis II to produce spermatids.
Spermatids, which are gametes, then undergo morphological changes (spermiogenesis) to become spermatozoa (sperm).
Need to develop flagellum for movement and an acrosome to digest egg membranes.
Finally, sperm are released from Sertoli cells through spermiation.

Hypothalamus releases Gonadotropin-Releasing Hormone (GnRH).
GnRH controls the anterior pituitary, which produces Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
FSH controls the number of spermatocytes.
LH controls the production of gonadal hormones, specifically testosterone, by interstitial cells.
Testosterone controls the rate at which primary cells become sperm (spermatocytogenesis).
FSH and LH together determine sperm production rate and quantity.

Oogenesis is the process of making oocytes, starting with oogonia.
Oogonia are progenitor cells that proliferate.
By ten weeks of embryonic development, the female ovary contains about 2,000,000 oogonia.
Some oegonia differentiate into primary oocytes.

Oogenesis aims to produce one oocyte, focusing on growth and nutrient storage rather than quantity.
The egg must store enough nutrients to support the first eight to ten days of development.
Oocytes get bigger and store nutrients - continued growth throughout process.

Primary oocytes undergo the S phase and prepare for meiosis.
During meiosis I, the primary oocyte forms two cells: the first polar body and the secondary oocyte.
The polar body gets rid of two copies of DNA.
If fertilization occurs, the secondary oocyte produces the second polar body, leaving behind the oocyte.
In reality, secondary oocyte becoming egg can't happen without fertilization; product is a zygote.

The menstrual cycle regulates oogenesis and prepares the endometrium in the uterus.

Involves hypothalamic hormones, anterior pituitary hormones, and gonadal hormones.
The hypothalamus releases GnRH, stimulating the anterior pituitary to produce FSH.
FSH stimulates follicles in the ovary to grow.
As follicles develop, they produce estrogen.

The uterus has three layers: the endometrium (inner), myometrium (muscle), and perimetrium (outer).
Embryo is parasitic - digests endometrium for nutrients.
The endometrium must increase in thickness and volume (proliferate), become vascularized, and develop secretory glands.

Stimulates ovulation, causing the mature follicle to release the egg.
Causes the remaining follicle to differentiate into the corpus luteum (luteinizing hormone).

The corpus luteum produces estrogen and progesterone.
Progesterone acts on the endometrium to increase blood supply and form glands, making it secretory.
Progesterone also inhibits the hypothalamus and anterior pituitary.

The corpus luteum produces estrogen and progesterone for about seven days.
When estrogen and progesterone levels drop, the endometrium degenerates and is shed as menses.

Requires Human Chorionic Gonadotropin (HCG), produced by the conceptus.
HCG stimulates the corpus luteum to continue producing estrogen and progesterone.
Female reproductive tract characteristics dependent on conceptus.
Female anantomy relies on external signaling dependent upon developing receptors.
Pregnant relies on HCG to grow, so the body needs to maintain that signaling to grow and continue.