GAMETOGENESIS AND FERTILIZATION

“Sexual reproduction is…?”

the masterpiece of nature”

What is the line of Erasmus Darwin

“Sexual reproduction is… the masterpiece of nature”

In mammals and flies, an individual’s sex is determined by the

chromosome

In mammals and flies, an individual’s sex is determined by the chromosome. But some are determined by the

environment

Types of sex determination

Chromosomal Sex Determination

Environmental Sex Determination

In vertebrates and arthropods, sex is determined by

chromosomes

XX gonadal cells activate the

Wnt pathway

XY gonadal cells activate the

Sry pathway

The testis will form what cells?

Sertoli cells and Leydig cells

two gonadal rudiments appear and remain sexually indifferent until week 7

Week 4

germ cells migrate into the gonads

Week 6

subset of mesodermal cells initiate their differentiation into Sertoli cells (if the fetus is XY)

Week 8

What blocks the development of the female ducts (in XY). Also, secreted by Sertoli cells.

AHM

This will develop to become the seminiferous tubules.

Sertoli cells

During Week 8: organization of testis cords from SC. TC will develop to form

rete testis

TC, undergo several rounds of proliferation, then arrest at mitosis then TC will mature to become the

seminiferous tubules

Germ cells migrate to the periphery of these tubules where they establish the?

spermatogonial stem cell population

Those that did not form the Sertoli cells will differentiate into

Leydig cells

Germ cells enter the gonad are organized in clusters or cysts surrounded by

pre-granulosa cells

Secondary sex determination in mammals occurs in two phases

within the embryo and during puberty

In females, this becomes differentiated into the clitoris

genital tubercle

In females, the genital tubercle becomes differentiated into the clitoris and this becomes into labia majora

labioscrotal folds

In males, coordination of the male phenotype involves the secretion of two testicular factors:

AMH and Testosterone

the gene related to doublesex, appears to initiate the testes determining cascade in many fish, amphibians, and birds.

Dmrt1

What regulates Dmrt1?

regulated by the removal of a particular methyl group from nucleosomes on its promoter, a reaction catalyzed by the enzyme KDM6B.

The seminiferous tubules contain two types of cells:

spermatogenic and sustentacular cells

Stem cells; develop from primordial germ cells that arise from the yolk sac and enter the testes during the fifth week of development

Spermatogonia

joins tight junctions to form an obstruction

Sustentacular cells

Sustentacular cells: joins tight junctions to form an obstruction known as the

blood-testis barrier

Leydig cells; secretes testosterone

Interstitial cells

It begins which is a diploid (2n)

spermatogonia

Spermatogonia undergo developmental changes and differentiate into

primary spermatocytes (2n)

Primary spermatocytes will undergo meiosis I and form

secondary spermatocytes (n)

Secondary spermatocyte will undergo meiosis II to produce four haploid cells called

Spermatids

Spermatids mature into individual

Sperm cells

A sperm contains two main parts:

Head and tail

contains a nucleus of a sperm with 23 condensed chromosomes

Head

a cap like vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte

Acrosome

In the tail part, this is the constricted region just behind the head that contains centrioles

Neck

In the tail part, this contains mitochondria arranged in a spiral which provide energy for locomotion of sper

Middle piece

In the tail part, this is the longest portion of the tail

Principal piece

In the tail part, this is the terminal, tapering portion of the tail

Endpiece

At puberty, certain hypothalamic neurosecretory cells increase their secretion of

gonadotropin-releasing hormone (GnRH)

This hormone in turn stimulates gonadotrophs in the anterior pituitary to increase their secretion of the two gonadotropins,

LH and FSH

stimulates the secretion of testosterone

Luteinizing hormone (LH)

indirectly stimulates spermatogenesis

Follicle-stimulating hormone (FSH)

Dihydrotestosterone development of the external genitals stimulates • Testosterone is also converted in the brain to estrogens

Prenatal development

traits that distinguish males and females but do not have a direct role in reproduction

Secondary sexual characteristics

The uterus consists of three layers of tissue:

perimetrium, myometrium, and endometrium

• Middle layer

• Three layers of smooth muscle fibers that are thicker in the fundus and thinnest in the cervix

• Coordinated contractions help expel the fetus from the uterus

Myometrium

• Highly vascularized and has three components

• Divided into two layers: stratum functionalis and stratum basalis

Endometrium

Branches of the internal iliac artery called

uterine arteries

branch off from the uterine arteries; arranged in a circular fashion in the myometrium

Arcuate arteries

branch off from the arcuate arteries and penetrate deeply into the myometrium

Radial arteries

supply the stratum basalis with the materials needed to regenerate the stratum functionalis

Straight arterioles

supply the stratum functionalis and change markedly during the menstrual cycle

Spiral arterioles