VetPrep Gametogenesis and sex differentiation PDF

Reproductive system L. Segabinazzi 2024 DIESTRUS Goals Remind you about cell division Meiosis x mitosis Understand the hormonal control for fetal sex differentiation Understand the defeminization of the hypothalamus Understand oogenesis and spermatogenesis Endocrine control of the estrous cycle Horm...

Reproductive system L. Segabinazzi 2024 DIESTRUS Goals Remind you about cell division Meiosis x mitosis Understand the hormonal control for fetal sex differentiation Understand the defeminization of the hypothalamus Understand oogenesis and spermatogenesis Endocrine control of the estrous cycle Hormonal feedbacks Understand feedback and supraphysiological stimulation “downregulation” Fertilization Maternal recognition of pregnancy in different species Vet Prep MM Sex differentiation and gametogenesis L. Segabinazzi 2024 Oogenesis Somatic cells Pairs of homologous chromosomes Each chromosome has one chromatid X Y Oogenesis Mitosis Chromatids get separated Generate 2 diploid cells (46 chromosomes) with 46 chromatids 2n & 2c Two identical daughter cells No crossing over Meiosis Special form of cell division that reduces chromosome Creates 4 haploid daughter cells Each genetically distinct Crossing over (Prophase I) Homologous recombination Not at random points: chiasma (chiasmata) Introduces variation into sexual reproduction Vet Prep MM Meiosis I Homologous chromosomes get separated Generate 2 haploid cells (23 chromosomes) but 46 chromatids N & 2C Meiosis II Chromatids get separated Generate 2 haploid cells (23 chromosomes) with 23 chromatids N&C Early embryo development Several mitotic divisions Early divisions occur without increase in cell mass “Cleavage” (or “Reduction Division”) Metabolic support provided by maternal secretions Zygotic protein synthesis begins at 2 to 16-cell stage depending on species (Totipotent cells) Totipotent – capable to give rise to any tissue Vet Prep MM Early embryo development Vet Prep MM An important event during embryo life is the differentiation from the morula to the blastocyst stage. During the morula stage, the cells differentiate into two groups of cells Inner cell mass (ICM) – polarized cells in the “Blastocyst” that will form the embryo Trophoblast – will develop the chorion (placenta – external portion) In addition, the cells from the trophoblast “pump” sodium into the intracelular space and because of the osmotic pressure, the embryo is filled with “water”, which forms the “Blastocele or Blastocoele”. Blastocele will later become the yolk sac. Embryology Vet Prep MM Embryo starts as a mass of cells that eventually form layers and will differentiate into the embryo proper and placenta. External genitalia Placental development Vet Prep MM The trophoblast is the external embryonic tissue and gives rise to the chorion (placenta). Extraembryonic mesoderm develops the allantois. Expansion of the allantois and fusion withthe chorion forms the allantochorion (placenta). Embryology: Sex differentiation Genetic (chromosomal) sex Zygote Gonadal sex differentiation ~6-8th week Phenotypic sex ~12th week Primordial germ cells migrate from outside the organism (yolk sac) into the organism through the hindgut to the undifferentiated gonad just within the dorsal body wall (also known as the genital ridge). Embryology: Sex differentiation Initially indistinguishable (about 6 weeks in large domestic animals) Reproductive system develops at the same time as the renal system. Vet Prep MM Mesonepfric = Wolffian ducts Paramesonephric = Mullerian ducts Embryology: Sex differentiation Vet Prep MM Embryology: Sex differentiation ✔Testis determining factor Testis development (Sertoli cells) ✔AMH Development of the Leydig cells – Testosterone – Male duct system Vet Prep MM Embryology: Sex differentiation Absence of ✔Testis determining factor ✔AMH ✔Testosterone Vet Prep MM Oogenesis Before parturition, the fetus’ oogonia (2n) starts meiosis I – gets arrested in Prophase I (duplication of DNA – 2n & 4C) Embryology: Sex differentiation Vet Prep MM Hypothalamus: Male vs. Female Female hypothalamus contains 2 functional areas for secretion of GnRH Tonic (arcuate ventromedial region; ARC) and Surge (preoptic area; POA) centers Hypothalamus is inherently female Testosterone during development “defeminizes” the brain Testosterone crosses blood-brain barrier and is converted to Estradiol Estradiol defeminizes the hypothalamus, eliminating the surge center Fetal ovaries produce Estradiol, but this does not cross the blood-brain barrier because it is bound to alpha-fetoprotein (@FP) Oocyte developmental events Vet Prep MM Most mammals are born with oogenesis at the beginning of Meiosis I – Primary oocyte Oocytes get arrested until puberty in prophase I (2N & 4C) Postnatal oocyte development Primary oocyte (Meiosis I) Duplicates the DNA (Interphase) Chromosomal complement is 2N & 4C Crossing over (Prophase) Meiosis resumed at the ovulatory surge of LH (Puberty) Meiosis I results in a Secondary oocyte (n & 2C) and a polar body (will degenerate) Polar body Oocytes Vet Prep MM Polar body Formed after meiotic divisions Important for reducing the genetic material Embryo development requires the oocyte’s cytoplasm Polar bodies degenerate Meiosis I – First Polar body Postnatal oocyte development Vet Prep MM Secondary oocyte (Meiosis II) The secondary oocyte is the cell that is formed by meiosis I in oogenesis. Thus, it has only one of each pair of homologous chromosomes (after crossing over). However, each chromosome still has two chromatids, making a total of 46 chromatids (2C). The secondary oocyte continues the second stage of meiosis (meiosis II), however, gets arrested at metaphase II; until they meet the sperm in the fallopian tube. Ovulation occurs during the second Meiotic division Meiosis II is completed at the time of fertilization. Results in an Ovum (n) and a polar body (will degenerate) Ovulation Release of the oocyte from the follicle Meiosis continues after fertilization (blocked at metaphase II unless fertilized) Fertilization occurs in the ampulla of the uterine tube Oocyte and cumulus mass moved into the infundibulum of the uterine tube by fimbriae Short window for fertilization (6-12 h) Vet Prep MM Fetal life Vet Prep MM Spermatogonium development in fetal life Primary germ cells originate near the yolk sac and migrate to the developing ovary (genital ridge) Differentiate to spermatogonium and multiply by mitosis Diploid cells (2N). Male mammals are born with spermatogonium that can undergo multiple mitosis ✔Testis determining factor ✔AMH ✔Testosterone Spermatogenesis Vet Prep MM Sperm function requires development and maturation through the testis and epididymis Temperature Overall 60ish days Lower than body temperature (34 °C/93 °F) Hormone-dependent Testosterone Spermatocytogenesis Formation of spermatid from spermatogonia Spermiogenesis Morphological change of spermatid into spermatozoa Spermatogenesis Vet Prep MM Inside of the seminiferous tubules First haploid cell – secondary spermatocyte Spermatid is the last cell after cell Division Spermatids undergo morphological changes to become a spermatozoa Spermatogenesis Vet Prep MM Outside the seminiferous tubules Inside the seminiferous tubules Sertoli cells - a "nurse" cell of the testicles that is part of a seminiferous tubule and helps in the process of spermatogenesis – produces inhibin and estradiol; Phagocytose residual cytoplasm Spermatogonium - undifferentiated male germ cel (2n) Blood testis barrier (Lamina propria) - Prevents autoimmune response to sperm Leydig cells - primary source of testosterone or androgens in males. Sperm cell Head Acrosome Miedpiece Tail Vet Prep MM Spermatogenesis Epididymis Transit takes about 2 weeks Sperm acquire capability for motility and fertilization Sperm are stored in tail of epididymis Vet Prep MM