Week 8

• FSH release from the pituitary is regulated by inhibin produced by cells in the ovarian follicle
• • The Testes are paired glands which may be outside the abdomen in scrotum.
  • Alternatively, they appear in abdomen and descend (via inguinal canal) into scrotum towards end of fetal development
  • In some cases, there is cryptorchidism / testicular maldescent
  • It is made of seminiferous tubules, which contain Sertoli cells and maturing sperm cells
  • Packed in between the seminiferous tubules are interstitial/Leydig cells
• Testis have dual functions with Leydig cells involved in testosterone synthesis, which is controlled by LH and Sertoli cells in spermatogenesis, under control by FSH.
• • Leydig cells synthesise testosterone while Sertoli cells synthesise and secrete inhibin and androgen binding protein (ABP)
  • Both cells produce paracrine growth factors (particularly members of TGFβ superfamily), which are inhibin (αβA or αβB) to suppress FSH and activin (βAβA or βBβB or βAβB) to increase FSH
  • Anterior pituitary gonadotroph cells synthesise and secrete gonadotrophins, luteinising hormone (LH) and follicle-stimulating hormone (FSH), which are Heterodimeric glycoproteins, of weight 25-36 kDa
• Gonadotrophins have a common gonadotrophin α-subunit (GSU) along with specific LHβ and FSHβ subunits
• LH stimulates testosterone biosynthesis in Leydig cells, while FSH stimulates Sertoli cells to support spermatogenesis as well as synthesise and secrete growth factors (e.g. inhibin)
• • The major endocrine stimulus to testosterone biosynthesis is LH, which acts via luteinising hormone/chorionic gonadotrophin receptor (LHCGR)
  • This is coupled to GS-AC-cAMP-PKA-CREB, involves acute stimulation of StAR-TSPO-VDAC, CYP (+ HSD) enzymes and chronic upregulation of StARD1 and CYP (+ HSD) genes
• • Biochemical Pathway for Testosterone Biosynthesis starts with mitochondrial uptake of cholesterol.
  • This is oxidised to pregnenolone by CYP11A1, and there are 2 options for downstream metabolism of pregnenolone.
  • The predominant pathway is species-specific, where the Δ5 (‘classic’) pathway is in humans, but Δ4 pathway is in rodents
• • The endocrine actions of testosterone are modulated by pre-receptor metabolism, where SRD5A2 reduces testosterone to DHT and CYP19A1 oxidises testosterone to E2
  • In liver, testosterone metabolised by SRD5A1 and 3αHSD/3βHSD to saturated androstanediols.
  • This is further metabolised by steroid sulphotransferase or glucuronidase
• Germ cell development in spermatogenesis is divided into five successive stages.

  1. spermatogonia A before becoming B.
  2. primary spermatocytes (leptotene, zygotene, pachytene and diplotene).
  3. secondary spermatocytes (diakinesis).
  4. round spermatids become elongated
  5. spermatozoa
• Meiosis in males starts at puberty (spermarche) and continues throughout adult life
• • Complete spermatogenesis and spermiogenesis depends on FSH + testosterone/DHT
  • FSH is more important in early stages of meiosis (via Sertoli cells)
  • Androgens more important in later stages (RS) and spermiogenesis
• • Spermatozoa still need to acquire motility and the ability to fertilize oocytes.
  • Both are acquired as sperm pass through the caput and cauda epididymides
  • So, sperm acquire “decapacitation factors”; added during storage (in cauda epidydimidis) but removed before fertilisation (in female tract)
• Ovarian glands secrete estradiol (E2) via secondary follicles of granulosa cells and the corpus luteum produces progesterone (P4)
• • GnRH secretion is pulsatile of 1 pulse/hr entrained by GnRH pulse generator to episodic secretion of LH & FSH
  • This also avoids desensitisation of GnRH-R, LHCGR & FSHR
• • The Ovarian Cycle starts with primordial follicle becoming primary follicle before secondary follicle (pre-antral/antral)
  • This is prior to Oocyte, granulosa, theca cells and pre-ovulatory follicle (Graafian follicle)
  • Then, ovulation occurs, involving release of cumulus-oocyte complex (COC)
  • This is before becoming Corpus luteum (luteinized granulosa + theca cells) and then Corpus albicans
• • Folliculogenesis starts with primordial follicles, which are oocytes + single layer of granulosa cells (GC)
  • This is before pre-antral follicles, which are oocytes + multiple layers of GC
  • This is prior to antral follicles, which are multiple layers of GC & theca cells + antra
  • Finally, Graafian/ovulatory follicles are formed, which are multiple layers of GC & TC and single large antrum
• • Estradiol (E2) exerts negative feedback on GnRH, LH & FSH and acts on endometrial cells and breast tissue as a mitogen to prepare endometrium for embryo implantation
  • It has diverse endocrine actions on organs like bone, brain and the whole cardiovascular system
  • It acts locally in follicle to upregulate LHCGR expression
• Endocrine support of follicle development and unction

  • Primordial and primary follicles are gonadotrophin-independent
  • Early antral follicles respond to FSH, Late antral follicles respond to both FSH (granulosa) and LH (theca), while ovulatory follicle responds to just LH
• Changing gonadotrophin responsiveness of follicles

  • Dominant follicle most responsive to FSH involves highest expression of CYP19A1, to make more E2
  • Negative feedback suppresses FSH drive to all follicles, so only follicle with highest expression of FSHR and LHCGR remains viable to ovulation
• Meiosis in the Female Initiates in utero and then arrests at/after menarche, where cohorts of oocytes recruited to develop monthly until menopause
• Endocrinology of Ovulation

  • Midcycle LH surge triggers ovulation of COC at +36h, which drives resumption of meiosis in prophase I oocyte
  • The Oocyte undergoes cytoplasmic and nuclear maturation
  • LH triggers exit from prophase I and progression through to next meiotic block at metaphase II
  • Midcycle LH surge even triggers follicular rupture to upregulate expression of proinflammatory cytokines and PTGS-2.
  • This catalyses synthesis of pro-inflammatory prostaglandins and leukotrienes, causing hyperaemia and weakens follicle wall
  • This upregulates expression of matrix metalloproteinases (MMP2 & MMP4)
• Functions of Progesterone (P4)

  • Exerts negative feedback (GnRH, LH & FSH) to alters viscosity of cervical mucus to block sperm passage across cervical os.
  • Inhibits endometrial proliferation and stimulates decidualisation-differentiation of endometrial cells
  • Increases blood flow through spiral arterioles and stimulates nutrient secretion from endometrial glands
• • Gonadotropin-releasing hormone (GnRH) is secreted in pulsatile rhythm, where parvicellular GnRH neurons in the preoptic area of hypothalamus have intrinsic pulse generator
  • Then, GnRH transported in hypophyseal portal circulation to activate GnRH receptors (GnRH-R) to stimulate expression of GSU, LHβ & FSHβ (via Gs-AC-cAMP-PKA-CREB), secretion of LH and FSH (via Gq/11-PLC-IP3-Ca2+)
  • But, GnRH-R is prone to desensitisation /down-regulation
• • Timing of sexual maturation is highly correlated in families, twins and ethnic groups
  • An estimate of 60-80% of pubertal variation being determined genetically
  • Attempts to discover important genetic determinants are via GWAS and investigation of disease models of absent puberty (HH)
• Ovulation of the Cumulus-Oocyte Complex (COC): Release of COC triggered by "mid-cycle" LH surge, GC and TC remnants of follicle form the corpus luteum (CL)
• Sperm undergoes capacitation and moves to the ampulla for fertilisation.
• Sperm penetration triggers completion of meiosis and cortical reaction as a block to polyspermy.
• The zygote undergoes mitotic divisions and forms a blastocyst with two cell populations: trophoblast and inner cell mass (ICM).
• The trophoblast, comprised of cytotrophoblast, syncitiotrophoblast (ST), and extravillous trophoblast (EVT), contributes to the placenta.
• The syncitiotrophoblast (ST) covers the chorionic villus, acting as a cellular barrier between maternal and fetal circulations.
• The extravillous trophoblast (EVT) invades the uterine decidua and spiral arterioles to remodel the maternal-fetal interfaces.
• The "Two Cell - One Gonadotrophin" model of luteal steroidogenesis involves the theca-luteal cell and granulosa-luteal cell.
• Maternal recognition of pregnancy requires the synthesis and secretion of an "LH-like hormone" by the syncytiotrophoblast (ST).
• Human chorionic gonadotropin (hCG) is similar to LH and plays a role in stimulating progesterone output and rescuing the CL.
• hCG stimulates the CL via the LH receptor (LHCGR) and its levels are elevated in the first trimester.
• Hyperemesis gravidarum is associated with the first trimester of pregnancy.
• hCG levels are hyperelevated in twin pregnancies, embryonic pathologies (trisomy 21), and choriocarcinoma during the first trimester.
• Progesterone, produced by the corpus luteum (CL) and placenta, maintains uterine quiescence during the first trimester.
• Placenta is an incomplete endocrine gland that requires the exchange of steroid substrates between the placenta, fetal tissues, and maternal liver to synthesize estrogens.