Reproduction and Development in Mammals

Reproduction, Development and Pregnancy

• Mammalian sex determination is regulated by chromosomes.
• Females have two X chromosomes (XX).
• Males have one X and one Y chromosome (XY).
• Testis determining factor (TDF) is located on the Y chromosome.
• Embryonic testes secrete testosterone, responsible for secondary sex organs and external genitalia.
• Absence of testes leads to female accessory sex organ development.
• Adult males have XY chromosomes; Adult females have XX chromosomes.
• Each has 46 chromosomes (23 pairs).
• Sperm and ovum contain 23 chromosomes each.
• Fertilization results in a zygote with 46 chromosomes (one from the ovum, one from the sperm).
• The presence of a Y chromosome ensures the embryo develops as male, even if the zygote also has multiple X chromosomes.
• An XXY zygote results in a male. A Y-only zygote (YO) will not survive.
• In female fetuses, one X chromosome in each cell is inactivated and condenses into a clump of nuclear chromatin (Barr body).
• The selection of which X chromosome becomes inactive is random.
• Embryonic tissues are bipotential before differentiation, meaning they cannot be identified as male or female.
• The bipotential gonad has an outer cortex and an inner medulla.
• The medulla develops into a testis under influence of the appropriate developmental signal.
• In the absence of that signal, the cortex develops into ovarian tissue.
• TDF is found on the short arm of chromosome Y, known as SRY.
• Errors during meiosis can place the SRY gene in the X chromosome.
• A single gene (SRY) determines maleness at the earliest stages of human embryonic development.
• The SRY gene produces a protein (TDF) that activates other genes necessary to develop male reproductive organs.

Formation of Testes and Ovaries

• Male sex is determined by presence of the Y gene.
• In rare cases, XX male babies are born.
• One X chromosome can contain a segment of the Y chromosome.
• XY females may lack the same portion of the Y chromosome found in XX males.
• The gene for TDF is located on the short arm of the Y chromosome (SRY).
• Errors in placing SRY in the X chromosome can occur during meiosis, during sperm formation.

Male Reproduction

• Primary male reproductive organs - Testes.
• Oval-shaped, suspended in a sac (scrotum) by the spermatic cord.
• The spermatic cord comprises the vas deferens, arteries, veins, lymphatics, and nerves.
• The testes descend into the scrotum just before birth via the inguinal canal.
• The interior of the testes is divided into 250 lobules.
• Each lobule contains seminiferous tubules (highly coiled, convoluted tubules).
• Interstitial cells are in the connective tissue surrounding the seminiferous tubules.
• These tubules unite to form a network of channels called the rete testis, which connects to the epididymis.
• The scrotum is a sac-like structure of skin and superficial fascia.
• Its function is to maintain the temperature of the testes.
• Temperature regulation is essential for sperm production.

Male Reproductive System

• Main structures: testes, reproductive ducts (epididymis, vas deferens, ejaculatory duct, urethra), accessory glands (seminal vesicles, prostate gland, bulbourethral glands), supporting structures (scrotum, penis, spermatic cords).
• Testes produce sperm (seminiferous tubules) and testosterone (interstitial cells).
• The epididymis is a coiled tube posterior to the testes; it is the first site of sperm maturation.
• The ductus deferens extends from the epididymis to the ejaculatory duct.
• The ampulla is the final enlargement of the ductus deferens.
• The ejaculatory ducts open into the prostatic urethra.
• Seminal vesicles produce seminal fluid (fructose, other sugars, prostaglandins, proteins, amino acids, citric and ascorbic acid).
• Seminal fluid forms 70-80% of ejaculate; it is alkaline.
• The prostate gland produces fluid that contains citric acid, calcium, coagulation proteins.
• Secretions form approximately 30% of semen volume.
• The bulbourethral glands produce a clear, slippery secretion for lubrication (pre-ejaculate).
• The urethra is a common passageway for urine and semen.
• The penis contains spongy erectile tissues, which help propel semen out (during ejaculation.)
• The erectile tissue is crucial for sexual function.
• DHT is needed for development and maintenance of the penis, spongy urethra, and prostate, unlike other organs that can develop without it.

Spermatogenesis

• Spermatogenesis occurs in the seminiferous tubules.
• The process involves 5 stages (Spermatogonia – mitosis, primary spermatocyte – meiosis I, secondary spermatocyte – meiosis II, spermatid – maturation of sperm, spermatozoa).
• Spermatogonia (germ cells) divide by mitosis.
• One daughter cell remains as spermatogonium, continuing to divide.
• Another daughter cell differentiates into primary spermatocyte.
• A primary spermatocyte undergoes meiosis I to form two secondary spermatocytes.
• Secondary spermatocytes undergo meiosis II to form spermatids.
• Spermatids mature into spermatozoa (sperm cells).
• Sertoli cells provide nutrients during these stages.

Spermatic Fluid

• Sperm is mixed with secretions of several accessory organs (seminal vesicles, prostate gland, bulbourethral glands), to form semen.
• Seminal vesicle fluid contains fructose for energy, prostaglandins to stimulate uterine contractions, and other proteins and sugars.
• The prostate gland's secretions contain citric acid, calcium, and proteins for coagulation.
• The bulbourethral gland contributes lubricating fluid.
• Semen is composed of sperm and the secretions of various glands(seminal vesicles, prostate, and bulbourethral).

Ovarian Cycle

• The ovary consists of an outer cortex and inner medulla.
• The cortex contains ovarian follicles in various stages of development.
• The medulla contains blood vessels, lymphatic vessels, and nerves.
• The ovarian cycle is typically 28 days in length.
• FSH and LH are critical for ovum release.
• The rise in estrogen triggers the LH surge stimulating follicle rupture, ovulation occurs.
• Follicle rupture releases the secondary oocyte.
• The empty follicle becomes the corpus luteum secreting estrogen and progesterone.
• The ovum is released from the ovary into the fallopian tube.
• If the ovum is unfertilized, the corpus luteum undergoes degeneration, and menstruation begins.
• The cyclical release of ova (usually one oocyte per cycle) is controlled by hormones.
• The cyclical nature occurs during female reproductive cycles.

The Menstrual Cycle

• Divided into follicular and luteal phases, each with distinct hormonal control.
• The maturation of the ovum takes place in several stages.
• The follicular phase begins with menstruation, when the endometrium of the uterus is shed.
• The anterior pituitary secretes FSH, which stimulates follicles to enlarge.
• The primary follicle matures into a secondary follicle and forms fluid vesicles, forming a secondary follicle.
• The secondary follicle grows into a Graafian (vesicular) follicle.
• The follicle cells secrete oestrogen, which stimulates the endometrium to thicken.
• The high estrogen level triggers the LH surge at the end of the follicular phase.
• Ovulation occurs (release of the ovum) on day 14.
• The ovulated secondary oocyte travels down the oviduct for fertilization.
• In the luteal phase, the collapsed follicle becomes the corpus luteum, which produces progesterone.
• Progesterone further thickens the uterine lining, preparing for potential pregnancy.
• If fertilization does not occur, the corpus luteum degenerates, estrogen and progesterone levels decrease.
• The endometrium is shed in menstruation.

Oogenesis

• Formation and growth of an ovum in an ovary.
• Germ cells (oogonia) migrate to the ovaries during embryonic development.
• Many primary oocytes are formed by birth and begin meiosis.
• Meiosis I is arrested around prophase I, then resumes at puberty.
• In each ovarian cycle, about a hundred oocytes begin their maturation journey.

Female Reproduction

• Paired abdominal organs - ovaries.
• Ovum produced by one ovary per cycle.
• Ovum is released from the ovarian wall into the abdominal cavity, then travels to the fallopian tube and is fertilized by the sperm.
• The oviduct (fallopian tube) facilitates egg transport (cilia and smooth muscle).

Uterus

• Hollow, pear-shaped muscular organ between the bladder and rectum.
• Layers: perimetrium (outer), myometrium (middle smooth muscle), endometrium (inner lining).
• The endometrium thickens during the menstrual cycle in preparation for implantation (if fertilization occurs).
• It sheds (menstruation) if the egg is not fertilized.

Vagina and Cervix (neck)

• A muscular canal that connects the uterus with the external environment.
• The cervix is the narrow portion of the uterus opening into the vagina, connecting the reproductive tract with the external environment.
• Physical barrier between vagina and uterus is a plug of cervical mucous.
• Connects the reproductive tract with the external environment.

Breast (Mammary Gland)

• Lobes (15-20). Divided by adipose tissue.
• Subdivided into lobules.
• Each lobule contains alveoli that secrete milk in a lactating female.
• Tubules converge to form mammary ducts.
• Ducts drain into the tip of the nipple.

Pregnancy Hormones

• hCG is essential for maintaining the corpus luteum during early pregnancy.
• hCG's levels rise, then fall as the placenta takes over progesterone and estrogen production, which are important for sustaining the corpus luteum.
• Progesterone maintains the uterine lining and regulates uterine contractions.
• Estrogen is critical for preparing the body for possible pregnancy and milk production in the breasts, and for the growth of the maternal tissue and the development of the foetal tissues.

Stages of Pregnancy

• Three trimesters (about 3 months each).
• The zygotes undergo multiple rounds of mitosis to produce specialized cells organized into tissues and organs to create an infant.
• First trimester (0-12 weeks): cellular differentiation to form organs, embryo develops paired somite segments, major organ systems, and limb buds form.
• Second trimester (13-28 weeks): foetus increases in size, bony parts of skeleton develop, maternal sensation of fetal movement.
• Third trimester (29-40 weeks): foetus grows significantly, circulatory and respiratory systems mature, maternal antibodies transfer causing temporary fetal immunity to diseases.

Labor and Birth

• Labour is the powerful uterine contractions necessary to expel the foetus.
• Oxytocin and prostaglandins cause labour contractions; they are produced within the uterus and the pituitary respectively.
• Activation of the foetal adrenal cortex initiates labour.
• Oestrogen enhances uterine sensitivity to oxytocin. The placenta cannot synthesise estrogen.
• The process of labour has multiple stages including cervical dilation (first stage), delivery of the baby (second stage), and then delivery of the placenta and membranes (third stage).

Lactation

• Prolactin from the anterior pituitary stimulates milk production.
• Prolactin-inhibiting hormone (PIH) (primarily dopamine) from the hypothalamus controls prolactin secretion.
• Oestrogen and progesterone inhibit milk secretion during pregnancy, except for colostrum.
• Suckling stimulates sensory endings that inhibit PIH and stimulate oxytocin secretion from the posterior pituitary, producing the "milk ejection reflex."
• Oxytocin stimulates muscle contractions causing milk ejection (the "let-down" reflex).

Other

• Ovarian steroidogenesis: cholesterol converts/pathways (Delta 5, Delta 4) for different reproductive hormones.
• Steroidogenesis: conversion process of cholesterol to other compounds, including ovarian hormones.
• Fetal Membranes and Placenta: membranes surround the embryo and the placenta facilitates nutrient, gas and waste exchange.

Additional notes on specific topics.

• Contraceptives: methods of birth control to prevent pregnancy & disease. Contraceptives can be surgical (sterilization) and/or hormonal, and/or barrier methods.
• Implantation: the process of a blastocyst attaching to the uterine wall. Factors (like hormone levels & enzymes) affect this process and uterine environment.
• Placenta: organ of exchange between mother and foetus during pregnancy, supplies nutrients and oxygen and removes waste products. It produces several hormones to support pregnancy.
• Amniotic fluid: fluid surrounding the foetus within the amniotic sac, helps sustain and protect the developing embryo.
• Hormones: hormones coordinate various physiological functions during the menstrual cycle & pregnancy (e.g, FSH, LH, GnRH, estrogen, progesterone, hCG, hCS...). hormones have specific target tissues.
• Menstrual cycle is affected by the hormone levels that promote growth and/ or the shed of the uterine lining.
• Pregnancy: process of carrying a developing embryo or foetus in a mother's uterus until birth. Involves critical hormonal regulation and exchange between mother's & foetus' blood systems.
• Human chorionic gonadotropin (hCG): maintaining the corpus luteum to keep oestrogen & progesterone levels high during early pregnancy

This detailed summary covers the core concepts discussed in the provided OCR output. It is structured to facilitate learning and recall, with a focus on key terms, processes, and relationships.

Explore the key concepts of mammalian reproduction, including chromosome sex determination and the roles of testosterone and embryonic development. This quiz covers the intricacies of male and female chromosomal configurations and their implications on reproductive biology, emphasizing fertilization and fetal development.